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background info: late model oil burning small block (340 rated hp). the consumption rate was unacceptable (a quart every 6-10hrs). so i built a pretty nice billy bad ass motor. i have it up and running but its not pulling the RPM it was expected to turn. AF ratios are good. timing should be acceptable at 34* WOT. i understand peak HP will come after my operating range. im just not sure the dynamics of how a jet pump works in regards to getting more RPM.
i think my motor should really be building power at 4000 (where its currently stopping) so i didnt think turning the pump higher would be a problem. the oil burning motor was turning 3900. from a dead stop last weekend it was blowing out the pump which it has never done before. the impeller is fresh so this tells me the motor is making HP but not able to sustain it to turn the upper RPM.
i hope half of this makes sense and i hope its in the right section. the plan was to reuse the impeller i have and not have to swap to a different impeller. maybe i need to downsize the cam to build more lower end HP. this motor should be a 450+HP motor.

383
9:1
AFR 190 vortec heads
comp cams XM276 112LSA
holley stealth ram IM/TB 36lb injectors
1.6 rockers
kodiak exhaust manifolds
pump hamilton 212 with 3.4 impeller

airjunky

iTrader: (2)

Maybe try those nz jet boat forums , those guys speak good English and pull some real magic with the small block - Hamilton pump combos .here mostly found in Alaska river fishing circles

SB

iTrader: (1)

Yup, it's a good engine combo....so...it's your tuning, rigging, or pump.

I know schit about jet pumps, so....I am with Air Junky. Talk to those who know.

ezstriper

how big/heavy of a boat ???

donzi matt

iTrader: (1)

You aren't overcammed for a 383. Did you degree in the cam or stab it in dot to dot?

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12.5 AF at WOT. seems pretty good to me.

21' about 4000lbs is my guess. im not actually sure.

based on the cam specs i believe my engine builder said its already got +4 degrees added into it. id have to double check with him.

the consensus says that i need to change impeller ($$$$$$$$$$$$$). because i shifted my peak HP curve up about 1000 RPM it wont pull passed that 4000 mark. the impeller limits the RPM on the motor. changing the impeller wasnt part of the plan.
the main thing i dont understand is my motor should still be making more than that stock motor in that same RPM range. which the RPM is indicating, but marginally. ive thought about changing to a smaller cam to make more low end and sacrificing the top end. ill never see that HP anyways with the pump i have. a pump manufacturer doesnt think even going to a smaller cam will make the difference.
with the impeller i have that 500RPM change from 4-4.5 is about 100HP at the motor. so its a big difference.
i think also because of the IM design with the longer runners than the original style IM that it has helped change the shift in RPM. either way the thing should be making tons of HP.

SB

iTrader: (1)

You had the wrong impellar to begin with. That older 340hp/350 made max power somewhere in the 4800-5000rpm range. It spun the impellar 3900 rpm.

It's like the common guy who builds a big badd azz motor and puts it in his Chevelle that still has 2.73 rear gears and a stock torque converter and he comes on and asks "why is it not really any faster ?"

FYI: Your current motor makes (should make) a schit ton more power at 4k than your old motor.

If your convinced it's tuned correctly, then it's time to work on your rigging which of course the impellar is part of.

We can't help you with wrong parts.

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thats exactly what i was told. you had the wrong impeller from the beginning. its what came on the boat from the factory. it still doesnt tell me why this motor wont turn it harder though.

whatever the motor is making at 4000 now is marginally better than the stock 383. it is what it is though. i might do a compression test on it just to make sure things are good. but for now its time to go play in the whitewater.

SB

iTrader: (1)

BTW: I know the XM276HR (224/230 at .050 112LSA) pretty intimately in marine applications. On most 383's with decent heads on an I/O boat max speed (hp) will be around 5200rpm or so. On a 350 , again with decent heads, will be 5400-5600rpm.

You'd be doing a disservice to remove that cam (unless you computed compression wrong and you really have 8:1 or something) and put in something smaller.

Even if you put in a typical 218/224 (used more in 350's then 383's, but still a good amt in 383's usually with less than ideal exhaust)) it will not make more power at 3k on your motor than the 276HR.

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Great information I really appreciate it. From the way this kind of explain to me is that we only have X amount of area to pull water into X amount of area on the blade to push water out so there's some sort a steeper curve there then in a propeller where its open