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-   -   compression ratio vs. pounds per square inch. (http://www.hotrodders.com/forum/compression-ratio-vs-pounds-per-square-inch-230755.html)

h0trod 03-15-2013 11:49 AM

compression ratio vs. pounds per square inch.
 
i just purchased a SBC 350 ci, the guy i got it from said he was planning on building a blower engine.

this is a fresh 010 block 4 bolt main.
edelbrock RPM heads.

im planning on doing a compression test on it today.

what kinda pressure can i expect for a 8:1?
how much pressure would i expect from a 10:1?

thanks guys.

delawarebill 03-15-2013 12:35 PM

motor
 
i'd expect 150# area.. but.... that motor is NOT broken in so those numbers will be lower.. the main thing is what ever u get the numbers should be fairly close to ea other... not 100 and 150...

MouseFink 03-15-2013 12:42 PM

The compression pressure (PSI) depends on several variables. The primary variable is the cam profile and valve overlap.

You can disregard variables and get a WAG at the PSI mathematically :

14.7 x CR = PSI

8:1 CR = 118 PSI
10:1 CR = 147 PSI

Example:
My 455 Pontiac had 12:1 CR with an average 215 PSI cranking (150 RPM). .
The hydraulic lifter camshaft was very mild CC-268H with only 39 degrees valve overlap. The short overlap quickly built up cylinder pressure for gobs of low end torque! That engine required at least 100% VP C-12 - 108 octane racing fuel.

A Ram Air IV camshaft with 87 degrees valve overlap in the same engine would produce an average 180 PSI. That much combustion pressure would be borderline on 93 octane pump gas with best performance with at least 100 octane VP racing fuel.

68NovaSS 03-15-2013 08:53 PM

Also, it doesn't matter if you're talking a 6-71 or 10-71 blower, you need to be looking at boost pressure, that's what will increase your static c.r. to higher effective c.r. Meaning an 9.0:1 static c.r., with 10 pounds of boost will net you an effective c.r. of 15.1:1.

MouseFink 03-16-2013 06:00 AM

When considering CR to boost pressure, the limit for 93 octane pump gas is 12:1 effective CR. Anything over 12:1 effective CR will risk catastrophic detonation.

Static CR.................8:1
Boost pressure ....... 8 PSI
Effective CR.............12:1

Experience has shown that an engine will make more power from low CR - high boost than from a high CR - low boost. Engines will differ in their tolerance to detonation so this rule is not hard and fast but it is a good place to start. It is easier to change boost than it is to change compression ratio.

454C10 03-16-2013 08:27 AM

It isn't the overlap that effects cranking pressure (compression test). The engine isn't even trying to make compression at this point in the 4 stroke cycle. In fact, the engine is trying to make vacuum during the overlap period.

What effects cranking pressure is when the intake valve closes. Long duration cams tend to have late intake closing points which will reduce the effective length of the compression stroke as the valve is still open as the pistons is rising to compression the air/fuel charge.

Since big cams have lots of overlap, people mistakenly tend to blame the extra overlap for reduced pressure on a compression test. However, more overlap does lower the idle vacuum.

68NovaSS 03-16-2013 11:03 AM

Quote:

Originally Posted by MouseFink (Post 1657229)
When considering CR to boost pressure, the limit for 93 octane pump gas is 12:1 effective CR. Anything over 12:1 effective CR will risk catastrophic detonation.

Static CR.................8:1
Boost pressure ....... 8 PSI
Effective CR.............12:1

Experience has shown that an engine will make more power from low CR - high boost than from a high CR - low boost. Engines will differ in their tolerance to detonation so this rule is not hard and fast but it is a good place to start. It is easier to change boost than it is to change compression ratio.

Yep, what he said ^^

MouseFink 03-16-2013 04:53 PM

Quote:

Originally Posted by 454C10 (Post 1657245)
Since big cams have lots of overlap, people mistakenly tend to blame the extra overlap for reduced pressure on a compression test. However, more overlap does lower the idle vacuum.

Correct, and I agree and the reason for that is as follows.

Cylinder pressure is generated on the compression stroke and not during the valve overlap period. However,the reason valve overlap causes a loss of cylinder pressure is because long duration camshafts always have more valve overlap and later intake valve closing points and the intake valve closing point during the compression stroke determines the amount of cylinder pressure that is generated.

That is why the duration of 218* @ .050" with the CC-268H "RV" cam in my 455 Pontiac engine had about 25 PSI more cranking cylinder pressure than it did when it was equipped with a Ram Air IV camshaft that had valve duration of 231* / 240* @ .050". After I changed to the Ram Air IV camshaft, I lowered the rear gear ratio from 3.08:1 to 3.90:1 to compensate for the loss of low end torque.

454C10 03-17-2013 09:01 AM

If you install 2 cams, both with 230 degrees of duration on the intake and exhaust but one has a 108 degrees Lobe Separation Angle LSA and the other has 114 LSA.

The cam with the 108 LSA will have more overlap and an intake valve that closes sooner. So the cam with more overlap will have lower vacuum at idle and higher cylinder pressure.

So a cam with more overlap doesn't always mean it will have lower cylinder pressure.

It is also important to look at the actual intake closing point (not at 0.050"). Stock cams tend to have long lazy ramps which also lowers pressure due to shutting the valve at the same point of a much bigger high performance cam with fast ramps.

MouseFink 03-17-2013 09:55 AM

The CC-268H single pattern camshaft in my 455 had .454" valve lift, 110* LSA, 39* valve overlap with 215 PSI cranking pressure.

The Melling SPC-8 Ram Air IV camshaft in the same engine had .480" valve lift, 114* LSA, 87* valve overlap, and 170 PSI cranking pressure.

I first used a CC-268H "RV" camshaft in my 455 Pontiac engine but the CC standard hydraulic lifters were noisy. After about the first 400 miles, I switched to a Melling SPC-8 Ram Air IV camshaft and Melling JB-951R limited travel lifters. That change resulted in a quiet valve train and reduced cylinder pressure.

I suppose the valve events made the difference in cranking cylinder pressure. I do not recall what the valve events were.

454C10 03-17-2013 06:45 PM

I would bet the cam with the lower crank pressure had a much later intake valve closing point.

DanielC 03-18-2013 08:14 AM

There is a lot of talk about measured compression ratio, cam duration, cam profile, ETC.

Why no mention of cranking RPM? A used starter, compared to a good fresh starter, and fully charged battery could possibly change the compression tester readings 20 or 30 PSI. A more accurate test would be a leakdown test.

If your rings are adequate, but not good, probably be better, as in more horsepower, and torque, AND better fuel economy, by putting new rings in the engine.

But that does not sound as "sexy" as a "high performance 3/4 race camshaft with chrome valve covers, and Super Thunderbolt high energy ignition coils"

454C10 03-18-2013 08:49 AM

Don't get me started on compression testing variables. I have two identical gages and one reads 25 psi more than the other. Compression testing is good to see differences between cylinders.


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