Atomic

iTrader: (42)

Its actually not an easy question...but since we are talking about prochargers and turbos, it simplifies it somewhat because the compressor sides are very very similar. Versus say a roots blower and a turbo which are worlds apart.

As Jake alluded to, its not really about pressure, but the mass of air you are moving. The more mass of oxygen in the cylinder the more mass of fuel you can put it and the more power you can make. Plain and simple. However, volumetric efficiency goes up with more boost. The engine becomes more efficient because the cylinders will be filled more readily with a higher pressure difference across the valve. So for max power you want max boost and max air mass..

That sounds great, but with a turbo the higher the boost pressure on the compressor side, the more pressure you have on the turbine side, which the engine produces on the exhaust stroke. This is why excessive backpressure costs a lot of power; you basically turned the exhaust side into a big air compressor for the turbo. However, high backpressure setups spool very well (think factory turbo cars), and the less backpressure the slower spool. To reduce backpressure you want a huge turbine on the turbo...most pure race setups have seemingly disproportionately large turbos. For instance, twin 80s on a 350ci engine. Great full race power, but forever lag, which is not of concern because this is a race-only setup. Obviously, most street vehicles compromise full race power for spool benefits. Also note that closer the boost to backpressure ratio is to 1:1 the more the engine behaves like a NA engine.

With a big procharger, you have a similar problem, the power needed to turn that big compressor comes directly from the crank. Instead of the engine powering the compressor through the exhaust stroke like a turbo, it powers it through the power stroke to do work on the procharger. You can see this graphically if you are familiar with engine cycle charts (otto cycle in this case), specifically the P-V diagram. With a turbo, the exhaust phases will be raised, and with a supercharger the power line will be lowered (keep in mind I mean from the ideal, with either it will be higher than the engine NA). The important thing is the difference between strokes. Basically, there is no free lunch to make extra power. The reason you get a boost is because the benefits outweigh the costs, ie, if a blower costs 100hp to turn and it nets you a 600hp increase, its a pretty good trade.

Then there is the subject of compressible fluids, which is why big compressors move a lot more mass of air for a given pressure. Lets assume the compressor isentropic efficencies are the same (say 70%) between two different size compressor. With the larger one, the output from that compressor is going to be the same temperature and pressure as the smaller one, however, the mass of air (and therefor oxygen) coming out of it is going to be greater than the smaller one. As you can imagine, the bigger compressor is going to require more power to turn, so the difference between out of boost and in boost will be quite dramatic with either super or turbo. Which is why big turbo cars, say a supra for instance, cant get traction until 4th gear at 120+ and are a dog out of boost.


As for your original question of would an 88mm turbo make more power than a D1, I would say almost certainly. An 88mm and an F1 would be fairly close I think.

smokeshow

iTrader: (16)

It is absolutely because of less effort. Let me explain it this way. I'm sure you've heard the old saying, 1/3 of the fuel energy goes to turning the crankshaft, 1/3 goes out through the radiator, 1/3 out through the exhaust. Just some rough numbers. Turbos make use of the heat in the exhaust, which is wasted anyway... Superchargers draw off of the power on your crankshaft that you need. Where would you like to take the power hit? Pulling 10% off of the crank, or 10% out of the exhaust energy, only a fraction of which robs power via backpressure?

Atomic

iTrader: (42)

Are you calling "big gains" 150-200hp? If so, thats not big at all, for these engines anyway. In theory 15psi of boost will double NA power...turns out to be more like 70-80%, so if you engine makes 400rwhp NA it will make 720rwhp at 15psi.

idahoblkss

iTrader: (9)

i realize turbos are more efficient and draw less power then a supercharger but im not buying thats where and extra 200hp is coming from.maybe 50hp??

lol .no not just 150-200hp from a base but and additional 150-200 hp on top of what im making now. the debate was that by adding a 88mm turbo to my set up i would go from 700-750hp to 900+hp , and going from say 750hp to 900hp is a big gain.

smokeshow

iTrader: (16)

Jarrett was showing 1100hp at the track with his 370 and PT88 on 20psi. That gain you spoke of is a big one...big not impossible in the least. Hell, foose04's latest track passes are showing at least 750hp, and that's a 4.8 on like 16psi...

Atomic

iTrader: (42)

Since procharger does not make or publish compressor maps, it will be impossible to bench race it, but I am pretty certain an 88 will make a lot more power than a D1.

Jake99

iTrader: (5)

There are so many variables you just can't put a number on it, just like a big wheel turbo makes less back pressure a huge head with 2in primary headers with open exhaust will do the same for a procharger setup, I don't dout you could make the same power with a f1, big heads/header, ect.. as a 88mm turbo with a nice ex wheel, but the average d1 stock head, average header setup won't compete with a 88.

kbracing96

iTrader: (26)

To give you a little comparison to you truck, my truck is pretty much the same thing, xcab 4x4, a stock 10.25:1 comp short block (ly6 with GM flattops) with some hand ported 317 heads. Isky Triple 12 cam. I have my turbo kit with a billet 75/75mm BW turbo with race cover and meth injection with a M15 nozzle spraying 50/50 Meth and -20* WWF.

I ran it this weekend, on 14.5ish psi and 14.5* of timing with about 95oct fuel and made a half a dozen pass's at 11.03 to 11.3, 122-124mph. Pretty much the same HP showing as you only with about 5 or so less psi of boost. Yours needs to make 5lbs more psi of boost to make enough power to spin the blower and propel the truck at the same time.

idahoblkss

iTrader: (9)

that is a good comparison ,and so far this has been a good thread discussion .pm sent.

Vortec350ss

iTrader: (20)

I really have nothing good to add here... but I find this stuff fascinating.

I wonder how these comparisons would look at lower boost levels where the supercharger is more efficient. At the same boost level with a good exhaust it seems that the blower actually pushes more air through the engine, its just the added drag on the belt is greater than the opportunity cost of moving less air through the engine with added back pressure from a turbo (and lets face it, the exhaust design itself looks like it wont flow well... stock manifold on the driver side and a log on the passenger?? My 1 7/8" ARH seem like they would outflow this by a ton).

At say 10 PSI where the average Joe tends to hang out, and where a blower can be much more efficient I wonder how that game changes. Take a 6.0 with ideal mods for both (cams spec'd correctly, best exhaust for both situations, free flowing intakes, and heads), which do you go with? For arguments sake lets set the ceiling at 10 PSI, then a simple increase in boost is out of the equation.