Phil99vette

iTrader: (11)

Everything plays a role in how a combo will run but with the technology available it just doesn't make sense to build a 8.0-8.5:1 cr motor. Its not the early 1990s anymore. Look at the small details, research, the road most traveled usually isn't the best one, just the easiest.

topend

There is no reason today with modern EFI systems to run a turbo engine below 10:1 . Anything below 10:1 compression is a dog!

If you could pony up for a turbo setup you could pony up for the fuel.

Blackpanther99

iTrader: (8)

100% agreed.

71 chevy

iTrader: (18)

my apologies. didnt mean to confuse you with facts.

40 ftlbs at the engine is a pretty big difference off the line.

eg.


(engine tq * converter multiplication * 1st gear ratio *rear gear ratio)

400 * 2.5 * 2.48(th400) * 3.73 = 9,250.5
440 *2.5 * 2.48 * 3.73 = 10,175.44

still think an extra 40 ftlbs is not a big deal?

NicD

iTrader: (7)

I guess you missed the part where I said to run another whopping pound of boost on the launch to get your torque back. Wow LS1tech sure isn't what it used to be.

I would recommend people search google and look up some engineering white papers if you want real answers to questions like this or hell even just pay attention to what manufacturers do as they have the resources and equipment to do the real testing. They don't choose things like this on accident.

Phil99vette

iTrader: (11)

In the real world 40 ft/lb of torque is the difference in a motor getting up on the torque converter and making the first # of boost and just stalling at 2800-3000rpms and not making a single # of boost.

NicD

iTrader: (7)

And that would indicate the converter isn't correct for the application, not to mention where the hell did this 40 ft/lbs number even come from? A point of compression isn't going to produce another 40 ft/lbs at 3000 rpm with a 6.0L like the OP has.

Phil99vette

iTrader: (11)

At the end of the day its all about optimizing the entire combination. Sure you can say that the converter can be loosened up work with a motor with less torque but what does that do down track? You end up with higher slip %. You build a new technology motor with todays compression and try to optimize the pieces of the puzzle and it will run better than the 8.5:1. You are not going to run enough boost to worry about the base compression.

If we were building a honda or DSM when its standard issue to run 30-35# of boost with a 5 speed its a different conversation. Most of these combos are going to run 10-14# of boost and if they go to the track they will probably run a spash of race fuel.

4 years ago, everyone said 10.5:1 compression would not work @ 35-40# of boost in a race LS motor on gas. While your talking about what can't be done, others are doing it.

ATVracr

iTrader: (5)

Ours was almost 13:1 but it didnt make 35lbs of boost.

NicD

iTrader: (7)

I'm not arguing any of that because it is all a system and of course it all works together.

I'm arguing that an increase of a point of compression isn't going to add that much power to the equation and certainly not enough to affect getting up on the converter.

As an example in every engineering equation I can find it shows the difference going from 9:1 to 10:1 compression is a theoretical 2.9% increase in torque. I say theoretical because obviously it varies with cam selection, etc but that's the best case scenario.

If the op's motor makes 340 ft/lbs at 3000 rpm he would pick up less than 10 ft/lbs of torque by raising his compression ratio a full point. If he wants to run pump gas with some meth and optimize it I would say it's not worth running 10:1 when he can run 9:1 more safely because he isn't giving up that much and could probably just run another psi or two to compensate while having a larger tuning window. I'm not talking about race motors running ethanol or race gas here.

Huron Speed

iTrader: (1)

This is good info. i agree. and that is the direction we will be going with the shop car build we will be starting in the near future.

-Casey

Phil99vette

iTrader: (11)

If your not going to build a 2000+ hp ls, dont use a 2.5" crossover like everyone and their mother does. Size the pipes for velocity.

gnfast87

Thanks for the great thread guys!

I didn't say in the original post what kind of driving I will be doing.
I'd like to drive it 20-30 times around town a summer. Maybe hit the strip
4-5 times per summer. I don't really enjoy turbo lag. That's why I was looking at twin TE60 turbos instead of the usual big PT88/4000 stall convertor setup. Our streets in Vermont from stoplight to stoplight aren't very long. I want a torque monster that is going move the heavy GN.

6L is going to be all forged. Hoping for 800 wheel horsepower. No race gas just 93 octane, big intercooler & alkycontrol dual methanol kit.

I'm definitely leaning the 10:1 compression route.

runsfromdacops

my motor is 10.5to 1. i have smallish rear mounted PTE billit 5858s so i didnt want to make them work to hard to make power. also with a higher comp motor you get a more fun car to drive in the the day to day off boost driving.

if you go lower comp you do open up the tuning window alittle so thats nice for your tuner but at the same time the car will be a dog off boost and youll have to push the turbos harder to make any power and spool time will be alittle longer just adding to dogness of the car

Twin turbo c5

Ive got 10.5 to1 compression with a twin turbo setup and it work great . but I've also had 8.5 compression with no issues on my other rides . To me either way works.

BAKED

iTrader: (14)

Mine is 11.5:1 but Im running e85 and an a2w. I dont see me ever going to a low compression setup.

Wicked69

Interesting thread. FWIW when my current 180,000 mile 5.3 blows (which has a little lag on the street) I want to use the flat tops from a 4.8 and put them in a 5.3 set up. The compression should be around 11.5. I would like to give that a shot to see how wicked the difference would be on throttle response. I do run E85 though and like trying something different.

PewterZCar

iTrader: (2)

I love my 11:1 boosted ls6. FWIW

gto_ua

when you have enough octane to suppress increased cylinder pressure and knock.

vmapper

iTrader: (8)

I find it odd that through this discussion, only one person brought up the more important aspect, that being, Dynamic compression (aka the CAM - valve events)
Something not brought up at all, how is boost achieved?
The OP mentions Turbo, but it just means a tighter tuning window and a wise cam choice.
A static 10:1 / DCR 7.4:1 on a centri blower is not as big of deal, as the peak cylinder pressure would be less vs a turbo or PD blower.

Tuning window closes with higher overall cyl pressures, eventually to a point of uncontrollable even with tame timing, at which point, higher octane is required. i.e. Meth injection.

There are people such as Aaron who have no issue tuning higher compression boosted engines.