Cam, Timing, and Rocker Ratio for Low RPM torque build HEMI mud motor

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Sandor27

New Member
#1
#1
1st post. Have done lots of reading here. I'm looking to build a high torque low RPM motor for a boat mud motor.
I have a stock non-hemi on the boat right now and it bogs pretty bad and will only run 2000-2300 rpm with a full load. I need more torque to get it up on plane so the motor can wind up. Prop is 7x4", so can't go smaller there (American prop, not Thai).

I bought a new hemi from HF today to build, so I can keep using the mud motor in the meantime. From my research here and elsewhere I plan the following:

Remove governor / billet flywheel / Billet rod (+.0xx to get deck to zero)/ Black Mamba Jr cam / 26# springs / header with RLV / mikuni VM22 / mill head / full port

Questions...

Flywheel timing: for lower RPM (sub 5k) do I need spark advance? PVL flywheel is stock at 28°, ARC is 32°. Is it better for lower RPM to NOT have greater than stock advance? Or does advance help at lower RPM on these motors?

Cam: The Black Mamba Jr. vs. NR 280-0211? BMJ gets great reviews. Have not read anything about the NR 280. Thoughts? NR 280-0211 is advertised as the ultimate 2k-5k torque cam.

Rocker Arm Ratio: Will I see any benefit in lower RPM torque from going to a 1.1, 1.2, or 1.3 ratio arm?

Compression ratio: I can mill the head easily (have bridgeport mill) and found the tables of height vs CR. What CR should I target? I'd like to stay on pump gas 93 or below.

Thanks. This site rocks. I love reading about your builds.

Sandor27
 
trinik7597

trinik7597

Active Member
#2
#2
There is only one place to get hemi ratio rockers and the spacer you will need to run them and that is Brad hill performance... check how far in the hole you are stock .. usually a +20 rod will get you to just about zero deck .. I would not mill more than 35 off of the hemi . I would run the mamba Jr
 
minibikin'

minibikin'

Sho-nuff
#4
#4
Timing advance will help lower rpms. Just remember, timing advance and compression ratio go hand in hand. More compression, less advance. I would say 10:1 with 91 octane would be ideal with the 32 degree ARC wheel. Also note, the only accurate way to set the timing is with a degree wheel. The ARC flywheel is machined accurately, but the keyway location on the China Predator has yielded base timing settings that vary up to three degrees.


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Prowler08

Prowler08

New Member
#5
#5
Ratio rockers add power across the board in TQ and HP at any rpm...
Just P&P the head and you can achieve good low lift flow which will help in getting the motor to respond quicker...
Also just by switching to Alky you will gain TQ and HP across the board...
 
S

Sandor27

New Member
#7
#7
OK, what are the forbidden words? I'm trying to reply and it's not letting me post. No profanities, no URL's, no websites, just some brand names?
 
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Sandor27

New Member
#9
#9
I meant that the post I was trying to enter did not have any profanities, links or sites. That I was just asking about some different options by brand name...

Here is a test:
NR Racing
Black Mamba Jr.
SK100 Slipstream Flywheel
NR 280-0211
Bill Hill
ARC
 
S

Sandor27

New Member
#10
#10
Piston is .023 in the hole at TDC, so the +.020" rod will be used.

I can only find two rockers for the HEMI: The NR Racing 1.2 or 1.3 and the Brad Hill.
At $110 or $190 for the rockers, I'll pass on these for now and just run a good port and polish.

I'm leaning toward the Black Mamba Jr. since it is $30 less than the NR 280-0211, but has anyone run the NR 280? Results?

Also, does anyone have experience with the SK100 Slipstream flywheel? $35 less than the ARC.

Thanks!
Sandor27
 
minikenny

minikenny

Member
#12
#12
It's just the anti-spam forum software like was already said. It does an excellent job weeding out spam, and an amazing job frustrating legitimate posters for their first 5 post or so. It's a tradeoff. :shrug:
 
Prowler08

Prowler08

New Member
#13
#13
The Slipstream flywheel is a okay lower cost alternative... They have made improvements over the last couple years...
The Mamba JR is a great little cam if you are looking for low end grunt...
Add Alky and you get extra TQ and HP for cheap...
 
BWL

BWL

Active Member
#14
#14
Yeah, the weakest point here is the prop. Most guys I know run a Black Mamba, Sr. on a mud motor, especially since you're chunking the governor. On the Thai props 5.5 inch prop are most popular on a built motor. I don't know about pitch and all that. They also run 1.3 ratio rockers. But again most of the mud motor builders I know run Thai-style motors and look for as broad an RPM band as possible.

