Ok so majority of compound boost the t/c feeds the s/c. Ofcourse when it's a pd sc that's mounted on the intake itself it's not possible but what about units where the sc isn't mount on the intake?


VW did it this was but why is majority of compound boost now the other way around?

Is it just because a bigger blower requires more cubes so using a smaller blower would be more efficient or what?


This btw is just for knowledge. I just don't understand or don't know why the turbo seems to always feed the sc,

 

correctly sized turbos work ok...and dont need superchargers

 

i hear some guys are feeding a turbo with a belt drivin supercharger on some diesel pickups, i thhought if lag is an issue than why not just run a belt driven supercharger??

 

It doesn't matter if the supercharger is first or the turbo, what matters is the device that flows the most air is first.

 

I think weve been over this before lol this was just a question that pop in mind and yes I agree with you. You don't need a super charger, but you also don't NEED a turbo lol couldn't resist.

 

Oh ok.. I thought it was something like b/c if you have a good size turbo and you try to put a bigger size sc then the engine might not be able to spin the sc (if it's too small)

Edit:

Btw if this was the case and the wastegate is receiving it's boost from the turbo itself then the whole compound boost where the boost multiples doesn't really apply anymore? I mean sure the turbo is compressing air that's already compressed but the wg will still open at whatever you have it set.

What I mean is let say your sc is pulled for 6 psi and your wg spring is 15. That means the turbo then is only at 9 psi or does it still carries over the whole equation so turbo is only really spoiling at like ~ 6-7 psi to get a total of 15? (very rough estimate but I hope you get what I'm asking.

So in a set up like this there no point of calculating the total # since the wg will open at whatever # it's set at?


Thanks

 

It depends on how you setup the wastegate...if the secondary turbo (smaller one) has a 15psi spring, and you put the pre-turbo boost to the top of the wastegate, the 2nd turbo will always make a pressure ratio of ~2:1.

You also have to remember that the pressure ratios multiply, so if both devices are making "14.7" PSI (2 bar of boost) the total boost will be 44.1 (4 bar of boost).

 

You've got a typo there. 14.7psi = 1 bar.

 

That's interesting. But if you reference the wastegate on the smaller one then wouldn't the overall boost only be at whatever the wg is set at?

And in your example you would see that 44.1 in the intake manifold and anywhere post secondary turbo correct?

Is there a multiplication difference if using two turbo vs s/c and turbo in a compound set up?

It seems like using two turbo yields higher total psi.

 

No, you have to start thinking in pressure ratios. If both turbos are making a 2:1 Pressure ratio, its 44.1PSI of boost at sea level. If either one was by itself, it would be making 14.7PSI.

Correct, although you may see some losses from intercoolers.

Nope.

Yes, the big advantage of compound boosting is PSI. That's why diesels LOVE compounds.

 

This I what gets me confused. If the wg is set at 20psi for example and it's reference after both of the turbo which in this case would see 44.1 psi at the intake track HOW would it see that pressure if the wg see 20 psi on it's way to 44 and opens up?

I mean if it can see 44 psi wouldn't the wg open way before it reach that since it's reference after both turbo?

 

Re: Just curious- why don't people use the supercharger to feed a turbo In compound

Because it multiplies pressure ratios.