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Lower compression pistons effects on ignition timing

Introduction to Engine Tuning

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I am interested to know what difference a lower compression ratio in an engine generally makes, if any, to how much peak ignition cylinder pressure is created from adding timing. The reason I ask is because I am rebuilding the bottom end of my turbo D16 Honda Civic, and the new pistons will drop the compression from about 9.5:1 to 8.5:1. I didn't choose the pistons (YCP Vitaras) for any other reason than they are widely available, widely used in moderate turbo builds and fairly priced.

Now, as per my previous forum post from a while back, my engine suffers from a timing limitation in that "excessive" cylinder pressure on ignition causes head lift rather than audible detonation, especially when running ethanol. Will the lower compression potentially mean that the cylinder pressure at an equivalent timing amount will be decreased due to a slightly larger combustion space? Would I have to add timing to compensate for this?

Obviously a good answer would be to put it on a dyno once its built to work out what is good, but I don't know any tuners in the area that are familiar with D16s and their limitations in timing. As far as I understand most tuners will just add timing until MBT and then back it off a little...most D16s lift their head before they reach MBT and will give no warning of doing so until it is too late, so I don't want to risk that happening because it means another head gasket and head stud set to buy on top of dyno time costs. I have, from research, worked out what I believe to be safe timing numbers on my setup, and have run it without lifting the head yet. This was after I lifted my head with timing that, although causing head lift, did give me a lot more torque (you could physically feel the difference).

So to summarize, will a lower compression ratio mean I have to run more timing to equal the cylinder pressures I am getting now with less timing and a higher compression ratio? And would I be safer to run more timing due to, from what I understand, there being less peak cylinder pressure?


In theory, all else being equal -

Less nominal compression will reduce peak, and overall, combustion pressure, because it's starting from a lower pressure.

Less compression pressure means slower flame front and less peak combustion pressure.

Again, IN THEORY, you can rum more boost and/or ignition timing - but you DO need to be careful.

TBH, I don't understand why you haven't address the head seal problem already, with the best studs and gasket you can find - it's more expensive on the fi5rst build, but it should be less than doing the build, then pulling the head and replacing the gasket, skimming the head, etc, if you do go a little too far for what you have.

BTW, you DID have the block, and head, lightly skimmed with the correct finish for the gasket you're using, didn't you?

Hi Gord,

Thanks for the reply. I lifted the head while already using ARP head studs, and when doing the repair made sure to use an OEM MLS head gasket and had the surfaces machined to 30Ra as per OEM specification. Removed timing, and no head lift since then. I am rebuilding though because I bent a rod since then (pencil rod vs 15psi). If you read the Honda forums, it is a very common issue for the D16 heads to lift even with ARP head studs and perfect machining, due to excessive cylinder pressures caused by too much timing. That being said, the ARP head studs and OEM gasket combo have been proven to work fine on decent hp builds, just with the limitation in timing being kept in mind. Without tapping the block out to run bigger studs I believe this is the best combo.

It will be interesting to see how much more timing I will have to run to make the same power now that I was making at stock compression ratio at 15psi. Is that possible, or because of the overall less cylinder pressure would also I have to run more boost as well as more timing to get to the hypothetical same amount of power as I currently had at 15psi?


Ah, sorry, I mis-understood.

I can't comment further, as I'm not familiar with the engines - seems like the next step is a $$$ one for taking care of the head seal!


I wouldn't try to recreate the same cylinder pressure that lifted the head, but yes to achieve similar average cylinder pressure you'd need more timing advance with the lower compression pistons.

I'd start with a bit less timing than you were running.

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