LaughaC

I've recently read that CIS manages fuel well up to 7 lbs of boost, and this makes me feel more comfortable with the idea of adding a centrifugal supercharger to my CIS engine.

Another article said intercoolers are not a worthwhile investment for such low pressure applications, so the first stage of this project is looking quite manageable.

Has anyone seen a chart depicting how much torque and hp increase can be gained from 5 lbs of boost without an intercooler?

Gary R.

Are you talking about a supercharger or a turbocharger?

84_Carrera

It'll depend on the engine specifically. On my 3.2L SHO engine, I was making 15psi of boost on my Vortech V1 S-Trim without a cooler at 9.0:1 CR, no detonation. When I built them for customers, I specified 6-9psi max at the stock 9.8:1 CR; after 10psi you started to worry about detonating.

As for the 911, being air-cooled, I'd imagine you'd want a cooler on there, but I don't have practical p-car specific experience in this arena.

LaughaC

Thanks, that's good info, Fred. My 911 has 8.5/1 compression.

Gary, I'm considering the simplicity of supercharger for better torque in daily driving.

Gary R.

My confusion is most likely my ignorance. I though you set a variable boost on (exhaust) Turbochargers and that Superchargers were pre-set for (intake) pressure at one given point. From what I had read in the past, a supercharger was much better as a bolt-on to a stock mid-to-high compression motor as it ups HP and Torque throughout the entire RPM range while a (single) Turbocharger "spools" up at higher RPMs and works best on low compression engines at higher boost pressures (which you seem to have). Looks like you can go either way, I would always go with a Supercharger myself.

DGaunt

You don't want to go higher than an effective CR of 9.5:1 on a single plug 911 engine due to the unfortunate locationof the spark plug. That probably means a physical CR of 8.5:1 at the boost you are looking at.

84_Carrera

Gary, a turbo is an exhaust-powered centrifugal supercharger, essentially, rather than a belt-driven centrifugal supercharger. This is nothing at all like having a positive-displacement blower.

On my old setup, I had an HKS blow-off valve after the blower, before the throttle body / intake. Ideally the MAF would have been before the inlet for the blower, but in my case it was after. The pulley diameter on the charger determines how fast it'll spin in relation to the engine RPMs. From memory, at the SHO's 6" crank pulley, the Vortech pulley I had was spinning about 45,000 rpms in order to make the 15psi of boost at engine redline of ~7200 rpms (I saw over 7400 on occasion... ). Vortech has a nice online calculator to determine the boost level & impeller RPMs for whatever your crank pulley & charger pullies are:
http://www.vortechsuperchargers.com/...pspeedv-1.html

My BOV was set to release intake pressure once I let off the gas (Vacuum seen at Fuel Pressure Regulator), not as a pre-set X-PSI of manifold pressure. I replaced the injectors (24 lb-hr stock) with 48 lb-hr Accel units to provide enough fuel up high, and the pump went from 112lph to 240lph. The chip overrode the factory programming to adjust the fuel injector pulse duration accordingly unless under boost.

A Turbo uses a wastegate also, sometimes instead of, a blow-off valve.

Gary R.

Thanks Fred, I learn more here each day...

84_Carrera

Here's a basic synopsis:

http://auto.howstuffworks.com/turbo.htm

One thing to keep in mind, centrifugal chargers are progressive, with low boost at low rpms, building manifold pressure up to (ideally) max. engine rpms / impeller rpms. I originally started with the same "15psi" pulley (either 3.25 or 3.125", I forget now) at 9psi, but had an intentionally-small K&N filter with an inlet reducer to restrict total airflow into the charger, which effectively limited boost from ~5000 rpms through redline at 9psi of boost. Inefficient way of doing it, but it was also a cheap way to do it.

Rather than reverse-engineer a charger setup for my 911, I'll probably look into picking up a later-80's 930 instead someday.

LaughaC

To minimize risk I'm only comfortable trying a centrifugal supercharger right now. A turbo would be too permanent, and I want to stay very conservative with the boost settings while using the '77 CIS fuel delivery system.