View Full Version : OT - Turbo Speeds
04-16-2007, 10:01 AM
I was reading a magazine article that said that one of the new diesels (Duramax, maybe) was going to be using a turbo that spooled up to 120,000RPM. That seems awfully fast! It also said that the mfr needed to spin it that fast to develop the amount of boost they needed. So I was wondering, what advantage does a turbo spinning that fast have over a slightly larger turbo spinning at a more reasonable speed? I can't imagine the weight being that much of a factor, and because of the tighter tolerances that would have to exist, I would think it would cost more to make. So, why that speed?
04-16-2007, 10:40 AM
As you stated, a big turbo will develop the necessary boost at a lower speed. However, weight is a factor, a large turbo has more inertia and takes longer to spool up, resulting in turbo lag. The small turbo may even overboost, but they use a waste gate to bleed off the excess.
04-16-2007, 03:20 PM
But there is also a point of dimishing (sp?) returns. A smaller turbo that has to spin faster to make a higher amount of boost will also heat the air more. Which is a negative side effect. So there will be a trade off.
The turbo on many of the factory equiped turbo vehicles will see 75,000rpm in stock/factory settings.
04-16-2007, 05:56 PM
Turbo choice depends on many factors.
You can narrow it down easily by weighing what type of powerband in general you want VS what the actual amount of peak horsepower you want to make is.
Common compressor maps. Learn to read them.
The amount of rpm a turbo will see depends only on how it is sized VS the engine, and what the wastegate holds the turbine speed too.
The reason for not picking a gigantic turbo that will flow more at the same amount of compression VS a smaller one spinning faster is responce. It's 2007. No reason for having a laggy POS from the 1970's. In today's world theres no reason to use a larger, more costly unit when a smaller one will provide more power at any point in the rpm range.
Be it air to air, or air to water. Very, very few automotive turbochargers are installed at this point in time without aftercoolers. No large diesel truck engines do. Heat is only an issue when you are simply fall off a compressor map, or the fuel can not help from pre-igniting upon entering the combustion chamber.
You also get into possible problems with larger compressers surging on some installs.