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Friday, December 26, 2008


The first rule of Mopar slant six turbo projects

There's a fair number of people on Mopar forums looking to build a turbo slant six who aren't quite sure where to start. I've seen a lot of posts where someone has a link to a random turbo for sale, asking, "Will this be a good turbo for my slant six?" Correctly sizing a turbo can take a bit of sifting through turbo maps and seeing which one you should start with. (If you're interested, the maps and calculator at Squirrel Performance are a big help.) But if you have no clue where to start, and somehow stumbled across this blog, here is my first rule for picking a turbo for a slant six project.

Don't go with a factory turbo off a small four cylinder and expect it to work well on a 225 cubic inch slant six.

Now, this is a rule of thumb, and like most of them, there are exceptions. If you've found a turbo map that shows that the turbo off some particularly high winding four is a great fit, then go for it Same goes if you have carefully thought through an application that needs the boost way down in the RPM range. But if you don't have such maps and just plain aren't sure, look elsewhere. What's wrong with a too small turbo? Well, having built my first incarnation of the Dart's turbo with the Mitsubishi TE04 off a Chrysler 2.5, here is a list of things I've learned from firsthand experience.

I'll start by showing an approximate turbo map of what you'd get if you were running this little turbo on a mildly built slant six that's running around six pounds of boost.

This is taken from the Squirrel Performance calculator mentioned earlier, and it shows the first problem. The goal with matching a turbo map can be approximately described as "keep that red line as close to that peak efficiency island in the middle as you can." And this one doesn't sail anywhere near the island. It's in the 60% and under efficiency range. At high RPM, it's off the chart entirely, quite possibly below 40% if you could actually hit 5,000 RPM under load. Which my engine wasn't able to do.

The reason my engine wasn't able to rev to 5,000 RPM is likely due to the next problem: The exhaust side isn't designed to cope with these flow numbers any more than the compressor wheel is. It becomes a major restriction at high RPM, resulting in an engine that can get a massive hit of torque at low RPM but runs out of breath way too soon.

That massive hit of torque may be what you're looking for - but probably not in the way it played out on my buildup. The turbine housing wasn't the only thing that didn't flow enough; the wastegate was so small it was barely able to control boost. With the wastegate plumbed straight to the compressor outlet, the boost could build up to 10 psi at 1,500 RPM. And that is below the torque converter's stall speed. That's what happens when you pick a turbo that's too small: The power comes on at an RPM too low to be useful, and it starts taking away power where you need it.

I wouldn't leave you with a bunch of don'ts, however. I'm going to wrap this up with where I would be looking if I wanted a secondhand turbo. Well, the first possibility is that I'll bet two of those little TE04H's would work nicely. But if you're going with a single turbo, you will want to either get one off a gaslone engine that's close to your own motor's size, or a larger diesel.

Unfortunately there aren't that many gasoline engines close to the slant six's size that used a single turbo. But there are a few - the Buick 3.8 V6, the third generation Supra Turbo, Nissan's 300ZX from the '80s come to mind. I haven't run the turbo maps on all of these to tell you which one would work best for you, but these are all ones I'd consider worthy of further research. And when it comes to diesels, there's a lot of large displacement diesels running Holset HX35s or HX40s you could get a turbo off.

I would mention the second rule of turbo slant sixes, but any long term readers of this blog could guess it. Namely, it's that you should plan to spend a lot of time on this project...


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