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Big bore short stroke or small bore big stroke

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Big bore short stroke or small bore big stroke

Postby FIRECHILD » Sun Dec 07, 2008 4:52 pm

The guy thats gonna put my motor together said I would be better off with big stroke . Its for a 3/8 mile asphalt track. I already ordered the crank,rods and pistons fo a big bore short stroke,just want some opinions,should I stick with what I ordered or call and see if I can cancel,he said he talked to a couple of guys that have built 2 bbl motors and they said I wouldnt make as much power. Any suggestions Thanks JEFF
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Postby xstfudonniex » Sun Dec 07, 2008 5:07 pm

smaller bores generally resist detonation better, therefore respond to nitrous and boost better, Bigger bores generally allow better cylinder heads because you can run bigger valves and they will not be as shrouded, the smaller strokers also allow for more RPM and less piston speed.
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Re: Big bore short stroke or small bore big stroke

Postby RW TECH » Sun Dec 07, 2008 5:19 pm

FIRECHILD wrote:The guy thats gonna put my motor together said I would be better off with big stroke . Its for a 3/8 mile asphalt track. I already ordered the crank,rods and pistons fo a big bore short stroke,just want some opinions,should I stick with what I ordered or call and see if I can cancel,he said he talked to a couple of guys that have built 2 bbl motors and they said I wouldnt make as much power. Any suggestions Thanks JEFF


There have been some claims about higher torque output with longer stroke 2-bbl engines with small bores but I've never seen it with my own eyes. Don't worry about it & run the parts you've bought. If they tell you that the longer crank engines make more power make them show you some data to support their claims.

What you really need to do with a 2-bbl engine is REDUCE LOSSES (friction, pumping, parastatic, etc.) and beyond that your big bangs for the buck are compression ratio and get the very, very, very, very best carb you can find.

I use Keith Dillard at Lone Star Motorsports for 2-bbls. because he's got a mountain of experience in dealing with restrictor plate NASCAR carbs & his 2-bbl stuff works excellently.
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Postby ProPower engines » Sun Dec 07, 2008 5:21 pm

Since you have rules to follow you leftout the real importants stuff like what heads are allowed cam hyd. soild,roller, intake, engine size limit and what carb allowed Crank stroke rod length..................
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Postby FIRECHILD » Sun Dec 07, 2008 5:50 pm

ProPower engines wrote:Since you have rules to follow you leftout the real importants stuff like what heads are allowed cam hyd. soild,roller, intake, engine size limit and what carb allowed Crank stroke rod length..................
Any cast iron head ported is allowed, solid mech cam,500 cfm 2 barrel must pass gauge,360 ci 12.5 comp. What im wanting to build 4.125 bore 3.250 stroke,347 SB chevy Victor jr ,sideways HVH ss carb spacer with a Braswell carb
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Postby 3V Performance » Sun Dec 07, 2008 6:17 pm

Is there a gear rule? or max rpm controlled by a chip?

If not then:

Biggest bore first for block used then match biggest crank to max cubic inch.
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Postby FIRECHILD » Sun Dec 07, 2008 6:53 pm

3V Performance wrote:Is there a gear rule? or max rpm controlled by a chip?

If not then:

Biggest bore first for block used then match biggest crank to max cubic inch.
No gear rule or chip.
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Postby FIRECHILD » Sun Dec 07, 2008 9:53 pm

Any cam recommendations?
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Postby sanfordandson » Sun Dec 07, 2008 10:13 pm

i prefer the longer stroke if possible on a 2bbl motor. we've done back to back testing involving stroke/ci and the longer stroke and ci won everytime. this was with a 7448 2bbl we've seen similar rseults with the 4412.
the last time we did a comparison we had a 4.155 bore 3.5 stroke 377 vs a 4.020 bore 3.75 stroke 380 same top end parts and similar rotating weights and the shorter stroke was down 70 lbs/ft and 30hp. the torque really helps if you have a heavy car.
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Postby af2 » Sun Dec 07, 2008 10:29 pm

3V Performance wrote:Is there a gear rule? or max rpm controlled by a chip?

If not then:

Biggest bore first for block used then match biggest crank to max cubic inch.


Then the cam to bring the equation together. The comparison using the the same top end and cam is not a true comparison.
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Postby hitchcock » Sun Dec 07, 2008 10:32 pm

xstfudonniex wrote:smaller bores generally resist detonation better, therefore respond to nitrous and boost better, Bigger bores generally allow better cylinder heads because you can run bigger valves and they will not be as shrouded, the smaller strokers also allow for more RPM and less piston speed.


This is all absolutely true. With the short stroke comes a reduction in piston speed and friction. This also increases piston dwell time because the piston is not moving at as high of a fpm rate. Lots of people run really long rods to try and gain this "advantage" I dont really think there is a lot of power there. So a larger bore flows better, but would you agree that there is friction picked up with the larger bore? Or is it a negligable difference?
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Postby MadBill » Sun Dec 07, 2008 10:57 pm

There's a graph in the old HP "How to Hotrod Big-block Chevys" book that shows GM data on motoring friction torque of a 4.25" x 3.46" 392 c.i. Vs a 4.09" x 3.76" 396 c.i. The short stroke engine has as much as 20% less...
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Postby hitchcock » Sun Dec 07, 2008 11:20 pm

MadBill wrote:There's a graph in the old HP "How to Hotrod Big-block Chevys" book that shows GM data on motoring friction torque of a 4.25" x 3.46" 392 c.i. Vs a 4.09" x 3.76" 396 c.i. The short stroke engine has as much as 20% less...


20% is a lot. Is there a good way to compensate for this, for us guy's running long stroke motors? Like anti friction coatings on piston skirts. And if one was to go that far is there an advantage by coating that alone and not everything else?
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Re: Big bore short stroke or small bore big stroke

Postby xenginebuilder » Sun Dec 07, 2008 11:24 pm

FIRECHILD wrote:The guy thats gonna put my motor together said I would be better off with big stroke . Its for a 3/8 mile asphalt track.

Had some experience with 2v and 2v with restricted spacers on a 3/8 asphalt track. I used to compare notes with other builders and racers who were very competitive (multiple track champions), the larger engines based on the 3/4 stroke, 383 and 400-406 SBC, would consistently show more power on the dyno than the 355's or 372's. But, at the track the shorter stroke engines would race better. This was maddening for some of the guys, because the "big" engines always seemed like they were going to be king kong, but were getting beat by the smaller engines on a regular basis.
My take on this is that typically you are gearing the car for a lot higher rpm than is typically tested on the dyno, a 2v engine will usually quit about 65-6800 but the really fast cars are hitting 7800+ at the end of the straight. The larger engines are harder to get to hook off the corner due to the fat torque curve, and then run out of steam about the starter stand. If you test the two engine combination's to the full extent of the rpm range that the engine really sees, you'll find out that although the peak power may be down at the highest peak, as the power rolls off the remaining power at 7800 may be as much as 40hp higher, making the average power from corner exit to corner entry better...JMO
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Postby Unkl Ian » Sun Dec 07, 2008 11:55 pm

Another consideration is the degree of banking, and how tight the turns are.

Do you know for sure what some of the fast cars are running ?
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