compression verses timing

[mx1986]

I was wondering which is better. I have 17cc domes on a 10 mil alky cub and it just seems like its fat the sparks r damp and no 3 to 9 o'clock burn. Plugs Still look brand new. Plus 5 timing and I have 39 pwk with clipard single pjets and a .112 main. Still fat but I'm pretty close. I have some 21cc domes that might do better. I talked to someone and they said that the engine would be fightin itself with that high of compression. Just wondering if someone could elaborate on that statement.

[n2otoofast4u]

17s arent that small. BUT you need to figure out your true compression ratio. If it was mine, Id turn the timing up.

[mx1986]

I am at 1100 sea level. So what's he talk bout engine fightn itself

[RadarRacing]

That would mean ignition or preignition would occur too early and try to push the piston backwards some before the downstroke. Different builders do different things. Normally 17cc domes would be small for a 10mil cub. That doesnt mean they are but "normally". Also "normally" we would run 7-9 degrees of timing or more. If you arent showing any heat on the plugs that may keep it safe with the 17cc domes and if you crank up the timing or lean it down you may have detonation problems.

 

Honestly you should CC the cylinders and check the true compression by measuring the trapped volume with the piston at TDC.

[mx1986]

It not detonating. But I think I'm pry losing HP Cruz I can't run as much timing. So the question in the begining more timing or compression. Or does it depend on the build. With the 17cc domes its pry running hotter and if I would change them to 21cc run Cooler and save on kickers.

[n2otoofast4u]

I missed the 10 cub part.

[possum]

If it was me I would ditch the 17's. And run less compression and run more timing.

[mx1986]

I here ya got some 21cc for it also. Think I'll try them

[J-Madd]

Depending on deck height you could need anywhere from maybe 18cc to 21cc domes. As mentioned, you need to know the true trapped volume. 19 cc domes on my 10 mil Cub makes it have 210 psi at 300 ft asl. The piston sticks out of the hole.

[mx1986]

What u run for Timing on that.

[possum]

What you got goin on there J-Madd that your piston sticks out?

[J-Madd]

The bottom of the cylinders have been decked. I can't find all of my notes, but I think the timing is probably at +8 or +9. Looking at the notes I have, I may have 21 cc domes in that motor now, not 19 cc. So it kicks 210 psi with 21 cc domes, not 19cc.

[possum]

Where did your port timing end up? how far are you out the hole?

[dirtydownunder]

Ahhh the age old question, compression vs timing vs boost.

[J-Madd]

Man I wish my notes weren't lost when I changed from BB to f'n iPhone. It's like 194 or 195*. If I measured correctly it like 0.064" decked off the bottom. And the pistons may be 0.010" or 0.020" outta the hole with a 0.020" base gasket. I don't necessarily recommend this configuration. It runs good but is hot as hell.