Originally Posted by 98Z28CobraKiller

I would do the 4.030 bore with a 3.8 stroke. Put 20 psi to it and run 8's nice and easy.

Originally Posted by TurboS10

That's what I figured out looking at what everyone else has done. 3.8 fit's better than a longer crank. GM must have had a reason for the taller deck on the ls7. For low 10's, lot's of people on a stock crank. I just can't see buying a forged 3.62 crank.......except for a class requirement or something. So 3.8 for me.

Originally Posted by chuntington101

Ok, Im ghoing to take the pragmatic apraoch here and ask, why cant you have big bore AND Big stroke?

Originally Posted by Shawn @ VA Speed

generally speaking, if you have 2 engines of the same displacement, the long stroke smaller bore will go faster at the track. the longer stroke gives you a much wider powerband with more usable power.

let's look at the pros and cons of each


short stroke large bore

pros

bigger bore allows the heads to breath much better
shorter stroke allows for a taller piston and better rod to stroke ratio
shorter stroke gives you a stronger crankshaft and doesn't load the bearings as hard


cons

bigger bore means thinner cylinders which equals more bore distortion and blowby
bigger bore means less area between the cylinders making the possiblity of blow head gaskets greater
shorter stroke raises the rpm range-higher rpms equals more valvetrain issues(which fi engines are harder on valvetrain components to start with)
shorter stroke means less tq, less tq means a looser converter and more slippage in an auto,also slower spool times
shorter stroke means smaller power band


small bore long stroke

pros

smaller bore-less bore distortion,more area between the cylinders,less head gasket issues
longer stroke-more tq,faster spool times,wider power band,lower rpm range,tighter converter. (better valvetrain control)


cons

long stroke-weaker crank,more bearing load,rod to stroke ratio not as good,shorter piston

compromise on cylinder head flow.



let's look at the cons of the long stroke engine


weaker crank-yeah maybe a little, but you have to be really cranking alot of power to figure this out-same with the bearing load. If you use the proper parts there will be no issues

rod to stroke ratio-who cares, if the pistons are made right and the cylinders are round you won't have an issue,unless you trying to do something stupid like put a 4.250 stroke in an iron block,but that's a no no anyway.

Shorter piston- once again as long as you don't get stupid and run the correct ring pack and design the piston properly ther will be no issues.


my 2 biggest likes on the long stroke is lower rpm(i don't like beating up the valvetrain) and more tq. Lets face it,unless we are racing a 2200lb car,we need tq. To the guys that say they have too much tq, i say no,you just need to figure out how to get the car to work better.

Originally Posted by blackonblacksls

If stroke is the end all for power, I think modular ford stuff would have alot more potential NA. I mean they have good flowing heads valve trains that can handle damn near anything with little to nothing done except for the basic spring and retainer setups. And over 4inches of stroke from the factory on some motors with just a 3.552 bore. There have been big bore vs stroker comparisons on the ford side and the bigger bore makes the motor as a whole work alot better. Meaning more power pretty much everywhere, with the strokersl doing little for power except for in the midrange.

Some of the fastest NA and boosted mod motor stuff is actually destroked from 4.1x to 3.75 and bored out from 3.552 to 3.75.

Originally Posted by LSX-coupe

What about a PD blower on a short stroke engine are we looking at a lot less low end torque? Reason i ask is that most PD blowers will make ungodly torque down low anyways. Pros and Cons the same then?

Originally Posted by Shawn @ VA Speed

everything can be done to excess-but you still prove the point that stroke is where it's at. they destroke them and still have 3.75 stroke with the same bore. if the benefit was more bore than stroke they would destroke them even more.

Originally Posted by blackonblacksls

Negative sir. The point was missed by yourself. If stroke was were it's at they could just use the forged factory crank and use the blocks with just a normal overbore. But instead spring for aftermarket cranks and spend the 2000 dollars on alum blocks just to spend another 1500 or so to have them sleeved just to make the same displacement they could have had by leaving the stock lengh stuff in there.

Originally Posted by Shawn @ VA Speed

since when does the ls7 have a taller deck?

Originally Posted by swift700

Here's another factor. All else being equal, a larger bore has a higher probability of detonation, because it requires more spark advance. There is more distance for the flame front to travel, and greater chance of a secondary flame starting in a far-off place... building steam and colliding with the primary front. A smaller combustion chamber lessens this risk.

Originally Posted by swift700

Here's another factor. All else being equal, a larger bore has a higher probability of detonation, because it requires more spark advance. There is more distance for the flame front to travel, and greater chance of a secondary flame starting in a far-off place... building steam and colliding with the primary front. A smaller combustion chamber lessens this risk.

Originally Posted by Shawn @ VA Speed

i understand what you saying, and you can def have too much stroke vs bore, but generally the fastest engines have a stroke that is equal to the bore or more stroke than bore.

Originally Posted by chuntington101

I am going to throw a **** load of gas on to the fire now, please dont think im trying to undermind your knowlage! However it popped into my head so i have to aks!

How come F1 engines use such a small stroke yet VERY large bores? Sames gose for bike engines. Is it simply because they dont NEED the torque?

Getting 750bhp out of a N/A 2.4ltr is still pretty impresive in anyones books.

Thinking about it its probably to get the best cylinder filling possiable.

Originally Posted by swift700

Interestingly, Mitsubishi went from undersquare to a square engine in the Evo X. It also makes more hp, though there were some head and exhaust improvements also. The engineers are always looking for that happy median.