You're headed in the right direction. What is the size/type of boat? That has more to do with speed and getting-on-step than the motor. 212s by definition aren't as strong by torque compared to 390s and 420s. I prefer the small-blocks though cause they're are very strong for their size.

If you build it even as you describe, you'll have a strong motor.

Alky is totally impractical for a mud motor. Miles away from land, and you need to be able to get gas form anywhere.
 
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S

Sandor27

New Member
#15
#15
My boat is a 1436 riveted mod-v. Not ideal for getting on plane, but my 9.9hp Johnson outboard easily does and gets me like 22mph, with planning speed around 14mph.

The aluminum Thai props are 5" pitch for the 5-7" dia range. My stainless prop is 7" dia x 4" pitch. Assuming a 15% prop slip, prop calculator tells me I need 4500rpm to get on plane. I'm going to keep the rpms conservative on this build under load to <6000rpm which should get me around 18-20mph. Right now I'm plowing at 7mph.

Alky is a whole 'nother can o' worms. Maybe next season's project!
 
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Sandor27

New Member
#16
#16
I placed an order for my parts. I went with the Mamba Jr. and since Isky had free shipping on >$200, I bought the ARC flywheel from him as well. The Slipstream wheel would have been $98 with shipping, so the $99 for the ARC was a no-brainer. Also got the +0.020 ARC rod, 26# springs, valve retainers, .027 head gasket, gaskets, etc.

I milled my head last night -.035" and started on the porting job. I'll post pics of the progress...

I calculated my compression ratio (if the lift does not crash the valves! - need to measure/clay once parts are in) will be 10.6:1 with a .027" head gasket, or 9.8:1 with the stock .046" gasket. My head measured at 22.4CC before modification and 19.3 after (syringe method). These #s take into account bore x stroke, -.003 on deck, as well as the volume in the head gasket. Should be pretty accurate.

Sandor27
 
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trinik7597

trinik7597

Active Member
#17
#17
You won't have any valve contact with that lift milling 35 but the 26# springs are not hemi springs and 275 lift may cause bind .. that is max lift on a hemi head with out seat work ... so just check it when you have it together.. ole did a nice thread on the hemi ports I would watch how much I do to them ...
 
BWL

BWL

Active Member
#18
#18
Sandor27 said:
My boat is a 1436 riveted mod-v. Not ideal for getting on plane, but my 9.9hp Johnson outboard easily does and gets me like 22mph, with planning speed around 14mph.

The aluminum Thai props are 5" pitch for the 5-7" dia range. My stainless prop is 7" dia x 4" pitch. Assuming a 15% prop slip, prop calculator tells me I need 4500rpm to get on plane. I'm going to keep the rpms conservative on this build under load to <6000rpm which should get me around 18-20mph. Right now I'm plowing at 7mph.

Alky is a whole 'nother can o' worms. Maybe next season's project!
Click to expand...
Yeah, you really are slogging along. I'm running 19 mph with a similar build, but with a MOD2 camshaft. Jr. will be much better. You're dead on with the speed estimates. Keep us posted I'd like to see how this turns out.

What rockers were you going to run?
 
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Sandor27

New Member
#19
#19
Stock rockers as the HEMI versions are $110 for the NR and $190 for the Bill Hill. So I will build as planned and see how it runs. If I feel I need a *little* bit more, I'll explore the rockers. Or possible Alcohol. Maybe just beer. :drinkup:
 
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Sandor27

New Member
#20
#20
Got more porting done. I think I am finished with the exhaust, but may go back and smooth out the intake guide wedge area.
I used my Dremel tool with flex extension using carbide burrs, small grindstones, home made sanding cones and felt with clay rouge.
For the cones, I cut sandpaper in a taper, 2" on one end down to a point across the short width of the paper. Rolled it up on a .100" rod starting from the 2" end and hot glued the tag end. Then threaded it onto a 1/8" drill bit to use in my dremel. Worked great and cost was free (had paper). Started with 80Grit went to 220, then 320, then 500 then 1000 then rouge. I left the intake at 80 grit. The exhaust short side is just radiused to eliminate the corner, the intake short side goes all the way to the valve seat. Any comment to my porting shape? Any improvements you could recommend? How to do it better (shape) next time? Thanks!
HEMI Exhaust Port.jpg HEMI Intake Port.jpg Screenshot_20160930-064406.png
 
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