Has anyone used a 4-7 firing order change in marine use?
I've been told by a reputable person (worked in Texaco reserch lab doing engine/dyno testing with fuels/oils and other parts but no marine knowledge.) that this keep #7 cooler and smooth things out and give a small bump up in horsepower. What about reversion? I would like about 550-600hp.

I have a NA (502mark IV) 509ci being bored out from the last disaster.
stock 4" stroke crank, 6.135 h-beams. Pistons size depending on bore looking for 9.4-9.5 comp. ratio.
cam was roller mechanical .575/[email protected] .581/[email protected] 108cl
rectangle port rpm air gap with 850cfm holley marine mechanical sec.
Dart marine 325 2.3in 1.88ex 121 chamber aluminum heads on order.
cyclone wet exhaust. Going to upgrade to msd ignition this year too.
bravo 1. Thanks in advance for help and info

4 & 7 are at TDC at the same time. But you would end up with the same phenomenum with cylinders 2 & 4.......adjacent cylinders firing sequentually. Plus you would have to have a custom ground cam to swap valve actuation for those two cylinders. Don't see it...

In a smaller engine, the results are not that noticeable. You might fnd 5hp in there.

In a bigger power application, you can gaing as much as 15 hp in a 750-800hp engine.

I have not tried this experiment myself. There are easier ways o make the power.

If you want #7 to run cooler, then plumb the circulation water to the front of the manifold with a #8 line.

You will get around the same 5hp increase in this engine.

GM must have seen some benefit to the cylinder/firing order swap. Look at the Vortec 496 marine motor. As for the intake manifold modification, see this thread for more info. The pic I posted is on an Edlebrock Air Gap manifold.
http://www.offshoreonly.com/forums/s...d.php?t=178265

For more info on what is best (cam design) for your motor, talk/write to RMbuilder. He is the cam guy (errr professional) with the right stuff.

That intake comes from Edelbrock pre-drilled for the water re route.

It can be done on most with a little effort

We have considered the 4/7 swap. However,if one is to use a custom grind,perhaps a double swap is worth considering.

Interestingly, GM is using the swap configuration to provide balance, when one bank of cylinders is off line.

It is my understanding,Nascar is still using the old original firing order.

The HP gain is only seen in the high, high rpm range and it is minimal.

I was running a 4/7 swap on the mtrr I just pulled. 580ci 750hp.
I couldn't tell you the difference/ comparison in pwr due to a totally new set up. Engine builder does most of them that way.

This was a big debate on hot boat awhile back. Basically guys that did back to back dyno runs, said it's worth a few HP way up high, and not really worth the hassle on a recreational motor. In drag racing where guys are looking for every last hp it's something to do.


Darrell.

I am running the double swap. 600 ci, intercooled 8-71's etc. Not sure of the performace gain on the dyno though. Idle does seem to be somewhat smoother. Using Crower cam, not sure if it is worth the $ honestly.