Rogue_Ant

Yeah, that does narrow it down quite a bit.
A major reason we didn't see more supercharged cars during that time-period was lack of digital engine management. Once the 80's came around, and OEMs transitioned from carbs to electronic fuel injection, plenty of supercharged cars crept up. Granted by the time OEMs got the hang of fuel injection, we were into the 90's. But plenty of supercharged examples from the 90's.

You can generally go up in impedance without (much) detriment. Going down is when it causes an issue.
The 951 & 944 injection driver is identical hardware. There might be a slight difference on the current limiting PWM control, but not enough for concern.

Butters944

This thread has perfect timing!!!

I've been dreaming about the exact same budget SC build for my 83 for weeks now... Can't wait to see how this plays out. Good luck, and I will be following your footsteps (but hopefully keeping AC) if you are successful!

odurandina

I was speaking to the extremely short lifespans of the Raptor unit's drive belts. this, despite Carl Fawcett's longstanding expertise added to painstaking efforts over a couple of years to reach 7~8 pounds of boost with the 968 variocam..... in the end, a toothed belt (double-sided?) solved the problem (to the point where the unit could finally be driven at sustained power levels).... but, even then, I don't believe anyone achieved stable boost going much past 4.5 p~5.0 psi.

that all being said, in Carl's final installment of the Raptor kit, short bursts of boost were still on the table. and the car's are able to keep the AC.....

nice car if you're not driving it hundreds of miles.... but, the one downer was the fuel consumption which plummeted from about 27 mpg to about 13~14...

V2Rocket

i wouldnt even bother with the 951 injectors, even the best of them is 23 years old now, and while they dont really wear out, why not just get a set of brand new, bigger injectors? i have 55# units to go with E85 on my SC and they were $160 barely used...

odonnell

What AC

Od, fuel consumption is kind of a big deal for me... now that you mention it.

Butters944

I'm trying to understand the fuel efficiency thing. What causes the increase in fuel consumption? I was under the impression forced induction is neutral or better for fuel consumption, guess I'm confused.

odurandina

what is causing the huge loss in fuel efficiency with belt driven supercharger as opposed to say, a turbo??

poor adiabatic efficiency; superchargers by their nature suffer from poor adiabatic efficiency due to the process of turning the impeller bearing (in the neighborhood of 40~50,000 rpm for centrifugal units!) thus, making a ton of heat, and carrying that heat directly into the supercharger unit itself, and, in turn, superheating and compressing the air... the results:

1. heating your engine and bay to near meltdown, and

2. ****ting an even hotter than normal exhaust charge out the back.....

and what made all this possible? that's right: nearly doubling the amount of gasoline passing through the nozzle when you step on the gas....

first; imagine a belt-driven centrifugal unit and all that extra gasoline–making all these added/unwanted kilowatts of heat but also the additional kinetic energy to drive belt (making even more heat due to the friction at the points of contact between the drive belt, pullies and the weight additionally applied to your pullies' bearings: the transferring of kinetic energy to spool your supercharger's bearing/impeller/vanes adds up to one big parasitic drain of your peashooter's limited power... then compressing the air very inefficiently makes even (MORE HEAT)....

but because we're talking about superheated air, that few extra psi of air only adds up to a small net % increase of molecules flowing into the engine.... and if this state of affairs wasn't mediocre enough, all this hot air makes your engine run even more hot – decreasing the overall efficiency of the supercharger even more.

but hope is not lost... roots blowers, by design, are more efficient at grabbing and compressing air, strong and efficient enough to run intercoolers, and efficient enough to make supercharging plausible for production cars.

by comparison, over the long haul, or even short time intervals, centrifugal superchargers really aren't very good (additionally, if the bearings aren't connected to a cool oil source)... but the advantages would be that they're easier to locate in the engine bay and plumb into the intake circuit, and they don't have to be fitted to replace the intake manifold...

however, if you're talking about the special class of centrifugal supercharger: exhaust driven TURBINES, you're in a new ballgame because they're piggybacking off the superheated/compressed gasses (free power) at the exhaust stage.

additionally, there's so much power available in reserve with turbos – they're able to push the superheated air (as a result of compressing it in the hot turbine) through an intercooler effortlessly.... you won't get a centrifugal supercharger to pass all that pressurized air through an intercooler without a significant pressure drop.... and while this problem also exists with your turbos – there's still plenty of exhaust power available in reserve for the tuner to compensate....


so, you kind of see why i'm not much of a fan of superchargers... but again, roots blowers do the work of compressing air efficiently enough to make running them for tens of thousands of miles, feasible.

.

V2Rocket

when youre in the driving mood related to MPG, it won't really matter if theres an SC or not. cruising throttle is so light that a simple bypass valve like the turbo cars use will suffice...

odonnell

I too have taken thermo classes... can I play? The turbo is efficient because it increases usable power from the same combustion process (far from isentropic... ~30% efficiency for most 4 stroke cycles). By increasing availability of usable work (i.e. exergy) you get more power. Basically second law of thermodynamics.

Supercharging doesn't dip into the usable mass flow work, so the overall net efficiency isn't as high, generally speaking.

With that being said, it takes higher flow injectors to accommodate the increased demand for fuel (fuel/air ratio needs to be correct, thus more air requires more fuel). If the increased demand for fuel outweighs the power gains from forced induction, you lose net efficiency.

A conceptual way to look at efficiency is what you want, divided by what it takes to get it.

MAGK944

Just reading through this thread, nothing to add but I cannot see close to 175hp with lightened crank, 951 valvetrain parts, balance shafts deleted, AC deleted cat delete, late headers and manual steering of course.

There is really nothing on that list that would get you above 150hp on a stock engine, fresh rebuilt or not. Am I wrong, missing something or just confused?

Butters944

I was thinking the same thing.

Rogue_Ant

The 944NA AFM is very well sized for the stock 944 motor...
Unfortunately, it is quite prohibitive for those wanting to increase airflow (hp).

A competitive 944NA, had a completely built engine. Higher CR, headwork, lightweight reciprocating/rotating assembly, ect. Was custom dyno-tuned and made 'only' around 145rwhp.

The 944 AFM simply acts like a restrictor-plate.

They switched to my NA-Tune (MAF), re-tuned the car (on the same dyno). The end-result was >170rwhp.
The more air you try to pull through the stock AFM, the more of a restriction it becomes.

odurandina

recalcalibrating:

Spencer + 3.0 968 engine + bigger Roots + Rogue = ?

.

V2Rocket

Dude.

odonnell

I was going off the 163hp rating. Is that optimistic? I figured about 5hp back from AC delete, 6hp back from BS delete, and the cam grind/springs/cat delete would file in a couple more at least. The lightened crank is probably good for at least 5hp, but I really don't know because I haven't had it dynoed.