Budman

BT,

For the most part, I like the cam. It seems to have real good midrange power, and it has a nice aggressive sounding idle, yet I can still idle at 600 - 800 RPM with no problem. I'm running it with Merc Silent Choice, which I use often, and so far no sign of reversion. However, as I mentioned, I really expected to see more speed out of this setup. Not necessarily the cams fault - could be a bottleneck elsewhere - perhaps the Q-jet & the stock exhaust are holding me back. Might try a Holley 750 this summer just to see if it makes a difference. Judging by the amount of midrange power that I am making, I think I might do well to try to move the power band upward in the RPM range - maybe with an open spacer under the carb, or even a single plane manifold. Also, I think I might be jetted a little on the rich side, but I would rather run rich than lean - less chance of torching something.

If I had it to do over, I might consider a cam with similar duration and overlap, but with more lift - maybe something in a hyd. roller. If you are running a performance exhaust, you can probably be a bit more aggressive on the duration and overlap and make a little more power.

bago Baja

Budman,
I'm running the stock 1988 7.4 / 330 merc, Bravo 1, 1.5 ratio, 23P Mirage @ ~4200 rpm's. Gives me ~ 57 on the speedo with two people and 1/2 tank of gas. I've talked to two others with this boat / engine combo who say they get 57 mph out of the stock setup. I'm trading my prop in on a labbed 21P with additional cup (Houston Propeller) as a first step with 4600 rpm as the goal. Hope to pick up a couple mph. I'm considering the same basic mods that you have done (except pistons, I'll probably stay with the flat tops) and would surely expect better than you have seen. I'm reading Dennis Moore's book right now. He agrees that large oval heads with oversized valves are the way to go along with a moderate cam and performance manifold. Dennis swears by the QJ and says it's good for 475HP on the BBC. I doubt your approaching that point yet. Dennis also stresses the importance of advancing ignition timing with performance mods if rpm's are increased over stock. Requires a new module on the Tb IV ignition system. He gives a procedure in his book for finding the right amount of advance at idle and WOT. blue thunder says the stock exhaust should be good to 500HP. Dennis Moore says "The biggest roadblock to improving performance with a marine engine is the exhaust." I don't know what to think. Just passing along what I've heard / read. What rpm's and prop are you running?

blue thunder

Budman... my other choice is a comp cam 226/234 @.050, 544/564 lift with a 111 LSA. Kinda like the tighter lsa and it has a slightly higher lift. Any opinions on that? I too am going to run the q jets and tune as Dennis Moore recommends. Hoping they will work out ok. Bago Baja... I think the stock exhaust will continue to work ok through the 400 - 500hp range, but certainly becomes a limiter at some point. Probably any hp engine would run better with performance manifolds where the exhaust pulses are seperated. It is all kind of debatable... make your best guess.

BT

Waterfoul

Budman, I have a high rise single plane intake if you want to try one. I could send it to you, you can give it a run, and if you don't like it, send it back. If you do, we can talk price at that time. It's a Holley Stip Dominator for oval ports.

Mike

formula31

Well, 12 years or so ago I also cringed at the cost of performance manifolds and then it was pretty much CMI or Gil. I went ahead and did all the engine stuff and ws disappointed. I finally got a set of Gils through CPperformance for $1400 and picked up 400 rpm. After that I was convinced. I think if you are gonna blow the motor with a procharger or a roots blower, the stock exhaust is fine to a point, but for NA with a performance cam, it really needs the reduction in exhaust backpressure and the seperated runners. Over 500hp NA, full seperate primarys are needed.

I have heard plenty of people with injected engines that put on aftermarket exhaust and did not see much improvement. I cant answer that one, but I know what it did on my carb motors.

BT, I cant wait to see how that cam works, I looked at that one too but was scared of reversion.

blue thunder

Formula... so you think I should try excaleagles 544/564 cam? I think I am there. What do you think of the imco SS risers with the stock manifolds? Worth anything?

BT

formula31

Wow, I dont want to tell you what to do for a cam. I stayed on the conservative side. The one you listed may be fine, Id keep the idle up a little though. Inconel exhaust valves too.

No, the problem really is the small hole at the top of the manifold and the open chamber that causes all kinds of cross pulse issues with a bigger cam.

bobby daniels

HEY GUYS I'VE DONE ALOT OF THIS AND THE CAM ABOVE IS A KILLER ONE ASK THEM TO GRIND IT ON A 110LC AND 4 DEGREES ADVANCED YHEN INSTALL STRAIGHT UP BE SURE TO HAVE 8.75 TO 9.0 TO 1 ,,IT IS NOT BIG ENOUGH TO HAVE REVERSION ,AND FOR EXHAUST A CHEAP WAY OUT IS CHANGE TOPS AND CUT THE WATER WAY DOWN GOING OUT I WELD UP THE WATER HOLES IN THE STAINLESS TOPS AND DRILL 2 1/4 INCH HOLES THEN SEND THE REST DOWN THE Y-PIPE OR ON THE DRIVE WITH A DUMP ,,,,REMBER YOU CAN'T COMPRESS WATER SO IN EXHAUST ITS BAD

ALSO THE QJET IS OK WITH WORK AND A DUAL PLANE INTAKE

HOPE THIS HELPS BOBBY

blue thunder

Thanks Bobby... great input. I only have somewhere between 8:1 and 8.2:1 compression at the moment. What affect would this have?

I have thru hull exhaust and am shopping for used imco risers to mix water further away, but may still use stock 4" risers if nothing is found.

Thanks for the help,

BT

bago Baja

bobby,
Please explain this cheap way out for exhaust. What are the tops? What do you weld up and where are the holes drilled? Is this a modification to the stock exhaust?
Thanks,
Tom