paaka937

This one goes out to all you prop guru's out there.
I have a 97 Formula Fastech 312 with twin 454's rebuilt to 460's and carb'd. Running Bravo 1's with 24P props. WOT I am hitting approx 5000-5200 RPM's at 67-70 MPH with 1 adult and half tank of gas. Not a whole lot else done to the engines as far as exhaust improvements except to remove the silent thunder box.

1) Does this sound about right as far as prop selection? I was under the impression 4600-5000 WOT was inline for these engines but I also don't want to over-prop the engines and burn up a drive/transmission. On the flip side I don't want to "over rev" the engines and burn one up that way either (pretty new to the performance boating scene and still honing in performance standards).

2) What kind of performance/ride changes if any would you expect if I switched to Mirage Plus props?

3) 67-70 MPH at WOT sound right for this set up? And if recommending changes what kind of speed increases would you expect?

Thanks in advance to everyone for the advice/opinions.

MissBehavin777

iTrader: (3)

Your a little short on info. Horsepower? Drive ratio? I have built twin 454's in my Fever I spin 28p. From what I hear Brett from BBlades is the man to talk to about props!

paaka937

Ooop's sorry about the lack of info...like I said still trying to figure out all the technical details. These were 454 Merc Mag's that were rebuild and bored out .30 over during the rebuild. In talking to the mechanic that rebuilt the engines he said they should be around the 460-466 cubic inch mark. As far as HP...I don't have a specific number but I would assume 350-370 HP with the rebuild unless anybody would like to correct me (all information is greatly appreciated). Gear ratio...I have no idea! The drives are bravo 1 drives that had nose cones put on them by the previous owner when he put supercharges on the engines (superchargers subsequently removed and engines converted to carburetor's but nose cones left on the drives). Unfortunately, the drive ratio was either covered up by a tie bar or worn off. I just recently had the drives serviced by Formula of Missouri so I can give them a call and I'm sure they would inform me of the specifics.
Not sure if any of that info helps but again thanks for the suggestions/info.

machloosy

Chances are, the drives are 1.5's. Depending on your ability to turn a wrench, it is pretty easy to pop a drive off. With it off, mark one spline on the input shaft w/ a sharpie, put the drive in gear and turn the prop once by hand. The number of input shaft rotaions gives you your ratio. 1.5=1.5, a little more than 1.5 would be 1.65, etc.
Next question, how many hours on the motors? If they are under 10, consider them still in their break in period, over 10hrs, you should be set to go.
If they are already broken in, you are propped too low, target 4800-5000 and start looking for a set of 25P or 26P mirage props. Once you have them, try them spinning in and out too see what the boat likes.
Good luck! And definitely talk to Brett @ BBlades

fleg1

Is your 24p a 3 or 4 blade? if it's a 4 blade I would go up to a 26p 4 blade and see what it does..

Generally a 3 blade will be a bit faster but a 4 blade will get you out of the hole quicker and give you a much faster cruising speed. The top end with a 4 blade will be slightly slower than the 3..

I would also recommend removing the nose cones, they could possibly be slowing you down and they really aren't effective unless your running over 80!!

Ask the engine builder what rpm you can turn the engines at.. He should know if you replaced the cam the WOT rpm should be on the cam card!!!

scarab31

454 mags were 365 horse stock, or at least they are in my Scarab. The previous owner had superchargers on them. What other work has been done...exhaust, cam or roller cam, roller rockers? You could easily be well over the horsepower your thinking you.ve got.

paaka937

Scarab31-
I really wish I could answer your question about additional "work" but to be honest I'm not quite sure. Form what I can tell the answer would be nothing...it appears to be stock exhaust and to my knowledge the only thing done to the engines would be the rebuilds with them each bored out .30 over. The majority of the modifications were done before I bought the boat and were subsequently removed (with the exception of the Holly carb's and lower drive nose cones) before I purchased the boat.
From talking to other formula owners and other 312 owners it sounds like the majority are running between 26P-30P prop's indicating that my 24P's are a little low. I am also finding out that the engines have rev limiters that I am probably approaching at the 5200 RPM range.

If anybody has suggestions for prop's to try out and ways of trying them out I am all ears...again the only thing I am cautious about is over prop'ing and trashing my transmission/drives.

bbladesprops

Your WOT number of 5200 is most likely acceptable. However it is also probably at the top of the desired rpm for the engine. The parts installed during the rebuild and a actual dyno sheet are the only way to ultimately make the decision. As stated above, I think you are fine at 5200 but 5000 might be better for longevity.

Formula FasTech hulls have always loved 4 blade Bravo 1 propellers. In fact in 1998 while I was at Merc Racing, I took a monthly order from formula for Lab Finished Bravo 1 props as they had become standard equipment that year for these hulls.

Most always we rotated the props out as these stepped bottoms can run quite well flat. The outward rotation maitains best stability.

I believe testing 26 Bravo 1 propellers and then Labbing them for 5000rpm is the best option for your set up.

Also, removing the nose cones could give you another couple mph.

Let us know if you want to test some stock B1 26's.

Brett