View Single Post
beepbeep beepbeep is offline
Registered User
 
beepbeep's Avatar
 
Join Date: Jun 2001
Location: Sweden
Posts: 5,564
P.S. Let's not forget that picking the right turbo trough compressor maps doesn't mean that we are looking for turbo where top-power plot ends up in center of max-efficiency island! Such engine would be good on a airplane, but cars rev their engines trough a certain range and you want your turbocharger to perform in optimal fashion troughout the range required by application.

With other words, working point will "wander" trough efficiency plot depending on boost, throttle and revs. A typical turbo would let you go trough max efficiency island where you need the power the most...around 3000 to 4000 revs. It will then wander to the left of the map, but it shouldn't go into overrev or surge limit.

To put it simply: if you pick smaller turbo it will pump more efficiently trough middle-range but start huffing more and more hot air at high revs. As we are not using CVT-gearboxes or are drag-racing with slushboxes, such character isn't always bad.

Highway perspective: my daily driver has a small Mitsu TE05-13b turbocharger. It boosts around 1 bar in middle revs, then tapers down towards 0.6 bar at 6000 revs as it runs out of steam. In this (extreme) case, my power curve is almost flat between 4000 and 6000 RPM. Torque is gradually declining. Engine produces around 220hp. If I drive my car against a N/A engine of similar output, I always run away as this torque allows me to dissipate more power in typical 4000-6000 RPM shift-range than N/A engine would. My "pull" would gradually decline as I climb trough the revs, but each upshift gives me almost same torque in next gear.

So,
For top end power: big turbo=low backpressure=efficient at high revs=less responsive=good for drag-racing
For good compromisse: slightly smaller turbo=efficient at middle revs=responsive=good in most cases
For torque monster=small turbo=high backpressure=monster torque low down=almost instant boost=needs to be boost controlled with boost taper at higher revs in order not to overspeed the turbo.

Hope this clears the choice parameters somewhat.
__________________
Thank you for your time,

Last edited by beepbeep; 08-11-2007 at 03:20 PM..
Old 08-11-2007, 03:16 PM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #32 (permalink)