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Old 09-03-2019, 09:38 AM   #1
Flusher
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Default Looking for a better understanding for my next build

I am in the process of refreshing my jet boat engine and have found some corners that have been cut the last time the previous owner assembled the engine. I plan on running this boat in NJBA and ADBA, so I am interested in running the engine closer to its potential. I am considering C16 or E85 if my fuel pump is capable.

My current combination as I pulled it apart is:

Big Block Chevy
*4.280 Bore
*4.000 Stroke
7/16 OE Rods
188 Heads
*2.19 / 1.88 Valves Intake / Exhaust
7.9:1 Static Compression
Crane Track Roller Cam / Lifters
*308 / 312 Advertised Duration
*268 / 272 Duration @ .050
.708 Lift Intake / Exhaust
*108* Lobe Centerline
*110* Installed Centerline
Littlefield worked GMC 6-71 blower, overdriven 10.9%
Enderle Mechanical Birdcatcher, 80A pump
Vertex OAC Magneto, timing locked at 39*

Changes i am making, with parts I already have, include:
Dart Big M at 4.530 bore
6.535 Oliver Rods w/CA 625 bolts
Crower 4" 4340 six counterweight crankshaft

My heads currently have Manley Severe Duty valves. The first valve I was looking for the part number on, I don't remember if intake or exhaust, appears to have a peculiar crack between the lock and the tip. It's possible that this could be a flaw in the hard chrome, but I don't want to take the chance. If I replace all the valves and springs, now would probably be be a good time to upgrade the heads.

Heads that I am considering are:
* Brodix Dragon Slayer 320
* Brodix BB3 Xtra 345

My questions are:
I have concerns that my blower is too small. I do not know what my boost was before. I understand that changes to cam, heads, etc. will change boost. How do I determine when I have reached the limit? Should that be based on IAT? Boost? Overdrive ratio?

Input on cylinder heads and camshaft would be greatly appreciated.

Thank you in advance.
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Old 09-07-2019, 12:35 PM   #2
Screwblower
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Default Re: Looking for a better understanding for my next build

Better flowing heads will drop the boost pressure at the same overdrive. Speeding up the blower up can have negative effects.
Better heads, increase compression , leave the drive for the blower at current speed.
Better heads, more compression , raise the chip, slow the blower down -option 2

You’ve seen motors make 1200hp at 20 psi and 1200 hp at 10psi, psi in the top of the motor is just a number.
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Old 09-07-2019, 01:32 PM   #3
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Default Re: Looking for a better understanding for my next build

procharger centri
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Old 09-09-2019, 09:29 AM   #4
Flusher
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Default Re: Looking for a better understanding for my next build

Quote:
Originally Posted by short1 View Post
procharger centri
I totally agree, a centrifugal is much better suited to jet boats. However, this is a nostalgia style build and for budgetary concerns, I need to work with much of what I already have. To the best of my knowledge, the quickest open blown gas jet boat had a centrifugal. Thank you for the reply.


Quote:
Originally Posted by Screwblower View Post
Better flowing heads will drop the boost pressure at the same overdrive. Speeding up the blower up can have negative effects.
Better heads, increase compression , leave the drive for the blower at current speed.
Better heads, more compression , raise the chip, slow the blower down -option 2

Youíve seen motors make 1200hp at 20 psi and 1200 hp at 10psi, psi in the top of the motor is just a number.
Thank you for the reply.

I'm struggling with uploading pictures, so here is a link to a screenshot of a data log that really illustrates what is happening with the peculiar case of a jet boat (https://www.instagram.com/p/e_VGQAoSwj/?hl=en).

The curves starting at the top left side of the screen is the RPM curve. The three little spikes occur when the rev limiter was released for the purposes of this test. Each of the three spikes represents a different design of inducer and the the affect that that inducer has on the pump's ability to quickly load up and pull the engine down. After the spike, the impeller is fully loaded and RPM remains constant throughout the pass. The black curve represents the current industry standard inducer. The green curve represents my 4-blade inducer prototype.

