Compression ratio and engine life

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Re: Compression ratio and engine life

Post by Alaskaracer »

ptuomov wrote: Sat Dec 02, 2017 1:56 pm Fair enough. Here’s what I think. If you increase compression, and hold everything else constant in the engine, the peak cylinder pressure always goes up. If you change other things in the engine at the same time, the peak cylinder pressure usually but not always goes up. Do you agree with that?

Not entirely. Too general of a statement. First off, the last part of your statement says "If you change other things in the engine at the same time, the peak cylinder pressure usually but not always goes up. My impression by that statement is you're assuming that cylinder pressure increases as a result of most changes, BUT..... You're trying to reword a blanket statement. I'll give you an example. My last engine in my dragster had a static compression ratio of 15.5:1 calculated. Want to try and guess what the cranking compression was? I'll help, it was 170 psi. Keep in mind, that my engine is more optimal than a street engine. On a stock 454 that I used to run in an old pickup I had, my cranking compression was 210 psi...and that was with 8.3:1 measured compression and a stock cam. What do you think accounts for that? The one thing to remember is static compression is nothing more than a difference in volume from TDC to BDC. That's it. DYNAMIC compression is what the engine actually sees, and is also partially responsible for cylinder pressure. The lower cranking compression was the result of camshaft events, not engine configuration. Change the timing of the cam, cranking compression changes. Change the cam itself, it will change again. Restrict the intake or exhaust enough to affect engine breathing, cranking compression likely will not change, but dynamic certainly will.
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Re: Compression ratio and engine life

Post by ptuomov »

Coloradoracer wrote: Sat Dec 02, 2017 4:09 pm
ptuomov wrote: Sat Dec 02, 2017 1:56 pm Fair enough. Here’s what I think. If you increase compression, and hold everything else constant in the engine, the peak cylinder pressure always goes up. If you change other things in the engine at the same time, the peak cylinder pressure usually but not always goes up. Do you agree with that?

Not entirely. Too general of a statement. First off, the last part of your statement says "If you change other things in the engine at the same time, the peak cylinder pressure usually but not always goes up. My impression by that statement is you're assuming that cylinder pressure increases as a result of most changes, BUT..... You're trying to reword a blanket statement. I'll give you an example. My last engine in my dragster had a static compression ratio of 15.5:1 calculated. Want to try and guess what the cranking compression was? I'll help, it was 170 psi. Keep in mind, that my engine is more optimal than a street engine. On a stock 454 that I used to run in an old pickup I had, my cranking compression was 210 psi...and that was with 8.3:1 measured compression and a stock cam. What do you think accounts for that? The one thing to remember is static compression is nothing more than a difference in volume from TDC to BDC. That's it. DYNAMIC compression is what the engine actually sees, and is also partially responsible for cylinder pressure. The lower cranking compression was the result of camshaft events, not engine configuration. Change the timing of the cam, cranking compression changes. Change the cam itself, it will change again. Restrict the intake or exhaust enough to affect engine breathing, cranking compression likely will not change, but dynamic certainly will.
The peak cylinder pressure is going to be impacted mainly (but not exclusively) by volumetric efficiency, geometric compression ratio, ignition timing, and burn speed. Do you agree? If you change nothing else other than geometric compression ratio, the peak cylinder pressure will go up, right?

If you make some other sensible changes that complement the increased compression, like for example making the IVC later, I'd expect the peak combustion pressure still to go up. In particular, if the bigger cam results in a higher VE near the peak torque rpm. But there's no guarantees.
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Re: Compression ratio and engine life

Post by Tom Walker »

Dynamic compression ratio can be influenced and affected greatly by r.p.m., as the volumetric efficiency changes through out the r.p.m. range of the engine due to the valve events dictated by the camshaft. The big one, intake valve closing.
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Re: Compression ratio and engine life

Post by Tom Walker »

Sorry, should have read the last post more accurately, I seem to have repeated similar information 😳
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Re: Compression ratio and engine life

Post by 89vette »

Coloradoracer wrote: Sat Dec 02, 2017 4:09 pm My last engine in my dragster had a static compression ratio of 15.5:1 calculated. Want to try and guess what the cranking compression was? I'll help, it was 170 psi. Keep in mind, that my engine is more optimal than a street engine. On a stock 454 that I used to run in an old pickup I had, my cranking compression was 210 psi...and that was with 8.3:1 measured compression and a stock cam. What do you think accounts for that?
I'd like to piggy-back on this thread in an attempt to figure out some common-sense compression calculations. I live in KC where barometric pressure floats in the 29-31 (inHG) If I convert from inHG to PSI, that means atmospheric pressure (1bar?) should fall between 14.2-15.2 PSI Even when you drop to sea level, it should be similar. The definition of 1bar is 14.5PSI.

