Milled Head? Pros Cons?

[brklynmutt72]

I am in the early stages of my rebuild on a 97 Challenger 787....sheared the rave valve in back cylinder and the chunks play ping pong inside the cylinder....so I got a used head and I have had a few people recommend that I mill the head for higher compression...? Has anyone done this before and what are the additional changes needed?..(Ex: larger jets , carb adjustment..etc )

 

[Sportster-2001-951C-Stock]

I think it's not a good idea, as long as the head is reasonably flat to begin with. The reason is, higher compression is going to move you closer to detonation unless other steps are taken, such as fattening up the A/F mixture and/or probably running $$$ aviation fuel.

Maybe if you plan to race your boat in a certain class, but to me even to get a 50RPM higher WOT wouldn't be worth the expense, I would tune for reliability/longevity (try to match factory as close as possible).

So if you do mill the head, are you planning on removing the factory airbox flame arrestor and shaving the throttle shafts, modifying the main venturi and installing a racing tuned pipe, these thinds would require recalibrating the carburetors as well most likely, so may as go all the way.

Even nitrous oxide is a possibility, but instead why not just start with a more powerful boat to kick off the modification journey, such as next year's 700hp supercharged puffer model?

 

[brklynmutt72]

That's what I thought...That's why I asked here...it was a suggestion I was unsure of...thanks for the info.

 

[Sportster-2001-951C-Stock]

As kids years ago we used to mill our heads on the flat sidewalk, and assemble the engines minus the lower ring to allow for faster revving. But these were junker motors we basically got for free and we were kids putting stuff together so we could ride through the woods. There was lunch money involved, and it didn't matter if the things blew up, in fact there were plenty of them we blew up on purpose you might say.

Were there any performance gains to be had over factory? Well, the only big one I can think of was we learned a 2-stroke will absolutely scream if you run it lean enough, that is it runs like a bat out of hell right before it lean seizes.

So what I'm saying is, to me the money spent on having a head shaved is better spent elsewhere, such as on a carburetor kit and not concentrating on cheaping out on aftermarket parts from a far away country known for making junk parts hat don't work correctly.

Also, don't be afraid to open those high speed mixture adjustments to the high side of spec, don't run these motors lean at full power. At least yours didn't roast a piston, sometimes those cheapo RAVE valves can break off and fall into the cylinder, that can also happen if the RAVE is bent and re-straightened. Those things take a real beating during operation.

 

[Dr Honda]

Milling the head for higher static compression is a GREAT way to improve torque, and overall HP.


Unfortunately....


On a seadoo... it's not that easy. Since we use an exposed top ring... our engines have a CRITICAL dimension in the squish band zone of the head. SO, "Milling the head" is not something a home shop can do.


TO do it on a seadoo... you will take 0.040" off the face of the head... and then you have to recut the squash band. It either takes a special cutting tool... or a CNC mill, with a ball end cutter.