WHAT IS THE "RIGHT" COMPRESSION RATIO FOR PUMP GAS?
 

I have to make my rebuild decisions soon. What I want is a stock rebuild for my 400 cid. We all know the stock 10.25 CR is too high for today's gas . . . . but if I go for say a 9.5 CR, will that be too high, even for 93 octane?

I am supposed to tell my engine builder what I want, but I am quite sure that I don't know what it is that I'm supposed to tell him.

I'm not even sure what questions I should be asking in this post. Do I want the highest CR I can get away with on pump gas? If yes, what is it? Do I pick a lower, safer CR that I won't have to worry about? If yes, what is it? How high is "too high?" And is there such a thing as going for a CR that's "too low?"

Next to cam selection (totally confusing), this is my most frustrating issue. I don't really care about performance as much as streetability and reliability.

Thanks for any and all opinions . . . . .

Your cam selection will have an influence on what will be a safe cr. For a street car, no reason to push it.

What heads are you planning on using? Any porting? I'd pick the cam to match the airflow of the heads and then match the cr to the camshaft and available gas. Picking the cr first is a cart before horse proposition.

They had compression as low as 7.5 to 1 on some engines in the mid to late 70's:gag:. I believe they were expecting even worse gas than they wound up with. But I think 9.5 to 1 should be fine with 93 octane if you have a good cooling system, and you got a good tune on the engine. An old "rule of thumb" used to be run the same compression as one tenth of the octane you were going to use, but there are still a lot of variables. Examples-8.9 to 1 compression for 89 octane, 9.3 to one for 93, etc. I have ran about 10.0 to 1 for years, with a mild cam, but I have a good cooling system, and I still have to mix in some octane booster, and have my "total" timing "locked out" at about 32 degrees. Im getting sick of this after about 24 years, as gas seems to be getting even worse, so Im having Mr P. Body put me a 455 together meant to run on 89 octane. He is using a slightly higher compression ratio than 8.9 though. Cam duration affects this as well, and I beleive you can get away with a little more on 400s than 455s (less swept volume)(stroke). Hopefully one of the "builders" will chime in tommarow to shed more light on the subject.:) Just offering up some ideas. Good luck with your build, and dont forget to install the best parts your budget can allow in the short block for years of troublefree service.

my 455 is 9.2:1 runs fine on regular but i use plus just because i think the cheap gas is just that...cheap.
my durango's check engine light comes on whenever i use regular. i dont wanna gack up my new 455.

agree.

My first 455 was 10 to 1 with iron heads, zero issues anyplace on pump fuel, even 89-90 octane.

I increased the CR on that engine to 10.48 to 1 with KRE heads, custom HR camshaft, zero issues anyplace.

The current engine is 11 to 1 and runs easily on 89 octane. I've even put 87 octane in it and no pinging anyplace.

For the average Pontiac "street" engine, no need to put a lot of compression into it unless you have goals for the project that include running really quick at the track, and you plan on taking the needed steps to effectively manage higher compression on pump fuel.

The cam is equally as important as the static compression ratio. The 9.5 to 1 limit we keep seeing regirgitated on this website is absolutely a retarded statement, IMHO.

I've had engines in here to tune at 9.5 to 1 that rattled like SLEDGEHAMMERS just about everyplace because of a poor cam choice.

I've built engines over 10 to 1 on iron heads and some of them are out there over a decade now with ZERO issues anyplace on a steady diet of pump gas.....Cliff

My 68 GTO has Iron Ram Air heads with 38cc Dished pistons for a likely 9.0:1, Cam is per signature. WOT on 87 Octane is clear.

I plan to add mild BOOST and deal with any ping using higher Octane.
Therefore any less Compression is BAD. Unless you plan for full-time BOOST
Any more compression is up to you, but NO BOOST.

Heads are stock 75.7cc #13s, I know cc will change after machining, head gaskets, etc.

I don't mind buying 93 octane, I don't drive it that much, but I did hear they are doing away with 93 and going to 92.

So I would shoot for midgrade 89 to premium 92, with no boosts, and not try to run on 87 because of F Rock's comment.

