i want lower compression

ok i got some 12.5 to 1 pistons(1197) and wanna have premium gas run in it not jet fuel. any way i have a ported head(needs some repair work) with some 410 cams. how to get the compression down around 10-1. do i go with a gpz head... or a gasket on the base with the head i have? where are the problems with these options.
thanks guys

Well I would go for the gpz head for sure. Maybe a bit more money but it will drop the compression and you will have a better head plus I think they have bigger valves but then again I have been told that you could shave the pistons if they are the wiseco as they have plenty of meat. I don't know the specifics but I would get the gpz head.

saxjonz wrote:

Well I would go for the gpz head for sure. Maybe a bit more money but it will drop the compression and you will have a better head plus I think they have bigger valves but then again I have been told that you could shave the pistons if they are the wiseco as they have plenty of meat. I don't know the specifics but I would get the gpz head.

First thing I thought of was shaving the top of the pots. Could also mill out the inside of the combustion chamber a shade.

Pull the head. Are the pistons level with the top of the head?
You can do two things. Pull the block and add extra base gasket, that will drop
your compression. (Puts the pistons further in the hole)
That is what I would do. Most coast effective.
Or you can pull the pistons and shave the tops off.
If you send the head out to have the combustion chamber reshaped for lower com. IT will only reck a perfectly good head

XENU

I second the suggestion to add another base gasket....chris3

FastKaw1 wrote:

Pull the head. Are the pistons level with the top of the head?
You can do two things. Pull the block and add extra base gasket, that will drop
your compression. (Puts the pistons further in the hole)
That is what I would do. Most coast effective.

How much effect does that have? I recall the gaskets in my rebuild set were only about .030" thick and probably compress to maybe half that thick under the head torque bearing straight down on the base gasket. I was just wondering how much it would actually drop the compression to add a second gasket.

It's bin 30yr. sines I figured this out. boy I hate to say that.
APEgeroge might be a better source.
A stock gasket is .030 Get a copper or alum. around .060 that will put the piston
.030 in the hole. and if I remember correctly 10-12% lower comp.
Make sure you use a good gasket sealer on copper gaskets.
You can stack two .030 to get .060 or any combination.
You wont to worry about valve to piston anymore.
Hope this helps

XENU

OPPS Sorry

APE JAY

its not put together yet.
but thanks on the suggestions sounds like i could go either way.

FastKaw1 wrote:

It's bin 30yr. sines I figured this out. boy I hate to say that.
APEgeroge might be a better source.
A stock gasket is .030 Get a copper or alum. around .060 that will put the piston
.030 in the hole. and if I remember correctly 10-12% lower comp.

Interesting, I wouldn't have guessed that much.

School Teacher wrote:

ok i got some 12.5 to 1 pistons(1197) and wanna have premium gas run in it not jet fuel.

FYI, there is a source for actual octane booster that WORKS because it contains tetraethyl lead. You can easily jack your octane up ten points if you want. It's called:

Octane Supreme 130

www.batterystuff.com/fuel-treatments/OS6q.html

www.kemcooil.com/specials.php

You can't use it if your bike has a catalytic converter, otherwise OK. I use it on mine.

The stuff they sell at Pep Boys or Kragens only raises it a very small amount because they can't put lead in it. It's pretty useless.

By compression, I assume you mean cylinder pressure. You could start by moving the exhaust cam to give them more overlap. That will bleed off cylinder pressure.

Jay