well I did look around and there is spotty info. But here is a conversation about the Herc 1 and Herc 2 3.7's. They talk about some performance issues. bread truck engines. - Cummins 4BT & Diesel Conversions Forums

Hope you get something out of it.

 

Star and tjc are right on with the exhuast thing. With older no computer diesels the more air you can get out of them and faster means they get more effiecent and means you can put even more air into them therefore more power.Of course there are always mechanical limitations of the design of the engine that will limit you as to how much you can do but pushing it to the limit is always fun.

 

Looking into these motors and people that have been useing them it doesn't seem that there is much to be had by huge exhaust. Must be some of those mechanical limitations. It appears that the Series 2 3.7 turbo is the engine to have. But the series 1 can be improved with the heads from the 2 along with the pump and injectors. One of the real limitations is the oiling system around the pistons on the 1 series but I would think that a well designed intake cooler would drop engine and exhaust temps along with add the denser air charge. Anyway good luck with what you have there. Blue is series 1 and red is series 2. The series 2 has output comparable to Cummins 4BT but not as much following or parts suport.

 

Cooler air gives you more compression as you actually have more more air in the combustion chamber. Same reason you lose compression the higher you are from sea level. So a cold air intake is one of the best things you can do to a diesel but you have to have enough exhaust to get rid of it the faster the better. Another thing that will help is insulating any pipe before (on this motor probably isn't ant) and a foot or so after the turbo helps get the gases away faster. I could see no gains for running duals on this motor.
Any gains will be in small increments due to the power you are starting with but if all combined gained 8 to 10 HP it would be like driving a different rig.

 

If You read the Gale Banks Memorial Library, He clearly states that there is a point of diminishing returns as far as exhaust diameter. Most of what He talks about is backed up by Dyno Testing. So I'd take His word over most web banter.

If My memory serves Me. There's nothing to be gained by going over 5" on just about any Turbo Diesel. It's all Bling after that. 4" is sufficient for most TD's, 3" for NA's

 

Nothing beats a quick hand calculation of RPM, displacement, volume of air, intake vs. exhaust temperature (without combustion / with maximum fuel injected and burned).

That will get you data on volume, pressure, and changes in pressure as the diameter of the pipe varies.

 

Exception to this rule:

In extreme cold climates, cold air intake can result in insufficient heat generated from compression to ignite and completely burn fuel.

The cutoff where cool air becomes a "wash" is around 0F, and then by -20 or -30F, there is increasing benefits to HEATING intake air.

At -40F or thereabouts, heating of intake air is required for most diesels.