Compression ratio versus displacement
 

I'm starting to consider options for a motor rebuild. One of the considerations is whether to increase displacement from 3.2l to 3.4l. Another consideration is the target compression ratio. So I have a question, which motor would have more torque and peak HP, a high-compression (10.3:1 or above) 3.2l with twin plugs, or a lower compression (9.3:1) 3.4l with single plugs. Assume the cams would be optimized for each design. I can find 93 octane gas with 10% ethanol easily in my area.

The 3.2 liter approach would potentially save me the expense of purchasing new cylinders, but this savings would be eaten up by the twin plug ignition. The 3.4l would require new P&Cs but I'd keep my current ignition.

What would you pick and why? Thanks for your ideas!

Steve

Choices.............
 

If you buy cylinders and pistons, plan on spending around $5k, maybe a bit more. That's what a set of KN Engineering slip fit 100mm cylinders and a set of 10.1:1 Mahle pistons cost me. On the other hand, you can buy the Electromotive XDi twin plug system for around $1300 plus plug wires. With the money saved, you can get a set of JE 10.5:1 pistons and still have some money left. Expect to spend a bit over $1K for cylinder head work plus the cost of drilling for the second plug. There are lots of small bits and pieces that will eat up a ton of money along the way. When I built my 3.3SS twin plug engine, I had about $11 - $12K in parts. The first thing you need to buy is a copy of Wayne's 911 engine rebuild book! Good luck!

You can put aftermarket EFI for the cost of the ignition stated.

I'd use "cgarr" for head work. Very cheap, fast turn around, and great work!

Generally more displacement increases torque, higher compression more HP.
I think torque is more important than HP from a usability standpoint.
I think Bruce Andersens book is a better book for hotroding engines.
It gives you all the combinations normally used.