This probably sounds like a dumb question, but I've been wondering for a while.

Why does a larger turbo make more HP than a smaller turbo at the same boost level? The standard answer is "flows more air," but isn't the amount of air flowing at a certain boost level determined by everything AFTER the turbo?

For example, I understand why cams and a freer flowing head would make more power at the same boost level. The motor will inhale more air and therefore the turbo will be pumping more air to maintain the same boost level. More air flow = more power.

But I'm not sure how a larger turbo can flow "more air" through the same motor without raising the boost level?

I can understand that the larger turbo might be more efficient and therefore the air coming out of the larger turbo would be cooler/denser, which would make more power. Is THAT the main reason that the bigger turbo makes more power at the same boost level?

Could someone please give the technical explanation on this one?

Thanks.

 

i would also like to hear this one elaborted upon.

 

^^^^same here

 

Based on what I've read, it's all about the "compressor density ratio" and not necessarily the size of the turbo. Let me attempt to explain.

Let's assume we have two different turbos, our "new fandangled turbo" and our "ole piece of crap" turbo.

New fandangled turbo achieves 78% efficiency pushing 400cfm of air flow at a pressure ratio of 2.0 (14.7 lbs of boost). At an ambient temperature of 75degF the charge air leaving this particular turbo will be ~224degF.

Ole piece of crap achieves 68% efficiency pushing the same 400cfm of air flow at a pressure ratio of 2.0. At an ambient temperature of 75degF the charge air leaving this turbo will be ~246degF.

Next, compare the final compressor density ratio for each turbo. Determining compressor density ratio will allow you to compare the actual density of the air leaving each compressor. When you do this you find that the new fandangled turbo puts out a charge that is not only cooler & hence less detonation prone but is actually ~3.3% more dense than the ole piece of crap. Assuming air fuel ratios are kept constant this will result in ~3% gain in power output. In reality the gain would probably be more since you could lean out the mixture and advance timing due to the cooler charge.

All formulas pulled from Forced Induction Performance Tuning by A. Graham Bell.

 

Because boost level does not have a linear relationship to CFM. You are confusing boost level for intake CFM. Think of boost as a measure of how hard the turbo is working/pushing and then you will understand that a larger turbo does not have to push as hard to move the same amount of air.

Sort of like a strong guy and a wimpy guy. The strong guy does not have to work as hard to his own proportion as the wimpy guy to move the same heavy object. The only problem with the strong guy is that he is harder to get motivated and consumes more food.

 

http://www.honda-tech.com/zerothread?id=1004578

 

Nice analogy!!!

 

you are making me hungry...

 

It isn't...not volumetrically anyway. Small turbo or large turbo at a given rpm, the volume taken into the engine is the same, and if the boost pressure is the same, the only difference is temperature...which can make a substantial difference simply because a lower temperature represents an increase in air mass, all else being equal.

Bingo.

 

Ok to put it into the simplist terms i know how.. I would say.. This usually with a larger turbo you get more lag off the bottom this is due to the turbo having to reach a higher flow rate to get into its efficiency range to make power.. look at it this way.. a t3 and a t4 turbo both can make 10psi of boost, but a t4 on a smaller engine will take much longer to get there, but... long before its maxed out the t3 will be blowing hot air and not make as much power as it becomes sort of a flow bottle neck.. This is why bigger ball bearing turbo's are the hotness as they allow you to run a bigger turbo and spool 500-1000 rpm sooner depenind on how big they are over a journal bearing.. Big efficient turbos are great for small engines that sustain hi rpms a drag car etc. I hope that made sense.. Its hard to understand when you start using cfm and temp's to a newbie or someone just learning.. give it time if you are serious about turbocharger knowledge there is a wealth of it on here on the net and in good books. Also a smaller turbo will starve a larger engine at higher rpms and boost levels.. Most usually start blowing hot air causing power loss long before that tho.

 

Thanks, Ted. You articulated the concept more concisely than I did.

 

very nice explanation. : )

 

i can't believe that people forgot to mention that usually on a well matched turbo the exhaust side is larger in addition to the cold side being larger also therefore alleiviating backpressure etc. it's like freeing up your exhaust more, and those are the "purest" gains from a bigger trubo, all the other gains are based on efficiency matching to your application (bascially the stuff said above).

 

Thanks for all the explanations. VERY helpful.