[70 Dream]

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 . . . . .

[bobzdar]

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.

[74Grandville]

Originally Posted by bobzdar

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.

agree.

[goatwgn]

They had compression as low as 7.5 to 1 on some engines in the mid to late 70's. 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.

[F ROCK]

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.

[Cliff R]

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

[Brent Flynn]

Originally Posted by Cliff R

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

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...

[screamingchief]

Originally Posted by Brent Flynn

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?

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.

[goatwgn]

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.

[Half-Inch Stud]

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.

[70 Dream]

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 . . . . .

[PONTIAC DUDE]

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.

[JSchmitz]

Originally Posted by PONTIAC DUDE

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.

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.

[Ragtop Man]

Maybe a better way to express it is that longer overlap cycles and longer exhaust duration drain effective cylinder pressure, all other things equal. This gets back to the dynamic CR issue.

What is clear from the thread is that if you choose fuel quality as the constant in the equation, you will have to vary camshaft timing with the CR ( as well as exhaust scavenge and port filling efficiency ) to arrive at a final answer for the combination

I could easily see 10:1+ static CR a long-cammed (041/744-like) car with a slightly rich mix, good scavenge and good ignition running just fine on 91 Octane Cali gas in a light car with good street gears, and be a stone pig in a heavier car with no convertor, gears or tuning.


What is also clear, at least as a free opinion, is that if your engine builder cannot give you a reliable recommendation for your desired combination and results that you need a real specialist Pontiac engine builder.

The cost is not that much more for the priceless knowlege that what they are doing not only works, but kicks ass.

[BruceWilkie]

Originally Posted by Ragtop Man

Maybe a better way to express it is that longer overlap cycles and longer exhaust duration drain effective cylinder pressure, all other things equal. This gets back to the dynamic CR issue.

Intake closing point determines number of degrees in compression. Overlap has nothing to do with it.
The overlap window is the time to purge the chamber of ex charge with fresh intake charge. Proper exhaust tuning(design) plays large here.

Exhaust opening point is touchy too. Too soon you lose power to CE losses, too late you lose VE to pumping losses on the upsweep. Ideally you want the exhaust to open as fast as possible right about the time the piston reaches max speed approx 75 deg ATDC on a 400. At high speed opening a bit sooner will still result in more hp as the VE increase with less pumping loss can exceed the CE loss. The more exhaust pressure you can relieve before the piston leaves bdc dwell the less pumping loss you will have.(ideal would be only swept volume on the upstroke side of exhaust event) Opening exhaust after max piston speed is reached offers no additional power from combustion(it does help emissions). Less pressure in cylinder on the exhaust upsweep also affects overlap requirements because a reduced pressure allows the intake charge to begin moving sooner into the cylinder. When exhaust first opened it sent a high pressure wave somewhere down the exhaust pipe. Behind that initial high pressure is a lower pressure which helps pull higher pressure (atmosphere) charge from intake across the chamber and purges the chamber. Thus the cylinder starts filling before the piston leaves tdc dwell. Closing exhaust too soon(or excess backpressure or wrong collector lengths) allows excess residual exhaust to contaminate fresh charge and hurts power. Closing it too late wastes fresh charge and kills off intake charge velocity that normally would help fill the cylinder. The piston does not suck the charge in, atmospheric pressure pushes it in.

And I am NOT discussing high speed scavenge tuning above. Same principals apply to granny's economotor.

Also keep in mind that although a 2.5" exhaust system can support near 500 hp, exhaust pipe on the end of your collectors makes them part of your collectors. Pipemax suggests a 3" od collector for a 406 with the 60916 cam.

I suggest spending the $40.00 for Pipemax for further details. It suggests a 406 at 9.7 compression 60916 cam 5200 rpm pk power at 400 lowest point - best case 432 hp.

Kauffman used to advertise a similar stock head good valve job 406 that dyno'd 418hp at 5100, 453 ft lbs at 4200. 72cc dports w/2.11/1.77 valve, 60916 cam, 750Holley and RPM intake.

Pipemax suggests similar combo needs 1.575-1.7" OD headers. Suggests 93-94 octane as well.

[Mr. P-Body]

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

[Cliff R]

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

[bobzdar]

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.

[aktattoo]

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

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

[Cliff R]

"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