max rpm low

[Kip]

I am looking for some advice concerning our max rpm. We have a 88 Supra Mariah with the 351. the prop is a 14 x 12.

At wot we are only getting 3600rpm and about 38mph. It starts and runs very smooth. I replaced plugs and wires and the fuel filter last year. I also checked the accuracy of the tach, timing, and that the secondaries on the carb are opening.

I have had this boat for about 2 1/2 years and am pretty sure it has always ran this way, but but was happy with it because it had so much more power than the IO's we ran around in. I really noticed what we were lacking when in a buddy's older mastercraft and could tell he had quite a bit more power.

I know the points are old and may have some cracks in them and I have no idea what dwell is and how to adjust it. Could either of these hold me back?

Thanks in advance for any suggestions you can give,

Derrick

[wotan2525]

14x12 is a big prop with a low pitch for that boat. I think it's probably what's limiting your RPMs. I bet you have a killer hole-shot, but having that prop repitched is probably going to get you a lot higher top speed.

I'd check with a local prop shop before I spent any money on a new prop... since.... I'm not a prop expert! ;)

[Century63]

Had a 351 in a 1988 Supra Pirata. Same issue. The dealer I had purchased it from had it all tuned when I purchased it. It had a 13x13 prop. It ran like that for 2 years. I replaced the engine with a 454 and had new stringers put in it and the old foam removed. Was told that the old foam probably weighed near 500 lbs. It was replaced with 35 lbs of foam. The only thing I could surmise was that all that weight was holding it back. Right now it runs 4200and 42 mph according to the gentleman that worked on it. I will be getting it wet this weekend.

Century63

[Kip]

Wotan. I spoke with the local prop shop and he thought I would pick up maybe 200-300 rpm by dropping down to a 13 in prop but still not putting us where we need to be. I do like the power out of the hole but I want more:D

Century63 did you have any other known issues with the 351? low compression or anything?

Thanks for the replies
D

[rludtke]

My Comp is equipped with a 13x13 prop, which I assume is the stock configuration. My max engine speed is 4500 RPM with this prop. This results in ~45 mph top speed. 14x 12 is a big prop, I think the 13x13 prop will get you what you are looking for. Presuming that your engine is in good working order, the fixed pitch propellor is the limiting element for engine rpm, their is nothing else you could change besides the gear ratio of the transmission.

The prop repair shop I have frequented several times has offerred me the oppertunity to try on several different props for comparison sake free of charge. I have not taken them up on the offer, as I am happy with the 13x13 configuration. Perhaps you could query your local repair shop to see if they could provide a simular offer? I have spoken to them about what the expected behaviours of different propellors should be, and they have been reluctant to say anything firm, as they say the have been surprised in several instances by the unexpeted results of trying out different props. Apparently theory and actuals are not in perfect harmony. The only way you will know for sure is to test a different prop.