Dio, budget 72cc kit has 215 psi compression

[teeoce]

In my limited experience, I have never heard of calculating compression that way. Can anyone verify this method? I'm not denying it I am actually very interested in this. It seems to be accurate. It appears to account for decreased ring efficiency and give a more true compression, assuming you know what the atmospheric pressure is in your location.

From recent readings I understand that the compression ratio can also be determined by putting liquid in the plug hole with the piston at TDC, and then dividing your cc (72 in your case) by that number.

A few comments/questions:

When setting the proper squish, does this not directly effect the compression? Which is of more importance?

The reason your calculations of desired gasket thickness are slightly off is because you assume that the bore is uniform throughout the entire length of the cylinder and head. In fact, the bore decreases to zero at the top of the head. So if you wanted to be more precise you would need to determine the actually volume of the head.

Just my thoughts, I love this stuff.

[MediumKahuna]

...o-ringing and chamber mods, I was able to eliminate gaskets altogether. My piston easily clears the exhaust port and comes within a whisker of the cylinder rim at TDC...

I've heard of guys grinding their cylinders into works of art but I'm scared to try it. Besides my piston is above the exhaust port at BDC and a little above the top at TDC.
Also the thicker base will advance my exhaust timing and I'm guessing that might be a good thing. What do you think? I'll go measure what the timing is now, brb.
The used Malossi head I've got has an oring but it looks baked and I've had no luck at all finding replacement parts after wearing out two keyboards in the search. I'm not sure what material the oring is made of but I suspect its some kind of special high temp stuff.
I think the kit I got was meant to use the stock head but I heard that it didn't hold up well. Besides my stock head is pretty chewed up.

[teeoce]

Also the thicker base will advance my exhaust timing and I'm guessing that might be a good thing. What do you think? I'll go measure what the timing is now, brb.

I think it would be best to adjust the base gasket, as previously described, so that the piston is just low enough for the exhaust port to fully open. At that point, you know your port timing is good and then you can adjust the head gaskets to the compression you desire.

[Wheelman-111]

Greetings:

What T said!

How do you "chew up" a stock head?

Scooterwerx sourced a Teflon-like ring made of Viton (TM-R) good to 450 F. or so. I commend the quality and attention to detail in all SW's work products. Listen to his advice, he is a Craftsman.

My head dome measured out to about 7.5ml with plug in situ. The scooter's, I mean... Wheelman's cranial capacity could be considerably less. My kit is regular Malossi 31-**** for SE/05 except for the gasketlessness and Werx-modded head.

As you know, according to the SAE compression ratio is the value given to the volumes in the cylinder at BDC over TDC. I have never seen it calculated as a multiple of atmospheric pressure under STP until you mentioned it, but it is kind of a handy guide. Thanks for that.

At least wo things skew your method, however.

1. The act of compressing the air heats it, so the recorded pressure will be greater than predicted.

2. Not all the air in the cylinder will be compressed into the CC - some escapes out the ports as the piston rises, even on 4-strokes, which can have considerably delayed valve closing timing and overlap.

I agree with the Forum member who suggested cross-checking with a second compression gauge - even my trusty old Snap-On needs calibration periodically.

[MediumKahuna]

...I have never heard of calculating compression that way. Can anyone verify this method?...

I know you can compute the Compression Ratio (approximately) from the Compression but I'm not sure my formula is right. I really want someone to verify (or tell me the right way) too.
The liquid method you mention yields the displacement divided by the volume at TDC. My formula yields the volume at TDC plus the displacement divided by the volume at TDC - slightly different.
I'm getting the total volume at BDC divided by total volume at TDC and the liquid method gets change in volume between BDC and TDC divided by total volume at TDC.
So lets call the compression "C"
CR is the compression ratio
D is the displacement
VT is the total volume at TDC
VB is the total volume at BDC
AP is atmospheric pressure (14.7 psi)
and try the math:
VB = D + VT
VB / VT = C / AP
(D + VT) / VT = C / AP
D / VT + 1 = C / AP
VT = D / ((C / AP) - 1)
CR = D / VT (by liquid method definition)
CR = (C / AP) - 1

so the liquid method says that the Compression Ratio is the Compression divided by 14.7 then minus 1 instead of just of just the Compression Ratio divided by 14.7 as I was thinking.

I'll buy it but, it would sure be nice to find someone who knows for sure.

[teeoce]

I believe what you said is correct as far as the calculations go MK. Still not sure if that is a proven method but I wouldn't doubt it. I hope that it holds true because I wasn't looking forward to measuring TDC volume with a liquid.

For simplicity, lets say that we had a 50cc engine with a TDC volume of 5cc. This would give us a compression ratio of 10:1. But I believe the way you were stating it would be instead of 50:5 10:1 you were using 55:5 = 11:1. But I believe you are now correct.

The thing I like about your method is that it uses the engine psi. This accounts for ring wear and other things that can offset the true compression if you were to use the "traditional" Displacement/TDC Volume method. I believe the traditional method is very theoretical and implies perfectly sealed rings. Keep in mind my studies of this topic are very young.

[noiseguy]

I've got these equations in an engineering handbook somewhere... probably easier to find them online.

Bottom line is that 215 PSI is too high based on experience. Sounds like the base gasket's too thin if the exhaust ports not uncovered and the piston TDC is outside the jug. Raise the jug with a thicker base gasket or two and run the stock head gasket and I'm betting you'll be back in range. You can keep the conversation in PSI without having to get into compression rations and the like.

On the "squish vs. compression," they are really two different things that get talked about at the same time. For good squish on a small engine like these, you want .7 - .5mm of head to piston clearance (that's about 0.028" - 0.020"). Doing this, you want to keep the compression at a reasonable number; if it's getting too high you can decrease by creating more volume in the combustion chamber. % squish gets into the diameter of the combustion chamber vs. the edge portions, and is more around how to shape the combustion chamber. I don't know anyone on the board that's actively experimenting with head design, probably b/c the scoots need broad tuning and this is best met by the stock head shape.

[MediumKahuna]

How do you "chew up" a stock head?

I got the bike that way. The whole bike was way moded to the max. You would have to work hard to do that much damage. There are little pits in the inside of the head and on top of the piston like chunks of something kicked it in there for a little while. Come to think of it there were probably chunks in the bottom end too but I can't see em now. My bottom end might not last too long anyway

At least [two] things skew your method, however.

1. The act of compressing the air heats it, so the recorded pressure will be greater than predicted.

2. Not all the air in the cylinder will be compressed into the CC - some escapes out the ports as the piston rises, even on 4-strokes, which can have considerably delayed valve closing timing and overlap.

I think number 2. is really going to mess up the compression ratio formula especially, when using it to calculate desired gasket thickness. I think I need to use the distance from the top of the port to the height of the edge of the piston at TDC instead of the stroke in that formula. I would still use the compression divided by atmosphere but not call it the compression ratio. I would like to know what compression ratio really means but I'm most interested predicting the compression that a given gasket change will give me.
I'm calling the portion of the stroke while the port is closed the Pressure Stroke or PS (let me know if it has a name)
So:
AP is Atmospheric Pressure
CW is Compression you Want
CG is compression you got

Gasket thickness to add = ((PS X AP) / CW) - ((PS X AP) / CG)

If I got 180 psi, I want 150 psi, my PS is 28mm and AP is 14.7
then I need to add 0.47 mm (0.18") of gasket.
I'll soon know if that works.

I agree with the Forum member who suggested cross-checking with a second compression gauge - even my trusty old Snap-On needs calibration periodically.

Sage advice, will do
And thanks a lot for all your help Wheelman