MPH estimate question

[Oldsquirt]

ALL THIS IS A GOOD READ, ESPECIALLY SINCE I'm STILL TRYING TO FIGURE OUT WHERE MY ASS IS.
The first thing you need is an rpm-to-mph reference from when Jim B had the boat. Then when you have a dyno sheet on the new motor you will be able to make use of Jer's JPC to do some calculations/speed projections.

[Oldsquirt]

They apparently don't publish the hp rating on the Indmar 502. They say it is about 415 though. I wouldn't be surprised to find out I was only putting out 350 hp. After all, that is what the impeller says I'm doing.
Not totally accurate. It is telling you that AT 4800 RPM you have 350 HP. That engine may well have 415 hp but at an rpm you cant reach with your current pump/impeller combo.
When I'm done. I'll try the AA to see what it does. I know ST likes the bigger impellers.
J
Keep in mind that running increasing larger impellers = increasing larger load on your engine at ALL rpms. Hope you at least have an oil temp gauge to accurately see how much extra heat that load is creating. A one impeller size increase is probably not too big of a deal, but several sizes bigger, you might want to start thinking about watching your oil temp closely and potentially adding an oil cooler (if you don't already have one).

[jbone]

Thanks for all the info.
I will post mph once everything is back together and running.
J

[LVjetboy]

OldSquirt has a good point. The custom drag curve I mentioned not hard to figure if you have a good tach and speed measurement. Here's how:
First record test temp, altitude and humidity. (Not required but a good habit for future reference)
Then run your jet at on-plane rpm and record speed. Then record speed at regular rpm intervals up to top speed. Say every 500 rpm.
Back home convert rpms to power with pump charts or JPC, etc. and your impeller cut. Then plot your drag curve (speed vs. power) with a program like MS Excel. MS Excel can add a trendline or curve fit of your choice and show the equation. JPC uses a 0.51 power curve which fits average jet boat trends best. But a 3rd order polynomial may be a better fit for heavy jet data in the power range most care about...so play with it.
Presto, you now have a custom drag curve and good speed predictor for future power upgrades.
jer

[FASTJTBT]

My Twin Turbo 540 Pushes My 19 104 At 4400 6lbs Of Boost Pump Has A Little Work Done By Papp. Good Luck

[jbone]

Back home convert rpms to power with pump charts or JPC, etc. and your impeller cut. Then plot your drag curve (speed vs. power) with a program like MS Excel. MS Excel can add a trendline or curve fit of your choice and show the equation. JPC uses a 0.51 power curve which fits average jet boat trends best. But a 3rd order polynomial may be a better fit for heavy jet data in the power range most care about...so play with it.
I knew I should have gone to school that day instead of surfing.
That sounds a little x and y for me.
Since the engine is out, I'll just have to wait and see.
Thanks though for trying to explain it to me.
J