Cliff, I am scared of a 10 to 1 compression, how can you run a 10 to 1 compression on 87 octane? Is the answer tuning? Cam? Please explain . . . . .

9.5-1 is a good compression to run with 93 octane.

8.8-1 for 87 octane


and be safe 90% of the time with Iron heads.

ALUMINUM HEADS I HAVE USE 10.5-1 AS A SAFE POINT, BUT 11-1 IF THE TIMING CURVE AND CAM CHOICE IS OPTIMAL


And just because you "can't" hear pinging, doesn't mean it isn't happening. have torn a few down with customers saying they never heard any pinging and damage was evident.

Pressing "the edge" is fine for a racer in a specific class, needing every "inch-ounce" of power. For a daily driver, it's not necessarily prudent to run "the limit". Fuel quality can vary from one geographic location to another.

IMO, the 3 or 4 HP you may not "gain" is irrelevant in the "big picture".

We keep engines under 9.5:1 with iron heads, so you can go anywhere and have confidence the fuel won't "kill" your engine.

FWIW

Jim

Right on the dyno we made 424hp/465tq with a Pontiac 400 with unported #16 heads. The compression ratio was just over 10 to 1, and we used a 228/235/112 camshaft. That engine was using a stock iron intake and Q-jet.

KRE did the same build with a set of their 65cc aluminum "D" ports and made 440HP (unported heads).

Drop the same engine back to 9.5 to 1 and put in a smaller cam and you'll be hard pressed to make 400hp, probably closer to 350-375.

One of our customers did the ever popular 400 cid with heavily cut 6X heads and the XE274 camshaft and made 380hp and close to 425tq. Still PLENTY of power for a decent burnout in a hard right hand turn leaving Dairy Queen!

As you can see, there is a modest power difference with more compression and the right camshaft......Cliff

Fwiw, I'm running unported 16's on an .030 400 with a 744 cam (224/236 @.050" 115.5 lsa) with no detonation on pump gas. Right around 10:1 compression.

There's no magic number as if you put the wrong cam in, it can detonate at 9.5:1. If you have mildly ported heads and a larger cam, it can run fine at 11:1.

What are your constraints? So far we have unported d-ports on a 400. What's the intake, transmission, rear gearing, etc? Or have you yet to select them? All of those factors will determine the rpm range of the engine, which will then influence camshaft selection, which will then determine your safe CR.

This might be enough to confuse you but, its an explanation of how things coexist in that realm.

http://www.thepontiactransampage.com...blueprint.html

"There's no magic number as if you put the wrong cam in, it can detonate at 9.5:1. If you have mildly ported heads and a larger cam, it can run fine at 11:1."

+1

Had a guy bring a GTO with a 455 here once that pinged everyplace but at idle!

It was 9.5 to 1 compression, Comp 224/230/110 hydraulic roller camshaft, 1971 HO heads. One of the WORST running 455's I've ever seen, or worked with.

The cam was simply all wrong for it. Basically WAY too good at cylinder filling at lower rpm's, so it acted like it was closer to 11 to 1 compression than 9 to 1......Cliff

"Pressing "the edge" is fine for a racer in a specific class, needing every "inch-ounce" of power. For a daily driver, it's not necessarily prudent to run "the limit"."

Exactly- why chance it, there are alot of other ways to pick up a few % HP that 1 more bump in compression will give you. Better head flow, cam swap, intake, carb, optimizing exhaust system.

We've had this debate before.

My personal thoughts based on dynoing my RAIV 400 that was 10:1 or so(0 deck TRW, no audible detonation-so all OK right? But it picked up 8 HP adding race gas-so it must have been detonating.

That's for sure. My buddy used to brag about running his iron headed 355, at 10.5:1, on 87 octane. He couldn't hear any pinging. So, I suggested looking at his plugs with a magnifying glass. "Wow, look at all the pretty silver aluminum specs on your plugs!" He runs 93 octane now.

If you are not worried about performance as much as playing it safe,just put the CR at 8.5 or so, and run it on regular 87 octane. JMO

BOBZDAR, I'm running a TH400 with 3.55 rear gears, I plan on using the stock heads, crank, and intake and exhaust manifolds.

