312 fastech motor upgrade
 

Thinking of swapping my base 7.4l mpi's out and installing 500hp efi's. anyone ever do it, any other thoughts. thanks guys

There was a time I was considering re-powering my 312.
I called the factory and spoke with them about it.
They knew of one guy who had put 500 efi's in his but I don't recall if they gave me his contact info.

Give them a call.

Local mechanic put them in a 312 about 3 years ago. Pulled 454mags that had 30P props (1.65 drives) and borrowed my 34P's. The boat owner loved it even though I thought the props were a little too big....they claimed the boat ran well past 80 but that was on the speedo. (probably was running 80 GPS)

i think 7.4's make the 312 heavy in the a$$ end, 500's would just make it even more. Neat idea though. Would probably get it out of the hole a big quicker.

The 500's can't weigh much more than the 7.4......but with 190 hp more per motor it's worth it!!! I had a 312 with 502 Mags, that boat was 8,000 dry from the factory......it can handle the weight of BBC's in the back!!

Pretty much the same block. There won't be a significant weight difference, as t500hps said.

With that, 80 mph on a 31 foot swim platform boat aint great.

I just test drove a 312 with the base 7.4MPIs at Robert's today and had this very conversation regarding the 312 and bigger motors. The original BoatTest.com report of this hull with 2 X 385 HP 454 Mags they got 72.4 MPH on GPS at 5,100 RPM with 22" props and stock Bravo 1s- 1/2 fuel 3 persons. The base 7.4 310HP model we drove today got 64 MPH GPS with 2/3 fuel and 5 persons 70 degrees outside temp. Given this - The 500s or 525s will get you well into the 80's The factory runs the props deep - switch to short lower units which will raise the props and easily get you 4 to 5 mph with that change alone. I think that with a 496HO and shorty lowers you could get 80+. And yes - I bought the boat.

Congrats...Robert is a good as they come to deal with (I bought my 382 from him)

So you think its worth doing the swap I have 400hrs and stock motors still pur but as always more power would be nice. boat is very clean just don"t know about return on $ spent down the road.

your boat will be easier to sell later...but you'll never get what you put into it. (so what's the speed increase worth to YOU?)

I think I'm gone track down a pair and just swap them, keep the boat I like and know what I have. Alot cheaper than new boat I guess.

PM "BoatDoctor". He owns a 312 with ''whipple" charged 7.4 MPI's. He see's 87+ MPH (GPS) everyday. Though IMO, you would need to freshen up the heads before installing superchargers.

Any idea how to reach him i tried sending pm and it won't go through as he does not execpt them.

Check your PM box for his contact information.

Need to freshen up more than the heads. Lowers in that set up, if not upgraded, will be toast in no time.

Speed Calculator
 

Here is a speed calculator app that will help you see what extra power will give you. Meant to post this the other day, but did not have the link. Be honest with the numbers you plug in and the estimates are pretty close.

http://www.go-fast.com/boat_speed_predictions.htm

Ditto on needing new drives/props if you go to big power - don't forget about steering upgrades as well. K-planes are a must at high speeds IMHO. Assuming you have bravo ones, they won't last long behind big power.

rubadub, that speed caculator seems to be right on, good info. My 312 has 502's with wipples and imco lowers, 29 pitch props 98.7 mph gps