Mercruiser cam question

[gadget73]

Hi all,
I'm in the process of repowering my boat with a longblock from marinepowerusa. I have a couple of questions about this new motor, and I need a response from the people who didn't sell it to me

First concern is the cam. Original engine is a 1993 5.7L Bravo engine (I can get the s/n if anyone needs it) that came with a roller cam. The new motor has the roller block, but it has a flat tappet cam. My concern is that this motor won't match the power output of the other engine, which is a low hour MerCruiser motor with the correct original roller cam in it.

My other related concern is the distributor gear. I don't know Chevys, but with Fords you cannot use a distributor gear from a flat motor on a roller cam and vise-versa. It shreds either the cam gear or the distributor gear, depending what exactly you do. I called their tech support and they assure me neither of these items are a problem but I'd expect that response from someone who just sold me a motor. I just want to be sure I'm not setting myself up for a huge failure that will somehow get blamed on me, and cause the warranty to be voided.

 

[MikDee]

Re: Mercruiser cam question

Welcome to iboats! You have a couple of valid questions, but to the extent of my knowledge, some of the earlier hi-perf flat tappet cams developed more power then the newer stock roller cams, this maybe the case here as the engine builder saw fit to install, and as far as I know, the dizzy gear is the same whether it's a flat, or roller cam, but I can't guarantee either of these statements. Hopefully, someone with more knowledge in this area will be along to help.

 

[TilliamWe]

Re: Mercruiser cam question

I do know that for a while Mercruiser rated engines with flat tappet cams and engines with roller cams the same HP. So as long as the cam they put in the long block has the right specs, it'll perform as well as you roller cam engine.

I am not aware of different distributors needed for roller cams or not roller cams. My 1987 and 1990 Chevy vans didn't have roller cams, yet my 1997 Chaparral did, and they have the EXACT same distributors in them.

 

[Mischief Managed]

Re: Mercruiser cam question

I do not know if a roller will fit in your engine or not, sorry. But I can confidently share the following:

Roller cams typically allow a lot more lift for a given total duration and longer duration at higher lift without causing excessive total duration, because the cam ramps can be steeper without causing excessive side loads on the lifters. That's good in a boat engine where excessive total duration kills low end torque, makes for a lumpy idle, and can cause water reversion in the exhaust system due to excessive overlap. I think you'll find you can make more top end power and a flatter torque curve with the roller cam, in a stock engine configuration. Roller cams work really well in boats.

 

[gadget73]

Re: Mercruiser cam question

Thanks for the info guys. I just wanted some assurance I wasn't about to install a time bomb in this thing.

As for the distributor thing, at least with Fords its not a different distributor, but a different gear on the bottom of the distributor. One has a steel gear, one has iron. I don't know Chevy motors all that well, so I wasn't sure if they had the same sort of deal or what.

Anyway, once it stops raining inside my garage perhaps I'll get a little more progress on this thing. Need to get the oil pan and oil filter assembly off the old motor yet, as well as the motor mount brackets. Probably I'll take those pieces to work this week and drop them in the degreaser tank for a good flushing prior to inspection. There is a growing puddle of oil on the garage floor that has me concerned that my pan might be shot. It didn't look rusty when I glanced at it during engine removal but who knows.

 

[ken_23434]

Re: Mercruiser cam question

In my experience, the roller cams are steel while the regular cams are iron. The roller cam will use a bronze distributor gear, but the distributor itself is the same.

I know from personal experience that if you swap your engine for a roller cam version and use the old distributor gear, you will find yourself stranded on the side of the road one day for no apparent reason (it was a car).

The issue is the stock gear is also cast iron. The bronze gear is stronger, I guess.

I am not sure if there is an issue using the bronze gear with a standard cam, though. I would not think there would be a problem, but I have no specific knowledge on that.

I would suggest asking that question of a high performance auto supplier. Someone like Jegs or Summit Racing. I think both of them have customer tech support.

If you are really worried about it and cannot find a definite answer, the gear is pretty cheap. Especially the cast iron version that you would be looking for. It is held in place with a single roll-pin. It's a very easy swap.

I don't think you will ever get two engines perfectly matched as far as power output. I would guess both of your engines are fairly close, unless you bought a special hi-performance replacement engine. If they put out slightly different, then a little input from the steering wheel should steer the boat straight again.