Sequential turbos - PeachParts Mercedes-Benz Forum

derherr65 · #1 03-15-2007, 11:52 PM

I keep thinking that sequential turbos would be interesting.
Now to clarify sequential turbo means you have 2 turbos, one small, one big. The small one spools up fast giving you low rpm boost. The big one kicks in at mid range somewhere and usually puts out a little more pressure than the smaller turbo. Result being power from off idle to max revs. This is not compound. One does not feed the other.
Do I have the room? I don't know. Do I?
Do I have the budget? Not yet.
Do I have the skills? Probably. I might have to farm out the aluminium welding.
Do I have the needed intercooler or pump mods to make all this turbo stuff safe and worthwhile? No.(Boy you guys are hard on me!)
But to the theory discussion anyway. (all mechanical parts, I hate computers in cars)

General idea:
Exhaust manifold lets out into a exhaust cut out. Cut out is actuated by a boost solenoid. Closed routes exhaust to small turbo, open routes exhaust to both. Both turbos exhaust into a large exhaust pipe.
Filtered air is routed to both turbos, but there is a one way valve or flapper immediately before each turbo. Both turbo outlets are plumbed to the intake manifold.
So I'm thinking the little turbo should be something slightly smaller than the stock KKK, maybe a GT1544, run at about 10-12 PSI. The big turbo should be a little bigger than the stock T3, a GT2259 maybe run at 16-18PSI.
So you would have lots of low end and enough top end that you'd need a myna pump to overfuel it. I think.

What are the pitfalls?

BioPOWER · #2 03-16-2007, 12:09 AM

I'm guessing you'd install them on your CD? That would be awesome!
I have always had a liking for the 300CD's, it's just something about having a two-door turbodiesel

.

Quote:

Originally Posted by derherr65

What are the pitfalls?

1. $$$
2. Engineering...solenoids, plumbing, etc.
3. Getting it to function smoothly
Wouldn't a VGT like on the new CDI's and TDI's also eliminate lag while allowing higher top-end PSI?
Just my 2¢

........I'm sorry, I just can't help imagining my 606 running twin turbos..

#3 03-16-2007, 12:20 AM

Instead of all that complicated junk leading up to a GT2259, why not just use a single GT2259V? The same response time (if not faster) as two turbos, much more tunability, cheaper, more simple, more efficient, lighter, and more reliable.

Twin parallel turbos were obsoleted by VNT turbos. Sequential turbos are only useful if you are going for high boost pressures with a very large turbo for the high end boost.

derherr65 · #4 03-16-2007, 12:21 AM

Quote:

Originally Posted by BioPOWER

I'm sorry, I just can't help imagining my 606 running twin turbos..

Lol! Well you know what I'm talking about then. Yes, a variable may be the better way. Almost certainly cheaper and simpler. I think a GTV23 was about ideal.

#5 03-16-2007, 12:34 AM

A VNT will build boost crazy fast. Unless you want to run more than 30psi of boost, I'd suggest the VNT route.

Here is the diagram you need:

derherr65 · #6 03-16-2007, 01:35 AM

Would be a lot simpler. I forget, what did you find the sizing of the your... Gt2256V was it?... to be?
What boost is yours set for?
When does boost hit 8-10PSI, wide open throttle, with the vanes fully closed?
What boost does it run as you pass 4000rpm, wide open throttle, with vanes fully open?

#7 03-16-2007, 02:15 AM

Quote:

Originally Posted by derherr65

Would be a lot simpler. about your GT2256V...
What boost is it set for?
When does boost hit 8-10PSI, wide open throttle, with the vanes fully closed?
What boost does it run as you pass 4000rpm, wide open throttle, with vanes fully open?

Right now I have it set for 13psi. It's small for people wanting big power, but it's great for a fairly stock engine.

I get 13psi by ~2000rpm at full accelerator depending on what kind of clutch play I use. If you were to power brake with an auto tranny, I have no doubt you could get full boost without moving. I'll get full boost in every gear before I shift at ~2800rpm (Except 1st gear where I usually hit ~8psi.) If I wire the vanes open manually, I get about 11psi at redline WOT.

I can take this turbo to about 18psi if I had the fuel to use that much boost. The GT2256V would be great if you want 150hp. A GT2359V will get you to the 200hp range. The very rare GT2559V will get you into the 250hp range. If you want the most power, a huge GT37V like Brandon has will get you 300+hp.

RUN-EM · #8 03-16-2007, 09:47 AM

be a better way to go. Use a belt driven supercharger for low rpm boost. Use a breather valve to allow supercharger access to atmospheric pressure.
Then have a large turbo feeding the supercharger for top end boost. The breather valve between the supercharger and turbo could be sprung to automatically close when boost is sensed. This manner would get rid of the off the line lurch of our cars as boost could be set from idle on up.

