Intake port matching on a BBC

[mofugly13]

I picked up a Mercruiser MPI setup to use with the big block 496 Ill be building in the future. I am going with the big oval iron heads, 781's or 049's. The ports on the MPI intake are rectangular. I understand that they should be port matched, ie. the transition from the intake to the head should be as smooth as possible. What can I do here? Is there a Mercruiser MPI intake with big oval ports? Does it matter all that much for the motor I will be building? I'm not going for a super high horsepower ripper, just a warm 496.

 

[Ryan B.]

I would have to say yes to gasket match porting the intake ports on the heads. Make sure the manifold is matched up also..
A little time with the die grinder/dremol will net you some free horsepower. Ive spent a few hours with the dremol tool on my big block heads for my car.

 

[mofugly13]

Thanks for the reply Ryan, I was gonna PM you the question, but I posted it up here instead. So, since the ports on the intake are rectangular, and I'll be using big oval heads, should I grind the big ovals out to rectangular size, and just make a nice transition to the 'bore' of the head's intake port? I have read that when matching the ports, you should only go 1-1.5" into the intake port on the head. Sound about right?

 

[Thunder]

PM Royken. he would know if Mercruiser made MPFI for oval heads. I know that GM marine used both type heads on 454 marine engines. But not sure if MPFI was ever made for oval heads .I know carb heads were both oval and rectangular.
If merceury has a manifold to fit oval heads that would be your best way to go. Using a rectangular manifold on oval heads isn't the best way to do things espically with EFI.
You would be better off getting a set of rectangular heads. They are a better performance head any ways.

 

[mofugly13]

Don't rectangle port heads flow too much for the low rpm torqu we look for? PM'ing ryoken now.

 

[Thunder]

I am sure with MPFI you will make plenty of low end torque with a BBC.
Boats are all about low end torque.

 

[ryoken]

doh, just seeing this one now...

just to answer for everyone else...

Merc ran the rectangles on everything, strange as it may seem, peanut ports included...

there are earlier mpfi intakes that are oval but aren't compatible with later systems iirc.. all are rectangle now apparently..

i was personally surprised when I found this out.. and as the others said, if it was mine and i had the heads off, i would certainly do some port matching...

as far as overfeeding, to much flow, my guess would be that it is soooo controlled by the cam needs and ECM that its not an issue and gets what it wants..

 

[mofugly13]

I thought boats were all about high rpm ZOOM, but I'm not a boat guy, so I claim ignorance. When I said that I thought the rectangle ports flowed too much, I was referring to the fact that they are soo big, the velocity of the air fuel mixture isn't what it should be, ideally, at low rpm's.

 

[Thunder]

Nope. Boat engines build nothing but torque. Most marine BBCs are all done well before 5K rpm. It takes massive ammounts of power(torque) to overcome the drag water puts on the hull and get on plane. Then to go fast it takes even more . Most boat engines build a lot of steady torque all thru their rpm band (Boaters call it operating range) and fall off quickly at higher RPMs.
The exceptions of course are hydroplanes tunnel hulls ECT. The hullls on those go fast boats are designed to provide as much lift as possible using both hydro and areodynamics to get them out of the water as much as possible to reduce drag. In a drag boat at speed The only thing holding you in the water is the prop. Without the prop drag boats would fly off the water . Even with props they often do. So without all that drag you can build a high revving motor.
But V hull boats need torque. Lots of it.
If you really want to build a torque monster throw a marine cam in and some rectangle heads on your BBC. Lets snap some axles.

 

[ryoken]

Thunders on it...

absolutely.... Lot of slug to be pushing trying to get a 36' Sea Ray up and on plane, even with twins..

For production boats, they are usually done by 4200, especially carbed motors.. The newer fuelies these days get pushed up another 500 or so on occasion. But only raceboats are running over 5 g's... whole other story there.

Most cruising is done in the 32 to 3500 range depending on what ya need to plane decently...

 

[mofugly13]

Sounds like these motors are just what we wheelers are looking for. Thunder and ryoken, thanks for the edumacation. I learnded me sumthin' new today.