I'm building an iron 402 to go along with my upcoming TTIX kit with 57mm .62 AR turbonetics. I know these turbos are a bit small for a 402 but it is what it is. I picked up a 402 rotating assembly with -6cc Wisecos. My math says with my 243 heads and LS9 gasket (~.040" crushed height) puts me at the same ~11:1 my stock LS2 is. Not ideal most would say. I'm trying to decide on a compression ratio whether I get there by dishing the pistons some more, running a Cometic head gasket or perhaps even swapping to 317 heads (but I think the latter will put me too low). My gut tells me ~10:1 - 10.5:1. I don't want to go down into the 9's because I don't want it to be a pig out of boost and I don't really see the point of going that low. I guess I need to pick a CR that allows the turbos to be in their peak efficiency but I don't really know how to do that . I guess I don't truly understand the difference between 20 PSI on 9.5:1 CR and 20 PSI on 11:1 CR. I know boost is just a measure of backoressure so thats not a good statement. Is there an empirical way I can choose the "best" CR for this setup? There is this BorgWarner turbo calculator bot thingy: http://www.turbodriven.com/performan...bot/index.html but I'm obviously too dumb to work it and I don't even see a compressor on there as small as mine.

An item of note is this UPP TT build. 11:1 408 @ 14 PSI = 818/859 http://forums.corvetteforum.com/c6-f...w-results.html

 

Drop the comp to 10:1 or lower. That 402 will spin up those little turbos instantly. Gotta be ready for when you order the bigger ones.

 

I think the biggest question here is, what fuel do you plan to run?

If you had access to e85 regularly, I'd leave it at 11:1 all day

 

93 octane with dual m15 meth nozzles

I know you want to cheap out and use your stock heads...lol.

I think you will be ok on those small turbos with all that meth. 11:1. Go for it, but you will need to drop it later on if you step it up.

That car will make tark so damn quick!

 

Yea, 93 octane with lots of meth, atucally one M10 and one M15 nozzle. Right now the Alky Contol PAC comes on at 3# at default duty cycle and the gain is at 5.5 but that was for my blower setup.

I want to set it up for these turbos. Later when I upgrade I'll re-do the CR whether it be with 317 heads or something else.

 

Bill MY 402 with twin 67 was 8.5-8.8 and they spooled instantly! So Its your car and it will be fine anyway you do it. But I prefer saftey over trying to get everylast HP out of a set up. So I'm with most guys on 9.0-10 is fine and it won't make a hill of beans difference in HP or spool time. But some one will chime it that it DID bla bla bla, but I will be willing to bet its almost inmeasurable on a set up like yours. On a big single or with two large turbos possibly, but none of the turbo's available with the TTIX kit will make a difference worth measuring on this motor IMO! I like the KISS method lol!

 

WOW didn't mean to shut down discusion on the subject lol! So feel free to give your opinions and experiences guys.

 

Just getting more traction on CF is all. Thought it would be the other way around.

 

Boost is not backpressure. They are related, but totally different measurements. Boost is pressure measured on the compressor outlet. Back pressure is pressure generally on the inlet side of the turbine. Some also use it to talk about pressure generated by the exhaust restriction post turbo. If you have an undersized turbine you can generate backpressure quite a bit higher than boost pressure.

I'd go 9.5:1 CR.

 

Sorry, what I meant boost is a measure of the the engine's inability to pump air. And I guess it's a lot more basic on a blower car. IE: A centri running 12# may drop to 10# when headers are added. I do want to get more knowledagble about turbo backpressure on both sides of the exhaust turbine.

 

Yeah my 402 on 15LBS made 848/95x. again with 8.8 CR.

 

Boost IS a measurement of resrtiction.. with higher CR you need to worry about cylinder wall pressures. That can be a major issue. But, I'd leave it alone and run E85.

 

E85 is not readily available around here and especially considering you will get about half the milage from the stuff. He'd have to keep drums around his house or make a 20mile trip to go fill up before every cruise etc... And the cost of the fuel system to support the power on E85 probably isn't in his budget at this time etc...

 

Gotta do what ya gotta do to make power reliably at that CR, e85 is the best way to do that. You actually don't burn very much more e85.. Kinda funny that people think that. Just keep a few 5 gallon VP racing jugs full, 20 miles isn't bad at all to fill up IMHO. Do what you like, but e85 is very awesome cheap priced fuel.

 

Well we could haggle about fuel economy and storing fuel, but none of that matters when he didn't plan on spending money on the fuel system etc... But your thoughts aren't any different than "Just run race gas" but the cost is cheaper. Im sure hes just going to keep doing 93 and meth for now.

 

throw the 317s on to lower the compression, But, I wouldnt be worried with 243s and high compression + Boost, i knw u knw how to tune LOL

 

What turbos? Was the car a T56?

 

I CC'ed one of the pistons, it actually came out to -9cc's, not the -6 like originally thought. With an LS9 head gasket, my 243's and figuring the pistons sticking out of the bore .005" after decking (won't know this for sure until I get my iron block machined) that puts me at 10.89:1. Not too bad but switching to 317's would put me at 10.18. I'll likely pick up some 317's and a nice turbo cam and build the whole long block.

E85 is fine but to me I get the same benefit from meth and it's on an as needed basis. With the blower I only went through ~5 gal every quarter. I'm guessing it will be more with the turbos but I'm still a fan of meth.

 

Precision 67's and yes it was a T56.

 

I would be running the 317s instead of the 243s on pump + meth but I would still rather have a lower compression ratio than that.