When the MEASURED 8.3:1 engine (above), 210psi/14.5PSI = 14.5:1 compression IOW, it's WAAAAY higher than I'd expect.

My 383 engine with 5.7" rods, a 269/269 .544/.544 cam, .040 quench, 20cc dish, and 4.030 bore, shows 10.3SCR and 8.4DCR using Paul Kelley's downloadable calculator. When I borrowed AutoZone's compression tester, all 8 cylinders landed just above 150PSI. Let's call it 152psi each. My cam is small PLUS it only has 10-deg overlap. (Though it shouldn't matter, .050 duration is 214/214).

My builder doesn't remember (and didn't record) the ICL. The card lists 108ICL with 112LSA. Due to valve-clearance issues (I decked my heads to 56cc (from the 65cc "standard" 195 Eliminator SBC AFR), it doesn't seem likely the cam could be mechanically rotated too far off.

Stock pressures from a 350 late 80's Corvette seem higher than mine...yet mine runs WAY stronger than stock ESPECIALLY in low/mid rpms. (It's a mega-ported long-runner setup with shortened tubes aimed at Superram intake tract length.


The calculations for "common-sense" cranking compression vs multiplication of actual "swept volume compression" don't make sense....nor do the readings I found in my engine. 152PSI/14.5(1bar) = 10.5:1 DYNAMIC compression. That's higher than my calculated static! But, it's lower than the MEASURED 8.3:1 shown at 210psi (by the poster above).

Ughhhh....I'm confused!!


BTW....170psi on a dragster is the ONLY thing that makes sense to me. BIG AZZ cam with lots of overlap/bleed-off!!!
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Re: Compression ratio and engine life

Post by superpursuit »

89vette wrote: Tue Dec 12, 2017 8:10 pm When the MEASURED 8.3:1 engine (above), 210psi/14.5PSI = 14.5:1 compression IOW, it's WAAAAY higher than I'd expect.

Your calculations seem to be way off. I have not heard of this method you use before but it seems it does not work out.

By your calculating formula all the early stock V8's ,and others, that had published and verified cranking compressions of 170 psi, by your method of 170psi/14.5psi = 11.72:1 compression ratio. Most of these early stock engine were around the 9.00:1 compression ratio. The way you are trying to calculate it is wrong and it seems like this is affecting your reasoning on this. If you look up tune up manuals on this you will find cranking compression ratios as well as static compression ratios. Apply your formula and you will soon see what I mean. I hope this helps.
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Re: Compression ratio and engine life

Post by Schurkey »

Compression-testing a warm engine results in considerable heating of the compressed air once it's had time to absorb heat from the cylinder, head, and piston.

Heating the air made it "expand", but since it could not expand, the pressure increased instead.

Some of the pressure in a compression test comes from heating the compressed air.
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Re: Compression ratio and engine life

Post by David Redszus »

The correct calculation of compression ratio is based on the ratio of cylinder volumes, not pressures.
It consists of: (chamber volume + displacement) / chamber volume = Static compression ratio.

The dynamic compression ratio makes use of the inlet valve closing point to determine displacement volume; all else remains the same. (Chamber volume + cylinder volume at inlet close) / chamber volume = Dynamic compression ratio.
Engine speed, throttle position, atmospheric pressure etc, have no bearing on DCR.

Compression pressure includes atmospheric pressure at BDC, DCR, and the ratio of specific heats of the fluid. For air/fuel vapors we use a specific heat ratio value of 1.33. Cylinder pressure at BDC x DCR^1.33 = Compression pressure.

This calculation will result in a value that is higher than measured values due to the effects of ring leakage at low cranking speeds. However, in a running engine the higher compression pressures will be attained. Or could be exceeded if inlet/exhaust pressure ratios are increased.

In an engine, it is the compression of air that causes an increase in temperature. Once combustion begins, it is the heat produced by combustion that further increases the cylinder pressure and the resulting force on the piston. During the compression cycle, negative work is being done by the piston. The reverse is true ATC and combustion ensues.

Any chamber combustion pressure rise before TDC is negative work and detracts from performance. Positive work is produced ATC by the area under the combustion pressure curve, not just peak pressure. Raising the pressure curve and positioning it correctly is the secret to making real power without rapid self destruction.
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Re: Compression ratio and engine life

Post by 89vette »

superpursuit wrote: Tue Dec 12, 2017 8:55 pm
89vette wrote: Tue Dec 12, 2017 8:10 pm When the MEASURED 8.3:1 engine (above), 210psi/14.5PSI = 14.5:1 compression IOW, it's WAAAAY higher than I'd expect.

Your calculations seem to be way off. I have not heard of this method you use before but it seems it does not work out.

