Tapped out at 5200 rpm's

[MidnightMantra]

Ok guys, got the engine broke in and made a few WOT passes. I am tapped out at 5200 rpm's. I am out of carburator at that point it looks like. At this time it has a 770cfm Holley vacuum secondary on it bolted to a single plane Dart intake. How much more carb do I need to get the engine to turn to at least 7000 rpm's?
Boat hull: Mantra/Semi-V 18'
Pump Details: Dominator, Semi-Pro grate, "A" Impeller
Engine Details: Chevy 468 C.I. , with aluminum rods, roller camshaft 310 dur/.714 lift, Dart Heads, 12.5:1 compression.
I do have a tunnel ram intake in the closet, was thinking of installing it and adding another 770cfm holley to it. What do you think?

[cal***boat]

Originally posted by MidnightMantra
Ok guys, got the engine broke in and made a few WOT passes. I am tapped out at 5200 rpm's. I am out of carburator at that point it looks like. At this time it has a 770cfm Holley vacuum secondary on it bolted to a single plane Dart intake. How much more carb do I need to get the engine to turn to at least 7000 rpm's?
Boat hull: Mantra/Semi-V 18'
Pump Details: Dominator, Semi-Pro grate, "A" Impeller
Engine Details: Chevy 468 C.I. , with aluminum rods, roller camshaft 310 dur/.714 lift, Dart Heads, 12.5:1 compression.
I do have a tunnel ram intake in the closet, was thinking of installing it and adding another 770cfm holley to it. What do you think?
Not that I'am a jet boat expert but from what I have read in here it sounds like the "A" impeller is holding down your RPM's and 5200 is about right with maybe 450-500 horsepower.
I'am sure someone will come in with a better reply.

[Squirtcha?]

I think you hit on it cal***boat. Not running out of carburetor.........running out of horsepower at the 5200 rpm. Smaller impeller may help, but only if you've got more hp to tap at a higher rpm.

[MidnightMantra]

Squirtcha,
This engine should be producing 600+ hp.
Engine Breakdown.
427 Tall Deck Block/o-ringed
454 Lunati Crank
454 Childs and Albert aluminum connecting rods
454 12.5:1 dome top pistons cutom made for the tall deck stroke
Crane Roller Cam / Solid / 310 duration @ .714 lift
Gear Drive
Dart Heads 2.3 intake/1.88 exhaust
internally balance
msd 6al ignition system
Holley 770cfm carb.
Is 5200rpm's all I am going to get out of this engine? Will more carb help? or do I need to downsize to a "B" cut impeller? Motor was built to turn 7500 rpm's. I ran this in my drag car and turned it to 7500 at the shift point. That was running a 1050 dominator on 100LL fuel also.

[sleekvino]

it doesnt matter how much horsepower you have if the pump cant take it it cant take it.id cut the impeller down or try the tunnel ram it looks like your cam can take it.

[FoMoCo]

Just my 2 cents. With all that power your motor is trying to build(yourspecs sound pretty good) you should be runnin a 950cfm or at least the tunnel ram with those 2 770's! I gained almost 400 rpms with the same motor when I switched from 2-600 cfm holleys to 2-750 dp holleys on my Tunnel ram. But just to be real, the goal of 7000rpms it pretty far fetched. With that kind of power you makin and you finally have enough carb far it, I would expect somewhere around 57-6000 rpms. If you decide to have your impeller cut down, try to remember that the more you take off the impeller, the more hole shot you lose!

[Squirtcha?]

Yupper I agree. It oughta be making way more than 500 hp. The question here is................at what rpm are those horsepower at?
That's the key. If you had a dyno run it'd tell ya at what rpm you're making the ponies at. Otherwise it's Desktop Dyno, or poke and hope.
My motor should be making 535 @ 5500 rpm, but since my impeller won't let me past 5200 rpm it's like they're not even there.
I just hate unused resources.

