Lance J

this is a link to 3000gt n/a with a 10.1 compression ratio. and he's running 10-12psi. now i have a 8.5 compression ratio. why cant i run a turbo kit, with intercooler, and water injection. 14psi should not be a problem. and did i mention 0 knock.

Lance J

http://www.3si.org/forum/showthread.php?t=288053

soory here it is

tammons

Well thats how the jap guys can run huge T04's on a small engine. Its doable, but it takes serious engine management. There is very little room for error running something like 10-1CR at 12 psi. You can almost think about it like a big shot nitrous system. One little mistake and boom.

The boosted compression would be almost 15-1 in this case, so you can imagine how a 15-1 NA race engine would run on 93 octane.

Now do that with cast pistons and you are asking for it.

8.5-1 is no problem with turbos. I ran my early stock 928 with twin t3 hybrid turbos, custom FI at 12 psi, but you must control the spark advance. My car ran fine on 93, but i tended to keep it at 8 psi for longevity. Also you have to make sure you dont have any detonation and keep the combustion temps down. I had mine running at about 1300F on boost.

BC

With enough control over fuel and timing, and enough *internal* combustion chamber cooling (i.e., methanol and/or water injection) I think its doable at 10:1 compression on a 928 2V to about 11 or 12psi. Margin of error? Of course smaller then at 8psi.

m21sniper

I agree with brendan.

An ideal setup should be able to handle as much as 1 bar of boost on 93 octane fuel on a 10:1CR.

Of course, the key word here, is ideal.

Lance J

in and ideal world. lol roughly what would 1 bar boost on my 79. put me at hp wise

tammons

If the head gasket would hold together at 16psi, you would be making between 500-600 hp and around 500 ft/# of torque, but the trans gives up at around 450 ft/#.

If you want good engine and drivetrain life just run it at 8 psi. 8 its fast enough to get you plenty of speeding tickets.

A 928 with the early 5 speed at 12 psi is a wicked fast ride.

SMTCapeCod

Great thread...I've got a nice pair of turbos to upgrade the 9Bs on my 3000GT VR4. And I've been lamenting the compression ratio in my Euro 928 because it would seem to make any substantial gain from forced induction...pretty dicey. The recipe for large turbos/large boost on the VR4 is pretty well established- selecting a larger fuel injector appropriate to intended boost and then adding electronic fuel control. The Unit on my car, a Pro-M, was purported to have Porsche fitments but they've since gone out of business. I"ve been wondering why people are monkeying with fuel pressure regulators when injectors and fuel control should deliver more sweeping and precise control of the AFR. Some of the VR4 are running large TD05s or Garretts and upwards of 30 lbs-- but not on pump gas obviously. Periodically someone's over-the-top project goes *boom*.

At the Boston shark party I was talking with a guy that had the Murf kit on his '89. He said he put down 465HP at 10lbs but was in need of more fuel. I remarked that $1.5K in turbos and mods puts my 3.0L in the same ballpark. He commented on the V-8s abundant torque...and I decided to foregoe any reminder that AWD makes quick work of any advantage from that....Don't get me wrong, I'm much prefer a blown 928 any day!

m21sniper

"in and ideal world. lol roughly what would 1 bar boost on my 79. put me at hp wise"

Assuming a perfectly efficient induction system(which you can't get), each 1 bar of boost effectively doubles displacement, so you'd have exactly double the HP and Torque you do now.

However in reality, the best one can hope for is about 85% efficiency, and getting there requires a lot of careful parts selection/design, and a lot of tuning.

But for a well setup car, a 219hp 928 4.5L V-8 running at one bar should produce 405 crank HP, plus the gain to 100% VE before boost is built, which is probably about another 20hp.

So about 425 crank HP total.

tammons

[QUOTE=m21sniperSo about 425 crank HP total.[/QUOTE]

That sounds a little low to me.

A 2.5L 951 running a vitesse stage 3 kit at 15 psi makes around 330 rwhp which at 15% drivetrain loss would be 380 at the crank. Add another 2 liters, good efficient turbos, a good intercooler and proper engine management, and I am sure you would be around at least 500-550hp at the crank at 15 psi.

m21sniper

It's simple math bro.

14.7psi is the equivelant to doubling atmospheric pressure, which effectively doubles volumetric efficiency- IOW, it doubles displacement(which obviously doubles output).

So if you start at 219hp with a 90% VE, the first step is to figure HP at 100% VE, which is a state of true zero boost/vacuum.

For a 219hp engine at 90% VE jumping to 100% VE would add 22hp, for a new zero boost output of 242 crank hp(This is the gain you're gonna see with an Eram as it forces the engine closer to 100% ve but does not actually build positive boost pressure).

Now double that for your 14.7psi of boost, and you end up with 484hp(and a theoretical displacement of 9 liters). But next you have to subtract system innefficiencies, which for a really good setup is about 15%.

That gives you a total corrected output of 411.4HP, and a net effective displacement of 7.65 liters.

This stuff is all covered in pretty good detail in Corky Bell's book "Maximum boost, theories of turbocharging".

You should check it out....REAL good stuff.

Herr-Kuhn

Have any of you ever seen a broken ring land on a cast piston? This is the one BIG reason not to attempt it. Hot day, too much ignition, high boost and a heavy right foot will do the trick...FAST.

m21sniper

Yeah, a good engine management system with full knock control, fuel enrichment, and spark retard would be ABSOLUTELY MANDATORY in a 10:1 car running 14.7psi.

BC

When aftermarket EFI is used, knock control is a per-car basis.

tammons

I have read that book a million times and it is a good book, so explain to me how vitesse is getting 330 rwhp out of 2.5L at 15 psi. Thats 380 at the crank. I had a race turbo builder explain the concept to me a while back.