switching top turbo on twins.

[RonA]

My twin set up is making 60 lbs of boost with an HX35 hybrid on top, with 65 to 68 lbs of drive pressure. If I switch to a slightly larger top turbo (HX40,HTB2, etc) does the air charge get cooler at the same boost levels? Or does it just allow you to make more pressure. I don't want to go over 60lbs total. I'm trying to figure out if I should try to add a little more air, or take the easy(cheap) route and back the fuel off. Have a little more smoke than i care for.

 

[Hohn]

Ron, I'm guessing that you could LOWER boost AND drive pressures with a larger top turbo without harming overall HP or getting higher EGT.



For example, an hx 35 and an hx40 can both make 32psi of boost--- but which is more efficient doing it? The 40 is. For a given amount of mass flow (kg/s), the larger charger doesn't have to spin as fast (assuming it's somewhere within it's map at all), and it will thus heat the air less and require less drive pressue to accomplish the goal.



IOW, a 40 at 32psi is actually delivering more O2 to the engine than a 35 at the same boost pressure. You would see this efficiency in air temps at the hat. Relatively cool temps at the hat are a sign of well-matched turbos, imo.



At 50-55psi, you are approaching the limits of efficiency for a 35 as the second stage. If you have a huge external gate, you can stretch the 35 a lot farther. Do you have the PDR gate?



Anyway, you asked whether or not a larger charger give you lower charge temps, or if it just allows you to make more pressure. You've already seen how it can/will lower temps. But a larger top charger doesn't allow more boost pressure, per se. It CAN allow more boost if you are exceeding the range of efficiency for the smaller one.



IMO, what you want in a top charger is one that is efficient at high pressure ratios-- 4:1 and higher.



When I think of twins, I think of a larger charger which is designed to flow high VOLUMES of air, but not necessarily at high boost pressures.



Thus, the total airflow across the small charger SHOULD be as close to constant as possible (in terms of kg/s) and this is determined by the large charger. IN this setup, the role of the small charger becomes converting high flow at low pressure to high pressure at lower flow (in terms of CFM), while the kg/s stays fairly constant.



Also, logic seems to dictate that the PR of the small charger plays a role in determining the size of the larger charger. If the small charger is efficient at a PR of 3:1, then you want the big charger to flow (in terms of kg/s) three times more than the little charger. This, because we have to feed the smaller charger three times more air to get it to be able to multiply pressure. In effect, the small charger is just trading CFM for PSI for a given kg/s.



Given this reasoning, we can see how most large charger would be considered too small. I dunno the kg/s of an HT3B, but it should be roughly 3 times that of the 35.



So- -my twins planning is going like this:



-- X amount of fuel is required for a given HP

-- Y amount of air is required to burn that fuel for a given EGT

-- Larger charger determines total system mass flow (KG/s), and should be matched based on Y above, adding a fudge factor

-- small charger determines spoolup, system efficiency, and how well the overall system mass flow is maintained in the face of a pressure/cfm swap.



All that to say that I think a 40 would do well in your case.



Feel free to disregard all this as I am not a bigwig in the HP world, and have ZERO experience firsthand with twins.



Justin

 

[Hohn]

OH, and don't sweat too much what the actuall boost numbers are. Things like timing and CR have much more effect on whether or not the HG hangs on. Also-- "all boost is not created equal" Some psi's are much cooler and efficient than others.

The less boost it takes to maintain a given EGT, the better.



JLH

 

[RonA]

I know that a 40 would work well as top turbo, trouble is they don't spool that well. I'd like to try one of these new whizz bang turbos. HTB2 or JRs new muffin fan or maybe even a ball bearing job. Just hate to go to all the trouble and have no reward at the end. Don't want to just increase the amount of boost pressure. 60 psi is high enough. I would like to run at the hp level I am at with same spool up and less smoke. I know..... picky,picky.

 

[SRadke]

That does not suond unrealistic to me Ron. You are a bit disadvantaged with technology not having the ability to manipulate timing as the modern computer controlled unit do but why should your goal not be possible? Tell me I'm wrong but if your smoking, you running high egt's with not enough air right? Seems like a no brainer. Trouble is you're working with a combination of parts that few people have done before so you're breaking new ground. Best of luck to you



-Scott

 

[Hohn]

Maybe J. R. 's new Schwitzer will be the way to go-- cheaper than a 40 too.



jlh

 

[RonA]

Egt is not bad. I am going to go back one size on the injectors and slide the plate back to the middle. That will be about 50 hp less. But still fine. After a couple of days it will seem normal again. By summer several other people will try some of these new turbos in twins and it won't be such a crap shoot for me.

 

[jwilliams3]

Go HX40, HTB2 or B1.

Answer to your question: Yes.

 

[SRadke]

I'm going to build a set with my Killer B2 on top before summer. Tough to compare a common rail to a 12 valve though.



-Scott

 

[RonA]

Don't want a 40 or a B1 starting to spool at 1700. I think Jeff's new one might be the ticket. HTB2 is too confusing for me. Bunch of variations. I'd get the wrong one the first 3 tries.

Ron