Cold/dry compression test

[Jimmy Jakobsson]

Hi! I did a compression test on my 493 stroker engine with about 400 miles on it, 231/237@050 camshaft, 10,04:1 scr, it have stand now for a few mounth during the off season, i just wanted to see what i have. The compression test show 200 psi on all 8 cylinders, i crank it over 5 times on each cylinder, the gauge swing up to 150psi on the first pulse, 75% of the final value, this must be pretty good on a cold/dry engine, i also skirt in some oil and do the test again and get the same on all 8. I also did a leak down test and that show 13% on all 8 cylinders, that makes me wonder a bit so i contacted Mahle engine components about it, i have their pistons and metric ring pack. He told me that he only use leak down numbers as a secondary information, especially on a metrical ring pack that relies on gas preassure to seal rather than mechanical tension and told me my compression test has great number with my camshaft and that i probably going to have many years of service from my big Mopar. What do you guys think?

 

[Don Frelier]

I think you are all set. Start driving it.

 

[69L48Z27]

Right on. Not much to think about when compression is even and that high.

 

[Jimmy Jakobsson]

Thanks guys for your answers, im going to start enjoy the car

 

[BSB67]

I'm always a little suspicious of cylinder cranking tests conducted with today's hydraulic lifters.

 

[Don Frelier]

Hopefully you have good fuel. With 200 psi if you have iron heads you may start getting predetonation once some carbon builds up.

 

[1966satellite]

Hi! I did a compression test on my 493 stroker engine with about 400 miles on it, 231/237@050 camshaft, 10,04:1 scr, it have stand now for a few mounth during the off season, i just wanted to see what i have. The compression test show 200 psi on all 8 cylinders, i crank it over 5 times on each cylinder, the gauge swing up to 150psi on the first pulse, 75% of the final value, this must be pretty good on a cold/dry engine, i also skirt in some oil and do the test again and get the same on all 8. I also did a leak down test and that show 13% on all 8 cylinders, that makes me wonder a bit so i contacted Mahle engine components about it, i have their pistons and metric ring pack. He told me that he only use leak down numbers as a secondary information, especially on a metrical ring pack that relies on gas preassure to seal rather than mechanical tension and told me my compression test has great number with my camshaft and that i probably going to have many years of service from my big Mopar. What do you guys think?

I don't think 200 psi is possible with 10:1 compression ratio.

 

[Don Frelier]

It is.

 

[1966satellite]

It is.

Atmospheric pressure is about 14.7 psi. Wouldn't 10:1 cr give 147 psi?

 

[Don Frelier]

It's not that simple. The temperature is changing as well. I know from experience you can get 200 psig on a 10:1 440. It's not a simple Ideal gas law problem.

 

[beanhead]

So much depends on where the cam is installed. Advanced will give a higher number. That cam may be closing the intake a bit soon for a 493, maybe not but I use it in a 440(bored to 452) and with a true 9.5 measured SCR it cranks 160-165, in at 106.5°
I'd try it with another gauge to verify. In the end, what matters is all cylinders are close to equal, and it doesn't rattle when you drive it, with full timing applied..

 

[Jimmy Jakobsson]

So much depends on where the cam is installed. Advanced will give a higher number. That cam may be closing the intake a bit soon for a 493, maybe not but I use it in a 440(bored to 452) and with a true 9.5 measured SCR it cranks 160-165, in at 106.5°
I'd try it with another gauge to verify. In the end, what matters is all cylinders are close to equal, and it doesn't rattle when you drive it, with full timing applied..

This cam close 63,5 degrees after ABDC, it is pretty mild for 493 cui. I get exactly 200 psi on all 8 and that is veryfied with a second gauge to. If the lifters have bleed down a bit its posdible the intake valve close slightly earlier and you get slightly higher value, but still they are all exactly the same and also no difference when adding oil

 

[beanhead]

...they are all exactly the same and also no difference when adding oil

That's a good thing!

 

[Jimmy Jakobsson]

One more thing, it shows 150 psi on all cylinders in the first pulse, thats 75% of the maximum 200 psi, i crank it over 5 pulses on all 8. Its a very good sign

 

[Jimmy Jakobsson]

Hopefully you have good fuel. With 200 psi if you have iron heads you may start getting predetonation once some carbon builds up.

I have not hear anything yet on 93 octane but i also only have 400 miles on the engine, the set up is: 4,15" eagle forged crank, eagle 6,760" H rods, Mahle forged flat top pistons, Mahle metric plasma moly ring pack file fit, Edelbrock e-street 84cc cylinder heads, Comp cam xe275hl 231/237@050 camshaft, Comp cam hydraulic lifters 822-16, Comp cam 3/8" pusch rods, prw steel roller rockers, King main and rod bearings set at 0,002" clearance both main and rods. Ignition timing 16 static, manifold vacuum to almost 30 at idle and 34 full at 3200rpm, the engine only going to be used on the street

 

[Splangj]

Wow, 30 inches of vacuum at idle! That makes your cam seem very mild, explaining your high cylinder pressure. Your engine sounds like it’s a very good build with those equal readings, someone did a thorough job!

 

[Don Frelier]

I think he was saying he has 30 degrees advance with the vacuum advance canister plumbed to manifold.

 

[BSB67]

Wow, 30 inches of vacuum at idle! That makes your cam seem very mild, explaining your high cylinder pressure. Your engine sounds like it’s a very good build with those equal readings, someone did a thorough job!

30 inches (hg) is impossible.

 

[Jimmy Jakobsson]

30 inches (hg) is impossible.

I was not talking about vacuum readings, i say the vacuum canister connected to manifold vacuum drag the ignition timing to 30 degrees when the engine idles. My vacuum reading is steady 15 hg/inch on idle

 

[Jimmy Jakobsson]

Wow, 30 inches of vacuum at idle! That makes your cam seem very mild, explaining your high cylinder pressure. Your engine sounds like it’s a very good build with those equal readings, someone did a thorough job!

Thank you very much, i have built the engine myself, the readings was very equal, exactly the same on all 8 and also that the gauge swings up to 150 psi on the first pulse is a good sign.