Why does a 66 spool slower than a 62?

[DESL PWR]

Given the same turbine housing and turbine wheel, why does a charger with a 66mm compressor wheel spool slower than one with a 62mm wheel?



Or is this not even true to begin with?

 

[Mhannink]

i belive the bigger wheel is heavier and takes more time the whole inertia thing that einstien(sp?) came up. think about it you have the same amount of power(exhaust pressure) but you have more weight. just like why our trucks are not as fast as a lighter car with the same power.

 

[raychem]

i belive the bigger wheel is heavier and takes more time the whole inertia thing that einstien(sp?) came up. think about it you have the same amount of power(exhaust pressure) but you have more weight. just like why our trucks are not as fast as a lighter car with the same power.

It is really not the weight of the wheel. Although the larger the wheel diameter the more mass it will have. The exhaust side has alot to do with it too. But the main reason for the 66 spooling slower than the 62 is the blade cut and the stages on the compressor wheel; lets not forget that the trim on the turbine side controls spool as well as turbine housing size.



I run a 66 and it spools to 15 lbs pretty quick... but until I hit 15 pounds my truck goes nowhere... then all hell breaks loose! So to say that the turbo is in it's map which varies according to compressor size and design. Larger turbos require more fuel to spool up, 500 plus hp for a 66 to work right so your HP target is very critical to charger selection.



A cam can work wonders as far as spool time is concerned and reduce it considerably.



Richard

 

[Forrest Nearing]

it's because of the compressor efficiency at different air masses... a larger compressor will usually be farther right on the map.



untill you get higher in CFM the compressor isn't going to be making pressure.



that's why head work and cams help turbo spool up. they allow you to move more air volume at a given RPM, and the compressor will start doing its thing sooner

 

[DESL PWR]

Hummm, so let me see if I understand, its kind of like the chicken and the egg. You need drive pressure to get the compressor going, but to get more drive pressure, you need more boost on the inlet side. And since the larger compressor isnt very efficient at low speeds its going to take a little while for things to get going.



With that in mind, you need enough fuel to develop heat which will increase the exhaust gas volume thereby building drive pressure to light things off as soon as you can.



Does that sound even anywhere close?

 

[Forrest Nearing]

you need drive pressure, but to get more drive pressure, you need more exhaust volume. heat helps, but you can only get so much heat by adding fuel. you need air with fuel to make heat.



more air you get through the engine, the better



it's not that the larger compressor isn't efficient at low speeds, it's that it isn't efficient at low CFM... put the same turbo on an engine that can move more air and the turbo will respond much quicker.

 

[DESL PWR]

Thanks Forrest, that makes sense. One more question. The result of poor efficency at low CFM (large chargers like the 66) is lack of boost. But, what is the result of poor efficency at high CFM (small chargers like HX-35)?..... High boost temps?

 

[Forrest Nearing]

they're operating out of their efficiency range... take a look at a compressor map. an HX35 is to the right or above their efficiency map. a 66 at low CFM is to the left of its map