I am beginning to fab up a single front mount truck manifold setup. I plan on doing a fmic, and my motor is completely stock aside from 317 heads and will get a custom ground cam. I bought a 66mm non-ballbearing turbo with a a/r of .60 and .81 on the hot side on a t-4 flange. Will this turbo be enough to make 600rwhp through a 6-speed? From what I figured it will not be efficient above 5krpm.

Here is a compressor map for a 66 and the air consumption chart for a stock LS1.



http://www.forcedinductions.com/consumption346.htm

Thanks in advance.

 

I think it can get there, but the backpressure will be high. You might consider swapping out the .81 for a larger AR, like a .96.

Mike

 

Newb question, can you just swap out the housing? If so, where do you get the housing? Also, the turbo I have only has a 2inch cold side to the intake, is that big enough?

 

I think the t66 will hit it, but it will be pretty high boost, maybe around 18-20 psi.

According to turbonetics the turbo is efficent to around 735 or so hp.


Good luck, Chris

 

You talking about the piping? That's too small. I was surprised when I found out that STS only supplies 2.5" piping on their cold side. I'm using 3".

 

I don't see it happening and staying togeather on a stock bottom end. Who's tuning of and what all is done to the fuel system?

 

You may get it there if you have a VERY free exhaust side.... I'd look at swapping out the housing as Mike said. Nice nick btw

 

dont think youll get 600rw with that turbo, definaly not with an .81 ar, you might get 550rw with a .96 ar, but i think the turbo will just about run out of steam before you get to 600, though you might make it, barely.

 

I mean the outlet on the turbo is 2in, I was going to run 2.5 piping. I am going to have Keith McCord tune it on E85, and I am not at all worried about popping the motor. I have just heard that 1atm (14psi) will in theory double your N/a hp, and I figured I would have 350rwhp N/a. Also, if the turbo is rated to 750, why would that be out of the question?

Thanks for the replies, keep em coming!

 

750 fly wheel horsepower, on a smaller engine that would use a higher pressure ratio.
you might just make it (with the .96 ar) but it would be close.
worse thing that could happen, is you build it and start tuning it, get all you can out of it, and later if it doesnt meet your full throttle standards, sell it and get a bigger turbo.

 

That was pretty much my plan, just to push this stock one until it pops, and build a 370 with a 76. Is there a way to increase my pressure ratio with a custom cam? Also, where can I find the .96ar housing, and will the cold side be big enough also?

Thanks again smokin

 

With a turbo that size, the dyno graph will probably have a strong resemblance to a Maggie 112 graph (near instant boost, then running out of steam up top). Big HP#'s are nice to brag about, and help at the track, but low end torque is a lot of fun on the street. I would definitely try it before deciding if you need to upgrade.

 

just keep your stock heads (i wouldnt get better flowing ones as they would lower the pressure ratio - less boost per power), and have as big of downpipe as you can fit.
the cold side should be fine, at 2", you could step it up right after the turbo, but i dont think it will matter to much.
if it was me id probably go 2.5 right after the turbo then get a ebay IC if you do the 3" core i think those have 2.5" inlets, if you go with a 4" core those have 3" inlets, so which ever one you get id get the same size IC piping and just step it right after the turbo.

 

It is mostly a street car, and I plan to leave enough room to be able to upgrade without redoing my piping. I would still like to look into upgrading to a .96.

 

I believe the only effective way to gain power by raising the pressure ratio is to reduce cubic inches.

Mike

 

This brings up an interesting engineering question I have never thought much about before. There would be absolutely NO reason to do it, as it would be much better and easier to properly size a turbo, but why wouldn't a large motor with a poor induction side and a good exhaust side mimic a smaller cube motor? As pointless as it would be, I am just curious about the actual physics involved.

 

I've definitely put allot of thought into this because the accepted train of thought didn't make sense to me.

Let's start with "why is a compressor more efficient at higher Rp's?" It has to do with gas density. Take one mass flow rate. . . say 70 lb/min (roughly 750 hp). At a 15 psi boost (about 2/1), the efficiency is 60%. At the same flow rate and 30 psi boost (about 3/1), the efficiency becomes a much more desirable 74%. The reason is that the higher pressure means a denser gas, which accrues less losses in the exit of the impeller and in the diffuser. Basically, the same mass flow rate at a higher pressure will take up less space, so the velocity will be lower and the losses will be less.

Keep in mind that this doesn't necessarily mean that the compressor takes less power to spin. Power to drive the compressor is a function of... head x mass flow / efficiency. So, if we're producing 50% more head at only 25% higher efficiency, we still haven't improved things.

Now, take a look at the exhaust side of things. . . assume you need 75 hp to drive the compressor. The turbine must supply this power. The power is regulated in increasing or decreasing the exhaust pressure supply to the turbine. If a turbine must produce 75 hp, then it will need X amount of backpressure whether it's being produced by 180 cid or 346 cid. The downside of the large motor is alot more piston area and stroke length that the X pressure is working against. It's alot easier to spin 180 cid against 50 psi of backpressure than 346 cid.

So, you decrease the cubic inches and raise the boost. The compressor gets a little more efficient, depending on where you're running it, but could be taking more power to drive also. The turbine has to produce more power, so the backpressure goes up a bit. However, the engine doesn't have to work as hard to overcome this backpressure since the piston area/stoke length is reduced. The net result is a gain in power.

Now, keep cid the same but reduce intake/head/cam flow and see the effects (heck, why not install a restrictive intercooler or just not open the throttle all the way?). The compressor gets a little more efficient, but could still be taking more power to drive still. The turbine has to produce more power, so the backpressure goes up. Now, you have just as many cid for the backpressure to work against, so you haven't gained anything there either. In the end, it's lose-lose. Having a dense charge in the compressor exit of the turbo, but knocking down the pressure before it gets in the cylinder is definitely NOT the direction we need to go.

In order to get the most out of a turbo, the best engine to build (in general) is one with small displacement but good flowing heads/cam. Take a look at the Supra's and 4.6 Cobra TT conversions. Both are turbo cars that have small displacement, good flowing heads, and produce tremendous power with a turbocharger. I'd be interested in building a 302 cid LSx using L92 heads and see just how much power a mid-frame turbo could make.

Mike

 

I would make sure that you have some good intercooling & meth. It'll probably spool quick, but get inefficient & start pushing pretty hot air at higher boost levels. I'm running what's basically a T66 on a 3.8L buick. I'm also running a lot of meth & 27psi.

 

That makes sense. Thanks for the explanation.

 

here is a guy talking about the effect of switching out the .81 to a .96 ar on a 76mm rear mounted turbo
https://ls1tech.com/forums/forced-induction/819658-81-housing-96-housing-swapped-again-feels-goooood.html