Blow-Through Twin Screw?

[Five9dak]

I have a 2.2L KenneBell supercharged 99 Dakota R/T 5.9L In its original form the kit uses a draw through stock type TB (my Holley flows 860cfm) on a restrictive long inlet that passes over the distributor at the rear of the motor. This inlet is rumored to flow only ~550cfm. In datalogs I have seen 7.2psi at below 3k rpm but it drops to 5.4 by 3300rpm and trails to ~5psi before the shift. I think this blower is really restricted by that inlet and switching to a blow through TB configuration with 4" blower inlet ducting and filter would show large efficiency and small boost gains with the same pulley.

NA the motor made 265/352 at the wheels, now it seems to have around 90 more ftlbs but perhaps only 40 more hp up top.

The kit is equipped with a bypass valve and actuator that I could re-use for cruising, and I think I would need a blow off valve to protect the TB. I have seen posts about using wastegate as a combination bypass/recirc blowoff on this forum before, thoughts?

Do the bypass/blowoff requirements change with a twin screw because they have internal compression unlike a roots? Will cruising outlet temps be too high without throttling the blower? I have never seen a twin screw blow through set up, but I have seen remote mounted roots like this.

What size BOV is required to protect my throttle body?

 

[black2003cobra]

KB prides themselves on selling integrated kits, and not just selling their customers the SC. If you suspect a restriction in the inlet, (which would certainly result in lower witnessed boost pressure), you might want to consider simply measuring the pressure drop from one end to the other. Armed with data, you could then contact KB and see what they have to say. That's where I would start, anyway.

 

[black2003cobra]

And it probably goes without saying, but log the pressure drop vs rpm.

 

[Five9dak]

I will take the data that you suggested once the truck is tuned in it's current state.

The inadequencies of this kit are well known and the kit is dicontinued and relatively unsupported by KB. It was designed a relatively long time ago and since many of kenne bell's kits have had upgrades offered, increasing the size and flow of the inlet manifolds. ("mamouth")

It would be interesting to see if this conception was true, and just how "bad" it is.

I will make sure to report my findings.

If you are interested, the kit was designed for 98-03 dakota/durango and ram trucks with 5.2 and 5.9 magnum v8 engines. The long manifold that goes over the distributor and into the back of the supercharger is the one in question.

Thanks for the response.

 

[DigitalGT]

Your bypass valve requirements aren't going to change due to the type of compressor used. Once the charge leaves the compressor, it is nothing but an air mass of a given temperature, density and humidity. The bypass valve doesn't care whether the air pump has internal compression or not. Centrifugal, roots, or twin screw, they're all the same in this respect.

You should certainly use a vacuum operated bypass for low load/high manifold vacuum cruise. If the one you have is of sufficient size, then I don't see why you need an additional bypass. Blow-through TBs may be a little bit more "risky" than draw-through units but as long as your bypass opens dump pressure back in front of the blower inlet, you'll be okay.

Using a turbocharger waste gate could be used as a bypass but I don't see the need for it. They're designed to survive in an extremely harsh environment and, simply by the nature of turbocharger control, they leak more than a good bypass valve will. If you look at an internally gated turbo, you'll see that the waste gate is nothing more than a flap door covering up a hole. It blocks the majority of the gas trying to get by but still leaks. External waste gates with plunger type valves are much better at leak control but still can leak a little. Keep in mind the designers of an external waste gate may have only had 1600° environments and thermal expansion in mind when testing and never thought to check performance when cold.

If you think you really need additional surge control, then it certainly wouldn't hurt to plumb a second bypass into the mix. Any good quality unit with about a 40mm valve will be more than enough.

I wouldn't be concerned about an increase in charge temps when swapping the TB to the other side of the compressor. You're still throttling the blower, you're just doing it differently. Besides, with the blower bypass still in place, you will unload the blower while cruising (because manifold vacuum will open the bypass) and temps won't climb. The blower has to be doing work to make the charge hotter.

I'm assuming your R/T is still fuel injected so with that in mind, where is the air flow meter? Is it draw-through or blow-through?

 

[Five9dak]

The truck's original EFI was speed density based. The factory coputer is still controller timming with a retard box and the fuel has been taken over by MegasquirtII running Extra 2.0.1 code. So I don't have to worry about the airflow meter.

I think I'll add a standard pop off type BOV for extra insurance against bent throttle shafts etc, but the bypass I have seems like it should fit the bill. I am still concerned about noise, so I may have to fit an auxilary large plastic throttle behind the air filter to block most of the noise from escaping there.

 

[Five9dak]

CI*PR*5200*VE/3464 = CFM

It is interesting to note that the often thrown around flow bench value of the inlet in question is 550cfm (from OP)

My logs show PR falling to 1.31 at the 5100 rpm shift point.

360*1.33*5100*.8/3464=

563cfm limit......

Once the truck is back on the road I still plan to take multi point pressure drop measurements to validate the suspicion.

 

[Five9dak]

After some more research terminator cobra/lightning style throttle bodies can be had from performance upgrade take off for $25 and flow 1089, the flange will support up to 1700cfm with an aftermarket accufab unit. I am going to cast a new inlet elbow that curves the opposite direction and has this flange. New throttle/kickdown cables will be easier to set up then the headaches associated with blow through.

1089cfm should support 10.9psi on the 360 at 100% VE. This will hold me over for a while with potential for a lot more boost with the accufab unit.