I've heard about turbo surge often with the bigger hp trucks, but just wondering exactly what it is, and what causes it? Anyone enlighten me?
It was also quoted as I remember it as the reason why the truck with 3 turbos never worked right. So what is it about multiple turbos that makes it more apparent? Thanks all.

 

i *think* it's when the compressor wheel is trying to push too much air at whatever rpm it's (the turbo shaft) spinning at. leading to a cavitation of sorts. mostly caused by an exhaust housing thats too small.

this is my very uneducated guess. hopefully someone will be able to give you a more definite answer.

dave

 

"Surge line
The map width is limited on the left by the surge line. This is basically "stalling" of the air flow at the compressor inlet. With too small a volume flow and too high a pressure ratio, the flow can no longer adhere to the suction side of the blades, with the result that the discharge process is interrupted. The air flow through the compressor is reversed until a stable pressure ratio with positive volume flow rate is reached, the pressure builds up again and the cycle repeats. This flow instability continues at a fixed frequency and the resultant noise is known as "surging"."

Stolen from: http://www.turbodriven.com/en/turbof...compressor.asp



brandon.

 

Well that should explain it!

 

So if I turn up my flux capacitor my turbo will work better?

 

So could you concievably cure that with tripletts ( lol that even made me laugh) by putting the turbos in a big-small-medium stage on the compressor side instead of the big-medium-small? that would keep the biggest amount of pressure on the medium sized turbo instead of a little compressor? I'm not planning anything like this before anyone asks, just curious as to how all this stuff works.
Do twin-turbo'd trucks have this problem if they're actually only running like 60-80psi? I know alot probably depends on the turbo you're using, but say like a ht3b and a hx40? or just with like hx35's as a top turbo?
I'm trying to put together twins for my truck, but there's alot I need to do before I actually do it, I've got the pipe and the turbos, just need headstuds, and to O-ring it, prolly some bigger sticks than the 370's because I'm using a lower turbo with a big exhaust housing, and of course a transmission that might actually hold all that would be good. don't think my stock tranny would hold up,,,,it might be fun when I have the spare tranny sittin ready to go out and launch it at 15 psi in 4wd, lol. Just see what kind of parts I can spit through the case. Maybe not, but entertaining thought nonetheless

 

If you did triples, you'd want to keep the order as big-med-small because otherwise you are trying to supply the med turbo with the small one, and it can't make enough air.

Surge has nothing to do with total system PSI and such. It can happen at 5psi or 50psi.

Surge is most common on turbos that use a very tight turbine side relative to the compressor size, in an effort to enhance spoolup.

Think of surge as "overspooling" where the turbo is spooled up to deliver more air than the engine can handle. In any case I can imagine, making the turbo laggier will alleviate surging.

On twins surging is rare because the small turbo (which is more likely to surge) has slower spoolup because it has to suck air through the big turbo (before the big turbo makes boost), and this creates a partial vacuum in the crosstube for a split second. Also, the pressure differential across the small charger is less (compared to a single setup) because of the restriction of the large charger.

So the bigger your big charger is, the faster the small one will spool, but the slower the big one spools.

IMO, you want both turbos in a twins setup to spool about the same time. That's a TRUE compound setup. Imo, your big charger should be making boost when the little one is only at 12-15psi or so.

jmo as a guy that doesn't have twins

 

I always thought the point of twins was to have a big charger for higher rpm boost and a small charger to take care of the lag that you would have with just a big charger alone. Don't some trucks that are strictly drag race or strictly pulling just use a big charger alone? Takes forever to spool up I'm sure,,,hence the reason for the huge external wastegates that some trucks use to bypass the top turbo once the big one lights? I figured there'd be some overlap, and you'd never truly bypass the top turbo, but thought the big turbo was responsible for most of the boost at WOT and high rpm. ?

 

Sorta. The little charger always plays a big part. With just the big charger you'd have a huge amount of airflow at 20-30psi and you needed less airflow (CFM) at higher pressure 60-80 boost. The big charger determines overall system flow capability. The small turbo serves as a "transformer" of sorts.

An electrical transformer can change a high voltage, low current situation and change it into lower voltage, higher current. For example, it could take 1000V, 20Amps, and convert it to 100V, 200Amps (assuming perfect conversion). in both situations, you have 20KW of power.

Same with the small charger. It can convert low pressure high flow to less flow at higher pressure.

So your larger turbo determines over all system flow (like the total watts of electricity in our transformer example). The small turbo determines the final appearance, and controls the ratio of CFM to PSI.

Ideally, they would compliment each other and both be working full time. You want as much "overlap" as possible, not so much a "handoff".

Ideally, you'd want the small charger to spool up to the point of max efficiency (say 20psi for an hx35), then STOP right there and continue doing the same amount of pressure multiplication as the larger charger spools up.

For example, you have the small charger gated at 20psi. This is a PR of 2.36. Then, as the big charger starts to make boost, the PR of the small charger stays constant (so it stays in the most efficient part of its map).

