Predator Hemi build question

fhpe77

fhpe77

Active Member
#1
#1
I've been running a stock internals Predator Hemi for about a year now. It's has the usual jet/e-tube/flywheel key/air filter/header stage-1 kit. I am impressed with the power this thing makes but have a serious engine building jones. Reading this forum does not help. :smile: To get my fix, I purchased another new Predator Hemi and a bunch of go-fast goodies. Here's the list:

Arc flywheel
Arc con-rod +0.020" (stock rod piston was 0.025" in the hole)
Black Mamba Jr. cam
26lb valve springs
SS valves/automotive style locks and keepers
Head/side cover stud kits

Prior to receiving the cam along with the included cam card, I had intended to have the head milled 0.035" which would reduce the combustion chamber volume to ~18cc. Along with a 0.027" MLS head gasket the static compression ratio should be 11:1. Now that I have the cam specs I plugged in the intake valve closing event for the BM Jr. (39 degrees ABDC) which results in a 10.2:1 dynamic compression ratio. I estimate cranking cylinder pressure to be >200 psi. This does not seem pump gas friendly to me. I was hoping to run 92 octane pump gas in this thing.

So my question is what static compression ratio should I be shooting for with this cam in order to keep the engine happy on pump premium?

-Ray
 
ole4

ole4

#2
#2
Your numbers look correct but I am not sure that removing .035 gets a hemi to 18CC do you know what the current CC's are for it with a plug installed? They vary on the three I have from 22.5 to 24CC's. You may well be right I am just asking because each .010 you remove will not be the same amount of cc reduction due to the hemi shape. Not sure if I am clear on this but as you mill the diameter of the hemi is reduced so if the first .010 removes .5 cc the next cut may reduce it by .7cc then the next .9cc and so on.
 
fhpe77

fhpe77

Active Member
#3
#3
Ole,

Thank you for responding. My head cut estimate assumes a linear relationship even though it is not. I did not cc the head yet but used figures posted on this and other forums. My assumption for stock combustion chamber volume is 23cc. The more I look at specs for other cams the more I wonder if the BM Jr.'s valve timing specs are in camshaft degrees. 78 degrees ABDC seems to make more sense to me. I believe you have posted a similar build using this cam. What compression ratio are you running?

-Ray
 
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ole4

ole4

#4
#4
Yes it is a Hemi head not milled but it is a custom grind but it is actually similar, 108 LSA, 236@.050 and 330 lift. I have the arc +.020 rod. I have not used the motor yet. I do have a BM JR on a rupp I did and it really runs nice power everywhere. It does run different than the SR, which sounds much cooler and has a strong top end rush. Both have Torque converters so bottom end is not a issue except for too much causing wheelies.
 
fhpe77

fhpe77

Active Member
#5
#5
I have not milled the head yet which is why I am trying to figure this out. The initial plan of shooting for 11:1 SCR seemed sound until I received the cam card and saw the intake valve closing event listed at 39 degrees ABDC. That seems like the intake valve is closing really early which is why I suspect 39 is camshaft degrees. Plugging 78 degrees ABDC into my spreadsheet results in a DCR of 7.85:1 and a cranking pressure of ~155psi. This would be fine on pump gas and seems to make more sense. I just need to confirm that Tim's valve timing specs are in camshaft degrees not crankshaft degrees. So what did your compression ratio work out to be for the custom grind build?

-Ray
 
ole4

ole4

#6
#6
I built it for a kart for my nephews kids nothing special. I still had .012 deck height and I stacked two metal gaskets so around 30 but I did not CC that head so probably around 9 to 1. Isky measurements are for .050 here is one I degreed with the stated IC and actual IC
 
fhpe77

fhpe77

Active Member
#7
#7
I will degree the cam once I get the bottom end together and recalculate using measured values. Back in the day I installed a cam in a SBC and the cam card provided numbers taken at both 0.050" and 0.003". It makes sense now. Thanks again.

