just wondering, because pettit claims 307 hp, but what would that number be like at 7,500 feet where i am at?

and how much do turbos and s/c's differ as to which one performs better in a given altitude?

thanks.

 

Hardly an expert, but theoretically, from what I know, forced induction creates it's own environmental conditions, namely the air density. I'd suspect that bringing it up to boost is slower, although probably faintly, but once at full boost, it is still regulated by the same mechanisms, regardless of how much extra work it has getting to that boost level.

 

turbos just spin faster to get to the desired psi while superchargers can only spin so fast. from what i've been told so turbos should be way better

 

A SC has a fixed gear ratio. So if you have a 9 psi wheel, at sea level you'll get 9 psi @10,000 rpm (or whatever the number might be). In the mountains, you might get 3 psi @10,000 rpm since the air is thinner.

On a turbo, you set boost based on pressure at the wastegate solenoid/boost controller. So if you set it to 9 psi, it will boost till it hits 9 psi then open up the waste gate. This means you get 9 psi at sea level, and you'll get 9 psi in the mountains too.

Turbo FTW.

 

^ yes....

 

ah no. with either you still get 9 psi of boosted pressure. you are just starting at a lower base number. both will make less final hp at the wheels because both are starting at a different pressure at altitude than at sea level.

 

Ah right, forgot about that. Boost is still 9lbs, which is to say, +9lbs, just that is 9lbs of boost on top of the ambient air density, which is different.

Does that mean a turbo will actually be reading higher boost on the gauge in high altitudes, because it needs to build up a bit more boost to push open the waste gate? I imagine that the waste gate isn't set on additional boost, as that would require a measure of current ambient to get the difference, but rather an overall pressure, which would be affected by the thin air? Internal engine air volumes and intake pressure would remain the same for turbos, just gauge reading it differently?

Or do I have this all wrong?

 

so the s/c will only get 9 psia, but the turbo will get 9 psig?

is the 300 hp posted on pettit's website WHP or BHP?

 

Why were the turbochargers developed? http://en.wikipedia.org/wiki/Turbocharger#History
It is very synthetic but it should help here

 

i read that already, and its also the same reason superchargers were invented.

 

Yes, i was going to edit my post with this: http://en.wikipedia.org/wiki/Superch...titude_effects

We will have to wait for somebody else to chime in!

 

Exactly right. They'll both develop their rated psi of boost, but will be starting with less dense (less oxygen rich) air therefore less whp. You can't fight the laws of nature/physics.

At altitude nitrous is a better way to go imho.

 

i just read rotarygods thread. it all makes sense now. i got it. gonna go with turbo if i keep this car.

 

Think of it in terms of PPO2 and it will make more sense.... power is based on the PPO2 the engine can breathe in and out... period.

If the PPO2 is 2% lower at altitude; but the compressor still compresses it to 8 PSI; what is the PPO2 at 8 PSIG? (Hint the pressure gauges are 1 ATA).

 

I read this when it was first posted and had a question. Initially, I didn't care as it doesn't affect me, but at second pass, I'd rather learn something. Now there are multiple types of superchargers. Does one (or more type) work on a positive displacement basis where at X RPM, it grabs Y CFM of air at ambient conditions and just crams it into the engine? Then a 9 psi wheel would basically be a wheel that would have the supercharger pull in (23.7/14.7) times the natural cfm of the engine thus creating essentially a 9psig condition (ignoring adiabatics)? Or does it have a wastegate like function. I would have thought a positive displacement system at decreased atmospheric pressure would generate less boost. IE. 12.5 psia atmosphere on a "9psig" system would actually only generate ~7.65 psig (ignoring heat) and thus 20.15 psia instead of 23.7 psia. A turbo or anything being throttled back to a reference gage pressure would generate 21.5 psia.

