timing limits on pump gas.

[maskmanLS6]

Im getting mixed opinions on how much timing i can add on pump gas. I have ported cylinders with a stock head, my compression is at 140psi. Im set at +4 degrees now and was considering trying it at +5. 2

[ClayAiken]

You might get away with 5. Just stay with 4. 1 degree's is not worth it.

[maskmanLS6]

You might get away with 5. Just stay with 4. 1 degree's is not worth it.

 

im kinda thinkin the same thing.

[maskmanLS6]

it depends.

pipes, porting, what pump gas you are using, and the dome design.

 

different pipes like different timing, same with porting.

and since you just have a stock milled head, it dont have good dome design.

 

just stay with plus 4 for now.

The head i have on there now is a stock unshaved head, i took the shaved head off because of squish issues. My pipes are fmf gold series and my port job is between a drag port and dune port. Ill leave it at 4 for now but will try 5 after mull eng. works the head.

[maskmanLS6]

I

if you have 140psi, with porting and a stock head, something isn't right.

 

a stock engine is around 130psi. with porting, it normally lowers compression.

 

I said the same thing. I think my cylinders were decked some time in the past. i didnt realise it until i had it put back together and noticed the piston was coming up a little high in the hole. The outer rim of it comes out of the cylinder just a tiny bit when at top dead center. About the height of a stock head gasket. When the piston is at TDC you can take your thumb nail and try to go across the cylinder onto the piston and it will catch on the top edge of the piston.

[RadarRacing]

Most people including me would tell you to stay at 4 degrees if you dont know what head you really have.

Back years ago we all ran 5 degrees with pumpgas motors. Detonation is as much of a factor with how hot a motor is as well as fuel and compression. Each degree of timing is a power gain up to a point. If you have the stock head thats not milled more than .020 or .030 you should be able to run 5 but watch it and listen for any detonation in case your setup doesnt like it. We dune raced with 5 degrees and would hot lap our bikes for hours but didnt have our jetting on the very edge back then.

[maskmanLS6]

Most people including me would tell you to stay at 4 degrees if you dont know what head you really have.

Back years ago we all ran 5 degrees with pumpgas motors. Detonation is as much of a factor with how hot a motor is as well as fuel and compression. Each degree of timing is a power gain up to a point. If you have the stock head thats not milled more than .020 or .030 you should be able to run 5 but watch it and listen for any detonation in case your setup doesnt like it. We dune raced with 5 degrees and would hot lap our bikes for hours but didnt have our jetting on the very edge back then.

 

Ill try 5 degrees on my next ride, its running pretty good at 4 with 93 octane.