-Ray
 
fhpe77

fhpe77

Active Member
#8
#8
Okay, so I got the bottom end assembled took some measurements. I used the dial indicator to find true TDC. Then I degree'd the intake lobe. My setup was a little sketchy with a printed degree wheel laminated to to thick card stock but the measurements were repeatable and appeared to be sufficiently accurate for my purposes. The intake lobe opening event at 0.050" was 12 degrees BTDC and the closing event at 0.050" was 41 degrees ABDC. Off +1 degree on each measurement compared to the cam card. Then I measured the intake closing event at 0.001" which showed 71.5 degrees ABDC. Plugging 71.5 into the spreadsheet resulted in an 8.35:1 DCR assuming an 11:1 SCR. This engine should run great on pump premium. Here's some pics:
20180122_212458.jpg

20180122_223400.jpg

-Ray
 
fhpe77

fhpe77

Active Member
#10
#10
Not powdercoat. Just your standard VHT high temperature engine enamel. The color is Chrysler hemi orange. :smile: The valve cover is going to be painted the obvious color, wrinkle black. I've been thinking about painting the blower housing wrinkle black and the starter hemi orange. My bike has kind of a rat-rod muscle bike theme going on so I wanted to carry that over to the new engine. Here's a pic with the stock internals Predator hemi:

20170510_194351.jpg

The terrible frame mod and bent left handle bar are both gifts from a previous owner. If I ever get those corrected then I will ditch the rat-rod motif and get some nice paint on there. Sadly the original chrome finish is pretty much gone.

-Ray
 
fhpe77

fhpe77

Active Member
#11
#11
I finally get it together and running. With the head cut 0.035", the combustion chamber measured 19 cc.

20180209_225625.jpg

This yields a 10.56:1 static compression ratio and an 8.02:1 dynamic compression ratio with the Black Mamba Jr. cam. I deviated from the paint scheme previously mentioned and kept the stock finish on the starter.

20180210_134900.jpg

20180210_134908.jpg

I decided to use the carb from my stock internals engine for break-in since I know it works. It's got a #37 jet and e-tube. This carb is temporary since I think the small bore is restricting the engine. For throttle hook-up I just modified the stock throttle mechanism with a direct link between the throttle blade and the throttle arm.

20180211_015650.jpg

Swapping in the new engine was easy because I was replacing an existing Predator. I took the opportunity to inspect and service the Max-Torque clutch. It seem to be holding up well with all of my abuse. As expected starting is more of a chore now that there is no governor to hold the throttle open. I have to crack the throttle a little to get it to fire up. It seems to need to idle higher than the stock internals engine and definitely needs a bit more warm up time. That has always been the case with any car engine that I modified in a similar fashion so it is not surprising.

I let it idle for 10 minutes and then shut it down to cool off. I rode the bike on the second start and it's noticeably different than the stock internals engine. It seems to have lost some low speed torque just off the line but once you cross 3000 rpm it becomes an complete beast. The torque curve seems linear and it pulls harder and harder as the rpm's climb. Reminds me of the LS3 in my car. I've tried a couple of top speed runs and I seem to chicken out at 44 mph each time. That's only turning the engine to 5800 rpm and at that point it is still pulling hard. I suspect that I could go a lot faster if i want to but I don't know if the bike is up to it.

Another difference from the stock engine is the sound. This modded engine is a lot louder. It's got all the usual singer sewing machine ticking noises at idle like a stocker but the exhaust note is louder. When loading it up during a run, it has that staccato high compression growl. I love it!

After the first couple of rides I noticed it was not sounding too good. It was knocking so I shut it down thinking I blew it up already. Next thing I notice is oil pouring out from underneath the bike. Turns out the side cover was loose and the gasket was partially shredded. I put it away for the night and pondered the problem. It dawned on me that the side cover studs were made out of stronger steel than the stock bolts so the stock 17 ft-lb torque spec was not stretching the studs enough to remain tight. A couple of days later I opened it up for inspection and found everything to be in good shape. I installed a genuine Honda side cover gasket and torqued the studs to 20 ft-lb. Problem solved!

I have been riding it a little every chance I get to break it in. The third oil change was performed today and there seems to be less metal coming out now and the oil still looked clean. For break-in I am using STP 30w single viscosity dino oil. It's cheap and tends to really stick to the metal parts. Next oil change will be some sort of 10W-30 full synthetic. I realize automotive oil may not be the best thing for these small air-cooled engines. So which oil is preferred in this application?

-Ray
 
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