Ok Guys, I've got a math test for you. (This is actually to help a friend out who is worried about his Compression test results).

Scenario:
He swapped in a LR4 (4.8 L Vortec) truck engine, stock heads, no porting etc.

Given that the LR4 has a factory compression ratio of about 9.5 to 1
Reference: http://www.new-cars.com/2003/chevrol...hoe-specs.html
(Lets assume it's from a 2003... it supposedly has 70k miles on it)

He did a dry compression test (did NOT add teaspoon of oil to cylinder) on all cylinders.

Questions:
1) What is the Maximum psi that he could expect to see on each cylinder
2) What is the lowest acceptable psi that he could see on any cylinder

I know his results and I have my answers to Questions 1 & 2 above in my head... but just wanted to get some feedback from the community... hopefully to put his mind at ease. (Please give some reasoning for your answer... not just a number... for example... x*y=z.... thanks!)

Bonus Question: What if this were a stock LS1 motor with 10.5 to 1 CR... same questions...

Any constructive input is appreciated.

 

not sure about the highest psi since a lot of the compression depends on the overlap of the camshaft but i would guess cold max 180ish. the Lowest acceptable cold i would think is around 120ish, anything lower than that and it will for sure be a turd in the low end. Test really should be done hot though or if not at least with some oil. They all should be around the same psi if not, then there is an issue

 

Assuming stock cam: 190/191 .466/.457 114 LSA

So how did you come up with 180 psi ??? Rationale? x+y=180... x*y=180... etc...


Looking for a calculated results... but thanks.

 

your question is pretty fucked from jump street. why don't you just post up the results and ask for help. By the way the numbers i gave you were from hand on experience compression testing different motors. Not some fucked up book math but real world ****

 

So you can't do math to calculate expected results?
You sound like the kind of guy who would put in a high lift cam and "Guess" that it's ok, hoping it doesn't smack your pistons.

Those numbers you gave were from on hand experience compression testing of a 4.8L truck motor? I highly doubt that.

If you think the question was that fucked up, why did you even try to answer it?

Go away dude, you're no help. "Any constructive input is appreciated."

 

Ok i will go away

by the way high lift has zero to do with smacking the pistons (peak lift occurs when the piston is further down in the cylinder), PTV clearance is all about duration. but my bad i dont know ****

 

Anyway.....

Anyone out there who can help in calculating expected compression (PSI) results based on a 9.5 to 1 CR?

No high school guessers please.

Thanks.

 

I'm interested to see how this math works

 

Boyles law: p1V1=p2V2 so if the compression ratio is 9.5:1, plug them into the volume figures. p1(1)=p2(9.5). absolute pressure of atm is approx 14.7 psia. p1(1)=14.7*9.5. p1= about 139.5 psia, or around 125 psi on a gauge. thats all assuming you have atmospheric pressure at BDC, your valve is closed at BDC, and you don't factor in temperature change (if there even is one). No clue how close this will represent an actual test. Thats my thoughts on the math

 

Nice. I'm diggin' the thought process. I'm a little stumped though... all over the place, we see people reporting compression test #s that are like 180+ psi, some as high as 200 or more.

180 / 14.7 = 12.25

So what's up with that? I don't know of anyone with daily drivers who claim to push their stuff that high for a CR...

The only thing I can think of is that maybe the pressure is increasing (P2) due to heat buildup as the air is compressed, or the air is absorbing engine block heat and expanding as it's forced into a hot engine... T1=85 degrees F , T2 = 400 degrees F. That increase in temp may be adding to the increase in P2 during the compression test???

Thoughts?

 

Boyles law requires the gas to stay at constant temperature, but everyone knows that doesn't happen because a gas heats when you compress it...which is how we can have diesel engines. Next easiest thing is to use the ideal gas law, pv=nrt. Nr is constant here so setting the compressed and compressed gas equations equal gives you p1v1/t1=p2v2/t2.

Numerator of the left side can be set to 14.7 but we have to assume an intake air temp, let's say 100c. We're solving for p2, we know v2=v1/9.5. Figure t2 is just below autoignition (detonation) which i googled as 280c. Convert your temps to Kelvin and i come up with 283psi. I didn't check all my units to make sure I didn't **** up, but it seems reasonable.

