rvida

Hi,

I have a 993 with the TPC / custom intercooler / custom computer running at 8lbs.... I have been toying with the idea of the 3.8 upgrade, but I think it will not add enough power to make it worth it. Somewhere I read about 4.0ltr. upgrade... Anyone have any info on it?
(I would also lower compression to bring boost to 10lbs..)

(Yes a'm also doing the LWF,clutch,gear upgrades...)

Thanks!

Rafael

-----------------------------------------------
'97 C4S
PSS9, TechArt wheels
TPC/Custom Intercooler 8+lbs/Custom Computer/etc...
HID conversion
Kenwood Music Keg, 300w amp, 2x8" subs, etc...
Integrated radar, tire pressure monitor
Plus other random goodies....
YES BITTEN BY THE BUG.....

Rick Lee

When I toured Ruf in November they had a 3.2 Carrera engine sitting on a bench, which they were boring out to 4.0l. I'm sure they can do it to a 3.6. Their labor rate is a very reasonable 70 Euros an hour. Check with them.

Lagavulin

It mostly depends upon what your final static compression ratio will be will determine the maximum amount of boost one can safely run detonation-wise.

For example, if you lowered your compression ratio to 9.0:1, you could run up to 21 psi of boost assuming an 85% efficient inter/after cooler without worrying about heat-induced detonation; that's a LOT of boost!

How much horsepower would that make?

Let's see how much horsepower your proposed 4.0L engine would put out using 9.0:1 cr. First we need to figure out the approximate NA hp of the engine, and then we'll apply boost on top of that.

Let's find out the stock hp/L:

285 crank hp / 3.6L = 79.2 chp/L

Now let's apply that output to a 4.0L engine:

79.2 chp/L x 4.0L = 316 chp

So the engine will put out theoretically 316 at the crank just by going to 4.0 liters. But we must impose a 4% hp loss per point of compression going from the stock cr of 11.3:1 to 9.0:1:

11.3 cr - 9.0 cr = 2.3 cr difference.

1 - (2.3 cr x .04) = 0.908

316 chp x 0.908 = 287 crank hp

We see that the compression ratio reduction will cost 29 crank hp. The question is, how much horsepower can be generated with 287 hp at the crank running 21 psi of boost?

Boosted HP = 'Pressure Ratio' x NA chp

We know the NA hp, but what is the 'Pressure Ratio' (PR)?

PR = (14.7 + boost) / 14.7
PR = (14.7 + 21psi) / 14.7 = 2.43

This means the blower is literally forcing an extra 1.43 atmospheres of pressure down the engine's throat, and the fuel computer is compensating by injecting the appropriate amount of fuel to maintain an air/fuel ratio of 12.5:1.

Now we have (..most) everything:

Boosted HP = PR x NA chp
Boosted HP = 2.43 x 287 chp = 697.41 chp!!!

This number assumes the fueling and ignition system is spot on, as well as the engine being mechanically sound.

Note that this does not take into account thermal losses etc, but it does give one a quick and dirty idea of how much hp to expect for applying 'x' amount of boost against 'Y' NA chp.

The reason I went through all of this is to show that you can run more than 10 psi of boost if you rebuilt your engine with a lower compression ratio, and in this case, 9.0:1. So if you do lower the cr, make sure that you take advantage of it by running as much boost as you can.

So don't make the classic mistake of lowering the compression ratio way down to 7.5:1 just to run a couple extra pounds of boost; your car will be a dog NA, and you'll be leaving a ton of hp on the table as a result.

notbob

Lagavulin, what is the max stock safe PSI for boost to run on an unopened 3.6?

Any idea what the optimal ratio would be for street? i.e. balance between lower cr & boost increase?

Lagavulin

According to my calcs (..which are really Corky Bell's calcs from his books), an 11.3:1 static compression ratio is too high to run ANY boost, even with an 85% efficient inter/after cooler.

With that said, I know there are some stock NA 993's running around with superchargers on them. However, I personally wouldn't do it unless I lowered the compression ratio with new pistons (..or machined the existing ones if the crown is thick enough).

Now that I'm thinking about it, I wonder if the factory rated compression ratio is a true static 11.3:1? For example, the 928 S4 32v engine is listed from the factory as a 10.0:1 static cr; however, the actual measured static compression ratio is 9.3:1 - 9.4:1. That 0.6 compression point makes quite a difference heat-wise in the combustion chamber; subsequently, one can run more boost safely from a detonation standpoint.

That would depend upon one's target horsepower, which in turn dictates how much boost is needed to attain that hp level. Knowing how much boost is needed, one can then determine the max compression ratio to safely run the target boost without detonation (..assuming engine management is up to snuff).

Using Corky's 1,075 max combustion temp guideline and an 85% efficient inter/after cooler we get:

11.0 static cr, safe max boost = 00.0 psi
10.5 static cr, safe max boost = 03.5 psi
10.0 static cr, safe max boost = 08.0 psi
09.5 static cr, safe max boost = 14.0 psi
09.0 static cr, safe max boost = 21.0 psi
08.5 static cr, safe max boost = 30.3 psi
08.0 static cr, safe max boost = 42.0 psi
07.0 static cr, safe max boost = 57.0 psi

Again, these numbers assume air/fuel and timing are optimized.

notbob

So from those numbers a 10.0 ~ 9.5 would be a safe combination of the two to generate good power but not be a dog NA?

Lagavulin

Yes, that is generally correct as one wants to run as high a compression ratio as possible for a given amount of boost.

With a centrifugal supercharger and it's comparitively 'soft' power delivery (..compared to a positive displacement blower, or even a turbo) one can get away with running more boost as it's not nearly as succeptable to 'tip-in' detonation at lower engine speeds.

With that said though, a positive displacement blower like a Whipple would be my blower of choice to help augment the 993's total lack of low-end torque.

notbob

How much boost can a whipple positive displacement make? Any idea on costs of one? Not to plumb it up or tune it just the base unit itself

garychios

A couple of questions. Is this a reall tt car? Or some kind of aftermarket turbo kit. Most of my friends with 96-97TT's are putting down around 500WHP (Like 550 to the crank)which propells the car into the mid 11's in the quarter mile. They did minimal work to the engine. They concnetrated on exhaust, turbos, computer and IC with pipes and intake.

I beleive a guy named eddie who made 1100 WHP on his 930 911 was on a 3.6 low compression motor too.

garychios

If your car is a c4s, sell it and get a real TT, it seems they are easier to modify boost with.