So, big charger makes 5psi, then total boost (at 2.36 PR for little charger) is 31.8psi
Big charger makes 10psi, total boost is 43.6psi
Big charger makes 20psi, total boost is 67.2 psi

So, with the big charger loafing at 20psi, and the small charger loafing at the equivalent of 20psi, you have 67psig of boost that is cool and usable, and the turbos are at maximum efficiency.

That's the magic of compounded turbos!

 

very well said hohn

 

Good explainations there, don't supposed you know this one, I have an HX40 and a 4lhr Schwitzer that I'm planning on using for twins, I'll prolly end up getting a PDR 40 for the top eventually when I have more $$$, but prolly stick with the 4lhr, I mentioned it to Paul at HTT and he said it would work fine for a twin setup, just hot rodding, occasional pulling/racing. It has a larger exhaust housing 26cm I think, but anyone I've asked specifics on this turbo doesn't really know much about it. It looks pretty similiar to a friends HT3b, but that doesn't mean much I know. So what would be the most efficient boost ranged for a HT3b or something similiar? Or how about the HX40? I've got just an HX 40 on there now and I can still hit 1500+ at WOT, and it's not slow to get there, that's my main thing is EGT's are out of control, I don't feel comfortable even at the drag strip making a full pass, did it once, but that's all the bravery I had. Already roasted one set of pistons and not too keen on doing it with the Marine ones,,,those are expensive lol. Curious as to your thoughts, thanks :-)

 

uhhhh... roger!

 

you brought back memorys of my old Aeronautic Enganeer at work.....or however you spell it

 

I know nothing about a 4lhr Schwitzer.

One thing to keep in mind about all these different turbos is that they are designed for different applications. But in almost EVERY application, they are not running high boost. This is because they are fitted to differently sized engines.

For example, an HT3B is fitted to large semi tractors, like an N14 Cummins. It moves a lot more air than a B5.9. But it STILL only makes about 21psi of boost. The HX35 that comes stock is designed for a smallish 5.9L engine, running about 20psi of boost.

The HX40 is designed for a C8.3 engine, running about 20psi of boost.

So in almost every transportation application for a turbodiesel, we see a max of about 20 psi of boost. The turbos are matched to engine size (and RPM), and sized to give 20psi efficiently at X amount of flow rate.

So, imo, you want a turbo to run about 20-25psi max to avoid excessive discharge temps in the hat. The only way to get around this is with custom turbos designed for MUCH higher pressure ratios than a commercial, off-the-shelf turbo. COTS turbos are designed for high flow at low boost. We need high boost, less flow.

We saw from my earlier post that 20psi at each turbo would give over 60psi of total boost.

Enter another variable: RPM

When you speak of turbo "size" relative to engine size the only size that matters is engine flow rate in CFM. There are two paths to CFM-- a larger displacement, or more RPM (ignoring volumetric efficiency).

For example: I have a 5.9L CTD. Let's say that the "best efficiency" of the HX35 is at 1800rpm at 20psi of boost. The 5.9L at 1800rpm is flowing 187CFM nominal. Now, take an HX40 designed for an C8.3L at 1800 rpm. The 8.3L is flowing 263 CFM at the same rpm where the 5.9L was flowing 187CFM.

How much higher rpm does the 5.9L have to spin to make the HX40 "think" it's an 8.3L? (263*1800/187)= 2530 RPM.

So, in short, the HX40 is NOT a very good fit to the B5.9L. It can be "doctored" somewhat by hybriding it (40 compressor with 35 turbine) to try and bring the turbo in sooner. But what happens here is the turbine side is mismatched to the compressor side. The turbine side chokes flow and hurts performance. So you can "cheat" a bit to help spoolup, but because the compressor is so much larger than the turbine, the hot side chokes too soon, and performance is less than ideal.

Far better, imo to run a smaller 35 compressor and a much larger turbine housing with an external gate. The lighter mass of the 35 wheel will help spoolup over the 40, even with a larger turbine housing. Remember, the stock 12CM housing on the 35 is roughly equivalent to a 3/4" exhaust pipe. Restriction? YOU BET!

So if *I* were building twins, I'd strongly consider a 16 or 18.5 non-wg housing on an HX35 up top with a HUGE external WG (44mm min) to keep the top charger from overspinning (operating at too high a PR as well).


Now let's talk about Big charger sizing. It's really quite simple. It depends on the small charger's size, flow, etc.

For example, our HX35 flows .46kg/sec. Note, Holset says this turbo will only support 280hp! Anyway, we want our '35 operating at a PR of 2.36 to keep it efficient. Therefore, imo, you want a big charger that will flow AT LEAST 2.36 times as much: 1.08kg/sec! While a hair too small, this makes the HX60 a pretty good choice at .95kg/sec.

I'd love to try a build a twins setup someday, of for no other reason than to see if my thinking is on the mark or not.

jh

 

But with a 4000rpm kit (that I have sitting in the garage yet to install) That HX40 might be more suited wouldn't it with the higher RPM? and isn't an HX60 roughly the same size as a HT3b? or is it bigger or smaller? thanks again, startin to get all this, it's still sinking in lol. very helpful info.