Help with Supercharging my 3.8

[Porsche964FP]

Hi Gang,

Great forum, I'm a mostly a silent reader here. I'm embarking on a project and I'd appreciate your collective wisdoms...

I'm in the process of building a supercharged engine for my narrow body 964 C4.

The car has 3.8 Mahle barrels, forged rods, ARP hardware, 993rs spec'd heads and valves (IIRC it's deep within my notes), titanium springs, retainers etc...

The supercharger I'm using is a Rotrex centrifugal supercharger, it'll be a C38 capable of up to 700HP,

Max flow rate: 0.63 kg/s
Max pressure ratio: 2.94

Peak input shaft speed: 12,000 rpm
Peak impeller speed: 90,000 rpm

Min inlet oil temperature: - 40°C
Max inlet oil temperature: +80°C

I'm planning on using aquamist to keep the system cool. I currently have Motec M84 management in the car. We're aiming to achieve a minimum of 500HP.

My engine builder will be rebuilding the engine to suit (currently he's putting together spec options) and so I would like some input on your suggestions on compression ratio, valve sizes etc also...

The car is used for track and occasional hard road use. Comfort and ease of drivability are of little concern - as it's a 90% a track car. However I don't wish to be waiting for boost all the time and suffer with little toque - looking for linear delivery/ dyno plot. Rotrex chargers are known for negligible lag and lots of torque.

To give an idea, a standard 3.6 964 engine with a Rotrex and Aquamist will reliably deliver 380HP.

Any help would be much appreciated.

Kind regards,
Frank

[Porsche964FP]

Here is the flow map for the charger I'm looking at...

[Tippy]

You have airflow capabilities to over 800hp, but parasitic drag starts to diminish these returns being a centrifugal SC.

I've used a couple centrifugals, and you can have nearly instant boost like a roots if you run an aggressive pulley ratio and tune a BOV to dump excess boost.

You just have to calculate you don't overrev the impeller speed. Very easy calculation. Take pulley ratio between crank to SC pulley and multiply times RPM of engine and transmission ratio internally of SC.

[Das Schnell Ei]

a half turbo!

[Porsche964FP]

Thanks for the info.

My main concern at the moment are my heads and cams - they're designed for the previous iteration of the engine and may not suit boosting...

Standard 964:
Intake port entrance diameter = 41.5mm
Intake Valve diameter = 49.0mm
Exhaust valve diameter = 42.5mm
Valve stem diameter = 9.0mm

My current heads:
Intake port entrance = 45mm 1mm less than 993rs Spec.
Intake valve diameter = 51.5mm 993rs Spec.
Exhaust valve diameter = 43.5mm 0.5mm more than 993rs Spec.
Valve stem diameter = 8mm 993rs Spec.

So what are your thoughts on heads and cams for this kind of forced induction?

I have some more question also. What is the standard compression of the 993 Turbo? What are head specs for the 993 turbos?

Originally Posted by Tippy

You have airflow capabilities to over 800hp, but parasitic drag starts to diminish these returns being a centrifugal SC.

I've used a couple centrifugals, and you can have nearly instant boost like a roots if you run an aggressive pulley ratio and tune a BOV to dump excess boost.

You just have to calculate you don't overrev the impeller speed. Very easy calculation. Take pulley ratio between crank to SC pulley and multiply times RPM of engine and transmission ratio internally of SC.

Cheers for that info re impeller speed. Great.

Originally Posted by Das Schnell Ei

a half turbo!

[Das Schnell Ei]

those heads will be fine for sure.... more power at a lower boost level

What cams are you going to run?

[Porsche964FP]

Originally Posted by Das Schnell Ei

those heads will be fine for sure.... more power at a lower boost level

What cams are you going to run?

Can you kindly explain your thought process behind the heads being good for this application?

I was planning on lowering the piston compression, but if you're saying that a low boost level based on current heads then perhaps I shouldn't?

Worth mentioning that the heads can be changed relatively easily to a better suited set if needed.

In terms of cams, I just don't know. As with most builders he's reluctant to tell me the specs I currently have, they're not all out sprint cams however.

[Das Schnell Ei]

All you have listed about the heads is the runner sizes, and the runner sizes are fine for FI...one would need to know the chamber size in CC to know anything about compression ratio

I was saying that larger runners will flow more air at a lower pressure ... so you can make more power at the same psi ..more cfm

lower compression with more boost is a better choice

What pistons are in the engine ?

