lowering compression

Cometic makes the best gaskets out there. They are made to order with at least 6 different thicknesses to choose from.

How do you know what your CR actually is? A 1200cc with a larger than stock dome is probably going to be more than 10.5 : 1. The larger displacement raises the CR up big time. Best to talk to your piston manufacturer and they can explain all this. A 1200cc block would have big liners so it was probably resurfaced and that is like installing thinner head gaskets.

Best to find someone with a Whistler and see what your CR currently is, that is the easiest method to get a true CR.

Things like sharp edges on the valve pockets can cause detonation, best to tootsie roll any sharp edges off the domes.

the only way to know your CR measure piston dome volume, piston deck height, head gskt thickness, cc cylinder head DO NOT go by CR listed by piston company are your cams degreed ? head gasket the correct one [ not hanging in cylinder } lot of different things can create detonation is timing set at 38 deg ? stacking base gaskets and thick paper base gaskets you are asking for trouble.

DFIGPZ wrote: the only way to know your CR measure piston dome volume, piston deck height, head gskt thickness, cc cylinder head DO NOT go by CR listed by piston company are your cams degreed ? head gasket the correct one [ not hanging in cylinder } lot of different things can create detonation is timing set at 38 deg ? stacking base gaskets and thick paper base gaskets you are asking for trouble.

+1, you shouldn't be having any detonation issues with your setup. Are you 100% the ignition timing is correct along with your jetting. I had a huge hole between 3000 and 3500 rpms. It turned out to be the ignition advancer springs. I did the same thing you did tighten them up, that actually created the hole. I bought a bunch of used ones and everyone of them did the same thing big hole between 3000 and 3500 rpms. I finally ponied up the $90.00 and bought a new advancer with springs and installed it, solved my problem. I left the springs alone and used them as is.

DFIGPZ wrote: the only way to know your CR measure piston dome volume, piston deck height, head gskt thickness, cc cylinder head DO NOT go by CR listed by piston company are your cams degreed ? head gasket the correct one [ not hanging in cylinder } lot of different things can create detonation is timing set at 38 deg ? stacking base gaskets and thick paper base gaskets you are asking for trouble.

A simpler way to measure CR is finding a shop that has a Whistler..... store.katechengines.com/whistler-compres...tio-tester-p174.aspx

To lower CR you are not asking for trouble if you use Cometic paper and aluminum base gasket set up.

PaintWorkz wrote:

DFIGPZ wrote: the only way to know your CR measure piston dome volume, piston deck height, head gskt thickness, cc cylinder head DO NOT go by CR listed by piston company are your cams degreed ? head gasket the correct one [ not hanging in cylinder } lot of different things can create detonation is timing set at 38 deg ? stacking base gaskets and thick paper base gaskets you are asking for trouble.

A simpler way to measure CR is finding a shop that has a Whistler..... store.katechengines.com/whistler-compres...tio-tester-p174.aspx

To lower CR you are not asking for trouble if you use Cometic paper and aluminum base gasket set up.

Cranking Compression And Compression ratio 2 complete different things aluminum base gasket is fine but stacking paper or thick paper base gaskets are trouble but hey do what you will

Lean mixtures will make a motor detonate. Sure you aren't lean on the bottom end? The more power an engine makes, the more fuel it needs, if you have improved it a lot it could just need more fuel. Plug gaps are a good way to fine tune spark timing, wider gaps start the burn over a larger area and detonation can come on sooner, Try reducing plug gaps a little.

DFIGPZ wrote:

PaintWorkz wrote:

DFIGPZ wrote: the only way to know your CR measure piston dome volume, piston deck height, head gskt thickness, cc cylinder head DO NOT go by CR listed by piston company are your cams degreed ? head gasket the correct one [ not hanging in cylinder } lot of different things can create detonation is timing set at 38 deg ? stacking base gaskets and thick paper base gaskets you are asking for trouble.

A simpler way to measure CR is finding a shop that has a Whistler..... store.katechengines.com/whistler-compres...tio-tester-p174.aspx

To lower CR you are not asking for trouble if you use Cometic paper and aluminum base gasket set up.

Cranking Compression And Compression ratio 2 complete different things aluminum base gasket is fine but stacking paper or thick paper base gaskets are trouble but hey do what you will

The number they advertise for the compression ratio of the pistons is just a way to sell pistons to stary eyed gear heads! :woohoo:
You want to know the actual cylinder pressure which is affected by things like decking the head or cylinder cam lift/duration/timing.Thats where horsepower is actually made/lost,not so much from fancy bumps on pistons.

Here's one of the first dyno run from a picture on my phone - was aming for around 12,5 on the A/F (25%-50%and 100% throttle) and got a bit closer than this
Also tried with old advance springs and play with the ignition a lot
cams are timed as specs (K410)
Carbs are Mikuni WM33 which have been renovated at Mikuni/Topham

SO if to lower it people would put a alu or copper base gasket???

one of the last with 50% and 100% throttle

DFIGPZ wrote:

PaintWorkz wrote:

DFIGPZ wrote: the only way to know your CR measure piston dome volume, piston deck height, head gskt thickness, cc cylinder head DO NOT go by CR listed by piston company are your cams degreed ? head gasket the correct one [ not hanging in cylinder } lot of different things can create detonation is timing set at 38 deg ? stacking base gaskets and thick paper base gaskets you are asking for trouble.

A simpler way to measure CR is finding a shop that has a Whistler..... store.katechengines.com/whistler-compres...tio-tester-p174.aspx

To lower CR you are not asking for trouble if you use Cometic paper and aluminum base gasket set up.

Cranking Compression And Compression ratio 2 complete different things aluminum base gasket is fine but stacking paper or thick paper base gaskets are trouble but hey do what you will

Cranking Compression (pressure) and CR are different but related. CR goes up and cranking pressure will go up. A Whistler tells you CR, not cranking pressure.

Lars ....call Cometic and talk to them about their base gaskets.

I am done way to many people in this what do i know I have only put hundreds of KZ Engines together from stockers to Drag motors that are 16.1 CR and lived its no wonder why good people like Larry do not want to get involved so you key board punchers have at it. :whistle:

yes thicker gaskets.
www.dynomanperformance.com/engine/gaskets-cometic.html

leon