Interesting data, concerning hp.

[Wolf1]

Heres some interesting data and such. Credit for info goes to ESC944 on 924board. Just passing it along.......

1 psi boost = .5 point CR (effective CR)

1 point CR = 2 psi boost (CYLINDER PRESSURE)
1 point CR = 2% HP

1 psi boost requires 1-1.5 octane (minimum)
1 psi boost = 6.8% HP max (1 ÷ 14.7 = 6.8%)

1 point CR = 3 - 5 octane
1 AF ratio = 2 octane
1° advance = 1/2 - 3/4 octane point
10° engine coolant (160° -180° range) = 1 octane
20° ambient = 1 octane
1 can NOS Octane Boost = 1.5 - 3.6 octane (see table)
1000' altitude = -1 octane point
1000' altitude = .5 psi (2" Hg)
6°F temp change = 1% air density
30% humidity = 1 octane
10° air charge temperature = 1% HP
20° charge temp reduction through intercooling = .5 psi additional boost with same octane
3/4 PSI DROP = 5% pressure (5% x 14.7 = .75 psi)
ENGINE CFM = CID x RPM x VE*
2 x 1728
*70% for the average engine 90% racing engines
10% HP increase = 7% AF ratio (based on 70% VE) or 10% AF ratio with 100% VE
10 psi fuel pressure = 8% AF ratio: 5 psi = 4% AF ratio

HP = CFM (int @ 28") x .257 x no. cylinders

10HP = .1 sec / 1 mph 1/4 mile
100 lbs = .1 sec / 1 mph 1/4 mile

[Wolf1]

14.7 psi of boost at sea level.

At 5000' in Denver, Colorado, this "boost" would be only 12.2 psi, as the atmospheres height at 5000' is only 99 miles high vs 100 miles at sea level.

A non supercharged engine will perform better at sea level where it has more boost (14.7 vs 12.2).

Without this atmospheric weight or "boost," the engine would not run.

Why? Because an engine creates a vacuum as it rotates thereby allowing the higher pressure atmosphere to rush in and fill the vacuum with 14.7 psi of boost.

Note: This boost does not show up on a typical 0-20 psi / 0-30"
Hg gauge as 14.7 but instead as "0" on the 0-30" Hg scale.

Now imagine on an ascending scale that the 30" Hg is 0 psi and the 0" Hg is 14.7 psi and you have an "absolute" pressure gauge.

Add the 20 psi to the 14.7 psi and the gauge markings would be 0-34.7. Floor the throttle at sea level and the gauge will read 14.7 psi and 12.2 psi at 5000' in Denver (with "0" inlet restriction).

With a supercharger and 6 psi boost, your new absolute pressure gauge will read 20.7 (14.7+6=20.7) at seal level and 18.2 (20.7-2.5=18.2) in Denver.

Here's how you determine if your engine is utilizing all of "natural boost."

If the gauge reads 4" of vacuum at wide open throttle, your engine is losing 13.6% of it's power or 2 psi of "natural boost." This is a simple equation that few really understand. 2 psi lost÷14.7=13.6% HP loss.

To eliminate the pressure losses, install larger non restrictive inlet components (filter, MAF meter, throttle body and/or inlet manifold). The
2 psi loss with a 6 psi kit (18.7 psi absolute) is 2÷18.7=10.7% HP loss.

This is a big number as 10.7% of the supercharged HP is 32 whopping HP on a 300HP engine. Whether fuel injected or carbureted, ALWAYS try for a "0" reading.

Now lets see... how do I remove the effects of pressure loss... well in an NA I could use simple sources of boost or pressure to equalize the loss, in other words, either you improve what you got or you supplement it.

Now we are not talking about runner length and plenum design for your intake, that is a long subject to cover, but basically the runners and plenum effect your power curve, meaning your engine can run out of air at higher RPMs or breath fine at those levels, but suffer at lower rpms. Torque, HP, Power band...

[Wolf1]

1 psi of boost is approximately 13RWHP.

A good 300HPengine is pumping out or sucking in approx. 450 cfm at 6000 rpm.

So if the supercharger pumps 850 cfm into the 450 cfm engine, the result will be a back pressure build up (boost). The more air the supercharger pumps in, the higher the boost and theoretically, the higher the HP.