But I am going to try to install a lower numerical (2.93) rear gear if I can before the rebuild.

Will use Sealed Power forged pistons for dishing, per PONTIAC DUDE's recommendation.

Still can't decide on rods, both sides continue to present persuasive arguments.

Cam will be final decision, based on the above?

All I want is streetability - would like to pull away hard from a light, and pass on a two-lane highway - just like they did when they were new 40 years ago.

These were pretty awesome cars in 1970, and I would be very happy to get back to 350 hp and 445 lbs/torque, those were pretty impressive numbers in 1970, and I think they are still pretty impressive even today for any car, new or old.

The problem is today's unleaded 91 or 92 octane v. 1970's leaded high-octane gas. Which leads me to my compression question.

Well, not to confuse things too much but I have a '67 firebird 400 with stone stock unrebuilt engine and about 120k miles with no detonation on 93. Pontiac used cams with wide lobe separation so in completely stock trim they can work on pump gas, but barely in some cases.

If you plan stock converter and shift point, I would run a crower 60916 or similar, 10cc dish just so you can run 91 if you can't find the good stuff and go have fun. Don't bother swapping rear gears, that car will run all day at 80mph and not break a sweat with those gears and a 27" tall tire. A 400 can operate at a higher rpm than a 455 with no added stress (and needs to to make the same power).

I would seriously consider an exhaust manifold upgrade or headers, the factory log manifolds will be your limiting factor - which is what came on a 350 horse engine in '70 I think.

I'm sure you'll get some differing opinions, but that's what I would do.

I used to run Hi-Compression, with all sorts of engine tweaks, but came to realize the Street/Strip is WAAAY better being compatible with the cheapest gasoline (87 Octane).

I simply cannot recommend the numerous tweaks that allow 11:1 with 93 Octane @ 25-30cents more/gallon, to be ping-free on Tuesdays& Saturdays, with audible-ping on humid days & rushhour sunsets. My recent adventure back into Hi-Compression was met with pulling the engine after Break-in for a single rod bearing failure. I could say it's too fickle, but I'm CERTAIN the gasoline is different than 10years ago, & 20 years ago.

BOBZDAR, forgot to mention tire size, I'm running 245/60/15s, 26.6" tall. I also have factory AC.

My reasoning for the 2.93 gears is not to save gas money, it's to keep RPM down, I go 60-65, it's pushing 3,000 RPM, that seems like a lot of needless wear and tear to me. 2.93s would reduce RPM by about 18%, or 1-(2.93/3.55).

In looking at the 1970 sales brochure for the GTO/Grand Prix - the 2.93 was standard in the GP with the 400 with air and TH400. They obviously had a reason for this, was it because the GP was more of a highway cruiser than the GTO, whereas the GTO was built for the quarter-mile?

My car left the factory with 3.23s because of the AC, I haven't a clue how it ended up with the 3.55s.

I am so appreciative of everyone's comments and viewpoints - every comment makes me think about something I had not considered before, that's the purpose of the Forums, right?

I'm just trying to lay everything out and think everything through, so I can come to a logical, sensible, decision.

Yes, I do want to be safe, no, I don't want to push it to the limit and have to worry about detonation, etc. So, I don't want to be anywhere near 10 to 1. BOBZDAR suggests 8.5 to 1. BOBZDAR, I'm going to throw this out to everyone and see what their take is. The advantages seem clear, but what are the downsides?

I'd comment some on this.

But these sorta "what CR ?" threads rarely ever go anywhere worthwhile or productive.
:sorry:

One side says do this,other side says do that,neither side will budge an inch off their position,the battle lines are drawn,warning shots are fired back & forth.

And eventually it ends up becoming an all-out p!ss!ng contest...

It easy as can be to tell someone else to "do this".

It's much harder to make sure that said fella does absolutely everything that will be needed to make such work as was intended.

The difference is,,,there is zero risk involved in just telling someone to "do this"...

And there is nothing but risk involved in actually doing such.

I can tell ya'll to go jump off a building all day,see that's pretty easy,and it's absolutely no sweat off my back.

But it's going to be a whole lot riskier for you guys to go out there and actually do such.