By your calculating formula all the early stock V8's ,and others, that had published and verified cranking compressions of 170 psi, by your method of 170psi/14.5psi = 11.72:1 compression ratio. Most of these early stock engine were around the 9.00:1 compression ratio. The way you are trying to calculate it is wrong and it seems like this is affecting your reasoning on this. If you look up tune up manuals on this you will find cranking compression ratios as well as static compression ratios. Apply your formula and you will soon see what I mean. I hope this helps.
The calculation I present was offered by a member of the Corvette Forum (where I normally hang-out). I hadn't heard of this method either -- though it SEEMS to make reasonable sense DESPITE the fact it doesn't seem to work. It bugs me because I can't figure out WHY it doesn't work!

This calculator http://www.not2fast.com/turbo/compressi ... sure.shtml shows my cranking compression SHOULD be above 200 psi...like the other 383 strokers I've noted. I'm trying to figure out if I performed the test incorrectly and/or checked out a faulty gauge?

The tester was similar to one of these: https://www.autozone.com/loan-a-tools/c ... 424906_0_0. Basically, just screwed the threaded end in the hole, tighted a hair beyond hand-tight, opened the TB blade, and turned the engine over. I removed my computer chip so it wouldn't fire. Each test took SEVERAL revolutions to hit 150psi...Somewhere in the neighborhood of 10 "chugs" of the starter. It SEEMED a bit too long to pressurize (compared to ONE other test I ran 30-40yrs ago) but they all came out the same.

Can't figure out why this SIMPLE task -- including what seems like straight-forward formula doesn't work as expected? It SEEMS there SHOULD be a correlation between PHYSICAL compression and PSI if you read Boyles Law: http://www.physlink.com/education/askexperts/ae518.cfm
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Re: Compression ratio and engine life

Post by user-23911 »

89vette wrote: Wed Dec 13, 2017 2:30 am

The calculation I present was offered by a member of the Corvette Forum (where I normally hang-out). I hadn't heard of this method either -- though it SEEMS to make reasonable sense DESPITE the fact it doesn't seem to work. It bugs me because I can't figure out WHY it doesn't work!

This calculator http://www.not2fast.com/turbo/compressi ... sure.shtml shows my cranking compression SHOULD be above 200 psi...like the other 383 strokers I've noted. I'm trying to figure out if I performed the test incorrectly and/or checked out a faulty gauge?




No different from any other online calculator........they're all wrong.

As to why this is wrong? (yes, I entered some numbers from my unmodified engines and it's wrong by quite a lot)
Varying heat loss with compression, ring leakage as well as an undefined IVC point.


One interesting point nobody ever seems to mention is that even if you've got fuel and air and spark but no compression, you don't get fire.
One of my friends was once trying to fix a dead engine....it wouldn't fire.
It's got spark and got fuel.
Does it have compression?
Yes.
How much?
Don't know, just put my finger over the plug hole.
How much on the gauge?
What gauge?
The answer was not enough.
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Re: Compression ratio and engine life

Post by MadBill »

89vette wrote: Wed Dec 13, 2017 2:30 am..
Can't figure out why this SIMPLE task -- including what seems like straight-forward formula doesn't work as expected? It SEEMS there SHOULD be a correlation between PHYSICAL compression and PSI if you read Boyles Law: http://www.physlink.com/education/askexperts/ae518.cfm
Did you see the part in your link that says: " temperature must be allowed to return to its prior value for Boyle's Law to hold true."?

Also, you should try another tester. I have encountered several whose Schrader valves have been replaced with normal tire valves; their much heavier springs will produce low readings in a compression gauge.
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Re: Compression ratio and engine life

Post by David Redszus »

This calculator http://www.not2fast.com/turbo/compressi ... sure.shtml shows my cranking compression SHOULD be above 200 psi...like the other 383 strokers I've noted.
The calculator is wrong; the pressure calcs are not too bad but the temps are way off.
I'm trying to figure out if I performed the test incorrectly and/or checked out a faulty gauge?
For a number of reasons, measured compression pressure will never match the calculated values; don't expect them to.
Reasons include a faulty gauge, leaky connection, temperature losses, ring leakage, unknown inlet pressure. Compression pressure calcs must be based on DCR and will show the maximum possible pressure.

One interesting point nobody ever seems to mention is that even if you've got fuel and air and spark but no compression, you don't get fire.
To elaborate Joe's point, you need energy to obtain ignition. That energy comes from a combination of heat and pressure. Under certain conditions, no amount of pressure will produce ignition without temperature. Conversely, very high temperature alone will not produce ignition without some pressure being present.

A higher chamber temperature requires less voltage to cause ignition.
A higher chamber pressure requires higher voltage to fire.
Both must be present.
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