Regards

Run-em
1983 300SD-aka- SPARKY THE DIESEL

derherr65 · #9 03-16-2007, 10:15 AM

I'm thinking a GT2359V would be about perfect.

rrgrassi · #10 03-16-2007, 10:24 AM

I think the new FORD POS 450 runs a dual turboed diesel. But being it's a FORD I did not pay really close attention.

Peter Plowman · #11 03-16-2007, 10:51 AM

Previously in the 6.0L Ford used a Garrett vgt (variable geometry turbine), it was capable of about 35 to 40 lbs of boost, but stock put out about 23lbs, Ford switched to using a compound turbo setup, now it runs a tiny turbo to produce boost from about 500 to about 1200 rpm, and the big one serves up boost to around 4k, usually they put out about 28lbs of boost, but some people have seen up to 40 when pulling. Sequential turbos are nothing fun to cut your teeth on, so if you plan to make a system using them I suggest you use a variable geometry which is a wee bit too large all by itself, to start out with, and then in a year or so you will be compelled by the lag to r&d a system that includes that larger vgt, with a smaller one of your choosing, also this will save you the trouble of having to buy another turbo besides big daddy and jr. Assuming you had to full intentions to make the system sequential, but wanted some time and expierence to make a fully educated decision. Otherwise, if you want to stay below 25 or 30 lbs just go with vgt (or vnt if you like)
Ford tech, but I'm leaving before this stuff gets too complicated for me. . . off 2 Navy!

winmutt · #12 03-16-2007, 10:53 AM

You can always do the supercharger/turbo combo.

psfred · #13 03-16-2007, 11:19 AM

Unless you really like rebuilding engines, limit boost on a 617 benz engine to 13.1 psi -- above that, compression pressures are too high for the crankpin bearing, and it you will get "significantly reduced engine life" -- meaning it will toss a rod MUCH sooner than normal. Don't know how long it will last, so if you are racing, it probably doesn't matter (ha ha), but if you want to actually drive it on the road, a year to two and it to stick one out the side of the block. You are also likely to blow a head gasket -- figure up the internal pressure. Stock with a turbo blowing 1 atm in there will give you around 700 psi before injection, something like 1500 after injection burn gets going. Double the pressure going in and you double the pressure inside, and you have 3000 psi in the combustion chamber .....

Remember, you are starting at 21:1 compression ratio, and it's a prechamber design, so there are limits on the amount of fuel you can burn, and how long you can inject, before you burn the prechambers off.

DI engines that have low compression ratios (16:1) require MUCH more boost to obtain the same effective compression ratio that a 21:1 engine has at 13 psi boost. Volvos require even less, as they run 23:1.

Peter

derherr65 · #14 03-16-2007, 11:36 AM

Hmmm. I was thinking 13PSI was a turbo limit, not an engine design limit. Naturally it wouldn't see even 13 PSI most of the time. Usually you're just cruising along. Higher pressures only when passing, on ramps, and the like.

I understand the compression ratio argument, but can you go into more detail on how a prechamber design limits on the amount of fuel you can burn? And why/how would the prechambers burn off.

psfred · #15 03-16-2007, 01:00 PM

A big enough turbo will produce something like 50 PSI, I don't remember what the practical limit is.

Prechamber engines actually burn the fuel mostly inside the prechamber, so you are limited to how much you can inject simply due to the fact that no oxygen will remain as the gasses expand and all you get is black smoke. Black smoke = unburned fuel, and hence loss of efficiency, and eventually loss of power as well -- over rich, and it doesn't burn hot, etc.

Longer injection time (more fuel from a given injection pump) results in higher temperature end of burn, and that can overheat the prechamber tip -- won't hurt anything else, but they get hot. So will propane injection into the intake -- it will pre-ignite is the mixture is ignitable in a 21:1 engine, it will get hot enough long before TDC. That will burn the prechamber AND the piston crown.

Slobbering low compression highly boosted diesels is an american thing, the European and Asians use high compression, low boost for the same power output per displacement, usually with much lower fuel consumption. US makers seem to have terrible problems keeping the engine together much above 18:1, they suffer from serious ring land failures and cylinder wall cracks. We have, at least, moved up to variable injection timing, that's a real plus, but only in the last 15 years.

It's easy to get more hp out of US pickup truck diesels because they are seriously de-tuned to keep the frame from twisting and the tranny working -- they produce MUCH more power in heavy truck form. This is not true of Benz diesels, they are pretty high on the power scale. Remember, the 617 turbo is based on a 1960's design and produced 150 plus hp with a four valve per cylinder head in 1972 (look up Merceds C111). That L6 Cummins is rated at 350 hp in heavy trucks....

Peter