Now this also explains why when adding say a larger Carb, you do see some power increases, the engine can breath better and the carb is no longer as much of a restirction and it is capable of suppling adequate fuel with that increased CFM.

[Wolf1]

More information, Credit to Kenne Bell:

Basically, this indicates how efficiently the supercharger utilizes the energy (HP) that "drives" the supercharger. A high adiabatic
efficiency means the supercharger consumes less energy (HP) from the engine to drive it, thereby leaving more engine power for
acceleration - and higher dyno numbers.

A low adiabatic efficiency means the engine must "waste" more of it's HP output to drive the supercharger. This is a very important consideration because the higher the parasitic loss/power consumption of the supercharger, the more HPyour engine must "waste" to drive it. This clearly shows up on a Dynojet when 2 superchargers are tested on the same engine with equal boost. Two superchargers may have comparable volumetric efficiency (VE) but one may have lower power consumption
(higher adiabatic efficiency).

Temperature efficiency is the difference between the temperature entering the supercharger as compared to the boosted discharge air
temperature. Superchargers with lower air temperatures are more "temperature efficient" than those with hotter discharge
temperatures.

Be aware that a temperature reduction AFTER the supercharger will not make more HP - as many experts would falsely lead you to believe. It's impossible.

Look at it this way. Your supercharger discharges 90 cfm of air in one revolution. Now let's trap all that air in a balloon. The oxygen in the balloon will always weigh the same regardless of how much it is cooled. Then how can this trapped airmagically acquire more molecules?

It cannot. So always remember - cooling air after the supercharger cannot make more power and therfore, intercoolers do not increase HP!!!

When will everyone out there get this right? The cooler air will, however, allow the engine to run more ignition advance and/or more boost on a given octane. Obviously, cooler air the supercharger increases power at the rate of 1% for ever 10ş.

Now that you understand the 3 basics of supercharging, let's assume that two superchargers have identical 1. volumetric efficiency
2. power consumption/parasitic HP loss and 3. discharge temp at 6000 engine rpm.

Further assume these superchargers take turns blowing air into an engine at 6000 rpm. Since an engine has no eyes or fingers, it cannot possibly distinguish between the 2 superchargers.

The engine only sees air flow/cfm and boost. Subsequently, the superchargers will both produce the same HP.

Why?Again, the 1. power consumption 2. cfm/boost and 3. temp are identical.

There is no "4th dimension" or unknown source of power that will make one supercharger produce more power than another. Let's summarize: If 2 unrestricted inlet superchargers, regardless of type or manufacturer, have the same VE, power consumption and discharge temp, then both will produce exactly the same peak HP. If more power is required, then raise the boost level. Boost is what makes the power. That's what superchargers do.

They make boost. However, it is very important to understand that they don't allmake the same boost at 2000, 3000, 4000 and 5000
- so they cannot possibly produce the same HP at 2000, 3000, 4000 and 5000.

For example: Centrifugal superchargers are also
very efficient, but where they produce the most cfm and boost. At lower engine speeds, the centrifugal produces proportionately less air and, therefore, less boost (engine back pressure) and power.

The Twin Screw produces approx. the same boost at any engine rpm. Every revolution of the Twin Screw delivers the same cfm or
liters per revolution i.e. 2L per revolution at 2000-6000.

That's why these superchargers are referred to as "positive displacement."

Unlike a positive displacement supercharger, the centrifugal might produce 1/2 L@1000 engine rpm, 2/3L@2000, 1L@3000, 1-
1/3L@4000, 1-2/3L@5000 and finally 2L@6000. That's because the centrifugal relies on engine speed to generate exponentially
higher cfm/L - and boost.

Boost is approx. 1 psi per 1000 rpm on a 6 psi centrifugal kit whereas the boost may be 6 psi at 2000-6000 on a positive displacement type.

The centrifugal boost curve is not exactly linear as indicated above, but it's close enough for discussion.

For example: Here's an actual test. 11 psi at 6000, 7 psi@5000, 4.5@4000, 2.5@3000.

What's important to remember is that the Twin Screws or roots pump out
approx. the same displacement or cfm per revolution at any engine speed.

Supercharger efficiency (actual displacement or cfm discharged) will depend on the overall design of the supercharger itself.

Besides VE, temperature efficiency and power consumption, you should obviously also consider a superchargers construction and it's reputation for performance, reliability and longevity.

