Visit The Dirt Forum for More Information

Author Topic:   Big hole Short stroke
A&M Motorsports
Dirt Maniac

Total posts: 117
posted March 17, 2005 06:10 AM  
Just wondering about really destroking a 4.125" bore to around 3.25" stroke to take some torque out and increase RPMs. Any body have any input or experience with this type
combination? Seems to me this wouldn't be as likely to blow the tires away on a bull ring track.

------------------
Artie Perilloux
A & M Motorsports

Kromulous
Dirt Forum Champ
Total posts: 796
posted March 17, 2005 07:01 AM  
I've been wanting to buid a 380, 3.5625" stroke, and a 4125" bore. Finanaces this year are low as usual, so we built a 360, same stroke, small bore.

Only thing is with the Big bore is, they need 18* heads or smaller (15* Darts) to really capatalize on the big bore, it unshrouds the valves. As well 18* stuff on a small bore is pretty much pointless.

I heard that sprint cars run 3.30 stroke and 4.125 bore, i think that makes a 360. I would imagine there very high strung, and dont run well until there wound up tight.

I think i would stick to anything with a big bore, and keep the strokes in the 3.50 to 3.625 range.

chomme
Dirt Freak

Total posts: 300
posted March 17, 2005 07:20 AM  
I've run a couple 3.25/4.125 motors. Great on a slick track with a smooth driver. Without going to 18* heads you will have a tough time on tacky. You really need to get the compression as high as you can (15-16:1+) to get any torque out of the combo. Which in itself is hard without going to a big dome which will negate the gains. You really have to get the combo right, you don't have as much margin for error as you do on a big cube motor. A good valve train is a neccesity also because you will have to turn the motor pretty hard. We usually turn ours in the neighborhood of 8500.

Kromulous
Dirt Forum Champ
Total posts: 796
posted March 17, 2005 08:29 AM  
Plus with 15 to 16:1 compression, you better be running some seriously good quality parts and a Dart big bore block.

15:1 and a stk blk at 8500 rpm will make a good Mod grenade i would imagine. We wont crank a stk blk 400 over 7000-7200 max.

I would think the best route would be with that short stroke, 3.330, would be to run some light weight flat tops, light rods and crank (40lbs maybe or lower), then angle mill the heads down to about 40 to 45cc on some decent 18* heads. So you can capalize on all the available rpm and torque.

Also, i wouldnt run the super small journal rods (1.88 Honda's), i would stay with 2.0 (chevy small journals) minimum.


A&M Motorsports
Dirt Maniac

Total posts: 117
posted March 17, 2005 09:39 AM  
With 40 cc heads I calculate 15.7 to 1 compression. I agree the stock 400 block isn't the way to go.

------------------
Artie Perilloux
A & M Motorsports

2nd2none
Dirt Forum Champ
Total posts: 755
posted March 17, 2005 06:17 PM  
my cousin built a 3.330" 4.130 bore 360 for his sprinter last year that had a used cup crank that weighed 38 lbs. The motor made good power but he said it was so explosive it was hard to drive.

One thing to ponder on: Super light crank, light rotating assy. He said it was so ON and OFF instant throttle response he had to really soft foot the throttle.

jtno6
Dirt Forum Racer

Total posts: 87
posted March 17, 2005 07:06 PM  
what wrong with a 331 (3.25 crank in a 350 block) or a 357(3.25 crank in a 400 block) these motors scream

chomme
Dirt Freak

Total posts: 300
posted March 18, 2005 01:22 AM  
My father-in-law has a killer Cola 3.3" crank for sale, not sure how much but he is always known to make deals. Anyone interested they can PM me and I will get all information together for them.

I am thinking of building a 360 with it next year but if someone wants it, it's sold!

Back to the Archives