92GTA

iTrader: (11)

Ok, check it out. I just bought a 1961 30ft Chris Craft 6 sleeper cabin cruiser. Right now it has twin 283ci Chevy based marine engines. 1 has 0 miles from a rebuild done in 1996 and the other is the original so needless to say it's a FUBAR'd solid block of corrosion, it can't even be disassembled to be rebuilt. So, I'm looking to replace both engines and I'll just sell the 0 mile rebuilt 283 on eBay.

So, the mounts, transmissions, linkages, etc, are all setup for a SBC block. I want a good combination of reliability (that why TBI), fuel economy (again TBI), and max TQ.

I figure I can get a pair of 350ci TBI engines complete with harness and ECM from a junkyard out of a couple of mid 90's GM trucks or SUV's like a Blazer or a Tahoe. I'll then rebuild both of them using duplicate parts to ensure both end up as close as possible in output.

So here is my real question. First, these marine engine's can't run more than 4100RPM max, cruising is 3600RPM. Anything more than 4100RPM and we get cavitation from the props due to the design of the boat. That and the transmissions and props are setup for that range. Also, it's obvious that for a marine application on a 12 TON cruiser, that I want the broadest TQ curve possible. I've been tuning TPI for years so I can easily adapt to TBI.

Question is, considering the above, what kind of intake and cam would make a good combination for max TQ in a TBI? Changing the heads is out of the question due to budget. Heck, I'd like to keep the stock intake too if just a good RV spec cam can give decent TQ.

Also, I thought about a 383ci stroker kit for even more TQ. My concern with that is the engine using more gas than the extra TQ is worth and the TBI system not being able to feed a 383ci displacement.

Keep in mind this has to stay TBI because of the flam arrestors that are required for the marine application will only fit onto a carb or a TBI setup. Also keep in mind the original Chris Craft exhaust manifolds must be kept because they have water passages in them to pre-heat the water before it enters the block. Plus I have to use all of the marine accessories, oil pan, etc. Even special marine spark plug wires and distributor. It's a wooden boat and I do NOT want to blow up, lol!

So yeah, any cam/intake suggestions are welcome keeping my application in mind. Also if anyone here has ever converted automotive engine to a marine use, I'd love to hear any and all feedback from you. Also if anyone can give me an idea of what the most TQ is I can expect from a purpose built TBI I'd appreciate it because I can weigh all this against a diesel conversion which I can't afford, lol.

Thanks! Alex

RBob

iTrader: (1)

First thing to check is engine rotation, many marine engines run the opposite direction then an automotive engine. In this case a reverse rotation cam is required.

For torque and lower RPM usage, go with more lift and less duration. To broaden the torque band more LDA helps.

My concern with a 383 would be the torque capacity of the drives. May find so much torque that things start breaking. If the 283's had enough torque, 350's also will. (You can feed a 383 with a TBI setup. BBC units have 48mm bores).

On the same note, if the 283's have enough torque, and fuel consumption is a concern, then maybe something smaller then 350's may be OK. But you can't beat a 350 for reliability, ease of purchase, and low cost to rebuild.

RBob.

kdrolt

I like the plan of using twin rebuilt junkyard engines, especially if you can get them from a old-person's Buick Roadmaster (probably better maintained than a copcar). The passenger cars will also be roller cam, while the truck TBI engines might not. The factory intake manifold is set up for broad torque so I'd stick with it but port it (mainly the plenum divider cut to allow better breathing, weld opened coolant holes closed, secondly port the lower plane runners as they tend) to not flow as well as the upper plane). You could also skip the plenum divider cut if you can handle the height increase of a spacer.

I'd swap out the peanut cam in favor of a 94-96 L99/LT1 Bcar roller cam (also used in 96-99 L31 engines) and keep the stamped 1.5 factory rockers. That engine will peak in hp near 4500 rpm, and have a very broad torque peak from 2500-3500 rpm. Porting the heads slightly will also help the engine be more fuel efficient at part throttle, and the swirl port intake will also help that out.

RBob - Good suggestion to check for reverse rotation. The screws might also be counter-rotating (one engine cw, the other ccw) so check the prop blades carefully.

Fast355

iTrader: (2)

Provided that both engines rotate the same way, I would pick up a pair of 305 TBI car engines. A mild cam such as a stock LT1 roller will push the peak torque up to around 3,000-3,500 and peak HP up around 5,000 or so. The increased torque of the 350/383 is not necessary, if the 283s could pull 4,100 RPM the increased power is wasted, IMO. The smaller engine will have greater efficiency at part throttle cruise due to decreased pumping losses. If you have enough engine to maintain 3-5 in/hg of vacuum while cruising at 3,600 rpm, the rings and valve guides will last much longer. Pull an engine below 2 in/hg for extened times and the rings wear rapidly due to the increased cylinder pressure.

Kdrolt, that same B-body LT1 cam was used in the L30/L31 from 1996-2002.

contact

is the goal, max torque at 4200rpm ?

naturallly, there wil lbe less torque at 3600rpm,
but you are throttled back, so who cares

if so, that needs a pretty healthy cam

are you going to change props,
if you have too much powerat 4200?