Compression

Did you know that when changing thickness of head gsk's. that there is a bigger change when going from a .060 to .050 than going from a .080 to a .070. A .010 change in head gaskets does not always result in the same amount of change in compression. As you increase the compression the .010 reduction results in a greater overall compression change.
Just food for thought. If you agree or disagree let me know why.

 

This is probably because as you bring the chamber down closer to the piston, volume taken up by the piston is not linear because the piston is not perfectly symmetrical and gets wider the farther down the piston you go until you get across the bore size of the piston.

I'm sure what you're talking about is all relative to the shape of the piston

 

Wouldn't it be more relating to the CC of the combustion chamber?

Heads will vary via make and brand... Sometimes you can take .005 off and remove 1CC from the head, other times it's 2CC or .5CC.

If you think of the gasket as part of the head, I would think it would have to be linear until you actually got into the tapered part of the combustion chamber.

Do you have specific information regarding this claimed non-linearity, or are you throwing this out for conjecture?

 

Actually WJ both!
I have been going under the asumption that the reduction or increase in compression on a hemi would be the same or linear with ea. change of .010 in the head gsk. However I am working on a new piston change for next year and using a compression calculator to come up with the actual compression for ea. .005 change in gasket thickness and noticed that as you raise the piston in the chamber the increase is not linear. I have not figured out why yet but think it might have something to do with the ratio of the cc change to the cc's left in the combustion chamber at TDC.
If you know why tell us all or maybe someone who does will read this and respond.

 

Sometimes it helps to think of things on the extreme side. If you had a head gasket so thick that the surface of the head was above the peak of the piston, then the compression change w/ .010" would be linear..as you went up...or down as long as you didn't get to the piston heighth. However w/ the piston in the mix, your TCD cc change is greater for every .010" change as you get closer to the deck surface because the piston is sticking farther and farther in the chamber filling all the gaps. Where as w/ the extremely thick head gasket case the piston top never breaks the plane of the head surface. Kind of like how a change from a .150 to a .155 jet change is a way bigger percentage change than a .100 to a .105 jet change. Confused now? me too...i'm taking a break

 

Makes perfect sense.

If you're not breaking the plane of the combustion chamber with the dome of the piston, then you're removing a perfectly circular slice of volume.

If the piston is at top dead center, then the dome of the piston combined with the size of the combustion chamber would change in proportion.

So in order to find out the actual compression, you'd have to know the given dome cc and head cc at 'x' deck height in order to calculate a compression ratio change, correct?

 

Buy a beaker and CC the cylinder at tdc is the best way Buy yeah, that's basically it. You'd need to know the cc of the piston dome, cc of the head camber, and how far the piston is in the hole, as well as how far down the first piston ring is, then obviously totall cc of the cylinder at BDC. Diamond has a good page for doing all this by just plugging the #'s in, ck it out:

http://www.diamondracing.net/cocalc.htm

 

Think about what you are asking ?
RATIO you are also making a ratio change of removing chamber volume .You are removing the same total volume of chamber with each .010 you remove but the ratio is not the same with each change.
To make a simple example of this, if your head gasket change got you a total loss of 10cc you remove 10cc from a 1000cc chamber that is a 1% change , now remove that same 10cc from a 100cc chamber and you made a 10% change by removing the same volume. The smaller the combustion chamber the greater the loss with removing the same volume .

 

Not to steal the thread, but along the same topic line.

Has anyone ever done any testing on compression ratio versus boost.

Assuming the same total compression ratio with static + boost....

Does a low compression high boost motor make comparable power to a high compression low boost motor?

I would think that it would, but that it would have a different torque curve and responsiveness. But, I don't have a means to test this, nor can I find anything online about it. I'm hoping one of you guys who race T/A can shed some light on it via actual testing done.

 

Compression Questions

Interesting question, try and post your question here http://speedtalk.com/
very good site for this type of stuff. I would think you would get lot of discussion on the subject.

 

Robert what you posted is pretty much what I have figured out. It is the ratio of the change to the combustion volume at TDC.
It doesn't matter where the piston is in the bore. If you remove .010 from the head gasket the piston will displace the same amount in cc's. Lets look at the extreme. Say you only had 10 cc's left in the combustion chamber and you removed 2.5 cc's with a head gsk. change. The ratio of 2.5 to 10 is far greater than 2.5 cc's to 100 cc's.
I have been doing this for a long time and no one has even mentioned this in the past. It is just one more thing you might need to consider when working out a new piston combination. If you are under the asumption that ea. .010 change in head gsk thickness gives you the same amount of increase or decrease in compression you have been assuming a falsehood.
Usually when ordering pistons you will give them the head gsk thickness you want say a .070 and the desired compression. As the weather and adjusted altitude change though and you make a gsk. change it is nice to know exactly where you are at.

 

An interesting note to this thread. Have any of you nitro guys actually figured out the compression change that occurs due to the amount of nitro that your putting into the cylinder. It's about .7 of a point with about 23 GPM. Even on an alcohol motor it makes a .13 difference in compression.

 

Mike,
I knew that it was a factor but didn't have a clue as to how much.
Where did you get this information? It would be very difficult to figure as if you are injecting 20cc's per turn into the motor how much is actually left in the cyl after the exhaust valve closes to be compressed and ignited? Lots of this fuel is blown out the exhaust pipe.

 

I have a program that calculates the compression with head gasket thickness changes and also shows the change for the amount of fuel. I was tuning a nitro car for a while so I made an excel program to help. Your right about the excess fuel loss out the exhaust. My program doesn't account for it. You could apply a correction factor to account for it but it would really be a guess-timate. My program has correction constants that you change to make the results match the actual computer information from your car.

 

warhawk,

I am not quite sure what you mean in your compression comparison. Volume is volume. If you have the same size bore in the gasket then the change from .080-.070 and .060-.050 is the same. For example if you has a 4.560 gasket bore the .080 gasket cc volume is 21.41. The .070 is 18.73. 21.41 - 18.73 is 2.68cc's.

The same bore 4.560 at .060 is 16.06 and .050 is 13.38. 16.06 - 13.38 is 2.68cc's.

So if you are removing the same 2.68cc's from each change your compression ratio will change equally.

Bob
Auto Race Industries
BuyItClub.com

 

Bob
You are correct about the .010 gasket change being the same for each change. This will occur regaurdless of the position of the piston in the bore.
However if you use a compression calculator program you will find that as you reduce the amount of area above the piston (cc's) that the increase is non linear and actually increases with ea. .010 change. The best I can figure out is that it has to do with the ratio between the .010 change and the remaing cc,s to be compressed. Check it out for yourself.

 

Mike,
Very interesting program but as you say and I agree it would be hard to calculate how much fuel is in the chamber at the time of compression. The best way would be to monitor the actual combustion pressure which can be done. There are just to many variables.

 

Bob ,yes you remove the same #of cc but it is a different "rate of change".
Maybe this is a better analogy.
If you had a engine with a dead flat head ,a flat piston at zero deck height and a .040" gasket and you remove .010" at a time ,the volume you remove can only be the the same amount , but your rate of change is not the same.

Take the .040" gasket and remove .010" and your rate of change from .040" to .030" was 25% of the chamber volume.

Now take the .030" gasket and remove .010" and now your rate of change was 33.3% of the chamber volume.
Now take the .020" gasket and remove .010" and now your rate of change was 50% of the chamber volume.
Now take the .010" gasket and remove .010" and now your rate of change was 100% of the chamber volume.

 

Robert,
Thats exactly what I have been saying only you used a good example.