Compression ratios?
Compression ratios?
(OP)
Sorry to open up a much debated question. Please bear with me.
I underatand how to obtain a ratio, but I am stuck on finding the piston dome volume. Easy if you have a dish or a dome only using the 1 inch down method BUT........
I have a piston which has a dome, different depth valve reliefs and both flat `lands` are at different heights (its out a m/cycle motor) so where do I take the 1 inch measurement from?? (obviously this will also alter the total burette cc......
If I take it from the crown (highest point) I dont think this will measure the dome, only the arear around the dome but I cant find the lowest point as valve releifs are rounded and slanted....
Any help, much appreciated.
I underatand how to obtain a ratio, but I am stuck on finding the piston dome volume. Easy if you have a dish or a dome only using the 1 inch down method BUT........
I have a piston which has a dome, different depth valve reliefs and both flat `lands` are at different heights (its out a m/cycle motor) so where do I take the 1 inch measurement from?? (obviously this will also alter the total burette cc......
If I take it from the crown (highest point) I dont think this will measure the dome, only the arear around the dome but I cant find the lowest point as valve releifs are rounded and slanted....
Any help, much appreciated.





RE: Compression ratios?
Norm
RE: Compression ratios?
One, I put one in a cylinder and did the 'one inch down'.
Next, I wrapped the piston with a sheet of plastic and took direct measure. Lastly, I took an assembled combustion chamber volume through a spark plug hole.
Averaged the three and called it "good".
If you ask the piston maker, they can usually tell you the approximate volume from experience...even they often do not know exactly. With large valve sizes and extra deep cut outs in a hemi, the volume of a large pop up can be cut in half, easily.
Rod
RE: Compression ratios?
its a 4 valve jobby.
The compression height is I think taken from the left hand flat, it is closer to the top of the barrell by about 1mm.
Thanks
RE: Compression ratios?
RE: Compression ratios?
RE: Compression ratios?
If it's "cc the piston with it positioned exactly one inch down in the cylinder bore. Fill the area above the piston with fluid, then subtract this volume from the calculated cylinder volume at one inch and you get the piston dome displacement in ccs. " then I'm going to stick with my original comment - pick any point on the piston at TDC, and move that point down an inch. (all the other points will be down an inch from their TDC positions too! Friggen amazing! It's almost like picking a datum or something!)
RE: Compression ratios?
Go to TDC, move 1" down an measure volume compared to the displacment from 1" of travel.
That gives the volume displaced from the chamber at TDC
Regards
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RE: Compression ratios?
But then again, if you take it from the crown aren`t you just measuring the area around the dome and not what the dome itself displaces? Arent I looking for the dome displacement to take from the head chamber cc, not the area around it as at TDC you have already measured a portion of that in the head cc??
I`ve tried the piston 1 inch down from the crown and filled the area with liquid from burette = 73cc.
Now if I leave the piston where it is and make a calculation from 1 inch from crown and then same but for distance from flat lands I get different results, i.e. 4cc???
Too much math is making my brain hurt!
RE: Compression ratios?
Maybe it's just a matter of semantics, but I prefer to think of setting to a position relative to the deck surface rather than TDC, because the two don't necessarily coincide once you've assembled the short block.
Norm
RE: Compression ratios?
Regards
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RE: Compression ratios?
If you start at TDC and move down an inch, then you can always calculate the volume at TDC as (volume in the head) + (volume in the cylinder at 1" below TDC) - (cylinder area * 1") + (fire ring area * head gasket crushed thickness)
Depending on design, the head gasket crushed thickness can sometimes be the hardest part... but it's probably more important for getting piston-to-valve clearance right than for getting compression ratio.
RE: Compression ratios?
It's obvious Pat has done this before and Isaac IS a math whiz, but, come on. This is really not rocket science. I've been doing this since I was 18 and I learned it from an engine builder that never even graduated primary school.
The one inch down deal is just figurative. If one inch down clears the dome, fine. If it does not, as in the case of many motorcycle pistons I have built, then two inch or whatever. My math sucks, that is why I always use three methods to find the end CR.
I tried a computer program once, but it was so convoluted that I got lost half way through. The easiest way on a hemi or pentroof is to just fill it from the plug hole (provided you already know bore/stroke/cylinder head volume)
Yes, Isaac. Valve clearance is most important...especially if, like me, you are using camshafts with radically different timeing and lift. But, it's pretty difficult to get the CR correct on the first try if you have nothing to go on, a previous piston or such. Valve clearance cuts just throw the CR calcs into a 'cocked hat'. Trial and error, very expensive T & E !
Rod
RE: Compression ratios?
RE: Compression ratios?
I guess static compresion means little anyway in the grand scheme of things once the motor is hot and running at xxxx amount of revs, but I couldn`t get that damn 1 inch thing outta my head!
As it happens I used over an inch last night to get the dome in, all clearances seem to be ok, i have .080 on the valve to piston.
Ivymike, that last post, your right, I had enough trouble with this let alone something of that nature, i`ll leave that to you NASA types!!
RE: Compression ratios?
Running engines on beds is not rocket science. All OEMs do it. It does cost a lot, but then so does type approval.
- Steve
RE: Compression ratios?
Clay sucks!
Rod
RE: Compression ratios?
Incedently, what does everyone use as fluid to measure the cc? I use an old barrell of red x thin fuel additive purely because I have boxes of the stuff and it seems to give me the same result as dyed alc.
RE: Compression ratios?
Heck, beer would work except for the froth and the wicked waste.
I have used petrol, but it evaporates a bit fast.
I have used cleaning solvent from the parts washer.
Use clay or plasticine for the piston to valve and piston to head clearance.
Regards
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RE: Compression ratios?
I've used ATF and water with Redline Water Wetter once in a pinch (dish soap 'bubbles') but I do not recommend it.
Pat, "clay sucks"!!!! I drilled an old piston yesterday for a generic roll pin just to see how it would work...I like it!
Rod
RE: Compression ratios?
RE: Compression ratios?
Valve cut outs in a hemi eat up lots of "dome"! I really would like 14:1 but the half inch lift at the valve precludes that as the dome gets too high and disrupts flame travel and power becomes erratic...At least that is what the 'experts' seem to think. I've use 13:1 on several hemi headed engines to good success, the Lotus twincam included.
(I use 14:1 on the Mini...but that's apples and oranges)
Rod
RE: Compression ratios?
I would think with a hemi with the plug in the middle, the flame would travel down each side of the dome no problem as it would be established well before TDC. t might work bettr if the dome promoted a bit more flame travel down the inlet side to offset the effect of the heat from the exhaust valve. How much is anybodies guess.
Fame travel over the dome can be a real problem with wedge heads
Regards
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RE: Compression ratios?
Rod
RE: Compression ratios?
wedge combustion chambers with inclined valves put the deepest portion of intake valve notch pretty well in from the piston OD.
http://members.cox.net/raunch/fabian%20r11.jpg
Hemis put the deepest portion out near the piston OD, and with big sharply inclined valves things get close to the ring groove real quick, requiring a lower ring placement.
http://
http://
http://jhau.maliwi.de/mot/img/gs80head06.jpg
When BB Chevys first came out, notching for aggressive cams would break into the ring groove. Some guys ran them that way, but I wouldn't.
http://www
RE: Compression ratios?
Rod