LS1: Bore Vs Stroke

[XJMK]

I was reading that it is more desirable to be undersquare (stroke is shorter than the bore is wide - which is how the LS1 comes stock) than to be oversquare (stroke>bore, ie stroked). This is because in an undersquare engine the power is made higher in the power band (above ~5000 rpm) verses lower in the stroker.

It seems to me that if you want something to be quick on the street (read: more fun in normal driving conditions) you'd want it stroked. If it spent more time at the strip you'd want it bored. Anyone have any input on this?

 

[wolfpackjeeper]

you have it about right.

 

[crazyjim]

you have it about right.

Ehhh not quite. Depends on how you drive.

The LT1 has more torque than it's younger brother LS1. This is because of the stroked nature you talk about.

However, the LS1 has a MUCH smoother power curve, and makes good power throughout the powerband.

 

[5-90]

As said, it depends.

Diesels make so much torque because they're hideously oversquare. An engine that is undersquare (bore < stroke) will invariably make more torque than an engine that is oversquare (bore > stroke) of the same displacement - simply due to compression. The higher compression also contributes to more compleat combustion - and increased NOx, if left unchecked (which starts wandering heavily into theory in short order...)

How do you plan to drive? If you want stump-pulling low-end torque, you want for the stroke to be greater than the bore, and (ideally) you want to have the longest connecting rod that the engine combination will support (this increases TDC dwell time, which serves to maximise the use of peak pressures.) If you want to wind up the engine nice and tight, you'll want the stroke to be less than the bore - so you can "run high" while minimising strain on the reciprocating assembly (why do you think Diesels typically have redlines down around 2800-3000rpm, at least for the larger ones? There's a good reason for that - and for why the larger Diesels have things like twelve-speed transmissions bolted to their backsides...)

A "square" (bore = stroke) engine is a compromise between the two, natch, and that's why it's so wildly popular (I'd have to check, but I think the Ford and Chevvy ~400ci engines are both "square" at 4.000x4.000. Maybe the Chrysler 400 as well - but I'd have to check my notes on that. The Chevvy 265 might also have been square at 3.480x3.480 - again, I need to check my notes on that.) Compromise isn't always a bad thing - but there's a reason that dedicated racing engines can wind up to 10,000rpm all day without ill effects - and that's mainly because the stroke is down around 2.5" or so. (a V8 displacing 180-195ci is fairly typical I think - again, I'd have to check. It's late, and my head hurts.) Racing engines tend to be grossly undersquare, since they're mainly after high-rpm horsepower output. Truck engines lean more toward being oversquare (think the AMC258, used in Jeeps for years - 3.750" bore x 3.895" stroke) to maximise low-rpm torque output. This means lower redlines as well (even the typical stroker is mildly undersquare - 3.875" bore x 3.895" stroke. This reverses if the block is bored out .030" or more - 3.905/3.935 x 3.895, then. But, it's not enough to cause great difficulty, and the increase in compression still helps low-speed torque output.)

 

[1blackz28]

Asuming all other variables are equal the real reason that a engine with a longer stoke will make more torque is due to mechanical advantage over the crank not compression. The longer connecting rod length gives more leverage over the crank pin than a shorter connecting rod if the energy behind the rod are the same. Think of it this way. The last time you tried to get a really rusted bolt off and had to use a breaker bar to do it, you used the mechanical advantage of the longer breaker bar to generate enough torque to free the bolt. You didnt suddenly become stronger, you just used a longer lever. All variables equal compression dosent change when compressing a given volume of air into another given volume of air no matter the shape of the container. For example I have a 10 cubic inch cylinder of air and I need compress that volume into 1 cubic inch of air. The shape of the cylinder dosent matter (long or fat).

 

[5-90]

Asuming all other variables are equal the real reason that a engine with a longer stoke will make more torque is due to mechanical advantage over the crank not compression. The longer connecting rod length gives more leverage over the crank pin than a shorter connecting rod if the energy behind the rod are the same. Think of it this way. The last time you tried to get a really rusted bolt off and had to use a breaker bar to do it, you used the mechanical advantage of the longer breaker bar to generate enough torque to free the bolt. You didnt suddenly become stronger, you just used a longer lever. All variables equal compression dosent change when compressing a given volume of air into another given volume of air no matter the shape of the container. For example I have a 10 cubic inch cylinder of air and I need compress that volume into 1 cubic inch of air. The shape of the cylinder dosent matter (long or fat).

Correct - but the longer stroke typically allows a longer rod, which allows more dwell at TDC. This, in turn, allows combustion pressures to build to a greater level before forcing the piston down (we're talking timing in milliseconds or less here!) and then taking advantage of both the pressure involved and the MA of the rod vice the crankpin throw.

I understand what you're saying about pressure generated as well - it does apply here - but it should be understood that there are multiple facets to the problem at hand (yes, a Diesel makes loads of torque due to compression. It makes loads of useful torque due to MA and structure of the reciprocating assembly. Both come strongly into play.)

 

[XJMK]

I originally was thinking of stroking it to a 383, but I'm not so sure I want to do that now. To do it right, from what I've seen I'd be dropping around $3500 for the 383, and I don't have the money for that.

I most likely will put in a decent cam with heads and exhaust to match, and put this plan on the back burner when I have some money to throw around.

The guys over at LS1tech can be opinionated, just like good ole naxja. Its refreshing to bounce ideas off people who aren't obsessed with a certain vehicle Thanks guys.