PaulDaisy

This is a purely theoretical question since I am not building a set of twins (yet). I see a lot of people using HX35 as the secondary turbo and I also see people swapping HY35 for HX35 for better flow at high RPM. But if the HY is smaller then it must spool up a bit faster than HX? Would it make a decent top charger then, well not for big HP but for towing twins?

GFB

The turbine housing is way to small to flow the amount of air that a set of twins (even a small set) would deliver.

j-fox

I was looking into it before I bought my compounds.
It may work for towing if you used an external gate, and got it tuned right. The primary must use a shall exhaust housing too.

gwhammy

I think it would make a good towing setup with a external wastegate. I've had a couple of sets of twins, one with the stock hx on top and one with a b1 on top. The hx spooled decent in twins but I didn't like the b1 setup at all. I do run tight convertors so that makes a difference.
I'm setting up to try a late model which is kinda like the hy on a set of twins this winter on a second gen truck. Bottom end spoolup is really key for me on turbos.
Someone on this board a few years back had a hy over some turbo and it was nasty up to around 475 horse they said.

HOHN

Paul, with an external wastegate, the HY35 would be a superb performer in a twins setup, given the high altitude towing you do.

I'd pair it with a smaller "big" turbo with a tighter housing. The Garrett GT4294 with the smallest housing would be a perfect match for that HY35.

They would work well for up to 450hp even in really thin air like you have.

But down at sea level, they will "overboost" and tend to feel stopped up on the high end-- and boost levels will be much higher than optimal, but you should still be fine.

If you built such a system, I'd not run any timing advance at all because the combination of excessive boost and advanced timing is tough on the headgasket.

With OEM timing and the twins setup I described here, you should be fine up to 55psi and be able to tow in that thin CO air.

JMO (see sig)

Big Blue24

Hohn, to give a slightly different opinion, I used to have the same mindset as you, small secondary to get everything going, small big turbo primary so it spools quickly and have lightning quick spooling twins.

I believe the legendary Nathan at MPI used to sell a stocker over GT42XX as a quick spooling setup. They battled drive pressure issues on several sets and ended up creating inventing the 3rd gen stocker over S400 75mm I think, with the larger 1.36 ar exhaust housing. I believe this setup spools even faster, has much better drive pressure to boost ratio, and flat out makes more HP.

I believe what everyone decided or thinks is that with the tight housing on the GT42, it restricted spoolup of the top turbo too much as both had too small of exhaust turbine wheels. Although the GT42xx had an internal gate, it had to stay shut till the 42 hit 25psi and getting it fully lit caused too much backpressure to where it choked the motor a bit, made a flat spot in the power band, and was awkward.

They did, however, build a few sets of 62/65/13ss and or 12cast over same GT42xx and experienced a much better powerband, better midrange pull with no flat spots, and great topend. All of this by just getting a bigger turbine wheel on the secondary. I hear that the stock 3rd gen over s475 makes nearly the same power as the 62/65/13 over GT42xx; has nearly the same drive pressure, and spools great.

Just like you I like to think about turbos and setups. I would try the s400 75mm 1.32ar housing with the HY35. That tight HY needs a loose bottom turbo to perform well in my opinion.

PaulDaisy

Thank for the opinions guys. Like I said, my question was more theoretical than practical, I thought the HY would be a faster spooling top turbo and you'd suggest a bottom one to fit with it. Perhaps the minute differences in low RPM spoolup between the HY and HX are not even worth considering, and practical minds (unlike mine) have tried it and said, nah, never mind, stay with the HX for the top - hence so many HX-35 based twins.
For the practical purposes, Hohn, I am very inclined to try BigBlue24's HE351 setup on my truck. The third gen stockers are quite inexpensive so the experiment won't cost too much, even if it does not succeed. From B-B-24's descriptions I hope to get more air and lower EGT's around 2000 RPM - precisely where I use the truck most when towing.
Of course, whoever watched my "high elevation" reports closely would say, just weld the wastegate shut on the HX-35... well, I may try that too once I get the time to hook up the boost gauge.

HOHN

Good post, BB.

I think we might be considering two different scenarios, though, but maybe not.

If the main concern was just flat out spoolup rate, then I'd say you are spot on. But I'm thinking more of spool threshold, because Paul tows a lot in thin air. That means you need the turbos coming in really early to keep temps from getting out of hand. Not quickly, but early-- if that makes sense.

The way I look at it, you have two turbines, both of which need a certain pressure differential across them to work. If I make the big turbine on the small side, then there's not as much delta P on the small turbo and it gets laggier. If I make the top charger smaller, there's less pressure fed to the bottom one and it will respond later, too.

So I can see how having either turbo be too tight will cause lag in the other.

I've never been a fan of the HY35, but I know firsthand that towing in thin air really changes turbo performance and what you want/need-- so I was thinking an HY might be more in its element? I dunno.

Paul, I think BBs 3rd gen turbo setup would work very well for you and would probably give you more air at 2K rpm right where you want it.

I'd also say to just block your wastegate with a pipe plug-- you won't need it because you aren't pushing the truck that hard.

Keep in mind that the turbo's spinning faster to make boost in the thin air, so if you're running something like 40psi in thin air, it's definitely overspeeding!

Justin