Dirteater19

Hey guys,

I'd like to hear from those of you who have lower compression ratio LS 2's and 3's and how they drive when off boost. How does the throttle response, torque and power feel compared to stock? Please post engine, C/R, what forced induction you're running, max psi, power and torque.

I'm trying to get an idea of what C/R I ultimately want to run since I do not want to lose too much off boost manners. Thanks.

EuroRod

My build is a 408 cid stroker motor with 9.6 cr Wisco pistons and a D1 ProCharger.
My drivability around town is fine, even with a fairly aggressive cam.

Pekka_Perkeles

...and how much ignition timing the tuner had on those off boost areas.

Otherwise that comparison is somewhat useless.

old motorhead

You're going to get better off boost hp, tq, mpg, and driveablitly with higher C/R....unless the guy doing the tuning is a dunce.

Pekka_Perkeles

Of course.

But that wasn't his original question.

old motorhead

Lemme put it differently. I've raised compression on an LS motor with no other changes other than tune. Off boost drivabilty, power, and fuel eco all went up.

This was basically an iron block LS2 with ported heads, mild blower cam, and 10psi of boost

Dirteater19

I understand all those factors go up with increased C/R. But my car is already supercharger and I'm rebuilding the engine with lower C/R in mind and am trying to find a balance between lower (and safer) C/R and driveability. This is mostly a street driven car out of boost 95% of the time, but I do take it to the track on occasion and enjoy a hard pull every now and then. I don't care about end hp and torque figures, I care more about engine engine safety and manners. For an example, I went with a fairly mild camshaft instead of the alternative.

So again, if anyone has real world experience with these engines anywhere between 9/1 and 10/1 C/R, id like to hear about how the engine behaves off boost and how much boost/hp/torque the given C/R is ultimately handling, preferably without meth or E85.

I want to run straight 93 octane pump gas for ease. I don't plan on going above 15 psi or 700 rwhp and 600 rwt. I'm leaning towards 9.5/1 but if I can get away with higher I'll gladly too it.

Patriot Six

10:1 LS7 here running 13psi. There may have been a slight decrease in throttle response, but not much. Drives great and I only lost about 20 lb/ft of TQ after dropping 1 point of compression.

TTZ06VETTE

9:1 LS7 with 850/850 RWHP/TQ. I have twin turbo setup with GT30’s. 16 lbs. boost. Run 16 degrees total timing with meth and about 25-35* timing in MAP areas out of boost. Start cutting back on timing 4200-4800rpm through peak torque. No impact on torque with turbos off Boost.

stevieturbo

Define "lower" and I guess whatever you then deem as "higher"


I'd suspect most people driving two with say 1 full point CR wouldnt even be able to tell the difference....maybe even 2 full points !

stevieturbo

Then that answers it for you. There is nothing in your post that even hints towards a higher compression offering any real world benefit at all. 9.5 will be ideal.

Lets face it....GM chose 9.1:1 for the LS9/LSA.....are people running around saying they're turds or crap off boost ? And you can fuel them anywhere and beat the crap out of them all day long.

Dirteater19

Lower than stock (10.9/1). Sorry, thought that was implied.

Dirteater19

Thank you for the feed back.

Dirteater19

Thank you as well, this is exactly the info I was hoping for.

Dirteater19

Yes, this is exactly what I was thinking. Everyone keeps telling me to go meth or E85 but the truth is it's ran fine for 3 years now at stock compression ratio with zero issues, it's just always been at the back of my mind. I figured if I go forged bottom end as well as lower the CR for some added safety I could worry less and beat on it some more without resorting to meth/E85.

stevieturbo

If you have E85 available to you, then it would be a no brainer.

But like me....it isnt. Regular pump fuel is all there is here hence build accordingly.

I've ran my CR at low 9's for years and I've run 20psi boost many times on pump alone. Although when racing or for speed events where traction may be better I would also use meth and for higher boost levels too.

But 90% of the time I simply do not need meth as I've access to more power than traction away.
Although I do live in a relatively cold climate so rarely have high ambients or charge temps to deal with.
Although if you're in a very hot climate that could be even more justification for adding a little safety into the build.

Obviously the LSA blower itself will resolve any concerns anyone might have about low rpm performance, but I'm sure there are many out there who just run a Crate LSX376 B15 etc which is around 9.0:1 and have no complaints and there have been users making over 1k with them

Pekka_Perkeles

With 2 points lower than stock compression I'd guess you could run up to 45 degrees of advance while cruising, since that's what stock LS7 has at lower cylinder airmass areas.

On the other hand, who cares if underhood temps are ok with your setup while cruising.

Dirteater19

Right now I run a vortech Si with 3.8" pully and stock diameter HB. It makes right around 11.8 psi at my rev limit. I don't need more power, it's enough, but if I lose some power after the rebuild due to lower CR, then I may go 3.6" or 3.4" pully to regain some power and hopefully have it come on sooner too. I don't think I'll go past 15 psi though since that's the limit of the SC so I'm hoping I'll be fine with 9.5/1 CR and 93 octane and a really conservative tune.

stevieturbo

All sounds like a perfectly sensible plan