# compression



## maktope (May 22, 2013)

I have my motor out doing some wellness checks and a cam swap. Currently my compression is sitting around 10.5-1. Ive read on these forms about lowering compression, they guy helping with the motor wants to raise it over 11-1. Can somebody give me the pros and cons for this? My current set up is a 428 (434 if I have done my math right). D port high flow heads out a 455 h/o. I put in a comp 268 cam.


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## BearGFR (Aug 25, 2008)

Pros: Your engine will survive on pump gas without destroying itself.

Cons: It won't.

11:1 is way way too high for pump gas and iron heads. You really ought to distance yourself from whoever's telling you this. They don't know what they're doing. Unless you plan to run the motor 100% of the time on real 100 octane (or higher) fuel, it won't live long. You can't get there with octane booster plus 93 octane, and you won't be able to pull enough timing out of it to make it by. Even if you could, it would be such a dog that it would completely negate any benefit of having that much compression.

With iron heads and 93 octane you really want to be no higher than 9.5:1. 

Is it possible to run more? Yeah, some do - but only if everything is perfect: that meaning cooling system, fuel management, diving conditions, altitude, alignment of the planets, etc. However for every one of those you find, you'll find a dozen more with destroyed engines who weren't so lucky. And for what? On a 428 like you're talking about, a full 0.5 of compression ratio is only going to make a difference of about 5-7 HP, tops. Worth it? Worth risking the life of the motor for a measly 5 HP? Your choice...

Bear


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## Chris-Austria (Dec 23, 2010)

I never thought of alignment of the planets.. maybe the alignment was bad when my 10.7:1 428cui iron headed engine died. I can only agree with Bear, ask the guy who wants to raise it: why?? Maybe he has got a nice new engine that he wants to sell after yours ran some time with +11:1


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## maktope (May 22, 2013)

Race gas isnt so much a problem. Right now I run 12 gals of 91 (highest avail pump in ca) and 6 gals of 108. Coolant system is great just had a radiator built everything new on the system. Guy helping is a track junky. Builds gassers for the strip and has a good rep. I think he is trying to steer me in that direction of a strip car only. Thats why I asked. And I tried to edit original post its a comp 272 cam. The 268 I ordered for my dads 66


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## maktope (May 22, 2013)

Ive read so much since ive bought the car sometimes it all burs together. I didnt grow up working on cars so this is a late start. And I decided instead of paying a shop I would do the work and learn. Thats where they guy helping me comes in. And the question. Because I ping already on pump gas. And every forum I can find everyone is lowering compression. I even went as far as ordering new heads. To also lower it then get the advice to raise it I was confused. Thanks for the info, I love my motor dont want to kill it. Lower we will go


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## geeteeohguy (Feb 2, 2008)

You can get by with 10 to 11:1 compression with aluminum heads, but bear in mind that aluminum heads are not as thermally efficient as iron heads and generally need to run about one full point in compression higher than a comparable iron head to make the same power. That said, the aftermarket aluminum heads are more efficient than the stock iron heads, and will yield more power out of the box. So, it's aluminum heads, bigger chambered iron heads, or dished pistons. I've played around with these engines and driven them as daily drivers for over 30 years, and have yet to find a way to run them at stock, high compression configuration on pump gas without pinging and detonation.


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## maktope (May 22, 2013)

Ive asked my dad about his. Different engines and all he laughs hands me a book and a wrench


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## Icefan71 (Apr 7, 2011)

What heads are on the motor? 455 heads on a 428 won't give you that much compression.


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## DSMTiger (Nov 17, 2012)

Just had my '66 389 rebuilt. I want it to run without problems on pump gas. I had it bored .06 over, put in hardened valve seats, dished pistons, and a slightly higher lift cam. Compression is now about 9.3/1. Sounds great on the run stand. Builder, who does a lot of Pontiac engines, says it will run fine on 91 octane without any loss from stock HP rating. Will dyno it next month when car is completed to see if it all works. Will let you know how it turns out. Will also give you more specifics from the "build sheet".

The engine is a stock tri power block with original heads, intake, exhaust manifolds, and original carbs.


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## maktope (May 22, 2013)

#64 heads are on it. The PO did a complete rebuild on it about 8k miles ago. But some of the parts stated were not the parts installed. So we took it all apart to check. And make sure there were no hidden problems. So the 64's are on there right now and I have edlebrock rpm performers 72cc in the trunk. Debating on installing them or not. Kinda want to see gow it goes on the dyno 1st


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## Icefan71 (Apr 7, 2011)

How did you calculate your compression? Unless I'm wrong #64 heads from a 455HO are 87cc's. In a .030 over 428 that gives you about 9.3:1 compression.


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## maktope (May 22, 2013)

No clue, he ran the test. And I believe you are right about 87cc. On top of the compression test the paper work from the rebuild says 10.5-1 also. If the heads wont get it there it could be another thing that doesnt match. I havent personally verified they are number 64


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## BearGFR (Aug 25, 2008)

Icefan is dead on. IF (and it's a big if) everything else in the motor is "Pontiac nominal" - the usual measurements - then 87 cc chambers on a +0.030 428 is 9.3:1.

There's no way to "test" compression ratio, at least not for us mere mortals. NASCAR techs have an instrument that can do it, but it's a very expensive tool - ($10,000 won't even get you close to buying one). Short of that, the only way to get to the actual compression ratio is to measure all the volumes, total them up, and calculate the results.
Anyone who tells you they can "measure" compression ratio with a plain old compression tester is someone you shouldn't allow within 50 feet of your engine.

Mixing 91 octane 2 to 1 with 108 is going to get you to around 96.6 octane - still nowhere near enough for 11:1. If you mix it 1:1 you'll get to 99.5. You might get by with that, but it could be borderline.

Out of the box, you'll make more power with the E-heads. at 72 cc's on a +0.030 428 a(again, everything else being nominal) it'll be at 10.6:1. _Probably_ ok, but personally I'd feel better with it at 10.5 or slightly less.

Which transmission and rear gear ratio do you have? If an automatic, is it a stock converter?

If it was me, I'd run the E-heads, go ahead and have the block zero-decked, and run pistons with a total dish volume (including valve reliefs) of 11 cc's. Combined with stock spec head gaskets (.045 compressed) that'll put you at 10.5:1 with an engine that will have excellent quench pad area characteristics.

Bear


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## maktope (May 22, 2013)

Trans is a m20. And I am putting in the new rear today with 3.55 gearing. I am not sure if you saw but I posted last night pictures of my old axle. And the info you guys are giving me is why I pulled the motor. Things just are not matching with what was said was done. Clutch was Chinese, power steering was advertised as a saginaw and wasnt. Had a Chinese crank so on and so on. I am not sure if my guy did bad math ti make it match what the PO's paperwork says or if there is more work done to it then stated. Honestly I dont even know if the paperwork is for my car at this point. Guess I will take a look at the heads while I am working on it today. Thanks for the info and help. Head hurts a lil now but I will get to the bottom of it


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## maktope (May 22, 2013)

As for the fuel ratio dead on bear. I blended for 97 octane. I went up by 2 octane per half tank untill I could run a whole half tank with no ping. At 97 motor was happy


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## maktope (May 22, 2013)

Ok verified they are 64 heads


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