Especially if I forget to remind you to use a parachute as well... :doh:

(please dont try this @ home kids)

LOL

Anyhow,if I were gonna build a 400 from the ground up with 74ish cc heads,at the minimum I'd probably be using the KB/Icon 14cc dished forgings myself.

:2cents:

Bret P.

So...is it longer duration, wider lobe separation, lobe profile or amount of overlap that enables higher compression engine to burn 89 or 93 octane? My engine in my Bird right now is .030 455 w/#62 heads ... i know my cam is .503 Engle from 1984, either 106 or 108LSA, and would love to try somethin else and maybe street drive this engine some...

What ^^^ he said.

Keeping the compression at 9.5 is perfectly fine for the stated use: Street car. You may give up 15-20 peak HP, but so what? Its a fun cruiser, not a performance street/strip car.

Don't worry about overreving the 400. 3.23s would work great in your car. I run 3.90s and my 400 loves 3000-3500 RPM. Sucks gas though.

The closer you run to 10:1, the more you have to bird-dog the tune. At 9.5 you can make a tuning mistake, discover it and change settings without much harm. At 10.5, a significant tuning mistake means you're sweeping your motor up with a broom.

In my mind, the two schools of compression thought break down like this:
The experienced tuners (and those builders with sharp customers) promote high compression because they're good enough to make it live and get big HP.

The hobbyists with less experience (or more caution) or those that have to stand behind the warranty promote lower compression, because it will result in less come-backs and damaged engines.

Anybody that is not a shark with engine tuning (or have one on speedial) should stay below 10:1 compression. Under 10:1 compression and a correctly chosen cam WILL live longer in the hands of a novice, IMHO.

And whatever you do, please buy some good forged rods!! I can't tell you how many twisted/broken connecting rods I've seen come out of MILD engines.

I can vouch for a low SCR in an A-Body 400. I must have read many of the SCR threads. Bottom line, after many cam threads and talking to everyone, ended up with 8.2-8.5 SCR. 256/262 cam with a Four speed 3.11 first gear with 3.23's in the rear. I run 87 octane everywhere, car never goes higher than 195* in 100* weather and never pings. Plugs are nice and brown they way they are supposed to look. I reguarlly go through a right rear tire!!!

There a lot of benefits to low SCR. I could run any amount of timing I want!! I run 18* initial and I dare you to find a car that starts faster than mine with no choke!!!

HTH,
Dave

Well,it sure aint overlap by itself,that happens @ the end of the exhaust stroke & the begining of the intake stroke,so nowhere near when the cylinder actually begins compressing the intake charge,the only way overlap can affect that is by it's influence on clearing the exhaust charge and initiating the intake charge,but that mostly affects charge density and such,so that mostly just affect the power output more so than it affects the cylider pressure and such.

Lotta guys like to focus on the intake closing event,that's mostly because of it's prominence in DCR theory and such,but it aint as simple as that either.

Wider LSA's tend to have later IC events,but not exclusively,that can also be affected by seat duration and installed centerlines as well.

Remember that later intake events from a wider LSA generally also means earlier exhaust events too (this can be very important,and largely misunderstood).

IMO,there is no one single feature of a cam that makes it "high compression" friendly,it takes a number of those features working together to make a cam like that,lots of seat duration,wide LSA for a later IC event (and thus earlier exhaust events too!),a decent I/E split to boost the lost overlap due to the wide LSA as that help exhaust scavenging initiate the intake charge,plenty of exhaust duration to help completely purge the exhaust charge before the intake valve actually opens,yadda,yadda,yadda.

But cams can only do so much on their own,there are a lotta other details that go into making a successful high CR street combo.

FWIW

Bret P.

X2 on Chipheads post. He covered most of what you need to know on there.:) To add one other thing-Hot, Humid climates tend to not be as "compression friendly" either, as Half Inch Stud reffered to.

Thank you, Chiphead, well laid out post, bro! That's the conclusion I think I was coming to, but you get the credit for coming up with it first.

There are so many knowledgeable guys on the boards, their knowledge is amazing, but gee whiz, as soon as you guys get going on "performance" is when I get lost. I am just a novice, I need things explained in A B C terminology.