[MidnightMantra]

I'm with ya there Squirchta on the Hp that seems like that aint even there. I was disappointed my first run at WOT, when I hit 5200 I was like in shock cause I know that the motor would turn 7500 without any hesitation. 5200 was like hitting a brick wall and going no further. So now, I am going to start by putting the 1050 Dominator on for this weekend and see if I can gain some more RPM's there. There goes the gas mileage. Also gonna pump in some 100LL AVGAS which should help also. Lets me advance the timing about 4 more degrees also. Realisticly I would like to get to 6200 RPM's, which I think will give me a good top end speed and not hurt the motor any. Gonna take it out this weekend for it 3rd time on water. I know it will be a long road to test and tune to get it where I want it. Got me a handheld GPS, gonna run the numbers this weekend for the first time on the boat. Im sure I aint gonna see what I want to see as for MPH. I am hoping for 75-80.

[Taylorman]

Originally posted by Squirtcha?
My motor should be making 535 @ 5500 rpm, but since my impeller won't let me past 5200 rpm it's like they're not even there. I just hate unused resources.
Why not cut your impeller to get a few more mph and add an inducer for the lost hole shot?

[Cas]

where's your peak torque?
I'm guessing it'll be somewhere around 4800?

[Squirtcha?]

I'm hearing ya hearing me. In order to turn 6200 rpm you'd probably need a C cut impeller. Maybe even smaller than that.
I was told recently (by some folks that know the stuff) that If I cut my AB to the B I'd probably pick up around 200 rpm. That'd be cool and get me closer to where my horsepower is, however like everything else in life, there's a tradeoff. Like FoMoCo said, "You'll lose some holeshot". Not to mention skiability in my case. We use my boat a lot for skiing and it will somewhat hamper it's ability to pull up skiers. Probably not too bad, but still not as crisply as it does now. In addition, around here (Arizona lakes) we all race from a standing start, making a good holeshot imperative.
You're also putting more wear and tear on your motor running at the higher rpm. It's not like you'll just be doing a couple quarter mile passes (at least I don't) you'll be running her pretty hard all day, every day.
Guess we both just need to figure out what it is we wanna do.
Personally I wanna whip some of those Cali guys that got me at FC and my only chance would be more horsepower, a bigger nitrous shot (same difference I reckon), or cut the impeller.
Hmmmn............................:confused:
Good luck with your quest for speed, and post up after ya get her out again. I'd like to see what happens.
P.S. Torque schmorque (sorry Cas ol buddy, but ya know I'm in the horsepower camp)

[MidnightMantra]

My peak torque is 637 lbs at 5500 rpm's.
Horse power at 5500 rpm's is 668 hp according to desktop dyno. Max HP is 759 hp at 7000 rpm's. With 575 lbs of torque.
This is with all the exact spec's of my engine build except for the port flow numbers. Im sure you can take 5% off of these numbers from the desktop dyno program.

[American Turbine Man]

You need a larger carb.
With 12.5 comp. and 100LL you run the risk of burning it down.
The jetpump puts more load on the engine than a car.
Be real carefull about adding timing.
Fatten up the carb from what you ran with it in the car.
If you are at WOT and the tach starts comming down get ot of the throttle.
With your power spec's I would run the motor at 6200.
Before you cut the impeller install the tunnel ram it will make more torque, then you will have a better idea of how to trim the impeller.
ATM

[396_WAYS_TO_SPIT]

I'm with everyone else on this. MORE CARB then record your readings. If not satified then cut the impeller down. Do you ski or pull tubes? If so then a small impeller will hurt you like everyone else said. I went from 750 cfm on my olds to a tunnel ram with dual 600's and she ispretty stout from 3500 and up. My friend is running a A/B cut impeller in his boat and he has a neck snaping holeshot.
When was the last time you went through the pump? That might help you as well.
But then again thats just my opinion;)
396

[Jim Brock]

You need more cfm's wether it is one big carg or a couple of 750's, you said the cam is 310 dur. is that advertised or at .050, what is the lob center, Jim

[manuel]

Here's a CFM calculator,
http://www.newcovenant.com/speedcrafter/calculators/intakecfm.htm
It said BIGGER for you!

[Cas]

Originally posted by Squirtcha?
P.S. Torque schmorque (sorry Cas ol buddy, but ya know I'm in the horsepower camp)
don't worry Dan, I don't hold it against ya :D
I'm finding a very interesting pattern in all the old Performance Boat magazines from the 70's I've been reading. When advertising the new boats, they talk about HP and when the article is about building engines or engine builders, they talk about TQ 1st, then HP.
"Build the TQ to the optimum peak rpm and the HP will follow" hmmmmmm :wink:
Anyway, I agree that Midnight needs more carb to let that motor suck and blow the way it needs to. I have a inquiry about this myself:
If say the 950 carb gets his motor another 250 rpms and that amount is enough to get the engine into a much higher power range. Is it possible that the engine could actually pick up 800 rpms? Did that make sense?

