# compression question



## Red 68 (May 24, 2014)

Is the compression ratio on a basically stock 400 with 72cc heads ok to run on pump gas? What compression is safe to run on pump gas.?


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## oldskool (Apr 26, 2015)

Depends on who you ask. Most engine builders recommend a max of 9.5 static CR with iron heads, and 91-93 octane gas. 9.25 is safer.

Now, there are too many possible variables to say exactly how much CR your particular engine may have. The "72cc heads" may actually have 75cc or a little bigger chambers. OR, at some point, they may have been shaved down to 70cc or even less.

And if the engine has had a cheap rebuild, it could have the cast pistons with 8 eyebrows and a bevel edge around the top. This will give the engine less static CR. Also, if the rods have been resized, the piston tops will be further down in the block. This will lower the CR.

But, if the engine has .030 or .060 over flatops, with the 4 stock type valve reliefs, and has had the deck cut down for zero deck height, it will have more CR. Then you must add in the head gasket thickness and bore size.

Here's a CR calculator. You can plug in your engine specs and it will calculate your CR for you.

http://www.wallaceracing.com/cr_test2.php

Then, if you have 9.5 static CR, you can run a cam with more advertised duration, which will bleed off some of the compression at lower rpm, allowing you to get by with less octane. This is what's referred to as "dynamic compression". A "steep ramp" cam will increase cylinder pressure, and make 9.5 static CR worse for pump gas. The CC XE series and the Lunati Voodoo series cams are the most popular steep ramp cams. They are said to increase cylinder filling by closing the exhaust valve quicker, allowing less fuel air mixture to escape, at low rpm, as it does when using cams with a lot of advertised dur. This is great for low rpm, low CR engines.

Just as an example: a Summit 2801 cam will give you less dynamic CR, than a Voodoo 256 or 262 cam. The Summit 2801 has 288 degrees adv intake dur. The Voodoo 256 has only 256 adv intake dur. That's 32 degrees difference. The 2 cams will run nothing alike. I'd venture a guess that the Voodoo 256 would cause detonation problems in a 9.5 CR 400 Pontiac engine.

But when I make such a statement, somebody usually comes on saying that's what they run, and it does great with no problems. So, I'll just say that I would not recommend the Voodoo for a 9.5 CR 400. But, I would recommend it for a 400 with only about 8:1 CR, or a little over. At 9.5 CR the cam choice is critical, for pump gas use.


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## BearGFR (Aug 25, 2008)

Hooo-boy... This topic tends to start wars.. 

There are too many variables in the mix for anyone to ever be able to 'guarantee' that any combination will always be safe, or not be safe.

There are many threads (and opinions) on the topic, so I'd recommend that you search this forum and others on "compression" and start reading.

Even on specific aspects of the topic, like the one oldskool skillfully pointed out, having to do with dynamic cr. It makes sense that closing the intake later results in less pressure and hence less tendency to detonate - but then - when the engine gets up into the rpm range where it is "on the cam" and volumetric efficiency is coming up, so is cylinder pressure. (after all that's the purpose of running a long duration / late timing cam - to shift the point where the engine is most efficient to a higher rpm so that it makes peak torque at a higher rpm - that's actually how a 'hot cam' makes more horsepower - remember HP is a calculated number and is (torque X rpm) / 5250 Change things so that peak torque happens at a higher rpm and viola - more HP - even though torque output is the same). The flow velocity at rpm is helping to force air into the cylinders and will start increasing pressure even though that intake valve hasn't closed yet. BUT-- you've got another friend helping you out, and that's rpm. As rpm goes up, there is simply less _time_ available for the engine to get into detonation.

Anyway, I'm not disagreeing that a late intake closing event can be beneficial, just pointing out that there are lots of things that go into this whole area of thought.

Another thing to consider. Compression _alone_ isn't worth all that much power. On a reasonably streetable 400, a full 5 points (x.5) of compression is going to make less than 10 hp difference, maybe less than 5 hp. So in the long run in my opinion anyway, pushing the limits just isn't worth the risk.

YMMV

Bear


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## geeteeohguy (Feb 2, 2008)

In a word, no. 72cc heads with a 400 will net you about 10:1 compression, and you will need a cool running engine to pull off driving with pump gas. 10:1 iron headed engines need octane.....probably about 95 at least. You can mix in race gas or av gas for a boost if you don't drive it that much. I do that with my '65. Or, you can change to bigger chamber heads. I did that with my often-driven '67.


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## Bensjammin66 (May 12, 2009)

Geeteeohguy is right. Even with the best cooling system ever, put 72cc heads on a 400 and youre going to ping with pump gas even with flat tops. I used to run big valve 72cc #16s on a 400 and I had to mix about 3 gallons of 93 to 1 gallon 110. $$$$$. Got some 6x-4s with either 92cc or 94cc chambers I forget but it ran MUCH better on 93 that way. I noticed a decent power drop though even with a valve job and mild port work on the 6x's.


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