I have a timing light from Kragen, and I think I have a spark plug wrench and a couple of rolls of friction tape in my tool box. That's about it.

I do understand your connecting rod comment. I am now sold on forged rods.

Anyone out there that thinks a low CR combo automatically = a complete slug has probably never driven a well tuned dead stock 455 H-O equipped GTO or T/A in their life.

They'll easily run with most any of the similarly equipped "dead stock" high compression GTO's or T/A's that were available back then.

Same wide LSA cams,along with the "same" most everything else for the most part,they just dropped the CR for the reduced fuel quality,the difference was like 25 "rated" hp between the '70 GTO 455 (360hp) and the '71 455 H-O GTO (335 hp),and honestly that was nothing that could'nt be recovered with just a little bit of careful tuning and such.

And as someone else commented on,with todays less than perfect fuels,one can often be much more aggressive with the timing curve on that low compression 455 H-O combo vs. the '70 455 high compression combo.

Honestly that '70 455 high compression ratio engine was severly undercammed by most hi-perf. folks standards.

The '70 455 high compression engine used the 067 & 068 cam based on trans (AT/MT)

The low compression 455 H-O also used the 068 cam.

Even the low compression 455 SD used the 744 cam.

So that '70 high compression 455 probably should've used the 041 cam w/1.5 rockers.

But then they would have had to really "fudge the numbers" to get it past GM corporate.

So ask me,it's more than just a wide LSA that dictates cam choice in any CR engine.

It's only when you start building beyond what the factory built,and specifically choosing and selecting a specific parts combo for maximum gains that you'll typically be able to exploit all of these different options & areas of potential gain to maximum effect as guys like Cliff R. talk about all the time.

And even then the results can vary somewhat on a case by case basis,like I said above the devil is definitely in the details,and not everyone gets all those details "just so" like others seem to be able to do with little to no effort.

Often it's a matter of weighing any potential gains against the effort involved in doing such that tends to make some guys approach their CR choices in a more conservative manner.

To many,a street car is'nt looking for every last .10 of ET or MPH.

Once that sorta dynamic becomes the primary driving force for a build,most folks would suggest that it is a race car that's pretending to be a street car.

To many a street car runs what it runs,it's more about having fun than running a number.

FWIW

Bret P.

Gotta agree with Chip and 1967Tempest.

I'm running on 87 octane with my 400 4bbl 4speed and 3.55s, with a CR of 9.5 to 1. No ping or any other problem..... well a small overheating issue tho but that's all and it's not the engine's fault per say. It's that water pump problem that a lot of us are having. Tho it stays cool so far with a 160 thermostat.

Everyone is talking about static compression..except for Cliff that mentioned a cam that was too good at filling a cylinder,everyone should really look at their cranking compression as one of the factors.My current engine is 180 to 185 psi and runs well on 87 octane with about 10.25 for static compression.

The problem with that is the very next engine with exactly the same cranking compression and static CR might be a completely different story.

IIRC your using the KRE-D aluminum heads so that has significant affect on that deal,as does most everything else.

Comparing iron head combos and aluminum head combos in terms of static CR is pretty much comparing apples & oranges,especially when the aluminum heads in question have a far better & more modern combustion chamber design than the OE iron heads have.

There are plenty of combos out there that will barely pump 140 psi and they'll still rattle their brains out,and there are other combos out there pump out over 200 psi,and they dont have even a hint of rattle,so there is no easy answer as to what will always work perfectly,and what wont ever work well @ all.

Even when trying to copy other "known safe" combos the results can be a bit hit or miss for others who choose to try following that very same route.

All it takes is missing one or two (or more) key details that the other guy took care of without even giving it a second thought.

FWIW

Bret P.

You just have to figure out how the cam plays into this deal. At that point it becomes more about the actual compression (dynamic) that the engine sees and not about the static compression ratio.

If you don't take anything else away from this thread, remember my first response:

"The cam is equally as important as the static compression ratio. The 9.5 to 1 limit we keep seeing regirgitated on this website is absolutely a retarded statement, IMHO."

