Hi guys,
I may buy an lq9 that was built few years back, never ran. Has a BTR stage 3 cam, wiseco pistons, eagle rods. Compression I’m being told is between 8 & 9. As it’s been a few years the builder doesn’t remember.

my power goal atleast for now would be max 650-700whp on pump.

Would a Precision turbo 7675 t4 .81 ar work good with this setup?

 

Compression is WAY too low. Those are 1980's turbo numbers. I wouldn't go under 10:1 on an LS with a turbo. Cam is also way too large. That engine is a recipe for absolutely no bottom end power and for power to come on like a light switch around 5500 RPM. It will suck to drive. A 76/75 with a .81 A/R is too small for a 6 liter, IMO. The turbo would start choking off right as you got into power. Something that size with a 1.25 A/R would be minimum, in order not to choke off the engine due to lack of exhaust flow.

 

Also, if it’s been sitting, was it properly prepped for storage? What do the cylinder walls look like?

 

S475 in a T6 would be a minimum.

 

It’s completely assembled now but from the pictures that I see, looks good. It’s stored in a shop with room temperature. About 3-4years I believe.

 

There is ABSOLUTLY nothing wrong with that a 9ish to 1 compression level. Which is the most likely. If the motors together and healthy, zero reason not to run it. Esp with those goals and pump gas.

I doubt its anywhere near 8:1 as they would have to intentionally run it with an odd combination to get it down that low. (big dish or huge CC heads). What heads are on it? Use a bore scope and see if the pistons have a huge dish. Can also do a static comp test since you know it doesn’t have some huge high overlap cam in it. Will give you a good idea anyway. 170-180’s is “normalish” in most cases.

You lose roughly 4% NA hp going from 10:1 To 9:1 and pick up roughly 6% per pound of boost. So all this bandwagon BS around night/day differences between 9:1 and 10:1 is nonsense. Its less than 1psi of boost difference. I’ve run 7.8 to 10.5:1 in turbo setups. The trade off is a MUCH larger tuning window and more power potential at like cylinder pressures vs a tiny bit better response. *IF* you run E85 and are starting from scratch, I agree something closer to 10:1 may be optimal depending on the goal. But its FAR from necessary for a s 700ish hp 6.0.

I’ve run an 8.5:1 5.3 (gen3 even) clicking off 8 sec quarter miles times and easily lighting off the turbo on the line. What more is needed for most? You just need the right converter/setup for the job.

.96+ T4 76/75 precision turbos have made more than your goal on 6.0’s. Depends if you want a drag car or a quick spooling street toy. Personally, I don’t’ believe a single T4 belongs on any LS. I’d look at the S475 T6. Or if you really want a T4, they modify the Borg 1.25T4 for the larger 87/96 exh wheel. Would prob be the same or similar price to the Precision unit and be 2x the turbo.

 

Since it's built is it .030 over or something like that?

Let's just say it's 8.5:1 with dished pistons. You'll be fine.Tuner should be made aware of compression they can probably add more timing in down low.

I'd only run a mid frame T6 in any new build, if you get the itch to go bigger you won't need to redo the kit.

 

"now but from the pictures that I see, looks good'.
That's reassuring.
Take it apart and do it right.