Hi all,


How much stroke is considered too much stroke with a tall deck engine? I see 4.5" but not as much 4.75". If rpm is less than 6500 max and most cruising is around 3750-4500, what's an okay fit? Not looking to go supercharged anytime soon so cubic inches are nice.

When Eddie Young was building my engines he had to start with parts I already had. One of the more important factors in engine design relative to longevity was rod angle which is rod length divided by stroke(according to him) He likes to keep rod angle above 1,5 so that the rod is not at a big angle at BDC. With the parts I had with a 4.625 stroke I was at 1.41 which he never would recommend for longevity. He wanted to keep max rpm under 5500 for this reason. Mercury used the 4,625 stroke in some of their N/A race engines. Comes down to how long you want things to last. It is a trade off.

There is lots of speculation and very little or no hard evidence.

Dart just took their block up to 10.4” deck height. I’d make it a 632 and go with it. An endurance application will need more effort in the valve train imo. That 632 won’t have to turn as fast to make the power.

Does anyone make an intake for that

The 632 has a lot of torque for a big boat and can turn RPM. Lots of NA drag motors that size.

I
seems like rod angle is typically mentioned. Anyone have real experience with an engine as far as service intervals and what you’re seeing as far as wear? Is it excessive wear on the rings or is it actually wearing on the cylinder bores? What are you refreshing and how bad is it?

some of these blocks can go really tall but I’m guessing you’ll need a custom intake or at least some spacers made.

I took the the engines apart myself that Eddie rebuilt. The pistons were slightly “egg” shaped. They were not the originals and someone had knurled the piston skirts to try to snug the pistons up in the bore. The bores showed wear but not crazy. From what I observed I would say that pistons would be more of a problem than the cylinders since it was obvious that the engines had been through at least what should have been through 3 sets of pistons but yet retained the initial bore. I know he will build you a 598 in a heartbeat but prefers not to build a 632. The cranks, rods and bearings all were good. The big chief heads, and valve train components were about gone.

If it gets all red and bloody, it`s too much stroking .

I have 632’s and told by my builder they should be gone thru every 250-ish hours. Torque monsters and sound great. Motors came in boat and I was told they were Zul motors.