Q on compression ratio

[purple69]

i am boring out my AS21 case to accept 94mm pistons. i was just wondering.......how will this affect my compression ratio? i am using stock heads that will be P&P. will i have to remove a ton of material from the combustion chamber to get a lower compression ratio (around 7-7.5:1)?


the way i look at it is that my deck height shouldn't change....and the chamber size will increase proportionally to the piston size.

i'm just questioning this because someone passed a comment the other day that made me second guess myself.

[Marc]

Any time you increase displacement the CR will go up if you don't do anything to compensate.
A typical stock 1600 with 52cc heads and .060" deck (8.75cc) will have a CR of 7.52:1. (396.16+52+8.75)/(52+8.75)
By increasing the bore to 94, .060" deck would yield 10.58cc; assuming that the chamber volume wasn't reduced at all when the heads were bored out (it certainly can't get bigger from that process as you are apparently thinking but it could get smaller), that'd give: (478.84+52+10.58)/(52+10.58) =8.65:1

There is NO reason to believe that your deck height will be unchanged - the case will need to be decked which would reduce it, but there have been reports of 94s giving out-of-the-box deck heights of well over .100" - you need to measure.

To determine the total volume needed in the chamber & deck to give the desired compression ratio, divide the cylinder displacement by one less than the desired CR - if you want 7.5:1 in a 1915, divide 478.84 by 6.5 = 73.67, about 11cc more than the example above. You'd either need to open the heads up by 11cc or run another .062" in deck height/head gasket, or some combination of the above. You could also have a dish milled into the piston top to add cc's there but 11 is a lot - you should be able to add 7-8cc safely, you get 1cc for every .010" depth of a 71mm diameter dish.

[vicsvw]

This make it easy to figure -> http://www.1800vw.bizhosting.com/motor.computer.htm

[purple69]

what is a good deck height to run?

i was thinking of some 0.060 cylinder shims, some copperhead gaskets probably 0.060 and also taking some material out of the combustion chambers.
i would like to unshroud the valves a bit but i'm not sure exactly how much i will take out.

[Chris V]

what is a good deck height to run?

.060" and below is considered to give greater performance. Anything over .060 is over .060 and doesn't matter anymore, eg .080 is no better than .120

So if you dished the pistons and reworked the chambers you could get away with .060" deck, according to Marc's statement. But remember, you will not know until you measure your case, heads, and P&C's.

[Marc]

.060" and below is considered to give greater performance. Anything over .060 is over .060 and doesn't matter anymore, eg .080 is no better than .120...

I might not go that far, I mean .080" probably is better than .120", but that's the concept - once there's insufficient "squish" to generate mixture turbulence, more deck can't really make it any worse.

...i was thinking of some 0.060 cylinder shims, some copperhead gaskets probably 0.060 and also taking some material out of the combustion chambers.

Remember that when it comes to the shape of the chamber at TDC it doesn't matter if you have .060" deck and no head gasket, or zero deck with a .060" gasket.

[purple69]

AH!!! i never thought of that.......i'll have to take that into consideration.

i don't even have all of the parts for my engine yet.......but when i get closer i'll see what my CR is going to look like. atleast now i have some idea of what is in store. Thanks a ton guys!!

[factman]

actually Mark i have read in some performance books that a combustion chamber with zero deck height is more efficient than one with some deck height. i would use the head gasket as it helps to seal the combuston chamber more just incase you overheat the motor, copper can expand a little bit and will conform more than the aluminum head will if it distorts a little

[Marc]

actually Mark i have read in some performance books that a combustion chamber with zero deck height is more efficient than one with some deck height. i would use the head gasket ....

I'm not disputing that point (although it is overrated IMO, important when wringing the last HP out of a drag motor but pretty insignificant for a stocker or most drivers) - what I'm saying is that the "deck height" volume is the same either way, regardless of whether there's cast iron or copper establishing the distance between the piston top and the cylinder head, the effective chamber shape is the same.
Head gaskets shouldn't be necessary and if you don't use them you don't have to worry about them failing. They aren't a new idea, they were tried 30 years ago and rejected because they caused more problems than they prevented...history repeats itself, though.

[general_lee_jr]

I run a mild 84:1 CR on my 1300cc rice burner motor. I burn jet fuel and everything else on the car is stock except my 4 105/15's ultra low tires.