lvjetboy

[tahitijet]

I'm sure this has been asked a few times but what is the average margin of error in the JPC? Just crusin today we were running 42.2 mph at 3300 with an A impeller. This equates to 100.2mph at 6057RPM. We had always figured the boat would run high 80's low 90's but low 100's seems to high. Your input Please? Thanks

[Oldsquirt]

I'm curious, but did you check to see how much HP would be needed at that rpm?

[Squirtcha?]

I see 700 hp with a Berkeley A impeller

[tahitijet]

yes 700.. My motor dyno'd just a couple short of that.. I used 700 cause 2hp only made a .some differance in MPH

[Oldsquirt]

AH, ok. So I assume by the rpm choice, 6057, that that is the rpm at which it made that HP. Did you get any other rpm/mph numbers while testing that you can plug into the calculator? If so, how well did they fit the curve?

[bp]

Finally, just because you dyno at say a corrected 700 hp doesn't mean you'll see that at the lake. Don't forget to adjust corrected power for DA. That's typically a big difference and seems few understand or appreciate what that means let alone how big the difference can be. My lake power is about 100 hp below max dyno power. Even with an impeller matched closely to peak engine power. Like OldSquirt alluded to, just because your max dyno was 700 hp, will your A cut match that peak or fall below?
Just some things to think about not related to JPC accuracy.
jer
are you saying that you lose 100hp just from D/A? what kind of D/A change are you talking about? that's pretty significant, but i don't know what kind of d/a you're talking about. i have documented runs from 800' to 4600', and i haven't lost 100hp.
if you're only talking about setting up your pump to run below peak dyno hp, different discussion. but generally that's what you're trying to do anyway.

[tahitijet]

thanks jer,
oldsquirt i put in the knowen numbers 42.2@ 3300 then knowen 700hp and it gave me speed and RPM. i don't have the dyno sheet right in front of me but I recall the motor making max power somewhere in the low 6,000 range is that a coniceidance?<SP>? or how it should be? No i didn't get any more numbers the water was horrible i just happend to have the gps on at cruise hit some half nice water and took a look at it and the tach. I will log a few more if i can get out early next weekend before the crowds.

[bp]

Bp I'm talking about DA for STP corrected dyno power vs DA at lake conditions. DA is DA and affects power by temperature, humidity and pressure. A bit more than just a pressure altitude. I'm also talking about peak dyno vs impeller cut. Those two combined can make a big difference between tahitijets 700 dyno and his lake performance. Assuming his quoted 700 hp is a corrected number.
Say the lake he runs is 1000 ft instead of sea level? Now he's only got 675 hp. And if his test run at 80 F instead of 60 F STP? Now he's got only 658 hp. But wait, instead of dry air there's 30% humidity? Now he's got 640 hp.
So he's already lost 60 hp from peak dyno. We're not done yet. His lake engine configuration may not be the dyno run max power setup. He may run more restrictive exhaust or intake on the lake compared to dyno run. My maximum dyno power was with Daves headers and no mufflers. I run the lake with mufflers and Bassett headers. Differences between dyno maximum power setup and lake running setup not always appreciated or acknowledged, wouldn't you agree?
But we're not done yet. He said he runs an A impeller. Will his engine turn that impeller to just over 6k peak dyno power...or fall short? If short could easily be 20 or more hp. So now he's got 620 hp or less at the lake. I'm sure you know many posts on this subject come from people who don't understand the difference between corrected and measured power. Let alone what standard power was corrected to or how that standard influences power. Or how an impeller cut can influence power and performance.
Yet you call me out and question my post?
My point Bp is that some, not saying you or Tahiti, don't appreciate the difference between dyno and lake running power. Once everything's factored in, that difference can be as much as 100 hp...with big power. Yet some insist on blaming any difference between lake performance and dyno on the dyno, lacking an understanding of what dyno corrected standards are or how that relates to lake conditions and power loss.
jer
call you out? no, that was on several other threads, but you never show up so why waste my time calling?
i still don't know if you're saying d/a alters hp by 100hp, or if you're saying something else.
what i can say is that i have over 350 passes, each one of which i have documented the d/a with my performaire II. i've made passes from 800' to 4600', and from the highest to the lowest, the greatest change in hp i've seen is 22hp, whether i compare it to the corrected dyno charts, or the uncorrected dyno charts, it doesn't really matter, the delta is about the same. this is based on wot rpm through the 1/4, and comparison to hp curves developed from the superflow data, both corrected and uncorrected. this is in "real world", practical application; not theoretical stp vs d/a.
how this might relate to a 500hp engine, or a 400hp engine, i cannot say - a percentage relationship??
if your point is that the hp applied on the lake will be less than dyno peak hp, once the engine is in the boat once all components are attached, or different components are installed that were neglected to be used on the dyno, i would agree.