The RPM is purely a function of impeller diameter. Cutting the diameter smaller will increase RPM everywhere in the run, at the expense of loading the pump faster. We refer to this as "burning out." This needs to be addressed by either back pedaling or with a launch controll programmed into the rev limiter of a Digital 7. Some minor tuning of the pump can be done with nozzle inserts, however any legit engine will overpower the impeller on the hit. A positive displacement blower makes this situation worse all around. This is the build and the direction in which I am headed, so I will make the best compromise possible.

My options of compromise, I would like to stay with either an AA or an A cut impeller. I will post up impeller horsepower curves when I get home later. Another option is to run a gearbox to underdrive the impeller. Currently, this is only done in the pro stock class.

Since I basically need to already do a lot of work to my current heads, I think it's best to start with an already good aluminum casting. I would like to avoid porting for this iteration. Because of how easily jet boats blast through the lower RPM range, I was contemplating the Brodix BB-3 Xtra 345. I don't have a problem not optimizing bottom end in favor of top end performance. I am learning towards the Brodix because they have an angle machined option, not just angle milling. If you would be so kind as to share your thoughts on this topic.

Any guidance as to what horsepower and RPM realities are greatly appreciated.

Thank you,
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Old 09-09-2019, 05:46 PM   #5
Screwblower
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Default Re: Looking for a better understanding for my next build

Just don’t put more crank arm in the motor.
Jmo, 345 BB3.
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Old 09-10-2019, 09:31 AM   #6
Flusher
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Default Re: Looking for a better understanding for my next build

Below are the as measured impeller curves for the sizes that I would like to run. I have included some of the lower RPM requirements to illustrate how little bottom end torque is desired.

By cutting the diameter of the back of the impeller, I can increase the RPM relative to horsepower. Horsepower per diameter is scalable, so fractional sizes are very accurate. By foiling the trailing edges, I can decrease the RPM a few hundred. The most common size impeller for a positive displacement supercharged application is an A cut impeller.

The impeller can be cut much smaller to tailor its characteristics to the output of the engine, however a smaller impeller could give a few more MPH, the sacrifice, early in the run, heavily outweighs the gain in MPH.

My questions are, is it possible to get the potential horsepower curve of my combination to intersect the below impeller curves? Can these curves be made to intersect without leaving horsepower on table, i.e. peak power is at 7000, but only turning 6000? What should I look at changing in my engine combination to better fall inline with the impeller curves?

Impeller requirements per shaft horsepower and RPM:

AA Impeller 9-3/32" diameter:
RPM HP
3000 109
3500 173
4000 259
4500 368
5000 505
5500 673
5600 710
5700 749
5800 789
5850 831
6000 874
6080 918
6200 964
6300 1011
6400 1060
6500 1111
6600 1163
6700 1217
6800 1272
6900 1329
7000 1388
7100 1448
7200 1510
7300 1574
7400 1639
7500 1707
7600 1776
7700 1847
7800 1920


A Impeller 9" x 30* diameter:
RPM HP
3000 96
3500 153
4000 228
4500 326
5000 447
5500 595
5600 628
5700 662
5800 698
5900 734
6000 772
6100 812
6200 852
6300 894
6400 937
6500 982
6600 1028
6700 1076
6800 1124
6900 1175
7000 1227
7100 1280
7200 1335
7300 1391
7400 1449
7500 1509
7600 1570
7700 1633
7800 1697


Thanks again for all suggestions, it's greatly appreciated.
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Old 09-10-2019, 09:50 AM   #7
Flusher
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Default Re: Looking for a better understanding for my next build

Quote:
Originally Posted by Screwblower View Post
Just donít put more crank arm in the motor.
Jmo, 345 BB3.
Thank you, but could you please explain why? Wouldn't the increased displacement increase power everywhere in the curve?

Looking at the above impeller curves, the RPM in a jet boat are not really relative to that of a car. It is very unusual to spin a jet boat engine into the mid to high 7s. I think that jet boat high RPM is not what you would consider high RPM.

Thanks again
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