I've had good luck with 9.2~9.6:1 on 91 oct/meth. I've ran both combos at 5800' and low elevation. 9.5 would be my vote for 91~93 octane/ meth street combo.

 

Depends on the power adder.

 

a built 370 with 11.2:1 with 12psi boost was a rocket, yes it did lift a head after we started to push on the timing. went from 16 to 23 in small steps and for every 2degs it felt like we added two more cylinders.
on the street we raced a proven 9s car on the bottle and walked him...... intell we pushed all the coolant all over the road.
I just had a 365 ls2 on 22psi and 23deg and it didnt feel as good as the 370 on the 12psi
def not idel but damn impressive

 

welcome to the dark side!

i also switched from a 4turbo,now thinking in going turbo,and from what i read looks like i'll be staying in 10.1 + 76mm for my street car

 

the only time I would run high compression is if I was using 100% meth. and when I say high I mean like 12.5:1 and higher. literally you get zero knock. EVER! No need to even air to air cool it. Straight out the turbo straight into the TB. No reason why you can't dump from a small pipe into a huge pipe than into a even bigger intake manifold. The pressure drop cools the air. Which is even more goodness! Now that beeing said. I cannot afford nor would I want to afford buying straight meth, race gas. So I just buy 91 and inject water/meth when that comes to a end. This is ideal for me. So this is why I prefer low compression. e85 is deffinetly something I myself might consider as it is not that much more expensive. I could then use 10.0:1 or 10.5:1 and get quicker spool, more power off idle, and more throttle response. In the end it will still not make the power that the 119 octane gives me by injecting water/meth on top of 91. And it will cost me more. And not make as much power in the end. Like everything else there is a trade off. So in the end what I'm trying to say is ideal compression ratio just depends on what fuel you are using and what kind of setup you have. Only thing is when you are racing the guys that are making the MOST power are not always faster then the guys making a larger power band. I guess the four banger mentality has turned me into a power tripper. GUILTY. let the flaming begin he he he

one more thing. Thus far I have yet to find a car running these large turbo's that is ACTUALLY using the turbo in it's efficiency range. EXAMPLE: a GT4294 has not even woken up at 20psi. Life starts closer to 35psi and higher. So why are you even using that large when you could run a GT4094 and have quicker spool and run it at 30psi falling in it's efficiency range and making more power than the 42 at 20psi? k off tangent sorry. end of rant. perhaps I shall start a new thread

 

I’m new to the turbo side but the compressor maps really confuses me trying to figure out what turbo would be ideal for a l92 370.

 

One thing I will repeat here and have heard allot on this forum. PSI is not a measure of power, it’s a measure of restriction. Is the GT4294 optimum 35 PSI while blowing through a garden hose or 5” hose? Manny people run different heads and intakes, some ported some not. I do believe you measure by CFM and not PSI(question)?

 

Lbs./min. Is the best measurement of airflow. Boost pressure is irrelavint.

 

You're suggesting running a single 4094 on an LS1 at 30psi?

 

It was an example. But yea why not? Ive saw them run past 45 psi on a 2.0 so why not on a bigger motor? Explain problem please.

You are thinking about it the wrong way. Choose a HP goal and do some homwork. You can figure it out. It's kinda a pain in the *** to explain how to read a compressor map. You just need to learn on your own like anybody else that understands them.

 

A 5.7l engine at 6000rpm and 30psi requires about 105# of air and a 4094s max is 80#. You should start a new thread with your theories on yellowbullet, that would be epic

 

What exactly does 105# mean?

 

I built a 8.65:1 370ci with stock 317 heads and a custom grind.
Engine has held together going on 7yrs. Now. Has seen 680rw on pump gas on many occasions. The antiquated turbo by todays standards is pretty much maxed out.
Although boost build is dam near instant,full psi by about 3000rpm. Turbo is an older tnetics t4 t76 and 38mm wg.
Imo, the lower scr is a more of a safety precaution, and has little to no effect on lag given it has been taken into account along with the rest of the build and appropriate matched components. There is virtually no lag with this combo of cam,ci and turbo used. It is safe on up to 15psig. On pump gas.And even on very cold nights, it seems to come in quicker.

Now i can see how a 8.5:1 4.8 would offer serious lag if were equipped with a s475.
I guess, just a matter of preference, i prefer lower scr but equipped with reasonable equipment and scr taken into account for the particular build.
It is my opinion that a lower scr may hold together longer than a higher scr. Even if give up some turbo lag time,for some added safety.

 

Pounds of air.

 

I ran 8.5 CP pistons in my JDM muscle on 93 and meth... I think V8's probably stay closer to 9-9.5 i guess... I cant imagine you guys running more than 10pds of boost if your at a 10.1 ratio with meth or nitrous... could be wrong...

 

With turbo sizes don't forget that some people also play around with turbine wheels, etc for faster spool, so what might be a higher boost turbo might have a smaller modified turbine wheel for fast spooling, have seen this before and bigger laggy turbos spool quick and produce a heap of power alot earlier than they normally would.

 

S4885.3, you MIGHT be able to hit 25 psi down low, but the turbo would quickly be out of efficiency and would not be able to maintain that pressure ratio up top. Now if you put two of them on a V8 you might be able to run it out a little further. It is just a matter of what that turbo can flow at x rpm at y pressure ratio. If it is out of its efficiency by 4000 rpm at 25 psi boost will drop off quickly.