Equally important is the manufacturers reputation for tech support and service.

For those of you interested in additional information and technical discussions, there are numerous SAE (Society of Automotive Engineers) papers written on supercharging.

[Wolf1]

Does the supercharger decrease my fuel mileage? Not under normal driving conditions. However, if you use the boost it will, of course, use more fuel. You can't make power without fuel.

What kind of performance increase can I expect A Supercharger Kit? Approximately 30-50%.

Why is a supercharger better than other performance modifications? A Supercharger is by far the best performance value of all. You can spend more money on exhaust, headers, cool air, chip, cam etc. and not get anywhere near the performance you would from the supercharger kit. You also can benefit from a lot of these in conjunction with your supercharger. A Supercharger is a power adder. So with the exception of higher compression, you can benefit from other performance modifications, especially anything that increases volumetric efficiency of your engine.

What parameters determine how much boost my engine can run on 92 octane?.
COMPRESSION RATIO - Lowering the compression ratio allows the engine to run more boost with the same octane.
For example, 8.0:1 vs 9.5:1 can tolerate another 3 psi. 3 psi gives 38HP. The 1.5 lower CR loses 3% (2% per point) or around 9HP in a 300HP engine. That nets 29HP (38-9=29). Now you know why OEM's use low compression ratios on supercharged engines.

AIR CHARGE TEMP - The supercharger itself determines how much boost the engine will accept. Some superchargers can require an intercooler. Others, don't need intercoolers because the discharge air temp is cooler.
1 psi boost increases supercharger air discharge temperature 10°-20°, depending on supercharger type. A 6 psi "rated" kit usually requires 8 psi "total" boost (+2 psi to overcome inlet and outlet losses). Consider two 8 psi superchargers: 8x10=80°, 8x20°=160°. The 160° supercharger should use an intercooler. An air to water intercooler will lower that 160° to around 80° (160°-80°=80°) or the 80° temperature of a more efficient supercharger without an intercooler.

BOOST - Both higher compression ratios and boost increase cylinder pressure and demand higher octane fuels to eliminate detonation. Fuel octane determines the amount of boost an engine will accept. One octane will support one psi of boost.
Example: If fuel octane is 94 or you add a can of NOS Octane Booster you can typically run 8 psi.

IGNITION TIMING - Retarding ignition timing will allow the engine to run more boost but not without a penalty. 4° retard=16HP.

AIR FUEL RATIO - You can't make horsepower without fuel. Richer air fuel ratios reduce power but permit higher boost levels on a given fuel octane. Leaner ratios make more power but need lower boost so a lean mixture with high octane makes the most HP. Supercharger kit tuning demands ignition timing, air fuel ratio, fuel octane, boost, compression ratio, discharge temperature and supercharger parasitic losses all be carefully considered in the design.
What causes detonation? 1. Lean fuel mixture, 2. low octane fuel, 3. excess boost, 4. lack of fuel, 5. advanced ignition timing, 6. vacuum leak, 7. "hot" spark plugs, 8. overheated engine, 9. excessive inlet temperature (underhood filter).

What is the most ridiculous aftermarket product?
Hands down, it's the "little intake fans" and the "throttle body spacers" with the bore grooves.

Right there on the list has to be those "underhood exposed filters" that suck in 200° underhood air instead of ambient (70°-100° air from the fenderwell).

10° air temp rise is -1% HP.

That's how dynos are calibrated for temperature changes. Does anyone really believe that all those OEM manufacturers designed expensive plastic inlet systems - for the last 20 years - that pull cool air from the fenderwell, hood or grill because they didn't have anything else to spend their money on?

Then there's those "factory calibrated" mass air meters that do not use a chip or re-calibrated processor with oversize injectors. They unwittingly reduce the voltage signal to the mass air meter to compensate for the larger injectors at WOT but they neglect all the driveability issues. The result is over advancing timing, lean mixtures, detonation, surging, bucking etc.

So if converting to a MAF, the idea is that it is sending a signal your ECU can read and use at all temps and RPM range. AFM can be tricked as can the stock ECU on the 944. With a 924, well CIS is a different animal, but principals are the same.

Why is knock protection so important? Knock (detonation) can destroy your engine if it is not negated. Suppose your wife or a friend unknowingly fills your tank with 87, or you get a "bad batch" of gas, the engine overheats, someone installs a pulley that raises supercharger boost, the wrong spark plug heat range is installed (too hot) or a vacuum leak develops and the engine leans out. That's when an engine needs knock protection.

