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Hemi predator gas build

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rkcarguy

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Hemi predator gas build w/pics

http://s874.photobucket.com/user/rkcarguy/library/?sort=3&page=1

The newest 12 pics are from last night.

Billet goodies and new valve train from ARC, I can post part #'s if you like.
28mm SS coated intake valve DJ-1128
25mm SS exhaust valve DJ-1121
26# white stripe springs and aluminum retainers with keepers
Mod-2 cam
6254 Billet rod

My paper strips and mic for measuring clearances. Go ahead flame away, they work great.
Oil the rod, crank, and paper, and insert the thinnest strip first. .0026" paper ended up being a bit loose on the rod, .0031" started to get tight before the bolts reached torque. Within the acceptable range, so we're done there.

Added oil hole at cam bearing in side cover. Can't hurt.

Rod installed in the block. .020" in the hole, ~.030" clearance between rod, top of block inside, and cam during rotation. These things are a close fit!

Grade 10.9 side cover bolts. M8x1.25x35mm long, with solid dowels. Some green Loctite and you can seriously forget about them. The stock bolts stretch like crazy.

Mod-2 cam and lifters in.

Side cover on. I re-use the factory gasket the first time around. If it's problematic, I'll replace it and check the rod bolt torque while I'm in there.

Flywheel slipped on for aesthetics, need to find the nut and starter cup.

My welded head pictures are also in there, but there is more seat cutting and work to do getting the new valves in there.
Note the dirty tissue. This was from grit and metal chips left inside the hole thru the crank and inside the wrist pin.
Also note, the hole thru the cranks rod journal is much smaller than my other 196cc clone, leaving about 1/4" thick wall which should be plenty.

More to come. My goal here is world formula-ish output as this will be a backup motor for my WF.
 
Great post.very informative.
It would not surprise me if its faster than a world formula.

What are specs on the cam
 
Mod-2 cam is about the same lift as a stocker but ALOT more duration. I'm going to cut and weld the stock Hemi rocker on the intake side to ~1.2 ratio. Should get me about .290 lift on the intake valve. Our track has 19 turns to exit and 2 decent straights, so I need a broad range of power delivery and don't want to get too wild with the cam.
Piston to bore clearance was .003" on this motor out of the box, and it had a nice cross hatch to it so I didn't touch it. Again, in poor boy style, you can carefully lay a .003" feeler gage along the skirt of the piston with it upside down in the bore, and see if it slides down into the bore. .003" went in, fairly tightly.
 
I think it has potential to be faster than the WF, but will it hold up? They really are cheap on the aluminum in these blocks compared to the WF.
I'm thinking about making a brace that will pick up the square pattern on the side cover, the one extra threaded hole at the upper top of the side cover near the cylinder, and the two 6mm holes in the head.
 
Looks good , be interesting on your results when finished.

Now about the rod clearance....awww heck someone has to give you a hard time; just messing with ya.

Seriously now ; Nice job on the cylinder head. I have had tons of hours put into cylinder heads from my old job which involved a lot ; and I mean a lot of head work and all my play toys ( since I have a hard time leaving anything alone even ported the head on my old Geo metro back in the day ) and will have to say that looks like a very nice job.

Good luck , let us know how it works out for you.
 
FYI, I went to our local mini-sprint/mini-bike shop yesterday to pick up a head gasket and looked at a bunch of his Dyno files for different motors.
He had a blueprinted WF run that was just under 15HP. He had just run a Hemi predator that is a similar build to mine, except he shaved the head a bunch and eyebrowed the piston, 275 cam, stock rockers and stock size SS valves, and PZ26 carb, which made 16.26HP on gas and 18.08HP on Alky.
One thing I noticed though was the WF had very broad power delivery of both TQ and HP that was a steady climb from lockup to rev-limiter. The Hemi looked more "peaky", coming in under the WF's graph until it spiked up above it on the high end.
This broad power delivery, likely explains why the WF is dominant on our 19 turn sprint track, but I think with a higher rev limit and lower(numerically higher) gearing the Hemi can be made to give it a run for the money.
 
I wonder what would cause the hemi to be more peaky. The fact that it has more cc, more compression, and methanol, which are all torque adders would suggest that the hemi would have a broader curve.

How does the 275 cam in the hemi compare to the cam in the wf?
 
