1979 100hp crossflow rebuild - performance upgrades

First of all, many thanks for all the help you have already given me, I have searched and read everything and anything I could find on here related to this motor and I owe you all for it.

I have a 15 ft Thundercraft ski boat, looks like a small Sidewinder. It came with a 1979 100 hp crossflow that needed a piston (light score, .020 should clean it up nicely) I threw on a 1975 85 hp crossflow that I had for the summer and am rebuilding the 100.

This is a ski boat and is used primarily for pulling skiiers/wakeboard/tubing. Since I have to rebuild it anyways, I am looking to try and build a good solid skiing motor out of it. Here is what I am thinking, I would appreciate some opinions from seasoned experts. I am not really looking for top end as much as a solid snap and a motor that will idle happy and run nice at part throttle when pulling a skiier.

- Keep the small 100hp carbs for better snap
- Wiseco pistons, bore the block .020 in all holes
- Use stock heads
- Boyesen reeds
- Bubble back exhaust cover to increase breathing

Here are my questions:

- Is it worth it to port the cylinders to 140 port profiles? Is this something that can be done with a dremel? Is this doable without too much headaches, or should I just find a late 70's 115/140 block? Where would I find the desired port profiles, should I just find a blown 140 block and measure from that (they aren't very common up here)?.

- Would the motor benefit from opening up the intake in the manner described by Dhadley (plenum)? I am thinking that increased RPM range (6500-6800) would be nice as this boat usually runs a short prop (15") for torque and getting 45+ out of that setup without changing props would be very nice, just for getting home.

- Is the bubble back modification worth the trouble of finding the housing?

Any comments? Thanks in advance.

I know I would probably be better served by finding a good used looper but this type of thing is fun to me and it's a lot cheaper. The lower unit is good and the motor is pretty clean. I suspect it blew up due to carbon as it was a gooey mess. The 21 pitch prop I found on there (and the abundance of ski rope) tell me that this poor thing spent most of it's time at or under 3K, pulling hard. The 85, with a 15 pitch prop, runs great right now.

Re: 1979 100hp crossflow rebuild - performance upgrades

The porting is very easy to do on that motor. Since the block will be completelt stripped to bore it they can open up the ports at the same time. All the ports (between the 79 100 and a 79 140) have the same center. They just used a bigger cutter (like a drill) on the 140. We open them up with a ball end mill. Just measure a 140 intake and exhaust and use those two cutters on the 100. Center up the cutter and pop open the port. A bigger cutter on the same center will make the new port higher, lower and wider.

Find the inner and outer bubble back manifolds. Don't worry about the (expensive) inserts.

You've got a solid plan. It'll work fine. You won't believe what that manifold will give you.

Re: 1979 100hp crossflow rebuild - performance upgrades

Might be beneficial to see if you could find a pair of 140 heads and go with 140 pistons... not sure how much more power that would give you. I couldn't find those heads used around here though and they are somewhat pricey ($235 CDN each) new.

Re: 1979 100hp crossflow rebuild - performance upgrades

Dhadley said:

The porting is very easy to do on that motor. Since the block will be completelt stripped to bore it they can open up the ports at the same time. All the ports (between the 79 100 and a 79 140) have the same center. They just used a bigger cutter (like a drill) on the 140. We open them up with a ball end mill. Just measure a 140 intake and exhaust and use those two cutters on the 100. Center up the cutter and pop open the port. A bigger cutter on the same center will make the new port higher, lower and wider.

Find the inner and outer bubble back manifolds. Don't worry about the (expensive) inserts.

You've got a solid plan. It'll work fine. You won't believe what that manifold will give you.

Click to expand...

Thanks DHadley! I was hoping you would chime in here, you knowledge of these motors is encyclopaedic. In your last comment, I presume you mean the intake manifold as it's apparently where the real gains are. The porting sounds easy. I think I will try and pickup a blown up 140 bubble back block, that way I'll have the port sizes as well as the exhaust parts.

I looked at an intake manifold last night and tried to figure out how to open the center web up. I presume we are talking about the separation at the carburetor (between both bore holes) in the manifold. It looked like I could mill this out pretty easily. Is there a thread that you remember that details the modification? How deep can I go before I plunge too deep? I guess I could always epoxy it, but I would rather keep it all alum. I am thinking of going down 5/8" and across and blending the passage in on both sides.

Am I out to lunch? Is this enough area? I am not very knowledgeable on 2 strokes but I would imagine going too big (affecting the intake volume) might have a negative effect.

Hoeser, as far as I know, the 140/115/100 pistons are identical on a crossflow. I want to keep the low comp heads to keep the motor smoother and less peaky. A rumpity idle might be cool on a race boat, but when I'm in the water and someone else is at the helm trying to get it all lined up I will pick a nice smooth low idle instead of a bit more on the top end. I agree that hi comp heads are the way to go for top end though.

Thanks for the help guys.

Re: 1979 100hp crossflow rebuild - performance upgrades

Yep, the intake will give you a bunch. We cut the wall out and go down far neough so it's about 1/4" thick where the wall is now. That wall is in line with the center rib on the gasket side.

You'll need the old style carb mounting gaskets. Send me your address and I'll cut a couple and send them to you. Use them as the pattern for what to cut and just go down far enough to leave it 1/4" thick. Don't worry about blending and smoothing, it's not necessary and won't make any difference. If you're going to mill the wall it'll be fine.

For acceleration you'll want the smallest carbs you can find. Remember, you're doubling the throat size (well, almost). Now each piston is pulling from two throats, not just one. If you use 1 1/4" carbs you'll have about 2 1/2" of carb throat feeding each cylinder. Cool, huh?

There's some work to be done on the carbs. Look at the gasket mounting side. Most have a hole in the casting (in the shape of a milk bottle) that goes up to the top and is open. You'll need to fill that with epoxy. On a stock manifold the wall seals it off. That'll be gone. It's very obvious once you look at it.

Re: 1979 100hp crossflow rebuild - performance upgrades

BTW -- you're right. All 3.500" crossflow pistons are the same. V4, V6, 90 hp, 140 hp, 200 hp -- doesn't matter, they're the same. But don't get caught up in the theory to bore it to .064" over to gain something. It won't. Just go with what you need - .020, .030 etc.