There is little if any difference between a 455 with a 204/214/112 cam in it with an 8 to 1 compression ratio than a 455 with a 234/244/112 cam in it at 10 to 1 compression. At least in terms of cranking compression, compression at low rpms (keep in mind here idle quality and low speed manners follow effective compression at low engine speeds) and the ability of that engine to effectively manage pump fuel on a daily basis.

Sit back for a second and calculate the power production from each of those engines, and convince me the jump from 8 to 1 up to 10 to 1 compression is just worth 6 to 7 percent more power.

Where folks screw this deal up is using a fast ramp cam ground on a tighter LSA, when they should have used a cam with more seat to seat timing, more overlap, and wider LSA instead.

We continue to hear all about how the cam events play into this deal. Meaningless if you look at them on paper (static). You've got to figure the dynamics of the engine process into this deal.

Another factor seldom if ever mentioned is engine RPM's. The N/A engine quickly looses is ability to fill the cylinders as the rpms increase. It will see the most cylinder pressure at or very close to peak torque. Choose a cam that pulls power down to lower rpm's, and narrows up the torque curve, and quess what? You've just dramatically increased the engines octane requirements and potential for detonation, all else being equal.

When you carry an engine to the dyno, and it cranks out 550 ft lbs torque from the start of the pulls clear out past 5000rpm's, with a damn near flat torque curve, the octane requirements for that engine are less than a much less powerful (average power) engine pulling the torque curve down with a BIG spike in peak torque right in the mid-range.

I see this ALL the time, and EXACTLY why the 455 brought to us to "tune" with the 9.5 to 1 CR and 224/230/110 HR camshaft was a TURD everyplace. It was WAY too good at cylinder filling at low rpms. We've built engines with higher compression and larger cams on wider LSA's with ZERO issues anyplace, and they made more power everywhere.

Being an engine builder and DIRECTLY involved with tuning butt-loads of these engines, we've moved to even wider LSA cams and finding even better results. The next cam for my own engine will be bigger, and ground on a 114 LSA. I'm expecting little if any difference in idle quality, low speed manners, or low speed power. I do expect to makes some additional upper mid-range and top end power and not have the car slow down anyplace. Well, it's good thought, we'll see how it works when we get it installed and do some testing?

As always, with this topic, we get PLENTY of theory, and great responses from folks who haven't built an engine in at least 10 years, or had a car running for at least 20.

If you aren't out there building these engines, taking them to the dyno, then installing them in cars and street/track testing, well, your comments are mostly opinion(s), or information you've "Googled" up someplace, or cranked out of your desk top dyno with all the free time you've got that could have been put into actually building and testing one instead.

Like everything else associated with this hobby, to really know what's going on with these things, at some point you've just got to get out from behind your keyboard and get your hands dirty....FWIW.....Cliff

Put the same cam in both engines, then it's only worth 6-7 percent power. The reason the low compression engine would make so much less power is due to it being way under-cammed for the amount of air that the heads can provide.

I've done this, went from 8:1 compression to 10:1 with everything else the same (unported d-ports) except I also went to a factory iron intake and picked up a measly 2mph in the 1/8 on my gtech. That translates to maybe 20hp (or roughly 5-6%)? The problem is with low compression engines people try to cram all the power in the low end so they have a ton of torque and then the engine is an absolute dog when it comes to actually making horsepower.

Pontiac originally used the 041 cam in the sd455 with 8.4:1 compression, a full 2 points lower than it was used previously and didn't lose much power - maybe 20-30hp if you do the gross to net conversion vs. the raIV (raIV was 375net, SD was 310 gross). If they had put an 067 cam in, it would have been down 100hp or so and wasted all the extra flow of the SD heads.

As far as low compression engines go, and I did cram everything into the getting power down low, what would be a dog?
Looking for some kind of baseline for what constitutes a dog or slug.

Thanks,
Dave

To say because Pontiac used a wide lobe sep in their 10:1 motors they were designed for 87 or even 93 octane is way off, what was even regular leaded back in the 70s? I think Sunoco 260 was like 102 at the pump. A little different in a 10;1 than tghe ethanol 87 stuff you might get now.

More food for the fight. LOL.
 

