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As the total says.
When everyone has used online dynamic compression ratio calculator.
How accurate has it been to the actual compression test preformed on the engine after it was built?
Using Wallace racing calcs my static comp is 11.45 to 1 and my dynamic compression ratio is going to be 8.8 to 1.
I know people are going to freak out at the 8.8 to 1 and say it's too high for pump gas. But from what I'm reading online and test I've seen preformed online by repitable companies you can push upwards of 9.2 to 1 dynamic compression ratio and not have destination problems.
But away from that I want to see real world results between online calculators and actual compression test results.
Thanks in advance everyone.
I used the same Wallace DC calculator. I'm at 11: 1 SC and my DC is calculated out the 8.52. The calculator says my cranking psi should be 175. I thought that seemed low. My actual cranking psi is 210. Seems more in line with 11:1 but who knows.....
How accurate has it been to the actual compression test
Never.
Compression doesn't work that way. Most especially, because in an engine it's an adiabatic process (for practical purposes at hand, little enough heat escapes during the process for it to be negligible) rather than an isothermal process (gases are allowed to either heat up or cool down after the process is complete and before "measurement").
I have ran 9.4:1 dynamic it was explosive power right from the go however, it needs to be tuned a little more carefully to get the most out of it. The stock computer is smart enough to pull timing to keep it from hurting itself while you figure it out, It's not going to need as much timing for sure, but remember timing is efficiency and sometimes less is more. If it's a daily driver though long-term, I will concede that mid-high 8 dcr is probably better especially if you want to experiment with different grades of fuel. I've backed my 11.8 to 1 street combo to 8.5 and it will happily run on regular 87. It's just the lower quality fuel smells pretty terrible at idle with an aggressive engine so I can use mid-grade if I'm highway traveling or premium if driving it around town. No noticeable difference in power between the fuels in my experience though.
From my fiddling around though and what I have gathered a slightly lower compression engine with a very early IVC event will produce a stronger idle to mid-range response than a higher compression one with a later IVC
You can do the high compression and the super early IVC. Just run a milder timing table, If you dial in the tune during the hottest part of the year when it's the hottest outside in the intake, air temperatures are hot and horrible. You'll have a safe, high compression combo. You can drive all year
I have ran 9.4:1 dynamic it was explosive power right from the go however, it needs to be tuned a little more carefully to get the most out of it. The stock computer is smart enough to pull timing to keep it from hurting itself while you figure it out, It's not going to need as much timing for sure, but remember timing is efficiency and sometimes less is more. If it's a daily driver though long-term, I will concede that mid-high 8 dcr is probably better especially if you want to experiment with different grades of fuel. I've backed my 11.8 to 1 street combo to 8.5 and it will happily run on regular 87. It's just the lower quality fuel smells pretty terrible at idle with an aggressive engine so I can use mid-grade if I'm highway traveling or premium if driving it around town. No noticeable difference in power between the fuels in my experience though.
From my fiddling around though and what I have gathered a slightly lower compression engine with a very early IVC event will produce a stronger idle to mid-range response than a higher compression one with a later IVC
You can do the high compression and the super early IVC. Just run a milder timing table, If you dial in the tune during the hottest part of the year when it's the hottest outside in the intake, air temperatures are hot and horrible. You'll have a safe, high compression combo. You can drive all year
I've always had my cars dyno tuned in summer for that very reason..
The engine isn't built yet so I still have the option to use larger combustion chamber heads to drop the static compression ratio lower to lower the dynamic compression ratio aswell.
But I Wanted to see how accurate the online calculators are compared to the actual real world test results.
I used the same Wallace DC calculator. I'm at 11: 1 SC and my DC is calculated out the 8.52. The calculator says my cranking psi should be 175. I thought that seemed low. My actual cranking psi is 210. Seems more in line with 11:1 but who knows.....
Do you know the specs of your eng and cam. I'd be interested is seeing how different they are from mine.
Mine is as follows.
3.903 bore
3.622 stroke
66cc chamber
6.3 inch rod
.040 gasket
0 altitude
0 boost
Cam is a comp cam 54 458 11 valve closing at 67.5@ .006 so add 1 degree give 68.5 ABDC
I lied, DC is 8.45 according to Wallace not 8.52 like I said above.
shitty memory....