The cam in the wf is a 308 not sure of duration and also has dual springs and a dome piston
but the hemi should be stronger.
 
i looked at the world formula cam specs and i calculated it out to about 243-251 @50 so probably not the cam. maybe the world formula head has a better flow profile than the clone.

but one thing that jumps at me is this. that engine made 16hp on gas. someone posted a very similar build on oldminibikes that made 20hp on gas with a stock appearing carb and 24hp on methanol with a tillotson carb so im wondering if maybe the clone had an issue when tested that they didnt notice?
 
Dyno power creep? The one thing I like about Brad's dyno results is I think it's very well calibrated to provide real HP #'s. It's pretty well known that the WF's, blueprinted and all legally tricked out, make real HP in the high 14's.
My take, being my WF came with an extra head and valves which I looked over, the valves are smaller and the ports aren't as big but have much smoother turns in them. The carb is 26mm, and flows far more than an S/A carb but is necked down thru the angled back adapter. All of this spells "intake velocity" to me.
We are limited to pump gas so I don't really care if someone made 30HP on alky I can't race with that legally.
Other factors are, the tested Hemi just had stock size valves and straight up stock rockers, not ratio rockers. I don't know the specs on the 275 cam so I can't comment there. All that was done to the ports was radius the short side and blend the port to the pocket. Finally, I doubt the compression is that high even with the head shaved a bunch.
I may spring for the $$$ to have mine tested when it's done, I'd expect maybe mid-high 16HP range due to the fact I have a welded head, 1mm bigger intake valve, and should be about .290 lift on the intake with the ratio rocker on the mod-2 cam.
 
Looks like 18.8 was the limit on gas if I read correctly?
That's young....he is on here and I PM'd him about my build. He thinks that with the RPM drop we see in the turns on our track that I'm spot on with the Mod-2 cam and cutting/welding some ratio into the intake rocker only. I found out some interesting things guys are doing with the WF on the tighter tracks that tells me they have too much cam from the factory for some situations....
 
rkcarguy said:
Looks like 18.8 was the limit on gas if I read correctly?
That's young....he is on here and I PM'd him about my build. He thinks that with the RPM drop we see in the turns on our track that I'm spot on with the Mod-2 cam and cutting/welding some ratio into the intake rocker only. I found out some interesting things guys are doing with the WF on the tighter tracks that tells me they have too much cam from the factory for some situations....
Click to expand...

You are right.the 20.01 , while on a sa carb, was on methanol.

Why don't you just do the black mamba cam which has similar duration to the mod 2 but more built in lift?
At 56 bucks its almost a no brainer and you don't have to mod the rockers
 
Same question as above, Why are guys using a mod 2 cam with no lift unless it is a cheater stock engine? I found my mod 2 cam build to be a little sluggish until the engine got wound up to maybe 6000 rpm and above (great on the top end) Are sprint karts dropping below that RPM in the tighter slower turns?
 
backyardhack said:
Same question as above, Why are guys using a mod 2 cam with no lift unless it is a cheater stock engine? I found my mod 2 cam build to be a little sluggish until the engine got wound up to maybe 6000 rpm and above (great on the top end) Are sprint karts dropping below that RPM in the tighter slower turns?
Click to expand...

Slowest turn at my track is about 28mph. With current gearing that's 3600 rpm
 
71 chevy said:
You are right.the 20.01 , while on a sa carb, was on methanol.

Why don't you just do the black mamba cam which has similar duration to the mod 2 but more built in lift?
At 56 bucks its almost a no brainer and you don't have to mod the rockers
Click to expand...

I have the Mod-2 cam on hand, and can cut/weld the rocker in about 1/2 an hour so I want to try it. My track is similar to 71's, our tightest turns drop a small block into the high 3000's rpm range. With the Mod-2 cam and billet rod being the only change in an otherwise stock GK-65, I went a little over a second a lap faster compared to the stock cam so I think it will work alright.
 
I raced last weekend and had a bunch of work to do on my kart, so this build has been languishing. I'll be back on it shortly. The head needs to go back to my dad for some more seat cutting to fit the new valves, a trip to the hardware store for some grade 10.9 flange head bolts and it will be ready to install. Seems like the hardware store is onto us lol, the high grade 10.9 side cover and head bolt lengths are the most expensive despite longer lengths and larger diameters being available.
 
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