Twin Turbos for Towing

Tate said:
30% bigger in what way? Compressor? Turbine? Flow capabilities?
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I have heard the ideal pairing is to have a primary to secondary compressor wheel ratio of 1:0.70 as measured at the effective cross section of the wheel. With smaller turbos 1:0.75 works well. This does not take into account efficiency or design of the wheel so I am a little skeptical. However that ratio seems to get you in the ball park based on others experiences.

Ghostman said:
Total mass flow is determined by the primary. The secondary provides flow in the lower rpms for quick spool up and can also recompress the primary flow if sized correctly. I would size the primary according to your fuel.

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I'm with Ghostman. There is a good post by JWT in an old, but informative thread that elaborates on sizing a little: - View Single Post - wastegating facts

I think it is also very important to size the turbine wheel and housing correctly. For towing you don't need or want something that is slow to accelerate but capable of high flow. Instead something that will be efficient in the range that you will be operating the engine will preform better. I.E. the 83mm s400 turbine wheel in a 1.1 housing would preform better that the 96mm/1.32 combination on an engine that spends hours at mid throttle.
 
White Knight said:
I have heard the ideal pairing is to have a primary to secondary compressor wheel ratio of 1:0.70 as measured at the effective cross section of the wheel. With smaller turbos 1:0.75 works well. This does not take into account efficiency or design of the wheel so I am a little skeptical. However that ratio seems to get you in the ball park based on others experiences.



2sgn080agree.gif
I'm with Ghostman. There is a good post by JWT in an old, but informative thread that elaborates on sizing a little: - View Single Post - wastegating facts

I think it is also very important to size the turbine wheel and housing correctly. For towing you don't need or want something that is slow to accelerate but capable of high flow. Instead something that will be efficient in the range that you will be operating the engine will preform better. I.E. the 83mm s400 turbine wheel in a 1.1 housing would preform better that the 96mm/1.32 combination on an engine that spends hours at mid throttle.
Click to expand...

I would say sizing it based on physical size is a waste of time, since it would be the flow capabilities that are going to dictate what is best. I would say it is a general rule of thumb, but definitely not the rule to ideal pairing.
 
Some light reading

In chapter 16 of [ame="http://www.amazon.com/Turbochargers-HP49-Books-Spark-Ignition-Applications/dp/0895861356"]Turbochargers HP49 (HP Books): Turbo Design, Sizing & Matching, Spark-Ignition & Diesel Engine Applications, Water Injection, Controls, Carburetion, Intercooling, ... Street & Race Cars, Boats, Motorc: Hugh MacInnes: 0075478001354: Amazon.com: Books@@AMEPARAM@@http://ecx.images-amazon.com/images/I/51yS3zmy2sL.@@AMEPARAM@@51yS3zmy2sL[/ame] by Hugh MacInnes he discusses sizing compound turbos and includes sample calculations. Here are a few scans from tat chapter if you are interested. The same formulas can be used for any engine.

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My 351 fed by a 475 tows like a champ. Leaves some to be desired over 2800 rpm for performance, but we are talking towing.
 
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