CR vs Cylinder pressure

[Mike T]

I changed a few things when I built my motor that would have affected the CR. I did not use an exact method of measuring the changes made but I guessed it to be about stock when all was done. I just went out and did a compression test stone cold and came up with 152 psi. I know this is not the correct way to check CR but wouldn't this be around 8 or 8.5:1 or about stock ?

 

[earlbrown]

No where enough information. Cylinder pressure is dependent on static compression and the intake closing event. You can have a 13:1 engine that reads low if the intake stays open well past BDC and you make a gauge read higher by having lower compression engine that shuts the intake ASAFP.

Timing chain stretch, long duration intakes lobes, retarded timing can all work to lower the PSIG reading on the gauge (and lower efficiency).


Don't feel bad though, based on my personal first hand experience I've noticed a 100% change of being wrong when someone guesses their final compression.

 

[Mike T]

Earl if I am understanding you correctly I think you helped answer my question. It sounds like the cam could bleed off compression but not artificially inflate it when cranking it over. Is this correct? I am just trying to determine that the compression ratio is not extremely low say less than 8:1.

 

[earlbrown]

Right. Static compression is a mathematical number compromised from the difference of volume at BDC and volume at TDC. In real life, it's a worthless number. Dynamic compression is the same thing but it's the volume at IVC (intake valve close) and volume at TDC. That ratio is the real life number. It takes into account that you don't start making cylinder pressure until the valve is closed.

The gauge is the PSIG reading of Static compression minus how much stroke is wasted with the intake valve open (not accounting for leakage).

If you'd like to learn some more about it, do a search for 'dynamic compression'.

 

[Mike T]

OK , I read your reply again and you are saying that an early intake valve closing can raise compression #s. Any idea how much this may raise a compression test? I'm not looking for an exact # but I am trying to determine if it is possible that my CR may be lower than estimated.

 

[earlbrown]

Your CR is most likely lower than expected. On the engines I've measured .vs what the owner thought, they were always wrong on the high side.

As far as how many pounds on the gauge as an indicator, you just can't get there from here.


If you just had to know what the static ratio is, I guess you could build a clear cylinder with cc graduations on it. Put the piston at TDC and fill it with veggie oil. Note how much it took then drop it down to BDC and see how many CC's that is. Static compression will be the ratio of the too.


Actually, scratch that. For that to work you'd have to be on a stand do you could tilt the engine to get all the air out of the combustion chamber.


If you know the EXACT piston dish and the Exact combustion chamber CC, and the exact head gaskets you're running, you can make an educated guess and get pretty close.


Does your engine have a big ass cam and/or high ratio rockers and a dog until the boost comes up?

 

[Mike T]

If you know the EXACT piston dish and the Exact combustion chamber CC, and the exact head gaskets you're running, you can make an educated guess and get pretty close.


Does your engine have a big ass cam and/or high ratio rockers and a dog until the boost comes up?

The motor has JE pistons that were a bit higher CR than stock, block deck is stock height, combustion chambers have been reshaped, large flat faced valves installed, heads surfaced .020, stock style graphite HG. After all of the changes the head chamber held just a few cc's more cooking oil than the stock configuration.

The cam is small 204/214 and I think around 450 & 470 lift, 1.55 rollers. The car actually responds very well considering that I have such a large turbo with only a 3k converter. It seems to pull decent on the motor alone before boost starts to come in.

The reason I'm questioning my CR is that after a dyno session the tech said that the CR must be much lower than 8.5:1 based on his experience.

 

[earlbrown]

I was thinking the JE's had more dish than stockers. That's only part of the equation as compression height plays a factor too. Not sure on that one. I know the pistons I'm making have a 22cc dish with infinite CH and it seems like those were going to give more squeeze than the JE units.

 

[Mike T]

I found a few specs, any educated guess would be great.

JE pistons 1.850 CH
29.5 dish volume
(.015 in the hole)

Heads 53 cc (after mods)

Felpro HG .063 (9441)

Stock stroke

 

[bison]

I found a few specs, any educated guess would be great.

JE pistons 1.850 CH
29.5 dish volume
(.015 in the hole)

Heads 53 cc (after mods)

Felpro HG .063 (9441)

Stock stroke

.020 over with 3.9" assumed gasket bore 7.52:1, .030 over 7.53:1.


Posted from the TurboBuick.Com mobile app

 

[Mike T]

.020 over with 3.9" assumed gasket bore 7.52:1, .030 over 7.53:1.


Posted from the TurboBuick.Com mobile app

Thank you bison. That is lower than I expected now I'm surprised at the 150 psi cold compression test. So the additional 5 cc in the heads & .015 down in the hole drops it a full point?

 

[bison]

Thank you bison. That is lower than I expected now I'm surprised at the 150 psi cold compression test. So the additional 5 cc in the heads & .015 down in the hole drops it a full point?

The cranking compression and mechanical compression are two different things. I just use the numbers you gave me to determine the CR. Everything has to be converted to the same standard of measure since you have SAE and metric measures in your info. Then the swept volume and clearance volume determine the CR. 10% increase in clearance volume or swept volume without increasing/decreasing anything else would be a 10% decrease/increase, it's all simple math.


Posted from the TurboBuick.Com mobile app

 

[Mike T]

I have been reading up a little since you & Earl have replied. It is hard to believe how much little changes can affect CR. I found that if my .063 head gaskets were reduced by .030 in thickness the CR could be raise by almost .5 & that the extra 5cc in head volume has lost about .3 in CR. I also found a CR/boost calculator online that shows 7.5:1 at 23 psi of boost would be around 19.22:1 under the right conditions. I know that figure is off a bit but even 15:1 would be kinda up there.

 

[earlbrown]

Is that the Wallace Racing java calculator?

From playing with the numbers, you can see how impossible it for people to guess their CR and get it right. It doesn't take much to move stuff around.

Keep in mind if you start thinking about other engines there's some relativity involved. A few CCs on our engines make a difference, a few CCs on an ocean liner mean nothing. It's also important to know that a CC carries less weight on a low compression engine than a high compression unit. If you're at 20:1 a CC will mean a lot more than taking one away from a 7:1 engine.

 

[Pronto]

I was thinking the JE's had more dish than stockers. That's only part of the equation as compression height plays a factor too. Not sure on that one. I know the pistons I'm making have a 22cc dish with infinite CH and it seems like those were going to give more squeeze than the JE units.

You can get JE pistons with a different dish that's more spherical. Mine are 20cc's.

 

[Mike T]

Is that the Wallace Racing java calculator?

Yes Wallace was one of compression/boost calculators & RSR was another that I messed around with. I also worked with a few different CR calculators and after trying a few different combos found that the # bison came up with is just about on target. I do see how a small error in any one measurement could really change the CR.

 

[Mike T]

You can get JE pistons with a different dish that's more spherical. Mine are 20cc's.

I could have sworn that my JE's were higher CR than stock also, but after searching their site I only found 29.5 cc dish listed. I did find that they now stock .010, 015, .020 & .030 over pistons and I would have used the .010 if they were available at the time of my build. Isn't JE now owned by Wiseco ?

 

[Pronto]

PMI owns them both. PMI is owned by Dover. http://www.pmi-brands.com/AboutUs.aspx

 

[Mike T]

IDK about one company acquiring all of those brands, seems like quality could suffer.

 

[earlbrown]

Customer service certainly did. That's one of the reasons I'm working with a Piston manufacturer to make 'custom' off the shelf pistons for our engines.