Dear Dave - Your Questions, His Answers

Piston SwappingI learned a lot from your answer in the October 2006 issue, where you calculated the change in head gasket thickness to lower the compression on an S2000. I've tried the same formulae to try and figure out high compression pistons for my Suzuki Swift GLX.

My car has a 75mm bore, a 90mm stroke and a compression ratio of 9.5:1. From your formula, my TDC volume is 46.78cc (47cc) and my swept volume is 397.65cc. If I want a compression ratio of 11:1, the TDC volume would be 39.7cc (40cc).

My question is: When I want to change from low to high-compression pistons, do I need to increase the thickness of the gasket? I plan to use a Honda ZC (D16A1) piston. Can I just directly swap this piston without modification to the block and head? Does it require a thicker metal gasket?Asri YaunBelait, Brunei Darussalam

First, for the non-Bruneians in the room, Brunei Darussalam is somewhere over by Malaysia. I Googled it. Now about those pistons. The head gasket is the least of your worries. From your calculations, to get 11:1 compression, you need to reduce your TDC volume by 7cc. The TDC volume you calculated is the total volume of the cylinder head, head gasket and piston dish. Your 7cc can come from any combination of changes in these three things, but the easiest is probably the pistons.

Using the Honda pistons might work, but only if a lot of things happen to be the same. First, the bore has to be exactly the same. That happens sometimes. Second, the pin height has to be the same. That's the distance from the top of the piston to the wrist pin. If the Honda pin height is higher than your Suzuki's, the piston will hit the head before top dead center. If it's shorter, the volume between the top of the piston and the top of the block will have to be added to your TDC volume, ultimately lowering the compression ratio. Having a gap here will also reduce turbulence in the combustion chamber, slowing flame travel, forcing you to run more timing advance and simultaneously reducing detonation resistance. In other words, it would be bad.

Next, the wrist pin diameter and method of installation needs to be the same. Some wrist pins are press-fit into the rods and spin freely in the pistons. Some press into the pistons and spin freely in the rod. And the really good ones spin freely in both and have some kind of snap ring at either end keeping the pin from just sliding out (those would be full floating pins). If the rod and piston aren't designed for the same size and type of pin, bad things will happen.

Finally, if all those things miraculously happen to be the same, you need to make sure the top of the piston is shaped properly to allow the valves to open. Most pistons have valve reliefs machined into them and if your new valve reliefs are smaller or in a different location, valves might hit pistons.

In other words, just figure out how big your current piston dish is and have new pistons custom-made with a dish that's 7cc smaller and just use the same kind of head gasket.