QwkTrip

I am failing reality check when doing a baseline calc of static compression ratio. I know stock engine is 11.0 compression but my math keeps coming up with 11.25. Can somebody check my numbers?

These are all nominal numbers, I have not measured anything on my engine yet.

Bore: 4.125 inch
Stroke: 4.0 inch
Gasket bore: 4.14 inch
Gasket compressed thickness: 0.051 inch
Piston valve relief: 5.3 cc
Piston deck height: 0.005 inch above deck
Cylinder head volume: 70 cc

With these numbers I come up with,

Volume at TDC = 5.215 cubic inch
Volume at BDC = 58.671 cubic inch
Static Compression Ratio = (58.671 / 5.215) = 11.25

Mordeth

Change head gasket bore to 4.15". Also, which calculator are you using?

http://www.wallaceracing.com/cr_test2.php

https://www.uempistons.com/index.php...55ff4c1df7db00

Josh B.

I got .054" on my dial calipers for the stock HG. If you leave the piston to deck at .000 on the wallace calc and go to a .054" it gives out a 11:1. The -.005-.006" PTD distance might have been a cheater type move by the engine designers to tighten the quench distance without going to a thinner stock HG, which OEMs tend to avoid.

Seems as though the factory SCR was higher than advertised, interesting find.

QwkTrip

That is too small of a change to make any appreciable difference. Looking for a bigger mistake in the numbers than that.

I am using a spreadsheet I created. Delivers same numbers as both the links you provided.

QwkTrip

I think 0.054" is the uncompressed gasket thickness. 0.051" is the compressed thickness, which is what should be used.

Josh B.

I measured the thickness under compression.

Mordeth

No it doesn't. Go to the links and enter the numbers. I would check your spreadsheet calcs.

Compression Ratio Calculator

Number Of Cylinders: 8
Bore: 4.125
Stroke: 4
Head Chamber (cc's): 70
Valve Relief / Dome (cc's): 5.3
Gasket Thickness: .051
Gasket Bore Diameter: 4.15
Deck Height: .005

Your displacement is 427.65 CID
The Compression Ratio is: 10.99

Josh B.

The deck height needs to be a negative value since there is piston protrusion, puts the SCR at 11.24 with a -.005" value.

Mordeth

Well that changes things. I did not know that.

QwkTrip

There seems to be about 2 cc of volume unaccounted for in order to get down to 11.0 compression. But where is it?

Another quick reality check....

It is fairly common to see 0.030" milled from heads. That would bring the head from 70cc down to 65cc (assumes 1 cc per 0.006" milled). Using the straight math says the new compression ratio would be 11.9 with stock gaskets. Including the 2 cc fudge-factor would bring compression to 11.6. Which is the real answer?

Mordeth

Mine measured 64.4cc after a 30 thou mill.

MTPZ06

This may help? https://www.corvetteforum.com/forums...ittle-off.html

QwkTrip

Thank you!

Mordeth

This was bothering me a little bit so I just went back through all of my notes from my log when I had my motor out. There was no piston protusion. It measured .001" in the hole. I then assumed .005" in the hole at temp. I am guessing it varies from motor to motor due to tolerances allowed from GM on calculated deck height and piston compression height.

I am sticking with my own calculations on the original static compression of my motor, which was derived from measurements.

0HP RESEARCH

The only way to know "true" deck height is to measure it using all of the real parts....otherwise you are just assuming. You will typically see notes on a lot of the aftermarket piston spec sheets as to what they assume 0 to be. Some aftermarket pistons will even build the pistons slightly under deck height as they assume you will have to clean up the deck surface when rebuilding.

Mordeth

I understand. But won't the deck clearance grow once the aluminum block is at operating temp? I have always assumed it would. If that assumption is wrong then I would like to know that. When I measured it cold I then figured that I need to make an assumption about changes to the deck height at full temp. This assumption means that my piston would "be in the hole" and not protruding when the engine is actually operating. If this is wrong I'd be happy to be corrected so I can correct my own measurements. I measured as much as I could accurately measure and then had to make some assumptions. I am guessing at some point there will always be assumptions, which can be made more accurate by additional measurements deemed reliable.

0HP RESEARCH

You are correct, everything moves as the engine warms and the piston is going to drop from the deck the hotter it gets.

You could figure 6-8 thou change as the engine warms (so if it's 5 out it would be 1-2 in the hole hot).

That being said, as the engine ages, carbon is going to build up and increase compression slightly too.


As far as we have ever seen, most printed stats are done on a cold engine, not assuming changes for temp.

Dan_the_C5_Man

I love this stuff.. I want to play..

So, understood that the block will grow, got it. But is it also true that so will the rod, and piston? And is it also true that the effective deck to piston height will be X at 1,500 RPM and Y at 7,200 RPM? (Rhetorical - we know these do move). Or is it in this case with the materials and quantity used, you wouldn't have a measurable change until 10,000 RPM?

There was a very cool article / PDF I read last year that was related to building LS engines, and they were going on about how machining a block in a shop at 60 degrees F would have a different result at 80 degrees F.

F1 engines can't even turn over cold, they have to be pre-heated first as I understand it.

The interesting thing is Everything is moving, all the time, just to varying degrees.

QwkTrip

Regarding changes of rod length, I think if you follow general guidelines for quench distance then the piston won't hit the deck of the cylinder head. Same goes for general guidelines of piston-to-valve clearance. That's not the answer you were looking for but it is probably a good answer for the practical minded.

I'm not concerned about change of compression with engine running. Just putting together a simple street engine so getting down to the gnats *** isn't important to me. I will error on the side of conservative if there is any question.