I have had better ET's with tighter LSA then wider LSA's at the track. ;)

Don't know about dyno results. Never raced on one. LOL.

Dunno man, it's relative. I had an 8:1 400 with 204/214 110 lsa cam and it sucked. Upped cam to 224/236 115.5 lsa and it was night and day.

Performance wise it went from sub 90mph 1/4 to 100mph (at 3700lbs or so). So for me I guess it's a dog if it's below 90mph in the 1/4? For somebody running 11's at 115mph they would probably both be a dog. Maybe it's relative to the parts you're using - if you're using unported d-ports, 100mph is probably good, especially at 8:1 compression. If you're using aluminum heads, anything under 110mph in the 1/4 is probably a dog.

If you don't care about 1/4 runs, then I guess if the car can't burn the tires or merge onto the highway with authority, it's a dog?

I see. Cause my low SCR car pulls pretty hard for a heavy car with 200lbs of car stereo stuff and another 100-150 lbs of spare parts, tools, chairs etc for shows. Revs fast and never has an issue hitting the redline.

If I had a cam that was too big, would it take a really long time to rev and then take a long time to get going, similar to towing a car or very under powered?

I ask only because I want to make sure that when I tune and build I make the right choices. Kinda part of learning how to do things the right way.

Dave

I don't think anyone was saying they were designed for that, but I think they're more tolerant of running 93 than if they had a tighter lsa?

Racing wise, a tighter lsa probably works well particularly if you're running a really narrow rpm range - which is Dude's area of expertise, so I think he would know. I know what's worked well for me and what hasn't - and I have 2 cars at 10:1 that will run fine on 93 octane and both have a wide lsa and are using factory parts similar to the OP, but then there's little margin for error. Based on my experience with a shorter duration, tighter lsa cam I know they would detonate themselves to death if that were the choice, I had to run 91 in an 8:1 engine with a small cam and it didn't run very well.

This is why I suggested picking the compression ratio after the rest of the combination was thought out, as the cam can have an influence. It seems like for his purposes and parts a 10-15cc dish will be ideal so fuel quality won't be a concern as 2 or 3 people have suggested. You don't want to worry about it on a street car if you don't have to. I don't know that going lower than that would gain anything other than the ability to run cheaper gas, which wasn't mentioned as a concern.

"I have had better ET's with tighter LSA then wider LSA's at the track."
Pontiac Dude

And Nolan Jamora from Isky Cams....

"The prevailing trend today is to use a wide lobe separation on small-tire cars. It's not uncommon to go as wide as 118 to 120 degrees to make the low-end torque more manageable. These cams still make great peak power, but the differance is where in the rpm range the power starts coming in. On the other end of the spectrum, you don't want to run a wide lobe separation in a heavy car with 14-inch-wide tires. You need the torque down low to move the car out of the hole. Generally, a 108- to- 110-degree LSA works well in street cars since they offer a good mix of low-end torque, top end power, and idle quality."

The same cam tech article has input from Chris Mays, a head engineer at Comp Cams and like Nolan one of the sharpest minds in the industry. Years ago it was Chris Mays that recommended I use a 108 lobe separation in my 462cid combo with my 254 at .050 solid roller in conjunction with my 3700+ lb car with 10-inch tires. At the time I contacted John Partridge at Bullet Racing Cams, he also stated the 108 lobe separation would be fine.

We do???? :confused:

I run one at 10.38:1 with a RA IV cam; runs fine on 93 octane. I run another (non-Pontiac) at 10.70:1 with a stock cam; runs fine on 91 octane, except when pulling the loaded 18 foot trailer. When pulling the trailer, I will get detonation if I don't use 93 octane, but no detonation on 93 octane.

Jon.

Well, your car sounds different than the OP's car, so your choices will probably be different. If it were me, I would probably not be as agressive on cam with a station wagon vs. my trans am or the OP's GTO unless I were trying to race it. My main concern was generating horsepower with the goal of going fast on a road course. I did 1/8 and 1/4 runs to test the engine and get some baseline so I could see how modifications affected performance as it's cheaper and easier than going to a dyno. I also had a 4 speed so had no concerns with torque converter, shift point modification etc, which is why I asked the OP that as well since they play into what you pick for a camshaft. All out horsepower and/or performance is probably not your goal, so you have to look hard at what you want the car to do and what your limitations are (parts wise) before making a decision.

