atb

So I'm kicking around the idea of bolting a rear mount turbo on my OB track car. The car drives awesome, but I'd just like some more ponies.

I'm new to boost, and have just started researching this stuff. I dusted of my copy of Maximum Boost that I bought years ago, and have started looking at sizing a proper turbo charger.

The Pressure Ratio is 1.48 bar.
The basic CFM is 399 AFR.

So here are the pressure maps for the following turbos:

T76


T70



I seem to land in the center island with both of these turbos when calculating for a 4.5L under 7 psi of boost.


I'm wondering if anyone has any input as to whether either of these turbos would be a good application for my car, and if not what might be a better choice.

It's a stock 8.5:1 US spec 4.5L motor, I'm looking for boost from 3.5K rpm to 6Krpm.

dprantl

If you're going to all the trouble to add a turbo, why plan for only 7psi? I would shoot for at least 12 psi on a 4.5 liter. A rear-mounted turbo is much easier to intercool, since the long pipe from the back of the car already cools the air quite a bit.

Dan
'91 928GT S/C 475hp/460lb.ft

blitz928

Here's a rear mounted turbo 928. Maybe this can be of help.

http://www.dragtimes.com/blog/porsch...-your-tailpipe

Courtshark

I agree with Dan; at 8.5:1 compression, you will need more then 7 psi, and with an RMT, the charge air will be sufficiently cooled/easily coolable before it hits the intake. I also recommend a water injection system for supercooling. At 8.5:1, and with a racing headgasket, you could run 25+ psi. And since you're willing to wait until 3.5k for spool, go as big as you can. A T70 is a BIG turbo.

123quattro

I think you'd be better off to picking your turbo based on how much power you want, rather than where it puts you in the efficiency island. A GT30R is probably a little small and might surge, but a 35R would probably be a good match. One of them could easily get you 500whp. Sizing based off of maps generally gets you a much bigger turbo than you need, which takes a while to build boost. OEM turbos are always off their efficiency maps to get quick spool up.

atb

Hi Dan,

Adding the turbo won't be trouble if I keep it simple. The RMT can be run without an I/C up to 7psi, and I'm pretty sure the 4.5L can handle 7psi without mods. I'd start at 5 psi just to make sure it all works without blowing stuff up.


I share shop space with these guys, I see them all the time. But I'm looking to do a homegrown set up instead of a turn key kit. They are the ones that gave me the idea for sure.


I'm not going for max HP guys here guys, just want to get an easy to put together system that isn't going to require any external cooling mods. If all works smoothly, then I'll starting upping boost and taking appropriate measures, but for now 7psi is all I'm looking for. So is the T70 too big? That's defiinitely the kind of information I'm looking for.

Thanks John, this is exactly the kind of info that I'm looking for. I'd like to see north of 350hp, but don't know how realistic that is with this motor under mild to mid boost. I haven't dynoed the car yet, but I'm probably only starting with about 200 HP to the rear wheels.

dprantl

A T70 is extremely large. I agree that a 35R would be good for a wide range of power for the 928. SAAB and VW are running their turbos sometimes to 90% capacity in stock operating condition. Those GT17's are working *really* hard, but the benefit is they still meet the power goal and have almost no lag at all which means they are at full boost even before 2k RPM. And they outlast the warranty period, but sometimes not by much

I don't think you should worry too much about blowing stuff up. The motor you are talking about is 8.5:1 compression and is just dying for boost. If it is in good health, I have no doubt it would run 12 psi without any modifications, especially when there are S4's running around reliably with 10psi on 10:1! Adding an intercooler is only a little extra work and worth it for that extra 5psi.

Are you going megasquirt? If so, are you planning on modifying your fuel rail for newer style injectors so you can run the cheap 42lb ones?

Dan
'91 928GT S/C 475hp/460lb.ft

Courtshark

Inefficient turbos ordinarily create excessive charge temps, which leads to predetonation, which leads to engine kaboom. In this case the charge air will be cooled because it has to travel a long way before it enters the intake, but the turbo will suffer in the process if it is too small and thus overworked. Sounds like diminishing returns to me.

atb

Thanks for the info Dan, I'll definitely look into the 35R.

Its interesting you mention about modifying for more fuel. Part of this "stage one" set up is running the stock CIS, so I guess I'm somewhat fuel limited in what I can run for boost. I don't think the CIS will work above 5-7 psi without leaning out. If I decide to upgrade, I'd just get a later model spider that has the bungs for standard injectors.

Courtshark

As soon as you feel the boost... you will want more! Trust me! It's like crack in that way.

dprantl

Adam,

Listen to courtshark. You don't want to just do a stage 1 and then "later" upgrade. It will take much longer and cost lots more too. Start with a good HP goal and build it for that goal and you will be happy. Shooting for 350hp at the crank IMO is not enough and I'm sure you'll be wanting more in short order. Heck, these days 350hp is becoming the norm.

Dan
'91 928GT S/C 475hp/460lb.ft

atb

Here's the map for the GT 35R.




Plotting the A/R and CFM barely puts me on the graph at 62%. Corky says that you want a minimum of 60% efficiency, so I guess that puts me in the game.


And with all you pushers around, I don't stand a chance.

Well, I'm gonna try to start small and learn, and then go from there. I've got too many projects going on as it is with my S4, but having too much fun with the track car.

hacker-pschorr

IMO CIS is going to be your limiting factor, not the engine itself. If I were you, figure out the next step beyond CIS then add the boost. It will make your life much easier.

You can blow any 928 motor apart at 5psi if not fueled correctly and with a bad timing map. This is why CIS needs to go, and while your at it a more modern, computer tunable ignition system would be nice.

Courtshark

I think that might be the first time anyone has said that here...

Lizard928

Adam,

It .48 psi boost (1.48 absolute), and almost 40 lbs of air an hour you are barely touching the bottom of the island on all of them. This is why the efficency is so low.

I would go for around the 35R with a ball bearing center.

I as well would recommend standalone EFI.
Heck I have a MSII controller and harness I could sell you. And seeing as how it's on the track car the install is easier too!
I could have you running on it in a day or 2.

I'll fire you off an email to discuss.