 

One thing of note is that you can increase the boost on a turbocharger very easily to reach a target power range while its more difficult to do so with a supercharger. They may both be tuned for 9 psi at sealevel, but when brought to the 10,000 ft you can increase the boost of the turbocharger to hit the same power you had at sealevel without making mechanical changes. Obviously there are tuning/reliability issues for doing so, but its worth noting nonetheless.

 

Remember boost is measured in PSIG typically; which is pressure above ambient; most mechanical gauges are zeroed at 1 ATA; so would actually read slight vacuum at higher altitude - while others would self correct but then the ambient pressure goes down....

So 8 PSIG at sea level might be 14.7 + 8 while (EXAMPLE) - 8 PSIG at 10,000 feet would be 12.1 + 8

 

Right. If you read my example, 14.7 + 9 = 23.7 for sea level. At 12.5 psi altitude, a regulated system of 9 psig is 12.5 + 9 = 21.5 psia. However, a cfm system would be 12.5 + 12.5*9/14.7 = 20.15. So what I was asking was would a supercharger generate 21.5 psia or 20.15 psia at altitude with a typical "9 psig" setup? This may vary by type.

 

Since a Supercharger is driven off the belt tied to RPM; it would produce the same PSIG; but not the same PSIA.

However; in an interesting turn of events; the turbo would produce the same(ish; very close) PSIA at altitude.

 

I don't care, as most of my driving is done at 90 feet AMSL. I can deal with that imperceptibly less dense air.

 

This is what I don't get. If a supercharger works on a CFM basis, then it simply grabs X volume of air and crushes it into Y volume engine. You wind up with a P/R of roughly X/Y ignoring heat. Your PSIA then is X*Atm/Y. When the rpm is fixed ratio, if Atm pressure decreases, PSIA decreases. Also, X*ATM/Y - ATM = ATM(X/Y-1) = PSIG. So PSIG is still proportional (and therefore affected) by local atmo pressure.

 

You are correct; now a turbo uses a fixed ATA flow modifier as opposed to RPM; so they don't have the same issue (technically they do based on lower ATA on wastegate actuator side with the spring; but it is nominal).

So use PPO2; 1ATA = .21 PPO2; 2ATA/1ATM (14.7 PSIG @ sea level) = .42 PPO2.

If PPO2 at 10,000 feet is .19; then 1ATM = .4 PPO2 regardless of the compressor source.....

HOWEVER; at 10,000 feet a 9 PSIG wastegate would actually make 9.xxx PSIG since the spring pressure doesn't change in order to equal .42 PPO2 the same as at sea level.

 

Right, the WG I know. It's merely a pressure differential of the manifold and a reference acting on the spring. So long as the reference pressure doesn't change, the spring will bring the manifold up to ~23.7 psia regardless. Against a fixed reference, it's really more of a 23.7 psia spring than a 9 psig spring.

 

Exactly.

 

A supercharger giving you 9 lbs boost at sea level will not give you 9 lbs more pressure at altitude. It would seem like that but you need to remember that it is giving you a set percentage more air from a ratio standpoint and not a set amount of pressure in psi. If we put a supercharger in space where we have no pressure in, we'd get no pressure out. We wouldn't get 9 psi.

If we had a standard 14.7 psi reference pressure at sea level and our supercharger was setup to give us 9 lbs of gauge pressure, that's going to be a total of 23.7 psi or a ratio of 1.61. However if we were up in the mountains and our ambient pressure was 11 psi, our same supercharger is still going to give us the same 1.61 ratio which equals 17.71 psi or a total of 6.71 lbs over our starting pressure. However pressure as compared to sea level would only be 3 psi!

A turbo is setup so the wastegate opens after a set amount of total pressure is reached. It doesn't matter what level you started at. When it gets high enough, it bleeds off. This means that as air thins out with altitude, the turbo will spin faster and faster to hit the required pressure. While you should theoretically have 9 psi at sea level or in the mountains, keep in mind that your temps will be different between them so while a turbo does much better than a supercharger at altitude, it still has some loss. It is the best option for altitude though.