[LVjetboy]

Forget torque. For jets it's all about power matching the impeller. If your peak power is 760@7000, by pulling the engine down to 5200 with a big impeller, you're loosing 180 hp in speed potential. For a small jet that can be 10 mph or more. Smaller impellers (may) loose some holeshot, but you make up for it in quicker acceleration once loaded. This because extra engine power on tap is converted to more thrust...the bottom line.
With a Dominator impeller, even if you cut to a C, you'll fall short of peak power...assuming your engine truly peaks at 7k. But you may gain 5 mph or more from cut size alone. Berkeley impellers are less aggressive than Dominator, and switching to a Berkeley C good for another 400 rpm. Depending on the slope of your engine power curve at that point, you could see a gain of 8-10 mph just by going from a Dominator A to a Berkeley C.
As for skiing, I've never had a problem pulling up single ski or no with a Berk B impeller and v-blocker, so I don't think that's an issue. But then I've never run as small as a C either.
When you look at the pump curves for impeller matching, remember those curves are based on actual lake power to the pump, not corrected power from DTD or a dyno run. Also as you approach peak power there's a point of diminishing return. So cutting to absolute peak power (if you could even determine that point accurately) not always the best trade-off. In other words trading 400 rpm for 10 hp not worth it.
Sounds like you're far from that point.
jer

[Wally_Gator]

I thought I would chime in on this.
The difference between a Berkeley A impeller and an A/B
is 200 rpm and so on down the line.
Each cut gives you less volume and more speed in the water exiting your pump.
While theoretically with enough horsepower, you can turn an A impeller at much higher rpms. The problem you run into is bowl and nozzle pressure. The A impeller with more volume will create more pressure on all parts involved. One A/B impeller will have less volume at the same rpm but will have a higher water velocity.
Just my .02
Walter

[wrightnow]

Originally posted by American Turbine Man
You need a larger carb.
With 12.5 comp. and 100LL you run the risk of burning it down.
The jetpump puts more load on the engine than a car.
Be real carefull about adding timing.
Fatten up the carb from what you ran with it in the car.
If you are at WOT and the tach starts comming down get ot of the throttle.
With your power spec's I would run the motor at 6200.
Before you cut the impeller install the tunnel ram it will make more torque, then you will have a better idea of how to trim the impeller.
ATM
I'm running 13.6 comp on 100ll and am having no problems, just keep the motor cool. While we had it on the dyno the motor produced more HP the cooler we ran it, so I run around 120.
Mark

[Taylorman]

Originally posted by wrightnow
I'm running 13.6 comp on 100ll and am having no problems, just keep the motor cool. While we had it on the dyno the motor produced more HP the cooler we ran it, so I run around 120.
Mark
Does that hold true for all engines?

[Some Kind Of Monster]

120 sounds a bit cool. Especially for a 100LL burning motor. I always thought that running a motor that cool can cause problems?

[LVjetboy]

Gator, the rpm difference per cut size also depends on impeller mfg as well as power level since pump curves are non-linear and all converge to zero. For example, at Midnights power (approx 520 hp) Berk A to A/B is 170 rpm...assuming no change in engine power available. But for his Dominator impeller, going from an A to A/B is only 135 rpm. How many extra rpms he picks up above that (from a climbing engine power curve) will depend on the slope of his dyno curve. If the slope is enough that he gains 18 hp by cutting A to A/B then he'd see a total 200 rpm gain w/a Dominator.
On the volume versus exit velocity...as long as you keep the same nozzle size, exit velocity and volume per second are directly related. In other words, by cutting the impeller you can't increase the exit velcity yet reduce the exit flow volume. They either both go up, both stay the same, or both go down.
But I think it's clearer to think in terms of total power to the impeller and not internal details of mass flow rate, pressure, volume or exit velocity. But we could go there if you like :) Everybody's like, no please anything but that!
So that's why I say size the impeller to match (near) peak engine power. Because the more power you deliver to your impeller the more thrust you get = more speed and acceleration.
jer

[Squirtcha?]