Note that I had to assume both temperatures here, which is why I'd trust 3rdcoast's experience over my high school chemistry in a second. And my math doesn't even consider valve overlap or any number of other variables.

 

Inertia may have something to do with it as well...

 

Assumptions like ideal gas and intake valve closes at BDC are not real world situations for this engine. You need a very detailed CFD analysis or else you're just guessing.

 

I think it may be a combination of gas compression AND heat absorption from the cylinder/pistons/valves/heads/etc that cause the psi to go above the 9.5 x 14.7 calc.

Found a good write up on the adiabatic process at Wiki: (which actually includes an engine example at 10:1 CR)
http://en.wikipedia.org/wiki/Adiabatic_process

If the test was done when the engine was cold (which my friends was), we could eliminate engine heat from the equation and go strictly with the heat of compression and pressure of compression to calculate (estimate) using pv=nrt.

As RezinTexas pointed out, we have to also consider valve opening and closing events.

(And for the record, I wasn't knockin 3rdCoast's real world experience in "Measuring" compression test. I'm sure that he's done the test hundreds of times and can speak to that. But I was looking more for the math and theory behind the question rather than just the after the fact results. He had a good point that when doing the test, the engine should have been hot and or with oil (actually once without oil and once with oil is the preferred method of testing... to rule out rings, among other things). It was when he stated that the question was f*d up that I took offense. I was merely asking for clarification on how he arrived at his results...again.. looking for a method of calculating it... rather than "here's a number: XXX" )

Thanks for the input guys. Appreciate it!

 

HRHohio no offense taken and i apologize for my wording. I'm rather quick tempered (hence my avatar) but im working on that. Some really good info in here, I like the math equations. I guess the reason i answered the way i did is that there are going to be so many variables in reverse engineering your compression ration that I thought i would just tell you what i know to be true. I'm interested to find out which formula ended up working for you and what eliminations/adjustments you had to make to the formula to make it make sense in your real world situation. im always in for some book learnen.

 

Hey, no problem dude. I snapped back too. I sense that you genuinely wanted to help out, and I appreciate that. It's all good.

I think that you and RezinTexas are correct in that there are too many factors (valve overlap from the camshaft, as you pointed out) to accurately calculate an expected result by 99% of the population, myself included.

Clearly, if we're talking strictly stroke, bore, piston shape (dome/dish), and head cc volume it would be much simpler. But just because a motor is 9.5 to 1 CR (Calculated) doesn't mean that the cyclinder will be getting the full 9.5 to 1 compression. If the valves are open any bit of the time, the CR will be lower. If both valves are open, overlapping, even more so.

When you calculate the CR with no change in temp, the psi calculates to 140 psi max. Take into account the heat of compression and the psi calculates out to 340 psi for a cold motor (using the formulas on the wiki page).

Obviously real world results, as 3rdCoast pointed out, are somewhere in between. Again, likely due to valves being open for a split second, and air escaping, optimal flow of heads, valve closing events, rings, etc.

There's probably a few engineers out there who can solve the problem with a +/- 10% margin of error, but they're probably working for GM, Mercedes, Ferrari, etc.

Oh... and his compression test results were 130 psi on several cylinders and as low as 100 psi on a few (no oil, and cold engine). Again, this was on a 70k 4.8L motor. He said it was too hard to do the test with a hot engine because the Turbo and downpipes are in the way. He's trying to trace a misfire... thinking that maybe he got a bad engine...

 

There are so many factors that a scientific formula will have so many different factors that it would be out of the realm of anyone without a background in engineering...

If I were you I would stick to comparing results to others (3rdcoast) in real world tests as experimental data is generally more informative then theoretical results. Almost any theoretical result is almost always backed up with precise experimental results...

So just some advice go with the real world numbers, it saves a lot of time and thinking lol...

Good luck to the op though I would be more concerned with all my cylinders being similar than looking for a specific number...I thought 4.8s were rated at 9.4:1 iirc but I could be wrong...

 

Yup. I hear ya. I don't mind the thinking part though, to understand the theory behind things. Helps with the application of it.

As for 4.8s, info on the CR depends on what source you go with... some say 9.4 Some say 9.5 (Source cited in first post). I've also seen 9.49 (Which is probably why we see both).

Looks like more wrenching...

Thanks everyone!

 

Yep good luck man!

 

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