MINI Cooper Forum banner

1 - 15 of 15 Posts

·
Registered
Joined
·
29 Posts
Discussion Starter #1
This might seem like a naive question to ask! But... I know how a turbo charger works and (since I bought my MCS a couple of months ago) I also know how a supercharger works. The pressure with a turbo varies according to whether its spinning or not - is it therefore true that a supercharger will always produce the same amount of pressure because it's always spinning?

Part of the reason for asking this is that I'm thinking of fitting additional gauges and the thought of a boost pressure gauge which never moves bores my to tears...!

Hope someone can help. :confused:
 

·
Mini Mod
Joined
·
9,227 Posts
andy_guildford said:
This might seem like a naive question to ask! But... I know how a turbo charger works and (since I bought my MCS a couple of months ago) I also know how a supercharger works. The pressure with a turbo varies according to whether its spinning or not - is it therefore true that a supercharger will always produce the same amount of pressure because it's always spinning?

Part of the reason for asking this is that I'm thinking of fitting additional gauges and the thought of a boost pressure gauge which never moves bores my to tears...!

Hope someone can help. :confused:
Boost pressure changes

read this
http://www.automotive.eaton.com/product/engine_controls/superchargers/whysuperchargers.asp
 

·
BMW Traitor
Joined
·
1,192 Posts
supercharger and turbos both achieve max boost at high rpms. superchargers work better than turbos at low revs due to the lack of exhaust pressure. turbos become more efficient at high revs than superchargers because they don't "eat" the energy from the engine.

to make it short, boost isn't always the same, vary the rpms and you can see the boost go up and down.
 

·
Brrrp Brrrp!!!
Joined
·
280 Posts
Barnabas said:
supercharger and turbos both achieve max boost at high rpms. superchargers work better than turbos at low revs due to the lack of exhaust pressure. turbos become more efficient at high revs than superchargers because they don't "eat" the energy from the engine.

to make it short, boost isn't always the same, vary the rpms and you can see the boost go up and down.

So is this to say that for the Cooper S, the boost will kick in till the next gear change? Or to re-phase, there isn't a cut off point for the super charger as oppose to the turbo charger where it'd only spool in at, say 4000 revs.
 

·
Mini Mod
Joined
·
9,227 Posts
The_Bster said:
So is this to say that for the Cooper S, the boost will kick in till the next gear change? Or to re-phase, there isn't a cut off point for the super charger as oppose to the turbo charger where it'd only spool in at, say 4000 revs.
Depends on the turbo or the supercharger. It's all in the design. But basically you select a supercharger that will give you the most efficient boost at the correct RPM. You can't overboost with a properly selected SC.

Not a turbo guy but design plays a part here too. You can get a turbo to spool in earlier but generally that's because the impeller itself is smaller and lighter. The downside it it could fade in the upper RPM range. Reverse that for bigger turbos.
That's what's behind the current trend and why everyone thinks the whole pop off valve squeek is so cool. Big boost....I must be fast.......... :rolleyes: Faster......yes. Fast...maybe.
Don't get me wrong. Turbos are awesome but on many car they're just like fart pipes. A lot of noise and not much performance.
And yes the same can be said for an SC that's too big or too small.
I raced against a guy years ago that stuck a big blower on his crap small block car. I beat him badly with my basically stock daily driver. He was stunned. I had more money :D

Read the link I posted above to help get a handle on how the bypass valve works in an Eaton blower.
 

·
Registered
Joined
·
227 Posts
Apply a little logic here. People are always talking about boosting their engine power with a new supercharger pulley. The reduced size means that it turns faster than the stock when driven by the same size drive pulley, so it produces more "boost." A supercharger is run from the engine crankshaft or some other shaft. A turbocharger is run by the exhaust gases. The higher the rpm of the supercharger or turbocharger, the more "boost" exists.

The engine manages the boost with a "waste gate" where it can vent excess pressure if it is not needed, so the engine can always REDUCE the boost at any given RPM. With both turbo and supercharger, the boost decreases as engine RPM decreases, but the supercharger, which is exactly fixed to engine rpm by the pulley will drop off with a different curve than the turbo run by the exhaust gases.
 

·
Mini Mod
Joined
·
9,227 Posts
Barnabas said:
Isn't the official data 12psi? I think I read that somewhere, but can't remember...
You're probably right. I'm going from memory from some posts in the past.
Andy has posted a ton of charts on actual values. Just looked up one thread and the numbers are 13.2 and 14 psi.
 

·
Registered
Joined
·
1,313 Posts
17 is between 15 and 19. :cool:



BTW, some clarification. Boost changes A LOT while driving. You will only see peak boost for a given RPM at WOT (wide open throttle). The vast majority of the time (99% ?) the engine is under vacuum, so boost is less than 0 psi? So, a boost gauge moves quite a bit and is just as interesting to watch in a supercharged car as it is in a turbo. It is not accurate to say that boost always climbs or always falls as revs climb. My last 2 cars had completely different boost behavior:

 

·
Mini Mod
Joined
·
9,227 Posts
Barnabas said:
Andy is a friggin machine. Do you know if the 17% pulley brings it up to around 18 or 19? Or maybe even 20's?
Good isn't he?

Here's a thread involving his 19%. Post #7 shows him getting just under 18 psi. Not bad. He could tell you what that approximately equates to in HP.
 

·
Registered
Joined
·
29 Posts
Discussion Starter #15
What a response!

Wow! Thanks for all your input - this is what MINI2 is all about.

Time for my next question... See the thread about choice of gauges and locations! :D
 
1 - 15 of 15 Posts
About this Discussion
14 Replies
7 Participants
andy_guildford
MINI Cooper Forum
Join our Mini Cooper Forums to talk about your new Mini. From reviews, mods, accessories, reliability concerns and more, this Mini Owners Club is full of info!
Full Forum Listing
Top