I would post a new thread if you're looking to modify your combination and get some opinions there. When putting a combination together, your limitations (parts wise, comfort wise etc) are as important as what your goals are.

Some might say "running the tight LSA cam caused pinging and switched to wide lsa similar cam and it didnt ping no more" .

This would lead some to belive it was the LSA that did this. Sort of. Actually it was most likely the intake closing point.
Intake closed point determines the number of degrees in compression.

Suppose you have a cam ground on 110 LSA installed at 106 ICL. Has a ping problem. Some would change to a different cam ground say on 112 LSA installed at 108 ICL. Both same/similar duration. Wider LSA and it doesnt ping anymore. Not pinging it runs better so they conclude that the 110 LSA was the culprit.

In all liklihood had they tried retarding their ICL to108 or maybe 110 icl with their 110 LSA cam they would have lost the ping problem and picked up power at a slightly higher rpm point.

The intake closing point being later lowers the dynamic compression because it decreases the amount of time/degrees that the piston is in actual compression.(True valve seat and seating time play an important roll here)
A big duration cam can get away with tight LSA like 106 and ICL at 102 because the added duration still creates a later intake closing point reducing degrees in compression.

Valve timing events are far more important factors than LSA or ICL. LSA ICL and duration at .050" cam lift are best used for checking/comparing purposes.

BTW dont let dynamic compression fool you.
For those that feel dynamic compression doesnt change....
At lower speed VE is usually not as great as at high speed. At low speed you will have a less dense intake charge less susceptible to pre-ignition and detonation than at higher speed where VE is greatest(usually at/near peak torque wot). At higher speed higher VE, the intake charge is much denser and being packed into the same volume. This increases the heat of the charge due to pressure difference of the charge density. (box of plastic balls vs same box filled with lead balls the volume is the same, the density different).

BTW Here is an excellent link on detonation vs preignition. http://contactmagazine.com/Issue54/EngineBasics.html

For the OP I suggest low enough compression you can "limp home" on cheapest gas available should you go somewhere need gas and they are out of premium.

Shoot for 9.2 to 1 CR and a 067 stock blueprint cam.

We ran fine at 10.25:1 compression and killed the engine at 10.75:1 compression with iron heads. We are now somewhere around 9.7:1 compression with a different cam and it is running as fast or faster than it did before and it is much more street friendly now than what it was also.

Excellent article!

X2!!

I liked this quote...from the article.. Probably the best thing to take from the read..

"For emphasis, the design compression ratio is adjusted to maximize efficiency/power on the available fuel. Many times in the aftermarket the opposite occurs. A compression ratio is "picked" and the end user tries to find good enough fuel and/or retards the spark to live with the situation...or suffers engine damage due to detonation."

I suggest printing the article and saving it as links sometimes no longer work.

Keep in mind diesels run in detonation, our gas engines arent built that strong but can withstand minor pinging for short periods without blowing up. (a tid rough on bearings though). Sustained heavy detonation(pinging) could lead to preignition but usually preignition isnt caused by just a little detonation. From personal experience pre ignition is ugly result and occurs fast and violent and gives little to no warning to react fast enough.

Don't forget the importance of the cooling system for SCR choice. Like Cliff said, there are a lot of "hands-dirty factors": if you know your car runs cool as a rule (your block/heads/radiator have no issues) then you you can bend the hypothetical limits a bit too. Anyone who has owned a LT1 '94 through '96 Firebird/Roadmaster/Caprice/Fleetwood ought to plain-amazed by the dead-reliable advantage of 10-1 SCR with a peanut cam on 87 octane (many with iron heads,) due to the reverse cooling. Those cars are proof-positive that, as many have stressed, COMBO overrides any hard/fast rule. As long as you don't cop out and say "Well, my combo has too many factors to figure out. . ." Any good scientist will RUN the experiment, be open minded about the results, and RUN it again and again until they've learned what's right. Then they don't have to take opinions from guys on forums.