thekidfan

For a tow set-up it would be great, up to maybe 550rwhp or so!!

Ph4tty

I've followed your thread about them with great interest since I've got a 64 already. Thats part of the reason I'm looking for a smaller alternative like the k31, even if it is an old school turbo its still bigger than a ht3b. I don't know if those two would be two close in size nor do I understand the downfall of having a poorly matched pair of turbos.

Idaho CTD

What people dont understand with twins is that you lose some efficiency due to drive loss. It takes hp to drive turbos so you typically only make about 85% of the efficency of the largest turbo. So for example a K31 flows 90lbs/min. At 85% that is 76.5lbs/min. A 64mm S-300 flows about 75lbs/min. See a problem here? The twins will typically run cooler but the drive pressures will limit the hp potential to about the same as the 64mm by itself. A 64mm turbo is really suited to a much larger primary and a K31 only really needs about a 58mm S300. The ATS set up is matched fairly well (just small) but they should have used the K31 as a stock turbo with the wastegated housing. So basically after a certain point going bigger with the small turbo does nothing for hp and hurts spool up.

Fishin2Deep4U

I am running a HX35 hybrid with a modded 14cm housing over a S400. It's Rip's kit with my top turbo.

I chose to go this route as I did not want the slower spooling S300. And teh peak HP difference is negligible.

Dave

kawi600

How much is rip rook's twin kit using the stock HX35? He doesnt list any prices =(.
What kind of power are you seeing with that setup?

HOHN

Good points. Another way to think of it is that the hot gases in the exhaust are doing work on the small turbine. In doing so, they lose heat and pressure (enthalpy). This means that there is less energy available to drive the big turbo.

It's not necessarily that the compressor flows any less-- a 90lb/min compressor is a 90lb/min compressor-- it's just that the turbine shaft power required to drive that compressor to it's delivery rating isn't there. So the operating range of the turbo will shift to the right (become more laggy) and the big charger will spool slower than if it was bolted directly to the manifold (used as a single).

This is why turbo sizing for twins can get pretty tricky, because every change you make to one charger affects the others.

For example, if you fit a smaller, tighter housing to the top turbo, you make the bottom one laggier.

This goes back to the big external gate. If you can bypass a lot of drive energy efficiently, then you'll hit that big charger faster and get it cranking.

A big problem with this is heat loss.

IMO, if you aren't having almost all your twins hot plumbing bathed in high-end thermal barrier coating, you're giving up a lot of performance and efficiency.

www.swain-tech.com Try the "white lightning."


Another thing I think would be great is if we could come up with a way to bolt the turbos together directly-- like have an adapter made that will mate the big turbo's inlet to the exhaust outlet on the small one. I think this would help immensely with efficiency and drastically cut lag.

Look at how the turbos are done on the new 6.4L p-stroke. This is the "ideal" imo as far as mounting and preserving efficiency.

Packaging is certainly a problem on our trucks, though....

jmo

Idaho CTD

Justin,

I do offer high temp coating on all the exhaust parts as well. Not only for function but for looks too. They keep the rust off as well as hold the heat.

A primary wont always flow in twins what it does as a single. Think about it and then tell me why.

Your are right with the heat loss but there is also a second turbine wheel in the exhaust stream causing a restriction ie. drive pressure. The higher the drive pressure the lower the hp all else being equal. That is part of what I called drive loss as well.

RowJ

See post #16!

RJ

RonA

I'd like to try getting the exhaust side coated. I was hoping the HTT SS housing was going to be ok, but it didn't work out. The new .8 is already getting brown.

Fishin2Deep4U

i have ceramic coated all my exhaust parts as well. Sure keeps things claen and much cooler under hood!

Dave

HOHN

This is a tough question because the concept of "flow" is somewhat misleading. I mean, the higher the "flow" the lower the PR, so the question of "how much does the primary flow?" is ultimately a function of "how much does the secondary flow?"

If the secondary is small, then the big charger isn't making much flow. Instead, it's providing a pressurized reservoir from which the small charger can draw.

It's a subtle distinction, but the difference between flowing "through" something vs flowing "to" something can have significant impact on how we figure things.

Personally, I avoid the concept of "airflow" if I can. Instead, all I care about is how much air I can cram into something. Flow is just a reflection that I have a pressure difference. The greater the pressure difference, the more flow I will have. In other words, we sometimes focus on flow as a means to create desired pressure when we should be focusing on the pressure as the means of flow creation. Flow is the rate at which we can eliminate the pressure differentials. Ideally, we'd be able to alleviate this pressure differential as fast as it would build up. We'd essentially have all flow and no pressure-- and we've come full circle.

So we start with the first restriction-- the engine. The smallish displacement and low RPM range means it won't swallow enough air to burn the fuel we want. So we turbocharge it and forcefeed it. The boost pressure that builds up is the difference in flow potential between what the turbo is providing and what the engine can handle.

So in a twins application, the compressor "sees" what's the next major restriction in line. Thus, the secondary "sees" the engine and intercooler as the main restrictions.

But the Primary only "sees" the secondary compressor and the cold pipe. To the primary, the secondary is a "variable restriction". This variable restriction is much different than the "variable restriction" that the secondary sees from the engine. The engine's air consumption almost linearly follows RPM-- varying about the VE and so on. But the air consumption of the secondary compressor varies from slightly more than the engine's rate to almost THREE TIMES the engine's rate.

So, the way I see it, the big turbo would actually "flow" MORE in a twins application than it would used as a single turbo. As a single, the rate of flow is limited by what the engine can handle-- and the flows stacks up and you get really high PRs. But in twins, the secondary compressor swallows more air than the engine alone would-- and you have a lower PR and higher overall airflow.

So all that to say that I'm not sure that "a primary doesn't flow in a twins application what it would as a single" unless we define the exact conditions we're comparing.

I've outlined conditions where a primary would flow more in a twins app than as a single-- can we think of conditions where the opposite would be true?

Thanks for the good question. You guys have no idea how much you teach me.

JH

DieselMike

Help me understand something here, I thought that when you had twins spool up would be faster. I still cant decide between an s-400 or a k-31 to put below my ps 62. I dont want to make low end spool up any slower, i tow on mostly flat ground, have some hills but nothing too bad. If i put in a helix 2 cam and an s-400 below my ps 62, would that help increase my low end spool up? My ps 62 does alright towing, but i dont want to make it worse. Im kinda leaning twards s-400 because well, more power. . .but not if it means slower spool up.

signature600

Spool is always a little slower than what your small charger will do by itself.

So, in my thinking, if you do a cam, and Twins, the spool "should" be close to the same! If you're towing a lot, I'd go with a K31 Later on you can throw an S400 on it and move your downpipe forward an inch or so

Chris

Jetpilot

Twins like singles are always a compromise...... There is no "one size fits all". That being said what is important is to understand the goals of the user. There are several twin kits available that fill the large percentage of the void needed. But for those that want something better a custom kit setup for the user is the way to go. No you can't just call and have one delivered in a couple weeks but the gains might be worth the wait.

Personally I like turbos from Forced Inductions or Bullseye. Both of the companies can help with turbo selection, problem is they don't make twins...... But there are guys out there who do a good job with custom setups, Nathan Wright (Idaho CTD) just happens to fit this bill.....

DieselMike

Maybe I should stick with a K-31 since I do alot of towing. With what I have now for mods, plus the twins and the cam, what kind of power do ya think I'd be making, also what kind of boost pressure would that twins set up make? Would I be making more boost while cruising vs. my single? Thanks for all the info guys.