Hi all,
Ive been a member here over 20 years but forgot my user name and password as I haven’t been super active in modding my ls cars for some time. I moved from street cars to motorcycles to track only to spec Miata and endurance racing so year, not a lot of LS going on.

anyway, I have been pondering this question for sometime and remembered what a great resource this section was. I am building an endurance race car where gas mileage is very important. Based on my projected weight, I expect that my maximum allowable horsepower will be between 200 and 225 hp.

The engine I used is completely open. The only thing that really matters is rear wheel horsepower.
obviously, less fuel consumption is better, and if I’m building this thing, I’d like to ensure the best bsfc I can.

My initial thought was a detuned 6.2. Then I thought an aluminum block 5.3 wousl be better with smaller bores which would allow more compression/timing. But part of me thinks 25% less friction and running a v6 would be better.

id love to hear your thoughts!

 

A GENV 4.3L V6 has an aluminum block and heads, so should weigh less than an aluminum 5.3L . Also, since it's shorter overall length, that moves the weight bias more towards rear (assuming front-engine). Plus you have less rotating mass and friction V6 versus V8. And since the bores are 3.92", the valves won't be as shrouded as a small bore V8. Not sure if there's aftermarket stuff available, but @ 285 SAE hp, even if you're talking 200-225 RWP it already meets or surpasses your goals. Downside might be the AFM (don't know how reliable it is on the GENV).

https://gmauthority.com/blog/gm/gm-engines/lv3/

Edit: Apparently there is some aftermarket support too.

 

I was *just* looking at that. My concern is the bigger bore size, which means less compression. But it's seemingly just the V8 with 2 cylinders chopped off. Higher power dumbed down is better to get a flatter curve and leaner mixture, but I'd imagine that with headers and some minor mods, it's a probably around 300-310hp. 225rwhp is probably around 265hp so we'd be taking 15-20% of the power out. That may work.

plus it seems like it's cheap AF. $1000 for a used one and it it wouldn't cost much to get everything rebuilt. Obviously, it will need some expensive stuff (high temp valves, ceramic coatings, etc) but this is probably 1/2 the cost of a 5.3 and 1/3 the cost of a 6.2.

 

Another question- do I want more or less overlap?
Obviously, in traditional MPFI, less. But with DI, fuel isn't injected till the end of the compression stroke, well after overlap is done. That could help cool the piston (and get all the exhaust scavenging I need out, as remaining hot exhaust is a killer for compression/AFR limits)

 

The LV3 already has oil spray cooling under the pistons. And the DI reduces temperature some, which is why the DI engines tend to tolerate more compression ratio.

Hottest part of the chamber is the exhaust valves, so if you want to do coatings, focus on those. Swain Tech for example, can do the exhausts (both face & backside radius) for $30 each. Should reduce the in-cylinder temps significantly to reduce detonation without a ton of investment. Also verify the quench distance is in the 0.036 to 0.040 to reduce detonation potential and increase efficiency simultaneously.

 

I don't have any specific math to back this up, but I would think running a cam with a super short intake duration to simulate an atkinson cycle combined with a high static compression ratio would help with fuel economy a lot. This would give terrible power numbers, so that would likely mean going with the 5.3 at minimum for displacement. A cam of this nature would obviously be zero overlap and likely have a lot of exhaust gas charge dilution, but I don't really see that being a problem in the scenario you are describing.

 

Would overlap be a bad thing with direct injection? Fuel isn't sprayed till overlap is long done with.

 

The only folks who would care about BSFC at 200 hp would be for a plane or boat.

 

The V6 sounds like a better match to me as well. Given the 225whp limit you could cam it to get a powerband that starts really low and stays flat all the way to the rev limit, which should make the car pretty fun to drive. Take any corner in any gear.

 

I'd say the V6.

For example, the LV3 makes peak torque(305lb-ft) at 3,900rpm. That's 226hp at that RPM. I'd bet the BSFC is pretty good in that RPM area.