Thread: Increase the compression ratio

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Hi,
just a couple of question about increasing the compression ratio (CR).

There are some facts thats influences the CR:

1. Head design -->chamber volume
2. Pistons design
3. bore diameter
4. the type of head gasket (thickness)

If I port my head, I will increase the chamber volume and therefore decrease the CR. How much is that usually? Which effect is bigger?

An other possibility to increase the CR is to machine my cylinder heads. But I heard this might cause trouble with the intake mainfold. It might not fit tight? Is this correct?

Is it not better to machine the block? In my Chevy 350 cui is enough space at the top before the valves will hit the pistons. I have never read something about machine the block to get a smaller chamber and better CR.
What do you thing about that idea?

Regards

James

if you are running stock dish pistons it would be a huge waste of money to machine the block and or heads to increase compression. you could slap a set of ..018 copper head gaskets for a quick up in cr/. but it probably would not be cost effective to do it for the small gain for all the bloody knuckle work needed to do it. bottom line is a new set of flat top pistons as being the best way to raise a street motor cr/. but then you will need to increase fuel and air intake and exhaust cappacities to do it right. so if you are asking for a cheap easy way to raise your compression to boost performance the answer is probably not! i hope this answers your question, if not let us know.

Originally Posted by James911

Hi,
just a couple of question about increasing the compression ratio (CR).

There are some facts thats influences the CR:

1. Head design -->chamber volume
2. Pistons design
3. bore diameter
4. the type of head gasket (thickness)

If I port my head, I will increase the chamber volume and therefore decrease the CR. How much is that usually? Which effect is bigger?

An other possibility to increase the CR is to machine my cylinder heads. But I heard this might cause trouble with the intake mainfold. It might not fit tight? Is this correct?

Is it not better to machine the block? In my Chevy 350 cui is enough space at the top before the valves will hit the pistons. I have never read something about machine the block to get a smaller chamber and better CR.
What do you thing about that idea?

Regards

James

James there are a few things to consider. First a port job usually has to do with the material around the valve in the port (outside of the chamber) there is usually very little taken out inside the combustion chamber except to unshroud the valves. If you are having machine work done to your heads do this first and then measure the volume of the combustion chamber. This gives you a concrete number to start working with. If you are going to any work to the block do this next but take some measurements first. When the piston is at TDC what is the distance from the block surface to the top of the piston where it meets the cylinder wall? Use this measurement to set up your quench area(along with the compressed thickness of the gasket). If you have the budget and some time next to consider is cam and pistons. Pistons are advertised to have a certain size/volume dome or dish. Before buying a complete set, try one and have the top face of it measured. I recently purchased a set of +5cc domes as advertised that actually measured to be -.3cc when accounting for the valve reliefs. Camshaft timing, especially intake closing is influential to DCR.
In short there are a lot of things to consider along with compression but time and budget are very limiting factors. Anything worth doing is worth being informed about. Form a plan and stick to the plan. From experience I can say that direct measurements are far better than "advertised". The guys and gals here have lots of collective knowledge and are always glad to help.
Chris

Got to wonder why you're going for raising the compression ratio??? Lots of ways to improve performance more efficiently then raising the compression...... Just curious, that's all....