Originally posted by wrightnow
I'm running 13.6 comp on 100ll and am having no problems, just keep the motor cool. While we had it on the dyno the motor produced more HP the cooler we ran it, so I run around 120.
Mark
Yes! Finally someone with a like opinion with regards to running engine temps.

[Some Kind Of Monster]

Squirtcha?
Do you run lower temps too? I don't have much experience to back up my comments, but I have always heard to run them higher than that. Have you had any problems? What kind of gains are you seeing? Any downside?

[Wally_Gator]

Originally posted by LVjetboy
On the volume versus exit velocity...as long as you keep the same nozzle size, exit velocity and volume per second are directly related. In other words, by cutting the impeller you can't increase the exit velcity yet reduce the exit flow volume. They either both go up, both stay the same, or both go down.
LV,
I originally stated on a berkeley impeller as you point out all manufacturers are different, even for the same model pumps.
You are right on the exit velocity, however I point out that an A impelller is loading more water into the bowl per revolution than a A/B impeller. Thus we see higher pressures at lower rpms. Holeshot. The smaller cut, will pump less volume per revolution and thus create less pressure at lower rpms. The advantage is that the water can flow faster with less of a load on the engine. Ultimately we see a limit by the bowl and the nozzle and what they may flow with safe pressures and at cost effective engine HP.

[LVjetboy]

"we see higher pressures at lower rpms. Holeshot"
Gator, that's a key point. How is holeshot affected by impeller size given a common intake, loader and jet boat weight? Not so easy to know. But race data shows rpms from idle to full throttle not much different than at top speed. Unlike a car, you mash the throtte you see max rpms real quick. That means what happens at "lower" rpms may not be a factor for balls to the wall in a jet.
By comparison, acceleration once loaded to top speed in a jet more straight forward. To me, holeshot's complicated by transient physics. How much air is being ingested? This depending not only on impeller cut but also on pump efficiency, and loader and intake design. Not to mention hull weight and speed...or nozzle size and losses.
From racer data, an impeller revs to near max power in a fraction of a second unlike a car so that eliminates some variables. Ignoring efficiency with cut size, the smaller impeller matched closer to peak power will build bowl pressure quicker than the big impeller matched to peak torque.
I have an idea that supports my post from Midnights data. But seems Midnight lost interest and I've been busy posting the audio/stereo thread, so maybe later.
jer

[MidnightMantra]

LVJETBOY,
I Haven't lost interest. I have been gone for the last 2 days since I started this post. I have read in detail all the information that has been provided on this post and I sure do appreciate everyone's input. Today I am going to take the boat out for another test run, I am going to install the Dominator 1050 carb and see what my results are from there. From there I will see where I need to make my adjustments for more speed and RPM's. I still have a tunnel ram I can install too. I have alot of area to make improvements, I still do not have a droop, place diverter or ride plate that can be installed. So I know I should be able to make some gains there also. I'm taking the GPS today to get a MPH fugure. I will post some results later today or tommorow and let you know where I am at. Thanks alot everyone.

[Wally_Gator]

LV,
I think we can definately go on for days on theory and actual practice. You have some very practical information to provide and it is interesting to hear it.
It isnot my intention to contradict or offend, I have offered some info from what the guys at my old work had been telling me for a long time. They are industrial pump designers.
Alot of the stuff went way over my head that they would go into. I was a network Administrator there, and being a gearhead as well, I would always check out their new prototypes and see what they were doing.
My point on volume was to she some light on how the cut of an impeller would affect rpms.
Keep up the informative posts..

