# 455 Compression Ratio



## thor7726 (Sep 2, 2012)

I calculated the compression ratio of my newly rebuilt 455 to be 9.71. Everyone says they can run up to 9.5 on 93 octane. Has anyone successfully run there motor on 93 with a compression ratio above 9.5? 

The motor is a 455 bored 0.030" over with the original #64 87cc heads. I'm also running a 041 cam with 1.5 rockers and a quadrajet. Will the cam help with the compression ratio?

Thanks for the help. Worried I should have gotten the Butler 0.06" thick gaskets...


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## oldskool (Apr 26, 2015)

*CR Calculation*

To accurately determine your CR, you MUST know the exact specs of all your components.

For example: the ONLY way to determine the actual chamber size of your heads is to actually measure 'em.

You can enter your specs into the Wallace CR calcultor and get a good estimate, IF all your info is correct. 

Compression Ratio Calculator - Wallace Racing

If the following were your specs: 455 + .030, 87cc heads, 6.7cc reliefs, .039 x 4.3 FP gaskets, .010 deck height

Then the Wallace CR calculator says: CR = 10.00

But, if your chambers are bigger than 87cc, or your deck height is more than .010, or your gaskets are thicker than .039, then your CR will be lower than 10:1


"...Has anyone successfully run there motor on 93 with a compression ratio above 9.5?..."

Yes. I've read of lots of guys who have run 10:1 and say they have not had any problems. If you are pushing the max, it is a good idea to make sure you keep your total timing low enuff to avoid detonation. And it is probably a good idea to run an adjustable vac advance, with it set to add only a minimum of advance. Most stock vac advance cans add way too much.


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## PontiacJim (Dec 29, 2012)

thor7726 said:


> I calculated the compression ratio of my newly rebuilt 455 to be 9.71. Everyone says they can run up to 9.5 on 93 octane. Has anyone successfully run there motor on 93 with a compression ratio above 9.5?
> 
> The motor is a 455 bored 0.030" over with the original #64 87cc heads. I'm also running a 041 cam with 1.5 rockers and a quadrajet. Will the cam help with the compression ratio?
> 
> Thanks for the help. Worried I should have gotten the Butler 0.06" thick gaskets...



Oldskool it right, to find the true compression its best to know exactly what your numbers are. I found that my 455CI had a deck height of .020" with the pistons I purchased, but from what I have read, stock Pontiac deck height is .023" on all engines. My 7K3 96CC heads are actually 98cc's. So there can be slight variables, but the difference between 9.7 and 10.0 ain't much.

The "041," if ground on the 113.5 Lobe Separation Angle, plus the long duration, creates a lot of valve overlap which lowers the *Dynamic Compression* which is what you really want to be more concerned about than the actual 9.7* Static Compression*.

Plugging your numbers into the Wallace Dynamic Compression Calculator Wallace Racing: Dynamic Compression Ratio Calculator, with the Intake Closing for the "041" cam as being 86 degrees, your Dynamic Compression is only 6.34 - which should work. (if you advance the cam 4 degrees, you raise the Dynamic to 6.56 -which some cams have built in) From my reading, getting your static compression in the 7.3-7.5 range is best with 8.0 pushing it on pump premium. So you may actually be on the low side and may experience a little loss in the lower RPM range (and this is why higher stall torque converters and higher gears are recommended), but will pick it up in mid-range to upper. So I think you should be OK as is - _my opinion.
_
And, as oldskool stated, you want to have your timing correctly set, initial timing, total timing, vacuum advance, and the advance curve. All of these are important. Your engine may want about 12-14 degrees initial advance, 32-36 degrees total advance all in at 2,500-3,000 RPM's, and around 12 degrees vacuum advance. You can play around with these numbers, but you want to get it right. Any pinging or detonation can damage or destroy your engine - even hammer your rod bearings to destruction if not addressed.


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## thor7726 (Sep 2, 2012)

Pontiac Jim / Oldskool, 
Thanks for the input! I measured everything except for the heads... I assumed they would be close to the 87cc cambers but on further research I should have checked them. The deck was .02", gasket compressed 0.45 etc. 

I'll need to do more research into the timing though and make sure I get it right.

Thanks again! I'll let you know how it goes!


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## oldskool (Apr 26, 2015)

"...The deck was .02", gasket compressed 0.45..."



If the deck height is .020, and the gaskets are .045 thick, that gives you .065 quench distance. I read that a lot of engine builders say you need to keep quench down to around .040, or not over .050. It is said that anything over .050 can increase your chances of detonation. So, it may be best to go with a little more compression, in order to have tighter quench. This can be done with less deck height or thinner Cometic head gaskets, or a combo of both. 

Then run a cam with more advertised duration, to bleed off some of the compression. This is said to reduce "dynamic compression". 

But hey, I'm no expert. Some of these other guys may have a better way to do it.


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