[bp]

jer, i will get back on this, just don't have time at the moment. too many chores. bumping this...

[bp]

one days set of chores done.. now to dig dirt tomorrow.. :yuk:
first, as we've noted before, dyno "corrected" means that the hp number is automatically "corrected" to standard atmospheric temp and pressure, which for all intents and purposes is 0, and never found anywhere (well, maybe new years day below sea level). the "raw" hp numbers are basically read at the atmospheric temp/pres etc., that exists in the dyno cell at the time the engine is dyno'd.
so, you have two numbers from the dyno at, say 6000 rpm. just picking out one of my dyno pulls, where the cell d/a was 1600', i see a 75hp difference at 6k, the corrected reading higher. now, the corrected is fine for comparison's sake, but for setup and planning purposes, not too terribly useful. AND, if what you've been saying is that the difference between dyno and lake beeing 100hp due to d/a is comparison between corrected and uncorrected, i think you're comparing apples and oranges.
now, once the engine is in the boat and set up, nothing much changes except d/a. in my case, with the impeller i've been running most all the time for the past 2+ years, i see AROUND 6k. by the way, there is no way i could turn an A impeller 6k, and i make considerably more than 700 uncorrected hp at 6k (well, maybe a cheap one with sloppy tolerances).
in any event, i am not trying to redefine the hp numbers, or come up with another specific hp number - i see no point in trying to do that. what i AM doing is looking at both hp charts as a curve, i.e., 5700-5800 was 9hp delta on both charts, etc. using that method, i have slightly less than a 40hp difference between 5500 and 6k, and i never drop near that low.
the pump is sort of like a dyno on the water, and while top speed can be affected by setup, early in a run when the pump is loaded and accelerating, if d/a were really causing a significant drop in hp, you would see that in rpm drop. what i am saying is that i do not see that occur.
increased d/a means to me that there is less o2 content, available to burn, per cubic foot of air being pumped through the engine. one thing that can affect performance on an n/a engine is not keeping the a/f ratio decent, allowing the engine to get real fat, when the o2 is going away, then also making sure there is sufficient fuel when it comes back.
in any event, i just do not see an up/down swing through the range of d/a like you're suggesting. perhaps in tahitijets case, the hp referred to is corrected? what does the rest of the curve look like, and what are the uncorrected numbers? and at what d/a?

[bp]

i run k&ns all the time, mainly because there is so much dust and dirt flying around in staging lanes. i've run without them, and i just have not seen enough difference to justify doing that (maybe 10rpm?), compared to what i feel is risk of getting dirt in my engine.
i have not made any passes with insert mufflers, so i cannot provide any performance numbers on those, but i don't think they provide a 30hp restriction by themselves. the water injection definately cuts hp, but how much? 20? dunno. later this year, i plan to track test with a performance muffler, and will have results.
i've been very clear in the past about the impellers i use. i run a legend B almost all the time. i have a heritage C that i use at phoenix.
it almost seems to me as though you've taken the corrected hp numbers from the few dyno runs you made, then subtracted from that number, based on assumptions, arriving at an arbitrary number. the number (650) that you have concluded appears to be based on part science (dyno) and a series of unproven assumptions.
the difference between 766 and 636 is 400rpm???????? i have no idea what you're referring to with that statement.
which is WHY i always go back to say i don't really care what the absolute specific hp number is - ballpark is good, but knowing the curve/trend is really important, where does it peak, where does it fall off, how does it affect real performance.
i'm more concerned about performance jer. i need to be able to call an 02 in the pits, then be able to go out and run it. but that means i need to KNOW the parameters involved. if i were losing 100hp when d/a went up 2000', i'd NEVER be able to do what i do.

[UBFJ #454]

For what it's worth, bp has his boat dialed in and Knows It to where he can make hardware changes that change his ET in the 1/4 by as little as 0.15 of a second ... I've seen him 'Fiddle' with the position of his petot sensor and gain or loose 0.15 on a run.
One of the reasons bp knows his boat so well is the time he has run it, his meticleous record keeping of motor and hardware setup data over time and his analytic nature ... If there's anyone that really knows his boat, it's bp.