[LVjetboy]

No problem Gater. I wish the pump mfgs had more info on cut size and efficiency. More later...
Midnight...I'll be interested in your results. I'm guessing you won't see more than 100 rpms with the 1050 assuming the 770 was in good shape and tuned...and same test conditions of course. I ran an engine similar to yours in DTD...I'll post later tonight.
jer

[LVjetboy]

Midnight, here's the reason for my original post. I ran an engine in DTD similar to yours as best I could figure. This 468 power peaks with 754 hp @ 7000 rpm. Torque peaks with 640 ft-lbs @ 5500 rpm...very close to your numbers.
Then I ploted the DTD power curve (dashed red line) If this engine turned 5200 rpm at the lake when hooked to a Dominator A, the actual lake power curve would be closer to the solid red line. Now where the gray Dominator A “impeller” curve intersects engine lake power available...that's where you run. Notice at 5200, the A impeller is absorbing near maximum engine torque but well short of peak power.
http://members.cox.net/lvjetboy/MidnightMantra.jpg
If you swapped your Dominator A for a Berkeley C (solid black line), you’d see an extra 90 hp applied to the impeller. The Berkeley C will run at 6500 and close enough to peak power to make any further downsizing a poor trade-off I think. For a typical small jet with no major setup problems or hook, an extra 90 hp can give better acceleration once loaded and about 6-7 mph on the top end. Cutting your Dominator to a D gives the same match, but I'm not sure how much you could cut from that impeller before taking a big efficiency hit. There’s little information on how cut size affects efficiency. So I can’t tell you whether the Berk C is better than a Dom D, but at some point, loss in pump efficiency may offset power gain.
So my approach would be to look for a good deal on a Berkeley B...that should get you halfway and be conservative. Try it out and if the rpms and speed go up as predicted, if so trim from there and sell the Dominator A. Keep good records of air quality and water conditions...apples to apples
My comment on carb sizing was based on running the two sizes in DTD. Because you're well off peak power, that A impeller holds you back from seeing the bigger carbs benefit. In fact, I got a gain of 10 hp by switching to the 1050 (at 5200) which is something like 40 rpms. You may not even see this on your tach or seat of the pants. This assuming both carbs are running right and tuned to the engine of course.
But if your impeller's matched and running at 6500 rpm, the carb swap power gain's closer to 25-30 hp or about 100 rpm.
Lastly, it seems with over 500 hp your 18' shallow v should already run mid 70's? Not sure maybe some here know your boat and the best setup.
jer

[Foggerjet]

This is good stuff. I'm learning alot from this thread. Some of y'all really know your stuff about hydrodynamics,(or so it would seem). It's common knowledge that the pump is where the speed is so all of us could benefit from this. Please keep it coming.
fog

[manuel]

Hey Midnight, where in Alabama are you ? or what is your home lake/river ?

[LVjetboy]

I think it's interesting too. You really need an actual dyno run for the best guess at a match...DTD's just a ballpark. In Midnights case tho, downsizing's a pretty safe bet for more speed. But your typical mild BB peaks much lower so you don't see the same payoff (unless you're WAY over impellered) since the power curve's already leveling off.
jer

[MidnightMantra]

Well I took the boat out friday to make a test run and check for a water leak I was having. Dang suction housing is killing me.
3m- 5200'd that sucker and it still pealed. Anyhow took it for a GPS run with the 770 cfb carb. Ran a 68.7 on gps at 5200 rpm's. Took the boat back home installed the 1050 cfm. Today I ran a 71.2 at 5500rpm's. My throttle response was a hell of alot better also. The fuel tanks were filled to the rim also. But it looks like I am going to have to change out impeller's and go down a size to get me in a better RPM range. LVJETBOY, I saw your diagram on the post and it is most interesting. What do ya'll think? Go down to a Berkeley "C"? I think if I can get my engine to turn up to 6000-6200 I will be in the ball park I am looking for. I still have to get a droop and a place diverter. Should I get those and test first before changing to a different impeller? I"ll be offline for the rest of the day but will get back with you guys sunday afternoon sometimes'

[Cas]

Jer,
Since you have a fairly close DTD for Midnight's motor, can you do up another chart using a B impeller and advancing the cam +4º?
I'd like to see what it looks like after moving the peaks down a bit.

[ChetCapoli]

I would get at least the diverter first and then cut your impeller down. I would look around for some loaders to try..in my boat i've seen up to 7mph difference in them at 5800 with a B(700hp)
CHET

[LVjetboy]

Midnight, thanks for posting your results. Like I posted above, my first choice would be a Berkeley B to be conservative, then if you're happy with the overall performance change consider trimming to a B/C...retest...etc.
Cas, I ran it again with +6 which brought the peak down but pays off by increasing power at 6000...which is close to where the Berkeley B matches. I'll post the chart tomorrow.
jer