4.00 bore
3.622 stroke
6.098 rod
60.2 cc chambers
.041 Head gaskets
11:03 SC according to Summits calculator
summit 8710 cam. 69 degree IVC
when I check your engine specifications I come up with 11.83 to 1 static compression as you didn't list any valve reliefs in the pistons so used 0 for pistons.
gasket bore diameter of 4.060.
Also the timing of your inlet valve isn't correct.
it works out to be 73 degrees unless it is advanced 4 degrees to give 69 degrees ABDC.
with the static compression you said 11.03 and cam ivc at 69 abc I get 8.48 or 176.46 psi.
I didn't provide all info. Didn't know you were going to check my static.
stock ly6 dished pistons with fly cut intake reliefs. + 7.1 cc volume. Pistons .006 out of the hole.
Cam is advanced 3 degrees.
I didn't provide all info. Didn't know you were going to check my static.
stock ly6 dished pistons with fly cut intake reliefs. + 7.1 cc volume. Pistons .006 out of the hole.
Cam is advanced 3 degrees.
with that info added it comes out to 10.84 to 1 static compression.
4 inch bore
3.622 stroke
60.2 cc chamber
7.1cc dish piston
4.010 gasket bore
.041 gasket thickness
0 altitude
0 boost
with that compression ratio and cam at 3 degrees advanced ivc at 70 degrees the dynamic compression ratio works out to be
8.25 to 1 or 170psi.
I'm not sure which calculator you are using but the ones I have used I get closer to 11 to 1.
With my pistons out of the hole are you using a negative number -.006 for deck clearance. Anyway there are many internet calculators that do give slightly different results. Some are more trusted than others.
Hopefully others will jump in and give their results to help answer your original question.
I'm not sure which calculator you are using but the ones I have used I get closer to 11 to 1.
With my pistons out of the hole are you using a negative number -.006 for deck clearance. Anyway there are many internet calculators that do give slightly different results. Some are more trusted than others.
Hopefully others will jump in and give their results to help answer your original question.
yep sorry about that. I missed the - for deck height.
Works out at 11.01 to 1 and dynamic 8.46 or 175 psi
I get by with a 383 SBC that cranks 225 psi on 93 pump gas all day long. Its about 9.3:1 DCR and 11:1 static. Runs 32° total advance at WOT. I read the plugs, ended up dropping 2 heat ranges compared to a stock L31 350 it was based on. With high compression you are likely to run into problems from the plugs being too hot of a heat range and acting like a glow plug. That engine combination moves 7,000 lbs everytime it runs and frequently moves up to 15,000 lbs.
I get by with a 383 SBC that cranks 225 psi on 93 pump gas all day long. Its about 9.3:1 DCR and 11:1 static. Runs 32° total advance at WOT. I read the plugs, ended up dropping 2 heat ranges compared to a stock L31 350 it was based on. With high compression you are likely to run into problems from the plugs being too hot of a heat range and acting like a glow plug. That engine combination moves 7,000 lbs everytime it runs and frequently moves up to 15,000 lbs.
Did you use online calculators to work it out before you built it.???
If so what do they say it should be as opposed to what you have on a compression test gauge???
Did you use online calculators to work it out before you built it.???
If so what do they say it should be as opposed to what you have on a compression test gauge???
Pat Kelley calculator showed about 20 psi less than what it obtained cranking over.
Even after the engine had minimal run time, it also had very equal leakdown readings. Do I trust the actual %, not really but as a comparison between cylinders, I feel it provides a good basis to locating issues like a valve that is not sealing correctly, cracked ring, etc on an engine that has been in service for a while or a fresh build.
Pat Kelley calculator showed about 20 psi less than what it obtained cranking over.
Even after the engine had minimal run time, it also had very equal leakdown readings. Do I trust the actual %, not really but as a comparison between cylinders, I feel it provides a good basis to locating issues like a valve that is not sealing correctly, cracked ring, etc on an engine that has been in service for a while or a fresh build.
thank you for that. I'm trying to find out how accurate the online calculators actually are. Have you tried the Wallace Racing one at all?
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Dynamic comp vs compression test gauge 
