Compression ratios? 4

[gtrack]

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.

 

[ivymike]

...and if you don't add too many, and you don't run too long, you can use 'em hot.

 

[evelrod]

Well, it's too late for this engine, but when a rebuild is in order, I'm going with another, local, piston mfgr. as I am not too pleased with these Venolia's (top ring is too far down) and the CR ended up at 12:1 and I wanted 13:1 or higher.
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

 

[patprimmer]

Rod

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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[evelrod]

I ran 13+ in the 80's but that engine was super critical on valve timing. Lift was only .450" so I had a bit more meat...still, if the timing was off even 2 degrees, the power and tq fell off a cliff. This new engine has cams with more lift and the ex cam is 30 degrees shorter (250 @ .050" presumably better mid tq.) So far I'm running them at 5 deg. retard for max tq, but hp is well off. It's a boxy little saloon and needs the tq, but I'm really getting too old for all this testing...I just wanna get it right the FIRST time...yeah, riiight! This is the third set of pistons in this engine this year. It's hard to just give up when I know I can do better.

Rod

 

[Tmoose]

re: ring placement dilemma

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 r11.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://www.jepistons.com/cat/je/auto/mopar/img/118758.jpg

http://www.moparstyle.net/history/images/HemiDrawing1.jpg

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.jepistons.com/cat/je/auto/chevy_bb/img/111596.jpg

 

[evelrod]

Tmoose, the hemi piston pictured is almost exactly what I ended up with if you deepen the notch another 0.060" (I'm guessing from looking at my piston v your photo). The cam grinder and cylinder head guy had a special set of pistons made up to give him 12.5 and, form looking at them, I think I can raise the pop up another 0.060" or so getting it closer to my ideal 13 to one. Anyway, that's a project for next year. I've gotta get the Mini ready for the November race at Willow Springs. Test session for the Lotus is in a couple weeks after which I'll have a better handle on what I may or may not do. It may end up ok for a vintage racer with an inexperienced pilot (my son). We'll see.

Rod