AV Gas, some S&F history.

[outasite]

I have to contradict. I run approximately 4° advance at about 7,000 rpm with AV gas.

My jetting was more determined by my exhaust tuning than the fuel.

4 degree advance from your"normal"timing would be correct. But not many including me, willing to bump my V6 timing to 29 degrees.

[Mark75H]

4 total. I crank at zero to make sure it doesn't buck against the starter.

[powerabout]

4 total. I crank at zero to make sure it doesn't buck against the starter.

that sounds unbelievable and we are talking a crossflow?

[FMP]

I think that's an additional 4* total

[phillnjack]

ahh I see
( that was in answer to the octane not the timing)

[dwilfong]

Had a interesting conversation to day with a buddy of mine at work. He finally got a piston stop tool and set the pointer on his OMC 225 V6. When he checked the timing with the light it was 28 deg. He thought it was wrong so he got a different light and it read the same.
He argued with me that the pressed on mag on the fly wheel had slipped and the timing was not 28.
Well It took me a few rounds to explain that it was right and the mag had nothing to do with it.
He has run this eng now for 2 years with the timing this high on 87 Octane gas.
Now the compression test comes in at 95 on one side and 105 on the other as he found out it has two different heads on it....LOL
He was told by numerous people that this eng would have melted down with that much timing.
Well odiously it did not.
So the moral of the story is?
Gust thought I would share this tid bit...................................

[powerabout]

Low comp, light boat, lots of revs?

[FMP]

And low psi might help

[Mark75H]

Crossflow. 4° total advance. cranking compression 275psi. That's why I know about gas, AV gas, jetting and oil ratio.


Can't go any higher on the compression because there is no more "room" in the spark timing.

[FMP]

I just learned something. It's having trouble staying in though.

[powerabout]

Crossflow. 4° total advance. cranking compression 275psi. That's why I know about gas, AV gas, jetting and oil ratio.


Can't go any higher on the compression because there is no more "room" in the spark timing.

Where would the crossover be from too high a comp and lack of advance?

[Mark75H]

When the compression is so high, that you can't have any spark advance. Torque would fall off because you couldn't match the pressure peak with the down stroke; the pressure peak would be too early. The rise needs to occur strongly ATDC and the peak needs to be when the piston and rod can apply the most force to the crank.

[outasite]

If your firing at 4 BTDC your peak cylinder pressure would be well after the optimin 14 ATDC.Ill be the first to admit, that I know little to nothing of cross flows.But my Merc would not like that at all.

[powerabout]

When the compression is so high, that you can't have any spark advance. Torque would fall off because you couldn't match the pressure peak with the down stroke; the pressure peak would be too early. The rise needs to occur strongly ATDC and the peak needs to be when the piston and rod can apply the most force to the crank.

Isnt that where you are now with fuel that cant deal with 275psi so you run retarded?
Time to run with a cyl pressure sensor so you can see where you are?
Ps pretty hard to get peak cyl pressure when crank and rod at 90 degrees, sure thats ideal and you might get it in a diesel but imho no chance in a 2 stroke

[Mark75H]

Isnt that where you are now with fuel that cant deal with 275psi so you run retarded?
Time to run with a cyl pressure sensor so you can see where you are?
Ps pretty hard to get peak cyl pressure when crank and rod at 90 degrees, sure thats ideal and you might get it in a diesel but imho no chance in a 2 stroke

I do NOT run retarded (ATDC),

I'm at 4° BTDC, 4° advance. The set up work for my motor was done decades ago and long proven to be optimal.

I dissected a few Quincy and Stevens motors and then consulted with O. F. My set up is all on his proven advice. Dyno and race tested; not guesses on my part. 275psi and AV gas; its right at the maximum usable balance point on my particular bore/stroke/cylinder head shape.