Smaller Turbo?

I've got a hankering for a little more low end oomph in the Mercedes ('85 300sd 3 L OM617 5 cylinder turbo diesel, 4 speed auto).

It's great on the highway, but around town it's a dog. It doesn't do much at all below 2k rpm, and I rarely wind it above 3k (I only ever get above 3k passing on the highway with the kickdown). About 3k is where it really starts to move.

I'm pretty sure the engine is wearing a 45 trim T3. I'd like to stay with a T3 if possible because of the rather unique fitting situation with the intake manifold.

I had had a Volvo 240 turbo that got a smaller watercooled T3 from a 740 (though I can't remember the size, might have been a 45 trim) and thought it was a nice improvement.

So my questions:

If I want a faster spool, do I need to change out the whole turbo or just the compressor side?

Are there smaller compressor sides for the T3 readily available than what I already have (and by readily available I mean like junkyard available)?

Should I just rebuild my current turbo and give it a bit more booooost? I'm not really sure what amount of boost is optimal on a 45 trim vs a 60 trim vs something else.

Is my logic all wrong? I've heard reports of boost coming on a tad earlier with my engine when the stock exhaust side is paired with a 60 trim compressor side.

VGT.

The first time I drove one of those engines I had the same thought. This thing would be better with a smaller turbo!

Unfortunately there's no easy way to make big strides while sticking with the t3 family of turbos.

There are a bunch of factors that influence how quickly a turbo accelerates/'spools'. I think for this case we can basically rule out the exhaust flow of the engine itself because making it flow significantly more at pre-boost rpms would be pretty involved.

The design of the turbine housing has a huge effect, but the question then becomes, is there more than one t3 turbine housing that can easily be bolted to the stock manifold? And even if there are, will any of them make it spool faster? Assuming you have something like a .48a/r housing already, there's nothing readily available as far as t3 turbine housings that will result in earlier spool.

As for the actual rotating parts of the turbo, the vast majority of the weight of the rotating assembly is the turbine section which is the turbine wheel and the shaft that goes through the center section. The compressor wheel is usually similar in size to the turbine wheel, but since it's aluminum it is always much lighter and has a relatively small influence on spool.

What's in your turbo is considered a 'stage 1' t3 turbine. There are no smaller turbines in the t3 family, so going smaller to lose weight would require you to change to a different turbo family.

As for changing the compressor side, there are smaller t3 compressor wheels, but they will not be easy to find at all and they will require you to change the compressor cover/housing to match the wheel as well, which will probably create some minor install/fitment issues. But, since the influence of the weight of the compressor wheel (especially given the same exducer/major diameter) is so small, you would probably not gain much by going that direction.

Installing a 60 trim t3 compressor MAY result in earlier boost because even though it adds a small amount of mass that has to be accelerated, it also equates to a larger compressor output at a given shaft rpm which may end up being a net benefit. Also, while you cannot shrink the stock compressor housing to fit a smaller compressor wheel, you CAN bore it out to fit a larger one, so it creates less (basically none) install issues.

But, the 'easiest' thing to do to speed up spool is to reduce backpressure after the turbine. The turbo operates mostly on a pressure differential across the turbine. You can look at it like a fraction or a ratio like 5:2. If you add 1 to the first number you have a slightly higher ratio, but if you take one away from the second number you have DOUBLED the ratio. That sort of explains how reducing backpressure is so important to turbo spool. I don't know how much of a PITA the downpipe design is on that engine but if it's not a major headache I would put a 3" exhaust on it as the first mod and get it up to 3" as close to the turbine housing as possible.

IF that doesn't get you enough, I think you'll be looking at bolting a completely different turbocharger to your engine.

In reply to Vigo:

Dude! That was an awesome post. Nicely explained

Vigo,

Is it ok with you if I repost your explanation on a Turbo Trans-Am forum that I moderate?

I'll give you full credit of course.

That is one of the best explanations I've seen in a long time.

Shawn

Yeah, what Vigo said.
Especially regarding exhaust, that one change can really wake up a turbo.

i had always thought that a smaller turbo would be better for my saab than the t3. my old NG900 SE Turbo used a T25 and made more HP with it kicking in at a lower rpm for a softer 'hit" of power. I always wondered if that would help keep the fragile transmission in the Classic 900 from self destructing

Vigo, thanks for the wall of information. I now have much thinking to do.

billet wheel; not smaller, but it'll spool like it's smaller

http://www.treadstoneperformance.com/product.phtml?p=2304&prodname=Precision+5431+CEA+Billet+50+Trim+Compressor+T3%2FT4+Turbo

I think that snail billet wheels may have some options as well

captdownshift wrote: billet wheel; not smaller, but it'll spool like it's smaller http://www.treadstoneperformance.com/product.phtml?p=2304&prodname=Precision+5431+CEA+Billet+50+Trim+Compressor+T3%2FT4+Turbo I think that snail billet wheels may have some options as well

That's almost what I paid for the car.

I think I'm just going to pick up a 60 trim from a Tbird and see what happens. I've heard good things. There shouldn't be any real modification involved, so going back to stock shouldn't be a big deal. If I could get my transmission to shift a little later (and I think I can), that would be nice. I found maps of both the stock turbo and the 60 trim. Maybe I can dig them up and someone can help me figure out what they mean. haha.

its an old merc diesel, its just slow. Its a great way to teach you patience.

On every turbo i've ever messed with, the more boost i ran, the earlier it started spooling and ramped up harder.

So... i think the solution here is more boost and a big exhaust.

fidelity101 wrote: its an old merc diesel, its just slow. Its a great way to teach you patience.

I really don't think it's that slow. Maybe about the same amount of fast as an NA Volvo 240. When it kicks down, it berkeleying flies. When I first got it, I didn't think the downshift worked. Figured out you have to floor it, then put the pedal like 2 more inches into the floor board. Also, I think it starts in 2nd gear unless you floor it. Not sure what that's about.

Anyway, I know it'll never be actually fast. It's all for fun. I'm sure I could do a bunch of work and still be overtaken by Chevy Aveos (happened yesterday ).

Swank Force One wrote: On every turbo i've ever messed with, the more boost i ran, the earlier it started spooling and ramped up harder. So... i think the solution here is more boost and a big exhaust.

^First step before actually doing anything to the Turbo. Need to get a MBC, wish I hadn't sold mine. See if I can't get my welding buddy to modify the downpipe, and go a bit bigger through the exhaust. I Might also remove the enormous shoot traps. I need to figure out the right balance between good flow and not having a horribly annoying drone on the interstate/have to yell at the person the the passenger seat to have a conversation. Although the S has a ridiculous amount of soundproofing.