Contour Enthusiasts Group Archives Forums Modifications Zetec Maintenance combustion chamber volume.

Now the compression ratio is nominally 9.7 for US engines and 10.0 for European engines, so I would expect the volume to be 57.1 cc? (Displacement = 497 cc per cylinder, so V * 9.7 == 497 + V therefore V = 497 / 8.7) Or 55.2 for Europe.

Actually the C/R for US spec Zetec is 9.6:1 so you might want to recalculate.

No problem. the actual volume would be measured once the engine is disassembled to eliminate any inconsistencies in each cylinder .
Mark.

I was informed by private message that the 9 cc discrepancy is accounted for by things like the thickness of the head gasket and the unfilled space around the piston. Which to me implies that the 47.7 cc figure may accurately reflect the shape of the head, but the 57.1 figure is more like what you should be using to calculate compression changes. Of course, if the compression ratio is 9.6 rather than 9.7, that makes 57.8 cc.

Since my own compression ratio was increased to at least 10.0 by shaving, I wonder how much was actually removed... the bore is 8.5 cm or a hair under, so a 1 cc difference is .0176 cm thick... boosting to 10.0 would reduce volume by 2.6 cc, making a shaving loss of: .046 cm, not quite half a millimeter. But it might well have been more than that, compression like 10.3 or something...

How high can compression go without knocking on 92 octane?

My best friends 78 280Z(yes, i realize I refer to this car waaaay to much but we work on it every weekend) runs 11.5:1 C/R on 92 octane. Sometimes 89 oct. No ping, knock, just pure tire roasting power.

Well in that case I suspect my C/R must be in the 11 range, because it can't use 89 octane any more except on cold days, and I'm pretty sure a good deal more than half a millimeter was removed from my head to correct warpage.

When the engine was reassembled, we measured an average of 160 PSI on a warm day at sea level, and that's with several cc of plumbing attached to the compression guage.

If air pressure was a hair over 15, then 160 would make a ratio of 10.5, and if there were 5 cc of slack in the tubing, that would make somewhere around 11.1 as the actual C/R... I think the plumbing slack may well have been more than that.

This is supposed to help give me tire roasting power, but somehow it's a bit less powerful than before the rebuild. Could it be that the computer just doesn't know what to do with 11:1 compression?

Your engine is still probably having trouble breathing, I know you have the CTA, but it may not be enough for 11.5:1 (jeeeez that's high), you reallt need to open up youe exhaust and intake some more. How many miles are on the car?

I don't know if it's as high as 11.5, it might only be 11.2 or so... but if you can sometimes run 89 octane with that 11.5 Z, maybe that's how high I am.

I don't know why you think it would have more trouble breathing with this compression than with normal compression. I wouldn't think these are related. It doesn't draw any more air than before. I don't think I can do much more with the intake than I am doing, except with a hypothetical new manifold. Exhaust will be dealt with at some point, I suppose, but I don't plan on doing anything drastic with it.

One question that occurs to me is whether high compression calls for any different spark timing. Would a chip's 91 octane tuning work better with this compression? Or would that just push its octane requirements up to 93?

It has around 57K, mostly not mine (it was once a fleet vehicle).

I realize now that my comment about breathing may not be entirely accurate. The Z got higher comp. via a stroker kit so it is pulling air in harder on the intake due to the longer stroke. You got your by decreasing combustion Chamber volume.You are on the right track about the Timing maps and fueling. If I were you I would look at warmongers post in the 3L conversion forum about the Greddy Emanage. It might just be the answer.