I live and breathe 1000 feet above sea level. So does my 440. On a standard day (aviation) the air pressure should be about 14.4 psi If my engine is 10.0 cr the cranking psi should be 144 psi minus loses to the intake valve not closing until part way up the compression stroke.
How are some people getting 170-190 psi compression tests?
1000 feet is not "alot" of altitude. Lots of things will effect the numbers on the gauge. The static compression ratio is one. The next major influence is cam profile. Altitude will some, but more importantly think of altitude like swapping iron heads for aluminum ones. Your 11:1 motor with iron heads is still an 11:1 motor with the aluminums. BUT with the aluminum heads it will behave like a 10:1 engine. Think of altitude the same way. Before jumping into an altitude discussion, how did you determine your are a 10:1 engine, and what is on your cam card?
Finally getting around to putting 452 aluminum heads and MRE roller tip rockers I Bought for my sons 70 Charger at the Nats in 2019.
Did a warm compression check: all 135-140 Psi. (1974 block +.060 TRW pistons ,2355? ,smog iron heads, purple stripe 484* cam ,I think, built two owners ago ,trying to find the records)
For comparison, pulled one plug on my 440 :180 psi cold ( +.030 SRP .010 down , Eddy 84cc heads 603 Voodoo cam appr. 10.1 CR)
How can the cranking pressure on a 10.0 CR engine be more than 10 times the air pressure where I do the test?
Carbon build up? Do I now have a 12.5 CR engine?
Im not following your math to calculate? The numbers from your compression test are not a direct relation to atmospheric pressure. You are not taking into effect cam profile.
If the atmospheric pressure in my garage is 14.4 PSI. The compression gauge reads zero at this point. Shouldn't a 10 CR engine produce 144psi on the compression test? ( 10 x 14.4 PSI = 144PSI) Maybe less with a longer duration cam?
Quote from: c00nhunterjoe on January 05, 2021, 12:09:37 AM
The numbers from your compression test are not a direct relation to atmospheric pressure.
/quote]
Explain please
No. It is not a direct formula. You are not taking into account cam timing and valve overlap etc.
I can see how intake valve closing time could reduce the pressure seen on a cranking speed compression test, but I can 't understand how the cam choice would raise the pressure above the mathematic ratio? :shruggy:
Supercharging could do it or turbocharging at operating speed.
The numbers I got (135-140) where good from a cylinder to cylinder variation point. When I get the heads off and look at the pistons I will have a better Idea of what direction to go with the new heads. Thanks.
If you are blowing 135 with a 484 purple cam, im betting the pistons sit further in the hole then you think.
I had the same question, but then looked more into it.
The process is not as simple as P1/P2 = V2/V1. There is a part of it that adiabatic, or at least partly so. As a result, when the gas is compressed (rapidly) the temperature of the gas increases and this means that the measured pressure exerted by that gas also increases. Apparently cranking speeds are fast enough and cranking speed (determined by how good your battery is also can make a difference).
:iagree: Yep, compressing air makes it hot and hot air expands which raises the cylinder pressure.
Quote from: c00nhunterjoe on January 05, 2021, 12:31:29 PM
If you are blowing 135 with a 484 purple cam, im betting the pistons sit further in the hole then you think.
Yea, I thinking that too.
Quote from: XH29N0G on January 05, 2021, 01:26:26 PM
I had the same question, but then looked more into it.
The process is not as simple as P1/P2 = V2/V1. There is a part of it that adiabatic, or at least partly so. As a result, when the gas is compressed (rapidly) the temperature of the gas increases and this means that the measured pressure exerted by that gas also increases. Apparently cranking speeds are fast enough and cranking speed (determined by how good your battery is also can make a difference).
I will look into it more. Thanks
I live at 6500' above sea level in wyoming. My 440 has 160lbs compression. Compression ratio is volume. If your motor is 10.5 : 1 when the piston is at BDC you have 10.5 times more volume of fuel/air than you do when the piston is at TDC. That's why dished pistons, valve reliefs, open chamber/closed chamber heads etc all make a difference on compression ratio. Why my charger has 160 psi at 6500 ft. Elevation