ltborg

This weekend with some of my autocross friends, we got talking about the minimum weights now in SM/SM2 and how big an engine you would want, etc. So that led to the idea of a 302 LS engine in a C5. You would loose about 140 lbs in the case of the 302 and we never agreed on whether this was worth the potential loss in torque. Anyone have any opinions/thoughts on this? While this is just bench racing, if any of you have some real experience with this (maybe even seen someone try it), it'd be cool to hear about it.

XPC5R

I've thought about it a lot, and actually have some of the parts to build it (I would get the same kind of weight break in XP). With older LS6 based heads and cam tech, you should have around 420-430 rwhp, and near 400 ft/lbs. That's more power than the tires can hold on most surfaces.

Easy build, using the 4.8 crank and rods (3.2 stroke) and LS6 or LS2 block. Compression heights were the same as LS6, so it looks like off the shelf pistons will work.

Biggest problem is chamber cc for good compression, with the short stroke, it takes a pretty small chamber to get over 10:1. Or it would require domed pistons.

It would be a nice screamer, with modest low end torque that would be easy to drive and modulate.

ltborg

The low end torque was the biggest concern. I was figuring if you went to the trouble of building such an engine, you'd go ahead and run it on race fuel with domed pistons and some crazy high compression which would probably help some.

In SM2, you'd only loose about 5% off your minimum weight, I'm not too familar with the XP rules but I would assume you would loose at least that if not more. Any idea how much that would aid cornering and braking? Sounds like you might be trying this so I take it the weight loose might be worth the lost torque? If you don't mind me asking, what size tires are you running and would you change if you get the engine built?

XPC5R

I think the 5% in weight (120lbs for SM2, 150lbs for XP) will be worth 2-3% on the track. That's 5% less weight transfer when cornering and braking, and 5% better acceleration for a given amount of tire traction.

In SM2, I don't think you can hit the minimum weight (2620), but in XP, with the traction control penalty, it puts my minimum weight at 2475, which I can reach with some work.

We run 315 front and 345 rear A6's now. At 2475 I would probably run 285 fronts and 335 rears.

ltborg

I thought the minimum weight in SM2 was 2733 (with an engine displacement of 5.665L)? The rule is 1600lb + 200lb/L right?

Solofast

If you could get the weight that low, you probably don't need nearly as much tire as you have now... I might be even tempted to run down to a 275 on the fronts and 315 on the backs and then run them soft to let them heat up more/sooner, kinda depends on if you need the 335's to put down the power... At that low a weight you need to be looking at what really works for some of the smaller/lighter cars...

It is all about having "enough" tire and keeping it working. Too much tire doesn't warm up fast enough and you end up throwing away a good bit of the early part of a run by just not having enought heat in them...

0ATI Performance

Yes, that is correct per the 2007 SM2 rules. A nominal 5.7L Corvette must weight at least 2740lbs. Also 2900lbs is the cap regardless of displacement. I think the original poster was proposing de-stroking down to 5.0L for a lower weight limit.

Be careful what you wish for......



Bob

ltborg

oh yeah, i see what you mean. missed that. thanks.

solofast, thats another reason i was thinking this package might help; it lets you decrease the tire size so you loose rotating unsprung weight. i know of some lighter cars (2000 lbs) that couldnt get 335s above 100 degrees. i have to agree that at that weight i think you could get away with 315s in the rear, but testing would be in order to confirm of course.

0ATI Performance

Keeping the rear tires hot should not be a problem, regardless of what size you put back there......

The current SM2 weight formula heavily favors small turbo engines, so for even less weight how about a turbo 2.0L Ecotec (like the drag racers use).... I heard they make tons of power, way more than you would need so with the right electronics you might make it manageable. Some of the fastest autocross cars have small turbo engines, even in SM2. Your effective displacement would be 2.0L + 1.4L = 3.4L = 2280lbs. The 4-cyl itself should help save some weight....

There is something not right about a "ricer" type engine in a Corvette, but the rules are what they are.......

Bob

XPC5R

I was calculating the minimum weight with a 5.1 engine. Good reason to use a solstice/sky chassis + 2.0 turbo

I am hooked on the V8 rumble though

I have tested the 285/315 combo last year for the same reasons, but the 315 rear lost too much lateral and forward bite compared to the 335 combo. The 285 front was still decent though, and could be made to work.

ltborg

thats good to know. doesnt look like your tires stick out too far in your avatar picture, can you minitub in xp or new rear fenders?

XPC5R

Those in the picture are 335's on 18x13. Sort of a maxi-tub. The 345's stick out about another 1/2", and have started rubbing on the fuel tank enclosures, so they are spaced out a bit more. I'll need to go to a small fuel cell and mod the tank surrounds to tuck them in tighter.