[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.

[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.

[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.

[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

[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.

[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

[Skip Fix]

"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.

[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.

[67drake]

Originally Posted by 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?

But then you say-

I don't really care about performance as much as streetability and reliability.

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

[70 Dream]

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.

[bobzdar]

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.

[Half-Inch Stud]

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.