[Tippy]

If you run low compression with high boost, this thing will make stunning hp I'd imagine. The good thing is, is the blower is so big, your parasitic drag will be pretty low.

I'd imagine this thing will make 600-700hp with 12-15psi of boost.

If I'm off base, someone call me out.

[Das Schnell Ei]

I think that blower will peter out at a little over 500rwhp

flows a little over a 1000cfm

[Porsche964FP]

Originally Posted by Das Schnell Ei

All you have listed about the heads is the runner sizes, and the runner sizes are fine for FI...one would need to know the chamber size in CC to know anything about compression ratio

I was saying that larger runners will flow more air at a lower pressure ... so you can make more power at the same psi ..more cfm

lower compression with more boost is a better choice

What pistons are in the engine ?

Apologies I thought I had mentioned my compression.
Currently I have set of 11. but for this setup I was planning on going lower. My fear is losing bottom end torque and having lag. However I guess that with superchargers and specifically this one that wouldn't be a problem... Your thoughts would be great...

So was thinking of possibly chucking in a set of 3.8 8. or 9. compression pistons.

Originally Posted by Tippy

If you run low compression with high boost, this thing will make stunning hp I'd imagine. The good thing is, is the blower is so big, your parasitic drag will be pretty low.



I'd imagine this thing will make 600-700hp with 12-15psi of boost.



If I'm off base, someone call me out.

Forgive the ignorance, 'parasitic drag' is about the lag?




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[Tippy]

Originally Posted by Das Schnell Ei

I think that blower will peter out at a little over 500rwhp

flows a little over a 1000cfm

Rotrex claims 720hp.

The airflow is good for over 800hp, but I assume the parasitic drag is what is the big limiter.

[Tippy]

Parasitic drag is the loss of engine power soaked up by turning the supercharger.

[Porsche964FP]

Originally Posted by Tippy

Rotrex claims 720hp.



The airflow is good for over 800hp, but I assume the parasitic drag is what is the big limiter.

I think he was talking about wheel horse power and not flywheel?

I assume Rotrex is talking about 720 flywheel...


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[Tippy]

Yes, they're referring to 720 flywheel. The compressor flows enough for over 800, but the parasitic drag pulls this down.

[Porsche964FP]

So what are thoughts on piston compression? I'm thinking around 8.

I've read about many 8. compression 1 bar turbo builds being streetable, which leads me to think that actually with this kind of minute lag charger I can get away with the same...

[Tippy]

7.5-8 seems to be the norm.

[copbait73]

I'm guessing the motor will struggle to make 500HP. If the size of a compressor made HP just bolt on two and make 1200HP-not.

This compressor is at least 3 points under a good Garrett GT plus the TURBOs turbine recovers some energy to drive it. SUPERs are great for linear airflow delivery, ( although a centrifugal SUPER less so) therefore predictable power delivery ...not superior overall HP. Here's a baseline, a very healthy, done right 3.6L based TURBO may make 600HP. A SUPER one will be less.

Your build considerations start like this;
What fuel do I intend burning?
Then what power do I want/need?
What boost level/ airflow is required?
What does this suggest my compression ratio needs to be? Get wrapped around the axle regarding dynamic vs fix if you like. In the end Octane (fuel) and C.R. are your bookends. No wrong - $s, octane then C.R. $s is your limit to buy broken parts.

[Tippy]

So a ported headed, bigger-valved, 3.8 with special cams will make only 475?

I'd burn that thing to the ground!

He has a serious blower with a serious engine.

What do you mean by 3 points less than a Garrett? Don't follow. GT what? 35, 40, 42, 55?

0.63kg/s is serious airflow!

[Porsche964FP]

Here's an example. A very good friend/ rennlister runs a standard 3.6 964 engine with a Rotrex supercharger - the car runs 380hp. I'll get the dyno plot shortly.

Having spent time around his car in the alps and most recently on track I can attest to it's pace.

I'm running hardware that allows me to run up to 8,000 revs - I'm not talking about simply bolting on a big charger to a small engine. I will be redesigning the compression, possibly heads and cams to suit. The charger was recommended to me by Rotrex on the basis of the engine I'm building.

I can't see why my proposed build would struggle to 500hp either?