[Wolf1]

At what point is intercooling practical? If the air discharged by the supercharger is too hot, the intercooler lowers the air temperature so the engine can accept that boost level without detonation. If the supercharged air is cool enough for 6 psi boost, then intercooling is not necessary. Intercooling is more practical with the less efficient Roots type superchargers at any boost level, as they heat the air more than a Twin Screw or Centrifugal. Therefore, the Centrifugals and Twin Screws, being more efficient, do not require intercooling in the 6-8 psi range, nor is it beneficial. However, for 12-16 psi racing applications with race fuels, a case can be made for intercooling.

[Wolf1]

Please keep in mind the EATON is a hybrid or improved design roots style supercharger.

It isnt a twin screw, but it isnt just straight rotors... so you have a higher effeciency roots, but it is still a roots.

Now for boost over say 18 PSI, you need to look to a twin screw up to about 25 PSI.

Beyond 25 PSI, well that is the realm of the turbo and centrifugal supercharger.

Turbos can be setup and run at any RPM and therotically are only limited by the amount of CFM they can supply the engine... so if sized right you could build insane levels of boost... 40, 50, 100 PSI... but not on our engines.

Now if you want old school high boost, well look at the 3-71, 6-71 and 8-71 "blowers" These blowers suck in the effeciency range and eat a lot of HP to produce HP. But they can easily hit 40 PSI.

Again all superchargers take HP to MAKE HP.

A turbo doesnt require hp and can supplier larger levels of BOOST, but it also generates an insane amount of HEAT.

A turbo in my opinion is the easiest to install and at the same basic levels of boost of most supercharges (5-7 psi) you would see more power from the turbo, since you have no loss of hp to drive the supercharger, but you do suffer lag.

Above 7 PSI and the heat gets ugly, even at 5 PSI, intercooling can be critical to substained performance on small displacement engines.

Now remember engines like the 924 have junk for volumetric effeciency, the head design sucks. You can still make power... but well the best thing would be to use a different head, or otherwise increase volumetric effeciency and use the supercharger to compensate at low RPM for a lack of good low end torque.

In other words if you heavily modify the 924 stock head, you tend to loose low end torque, but superchargers make loads of low end torque... so...

[fast924S]

WOW some crazy info there, I Dont agree that a MAF upgrade is bad but to each his own

[Wolf1]

Originally posted by fast924S
I Dont agree that a MAF upgrade is bad but to each his own

I think what they are referring to is the aftermarket MAF sensor housings for Mustangs/Camaros etc.
These MAF housings use your original sensor and by reducing the airflow across the sensor wires tricks your EFI system into shortening the injector pulse width to compensate for the larger injectors. They work great at wide open throttle, but driveability suffers in the "normal" range.

[auh]

My head hurts now...

[fast924S]

Ahh I get it now

[pokey]

My head is about to explode...

[DannoXYZ]

Boost and power are not linearly related. At higher boost-levels, say 20psi+, you get a lot more power increase for each additional PSI. Going from 15 to 20psi may get you +60-70hp, but going from 20psi to 25psi will get you an additional +100hp.

"Be aware that a temperature reduction AFTER the supercharger will not make more HP - as many experts would falsely lead you to believe. It's impossible. "

That's correct. What happens is that the CFM flowed remains constant even with a temperature drop. But that air will be at a lower pressure now with intercooling. So if you're running 10psi without intercooler, that same CFM will be down to 7psi now with intercooling for the same volume flowed. So you won't make more power, but will make the same power at a lower boost-level. Then... you can crank the boost back up to 10psi and flow more CFM for more power.

[ERAU944]

LOL the 924 head is junk??? tell that to porsche - they only designed the thing! (albeit not for forced induction, that's what the 931 head was for)

[fast924S]

Well besides the AFM in the 924S and 944 the head is the biggest restriction.

[DannoXYZ]

The head on my 2.3L 190E has ports that are significantly larger than the one my 951... which needs to flow twice as much air... hmm....

[fast924S]

Yea put port sizes and air speeds are related, it all depends on how the intake is setup, A smaller port could flow more air if tuned for it. Also depends on what RPM you want your power curve to be