Got a question for the turbo experts here. iv been doing a lotta research and reading on twin/triple turbos and one thing I never see is how to properly size the turbos or if it even matters. for example on a compound setup, i heard somewheres that the big turbo should flow 2.5-3 times more air than the small one. but I can't find out why or why u shouldn't run two the same size? same with triples. almost always the kits u see r two big ones feeding a smaller turbo, with the exception of one build, the rat rod budget triple turbo in dpm. there the guy used all the same size chargers but all different housings. so the question is, does it even matter what turbos u use and if it does, why? What would make one setup better than another? turbo experts please chime in thanx

 

Those questions would take a very detailed answer.

But in general on a twin setup you want the primary, large or bottom, turbo to be sized to move all the air you need for your power level. The secondary, small or top, turbo is sized to meet spool up needs, and to match the primary output for the desired PR or lb/min for the power level.

In triples you run 2 turbo's in parallel feeding the 3rd in series. You size the primaries (parallel) so each provides half of the lb/min required for your power level. The secondary works the same way as the twin setup.

 

can u explain the last half of that statement please? how is the secondary supposed to match the primary output? the rest of it makes sense........

just one more question tho. if the two primaries in a triple setup provide half of the air required for a given amount of fuel, basically the two are doing the job of one primary in a twin setup right? so what is the benefits of triple over twin? quicker spool? cause u aren't moving any more air for the same given setup of motor....

 

You need to size the secondary to be able to bring the boost up to the level required and be at the fat part of the MAP, i.e. where it moves the most air the most efficiently. The secondary doesn't increase the airflow at all, it just accelerates the air.

A primary may move 110 lb/min, but it's only capable of doing so at a 3:1 PR, which makes about 30 psi of boost. The cummins head cannot flow 110 lb/min at 30 psi. It needs a greater pressure differential between the cylinder and manifold to get the air thru. The secondary takes that 110 lb/min at 44.7 psi and compresses it at 3:1 again, which makes for 110 lb/min at 90 psi, and that's enough of a pressure differential to get the air into the cylinders.

Those numbers are arbitrary to make an example. Most twin setups run lower boost and PR. The lower the PR the cooler the air, the cooler the air the more 02 per intake stroke.

The two smaller turbo's in parallel should spool quicker than a single primary for the same lb/min.

 

Right. but now this question.....doesnt that mean that the secondary hasta b able to flow 110 lbs/min too then cause otherwise it will b a bottleneck no? therefore making it the same size as the primary? or am I just not following something here? If the primary moves 110 @44.7 psi to keep that flow u gotta have the secondary b able to move that right? I understand the compounding of the PRs but now can't wrap my head around the flow part dn mean to b annoying just trying to learn and understand

 

Its easier to understand if you look at flow in terms of CFM. IIRC, its about 15:1 CFM to lbs/min, so 110 lbs/min is about 1650 CFM. When at a PR, output would be around 550 CFM output. The secondary needs to be able to consume that amount. Which works out to about 37 lbs/min. The mass flow rate stays the same, but the volumetric flow will decrease because it is compressed.

 

Because the secondary's input is already compressed it not limited by it's normal MAP. The flow does matter, as you can't put a tiny turbo with a 4:1 PR as the secondary, but you can't read the MAP. If the secondary has 1/2 the CFM of the primary (just a ballpark) it should be able to move the compressed air.

 

Ic. Tate that makes sense.....I forgot that with pressurization volumetric flow is decreased. makes sense now. so if the primary has a PR of 3:1 like in ur last example and the output is 37 lb min then the secondary should b rated to flow 37 lb min in the highest efficiency of its map for best results right?

 

Depends on the application. There are benefits to running the turbo on the high side of the map, as you can size it smaller, and will therefore be a quicker responding turbo. The higher the efficiency, the less temp. increase there will be with the compression, the less pressure will be required to move the mass of air. Generally, the larger the compressor, the more efficient it becomes. Larger it is, the slower responding it is. A race truck vs. a street truck will have different want the larger secondary to keep the efficiency. A street truck would go for a smaller secondary to maximize response.