StrikinLightnin

At present my engine is a 502 with Brodix Plus2 CNC heads.
The cam is somewhere around 598in/615ex lift.
I bought the boat already setup like this and that's all I know about the cam until I pull it out and get numbers.
I want stroke it out to 540 and use the same heads with MAYBE 741 camshaft.

jryan26

Mine are 540s with stock hp500 heads. No reversion. I was worried about it also.

plumbers crack

Im running twins with gil exhaust wet,no reversion y pipe and silent choice 509 cubic inch

Cignificant

502s, GM rect port heads, CMI elbow tops, silent choice, 741 cam, B&M blowers, no reversion..

greencard

iTrader: (2)

a 741 with factory mag exhaust WILL have reversion. I learned the hard way!

HPJunkie

502 , Brodix bb2plus ported heads, 741 crane on 114 l/s, EMI with ss risers and silent choice and procharger no reversion.

PatriYacht

Bigger engines will tolerate bigger cams and revert less. A 540 with those parts may work ok. Keep your idle up. If you spend too much time at 6-700 rpm, that could give you a problem. But don't be surprised if it happens. My 540's with the next size larger cam ( Crane 651) and AFR cnc heads reverted like crazy. If I loosened the tailpipe- manifold connection, water poured out in a steady stream. And my tailpipes didn't dump the water until after the transom.

rmbuilder

Some (not all) of the posters of the 741 not experiencing reversion are forced induction applications. The comparison is not apples to apples with N/A and should be addressed in cam selection. In a N/A motor the probability of reversion is significantly higher than forced due to the differences in the pressure differentials during the portion of the overlap period ATDC. Exhaust reversion (not intake reversion) occurs after the piston reaches TDC (beginning of intake cycle) and is descending in the bore, reducing pressure across the piston crown, at a time when both intake and exhaust valves are open. At low RPM (N/A) the pressure differential is:
Intake column> combustion chamber< exhaust tract
Allowing both exhaust gas and water back into the combustion chamber
As RPM increases the inertia of the intake and exhaust columns increase so:
Intake column> combustion chamber> exhaust tract
Negating exhaust reversion.

At low RPM in forced induction applications:
Intake column> combustion chamber> exhaust tract
The intake column is pressurized increasing the fill rate/pressure of the combustion chamber (blow through) and effectively restricting the exhaust charge/water from reverting past the exhaust valve back into the combustion chamber.
A cam profile that shows no sign of reversion in a forced induction application will have a much higher probability of exhaust reversion in an N/A application.
Bob

US1 Fountain

iTrader: (1)

Wouldn't it be just considered a NA motor at idle since there is no boost? That idle is when the reversion is a factor, correct?

rmbuilder

US1 Fountain,
Not necessarily. With a roots type supercharger for example, even at very slight throttle openings when the blower is technically not "making boost" the rotation of the rotors generates enough energy in the intake column to show an increase in the VE of the engine which counters the tendency to revert.
Bob