318 to 350 swap
Posted: Mon Jan 08, 2018 1:32 pm
This past year I decided to replace the original 318's that came with the boat with a couple Chevy 350's. I had replaced a lot of parts on the 318's, manifolds, electronic ign., alternators, starters, wiring, etc. The motors ran fine, but I was looking for a little more power and part availability. I ended up scrounging around and found a late 80's 350 with Marine Power trim on it. Then I found a late 80's 350 with OMC trim on it. Then the fun began. As almost all 28 express owners know, the engines are a very tight fit, much more so than the 32 ft models. I started out by yanking out the old engines, cleaning the engine bays, then tried to dry fit the Marine Power enigne to see how it fits.
First problem was the exhaust manifolds were to high and they hit the floor bracing. Then the engine mounts were not long enough and would need at least a 3 1/2" spacer underneath mount and hull stringer. The thermostat housing was also hitting the floor bracing. So, after taking some measurements, I found out that the Crusader log style manifolds would fit under the floor. I found a few older Mercruiser thermostat housings that fit, but the had 4 outlets and I only needed 2, so I plumbed the extra 2 hoses into the manifold just before the elbow. These would have been used if I had closed cooling, but I had raw water so I didn't think I needed them, but in the mean time I will leave it like this. I ordered some longer engine mounts from Marysville Marine and was able to get close to the height I needed. I also ordered another crank shaft driven raw water pump for the OMC engine to match the Marine Power. The velvet drive tranny's bolted right up to the GM bellhousings after installing new flexplates on the flywheel. Set the engines back in the hole, and see that the larger inboard oil pan was hitting the bracing on the hull between the stringers. Pull engine out again to trim off some bracing, I don't think I removed enough to hurt anything, I hope. In and out with engines a few more times to get everything fitted right. Rebuild both Rochester carbs. I am sure I skipped a few steps in here somewhere.
But the boat is now ready to hit the water. Drop it in and idle around for a little while, then put it up on plane. The old 318's would run up to about 38-3900 rpm's, at around 37-38 mph. I left the same props and shafts on for the 350's. The 350's will run up to 4,200 rpm at about 39 mph. Being that I am running the same props I guessed that my cruise speed wouldn't change much, and it didn't. I would cruise at about 2,900 rpm with the Chryslers at around 26-29 mph, this would be right before the four barrels would kick in. With the Chevy's, I can cruise around 3200 rpm before the four barrels kick in at around 31 mph.
I liked that part as I primarily take this boat over to Isle Royale Nat'l Park which is about 60 miles of open Lake Superior to get there. I only made one trip there last summer, but I was very happy with the end result. I did have one hiccup though. After the initial sea trail, we went on a 60 mile round trip overnighter. Had no problems. Then the night before the Isle Royale trip, I went to the Marina to fuel up, and as I was getting the boat back on plane, it started to vibrate real bad, I brought it back down to idle to check on engine, saw nothing, and put it back on plane. This is when it started to shake bad and then all of the sudden the engine started to free wheel. Uh oh. Blew up Tranny? Nope. I sheared off the prop shaft right behind the strut, loosing the prop also. I was really bummed because I have yet to get any spare shafts or props. I called a guy I know that has an older 28 express he wasn't using, and he gave me his spare shaft and prop. He is an AWESOME guy! So we pulled the boat out, removed the old shaft, stuck the used one in and the prop. Put it back in the water at 11:30 at night. I did a few test runs up and down the canal, good to go. After examining the old shaft, it looks like it had been half broke for awhile, and finally gave up the ghost.
Next spring I plan to get two new shafts. And after talking to a representative at Acme Props I will order up a new pair of props also. The Acme Props are designed like the Michigan wheel djx props for less than half the money. After giving him my boat info he gave me a recommendation for a prop that is different than the 14 x 12 Michigan Wheel I have on there. I will post results when I get her back in the water. Which is going to be awhile, we have got over 150 inches of snow in this part of the Upper Peninsula so far. We average 250 inches.
I will also try to post some pics of the engine swap shortly.
First problem was the exhaust manifolds were to high and they hit the floor bracing. Then the engine mounts were not long enough and would need at least a 3 1/2" spacer underneath mount and hull stringer. The thermostat housing was also hitting the floor bracing. So, after taking some measurements, I found out that the Crusader log style manifolds would fit under the floor. I found a few older Mercruiser thermostat housings that fit, but the had 4 outlets and I only needed 2, so I plumbed the extra 2 hoses into the manifold just before the elbow. These would have been used if I had closed cooling, but I had raw water so I didn't think I needed them, but in the mean time I will leave it like this. I ordered some longer engine mounts from Marysville Marine and was able to get close to the height I needed. I also ordered another crank shaft driven raw water pump for the OMC engine to match the Marine Power. The velvet drive tranny's bolted right up to the GM bellhousings after installing new flexplates on the flywheel. Set the engines back in the hole, and see that the larger inboard oil pan was hitting the bracing on the hull between the stringers. Pull engine out again to trim off some bracing, I don't think I removed enough to hurt anything, I hope. In and out with engines a few more times to get everything fitted right. Rebuild both Rochester carbs. I am sure I skipped a few steps in here somewhere.
But the boat is now ready to hit the water. Drop it in and idle around for a little while, then put it up on plane. The old 318's would run up to about 38-3900 rpm's, at around 37-38 mph. I left the same props and shafts on for the 350's. The 350's will run up to 4,200 rpm at about 39 mph. Being that I am running the same props I guessed that my cruise speed wouldn't change much, and it didn't. I would cruise at about 2,900 rpm with the Chryslers at around 26-29 mph, this would be right before the four barrels would kick in. With the Chevy's, I can cruise around 3200 rpm before the four barrels kick in at around 31 mph.
I liked that part as I primarily take this boat over to Isle Royale Nat'l Park which is about 60 miles of open Lake Superior to get there. I only made one trip there last summer, but I was very happy with the end result. I did have one hiccup though. After the initial sea trail, we went on a 60 mile round trip overnighter. Had no problems. Then the night before the Isle Royale trip, I went to the Marina to fuel up, and as I was getting the boat back on plane, it started to vibrate real bad, I brought it back down to idle to check on engine, saw nothing, and put it back on plane. This is when it started to shake bad and then all of the sudden the engine started to free wheel. Uh oh. Blew up Tranny? Nope. I sheared off the prop shaft right behind the strut, loosing the prop also. I was really bummed because I have yet to get any spare shafts or props. I called a guy I know that has an older 28 express he wasn't using, and he gave me his spare shaft and prop. He is an AWESOME guy! So we pulled the boat out, removed the old shaft, stuck the used one in and the prop. Put it back in the water at 11:30 at night. I did a few test runs up and down the canal, good to go. After examining the old shaft, it looks like it had been half broke for awhile, and finally gave up the ghost.
Next spring I plan to get two new shafts. And after talking to a representative at Acme Props I will order up a new pair of props also. The Acme Props are designed like the Michigan wheel djx props for less than half the money. After giving him my boat info he gave me a recommendation for a prop that is different than the 14 x 12 Michigan Wheel I have on there. I will post results when I get her back in the water. Which is going to be awhile, we have got over 150 inches of snow in this part of the Upper Peninsula so far. We average 250 inches.
I will also try to post some pics of the engine swap shortly.