Super Jet Will increasing stroke change your timing

[Cannibal]

today I checked a std stroke engine at 30 degrees btdc the piston was .215 down on the indicator
on my 12mm stroker 30 degrees was .275 down on the indicator.
its around 4 degrees btdc lower than a standard crank so on this engine if i ran the stator lined up with the mark It had 4 degrees of advance in it.
so now when going to set timing on this engine do i compensate 4 degrees so its firing truly at the right time according to the crank?

Im going to give the epic a try. When I put my curve in for it should I offset the timing 4 degrees due too the piston location compared to a stock stroke?

You are using a piston travel to degree correlation chart for a stock stroke engine. You have a stroker and cannot use that chart any longer, throw it away.

Your piston is located .06" lower in the cylinder, but NOT 4* lower. Piston height is not measured in degrees, crank rotation is. You said it yourself, you measured both at 30*, not one at 30* and one at 34*.

Yes, the piston is lower, but is moving faster, now that piston has to cover that extra .06" in the same 30* of crank rotation because they both are at the TDC at 0*.

What you have to realize is that as the crank rotates from TDC to BDC, it rotates 180*. Stock stroke is 68mm and you have a 12mm stroker, you are now at 80mm.

That piston needs to travel 12mm farther in the same 180*.

If you take 68mm and divide by 180, you get .377mm of piston travel per degree.

Now do the same with the stroker: 80mm / 180 = .444mm.

So you can see that the piston travels .066mm farther in your stroker for every degree that the crank rotates.

Keep in mind these calculations are just for demonstration purposes. The piston pauses for "X" degrees at TDC and BDC, but the longer the stroke, the more degrees your piston pauses. If I knew how many degrees stock and stroker pistons stayed at TDC and BDC for, the calculations would be closer.

I unfortunately cannot give you advise about how to set up your stroker engine ignition curve because my experience is theoretical, not practical, but there is plenty of people running strokers that can help, or you may need to consult a builder. If you tell them what you have, they can program an ignition for you.

Good luck!