Box Stock

need some engine help

T

Time Bomb Tom

Member
I'm new to the whole dirt racing kart thing and will be running a couple karts on a buddy's backyard track. Just need to know if what I plan on building will work for me. Non Hemi predator with billet flywheel, .020 billet rod, 18 lb springs, .625 carb on methanol. I only plan on turning it 6000-6500 rpm, so what cam would work best for me? I can do any machine work that is required. Any ideas or comments on this would be appreciated..

thanks
T

forgot to add, other engine gets the same parts and is a 196cc greyhound
 
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I would look for a black mamba Jr cam from smallenginecams.com, with 26lb springs, mill your head .050-.060 for more compression, and bump the timing to 34 degrees. If you decide to go with a bigger carb later on that will help you out alot also and you'll already have a good cam and springs to back it up. I would try to find a .880 or .930 big pipe to use also if It were me. Those 18lb springs are more of a cheater stock spring, if you want to keep them I would just use a mod2 cam from dyno cams
 
What kind of hp should it make? Would it be better to run a Mikuni 22 on race fuel instead of methanol?
What about running the Dyno Cams CM cam and 22lb springs? Would i need longer pushrods and would stock rockers work?
 
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Stock rockers and pushrods are fine until you get into higher hp range. With what I mentioned and a Mikuni 22 on alky you should see around 15hp, little less if you use gas. Don't know much about the CM cam
 
I would assume replacing the lifters with a new cam is a must on these engines too? Are the standard clone lifters ok or does the black mamba jr require the billet pieces?
 
I would not worry about changing lifter. Load the cam of your choice. I like the mod 2 from dyno cams. It has a wide power band that would work well with the 22 pound valve springs. I stay away from the high lift cams seen to may guy invest lots of money only to jump on the gas at a standing start and bend push rods running in the back yard. If you want to race it that's a different engine build all together. If your predator can take clone lifters and you really want to change them the box stock project has light weight ones that fit a clone. Any box stock project dealer like myself can get them for you. Just make sure they fit before you buy them. Fly cut the head .050 and put in a head gasket the same thickness that has sealer on it, also can get from the box stock project. This allows room for crank flex and seals better then a factory gasket. And more compression slowly. Ps don't for get to have .003 to .004 crank to connecting rod clearance.
 
A good clone or non hemi head with Burris .14 gasket. This will bump up your compression and lighten valve train allowing you to run a lighter spring. 10.8 valve spring cl2 cam (because of the higher lift). Reduce ring drag, clearance bore, leave the stock rod just give it some clearance, relieve the crank bearings, timing around 34-36 with a billet flywheel and you'll have more than enough power to play in the back yard for little money.
 
W5R said:
I would look for a black mamba Jr cam from smallenginecams.com, with 26lb springs, mill your head .050-.060 for more compression, and bump the timing to 34 degrees. If you decide to go with a bigger carb later on that will help you out alot also and you'll already have a good cam and springs to back it up. I would try to find a .880 or .930 big pipe to use also if It were me. Those 18lb springs are more of a cheater stock spring, if you want to keep them I would just use a mod2 cam from dyno cams
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if I go this route, what head gasket do I need to buy?
 
Ok, piston pops out of the block .005 with my +.020 rod. Piston is the stock dish, so with a .014 head gasket and .050 shaved off the head, what will my compression ratio be?
 
You should be using a .025-.027 head gasket at least, with the piston popup at .005 and a .014" gasket you run the risk of the piston smacking the head as the rod breaks in more and more
 
I would go as close as you could to .030 clearance with .005 popout. Below is a 70MM gasket, .036 thick.

Predator 70MM .036 thick head gasket

Using this calculator SCR Calculator

I am getting an SCR of 11.23 and that's guessing your head is roughly 18cc's(if you are running a 22cc head, milled .050).
2.756 Bore
2.165 Stroke
.014 head gasket
Deck height -.005
Piston top volume 1.8
Combustion Chamber 18cc's

I have no clue what the cc of a dished predator piston is, I found 1.8cc online, not sure how accurate that is but that's the piston top volume I used.
 
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ok, .030 head gasket is on the way so I should have plenty of clearance. what is the max I can mill off the head before I run into rocker arm problems and misalignment? Already took .050 off and its just touching the fin.

I happened across a last chance parking lot sale at HF and picked up another predator and a hemi. Hemi is getting a small cam, 18# springs, 8 degree advance key, header and high flow filter. it will be going on my daughters yard kart and is going to give the old torque convertor some hell.
 
Time Bomb Tom said:
ok, .030 head gasket is on the way so I should have plenty of clearance. what is the max I can mill off the head before I run into rocker arm problems and misalignment? Already took .050 off and its just touching the fin.

I happened across a last chance parking lot sale at HF and picked up another predator and a hemi. Hemi is getting a small cam, 18# springs, 8 degree advance key, header and high flow filter. it will be going on my daughters yard kart and is going to give the old torque convertor some hell.
Click to expand...

Do you know how many CC's the combustion chamber is currently as you are getting close to the max CR for pump gas, not sure how much more you are planning on milling off, but each engine/head is different. One thing to keep in mind if you run a high lift cam, you may be closer than you think on Piston to Valve clearance milling say .080 total off the head. I think what you are asking is about rocker arm geometry, if you need shorter pushrods to ensure the rocker tip is hitting the valve tip correctly?

Comp Cams Video
 
I will probably be running 100 octane low lead aviation fuel as I have access to it. Parts are all here and I will begin work later today on the engine. Still need to slick up the head after milling it. Is there any certain special tool for compressing the valve springs? It has the automotive style keepers on this engine unlike the clone. I don't think my spring compressor will close up far enough to catch the retainers unless I use a flat washer or something else to make up the gap. Any idea what spark plug I should be running for this? It had an Autolite 3910x in it.
 
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