Hey everyone, I was advised to post this in the general turbo tech section to get more eyes on it.

I'm considering a mild small block 408 for a B body Plymouth. I already have the 408 from a dart I purchased. It's a standard fare 360 based 408 with Edelbrock heads. I would estimate around 425hp/450 ft-lbs.

It's not studded and it has no special block or parts other than it's a stroker. These blocks are generally regarded as being limited to neighborhood of 700 HP max. I am considering turbocharging this and putting it in my belvedere.

I have a set of Magnum Dakota exhaust manifolds I want to use for this. My intention is a responsive stroker with a mild cam and boost levels, to achieve in the neighborhood of 600-650 HP at the crank. I would keep the rpm below 5500 and rely on boost and displacement to get the power and torque.

So question is what lower cost turbo would get me to that power level without back pressures that would be severely damaging to the engine or turbo?

From everything I have read, at that displacement level I shouldn't need too much boost to get to 600, but at the same time that displacement is a lot of flow on the exhaust side. On the other hand, I don't want the lag nor do I need the power associated with a 1000 hp+ turbo.

To be honest I'd rather err on the side of the turbo being a little small if it gets me a more responsive combo.

Looking forward to seeing some suggestions.

 

Your 408 doing 425hp @ 5500 means it's running at 86% efficiency. Which means you'll need 9psi in the manifold to shove the 65lbs/ min of air needed for 650hp. Assuming 4psi is lost thru intercooler and such, that means 13psi at the turbo for a pressure ratio of 1.9.

Your standard s476 with 96 turbine would work if you want to do a single.
If you want response with better up top flow on the exhaust side as well, go twins with a pair of 7.3 Powerstroke turbos would work great. You can get them for $200 all day. Plus they're already wastegated. For even better spool pipe 2 cylinders into each side of the divided housings.
There's aftermarket options as well, drop in ball bearing center sections for $400 each, or complete ball bearing next size up turbo for $1400 for the pair.

 

Are there any examples here of people using these turbos for applications like this?

 

No clue, you'd have to search to see. What are you curious about?

 

I did search around. Found some single turbo applications. Nothing seems very compelling. I'd be curious to see if it would work well or not.

 

Search S475, not 476. The 476 isn't nearly as common. The S475 works quite well on engines making your na hp. It's a super common turbo.

 

I'm not super informed when it comes to LA engines, what fails at 700 hp? Many times, it's low speed torque that kills things, and rpm can be your friend.

 

Look here: https://www.theturboforums.com/search/4859816/?q=s475&o=relevance&c[title_only]=1&c[node]=234 259

 

The s400 96/88 turbine has an outlet area of 6,079mm², doesn't matter if you know what means, but no matter what all your exhaust flow has to go through that area.

2 gtp38 turbos from a 7.3 have a combined area of 6,633mm², or 10% more. Now the interesting part, a turbos rotor weight quadruples when you double the size. So in this case, the gtp38 turbines are 54% the size of the s400 turbine, which means they are 30% the weight. So by going twins your picking up 10% bigger flow area on the exhaust side, while having a rotor that weighs 40% less.

What I just told you most people don't know, people barely know how to size a single turbo, yet alone 2 to do the same job as one.

 

But you're driving it with 50% less CID, aren't you?

 

Correct, but look at it like this. Each bank (50% of flow) is driving 50% of the s400 turbine as a single.
With twins, that same bank is now driving a turbine that is 30% lighter. So even though the flow is halved, spool up is faster because the flow doesn't have as much weight to get spinning.

 

The powerstroke turbos are also an older design and likely less efficient, maybe not enough to matter in such a mild application.

I am not sure I buy the weight argument, if we look at a solid cylinder your weight tracks, but I think you would actually have to weigh these to see the difference, design is different, larger relative shaft on the smaller turbo, different blade count etc (maybe that's why you claim 50% in one post and 40% in the next). I get the point... but the s475 also has a better drive ratio, more turbine blade area to take advantage of exhaust energy, better compressor design.

The PS turbo I am thinking of is 58/82 on the compressor, 76/63 on the turbine, which is not really a great drive ratio. It was fine for the PS diesel, but it isn't a performance gasoline turbo by todays standard.

I don't think there will be much of a difference in spool. You would get the cool powerstroke turbo noises though.

The powerstroke turbos may be easier to package, they take a comparatively tiny downpipe and the s475 has a monster footprint.

 

I have both an s400 and gtp38 turbines, but the gtp38 is in a turbo and I don't feel like taking it out. So I instead used the gt62 turbine I have, which had 2 negatives going towards it for this "test".
1) it's mixed flow, so the turbine inducer is chopped off, less material in the turbine.
2) it's a boreless shaft, so alot less shaft material than the s400.
Pic of the 2... #ad


Weight of the gt62 was 88.9oz #ad

S400 was 33.6 #ad


Gt62 is 125mm exducer which works out to 12,271mm²
S400 88mm exducer is 6,082mm² for an area difference of almost exactly 50%, so the s400 should weigh 25% of the gt62.
33.6÷88.9=37%, yes that isn't 25% but like I said earlier, we're missing a good chunk of shaft and the inducer to the gt62. With both of those I'd bet it would be closer to ~27%. Like you said, blade count and stuff varies things to some degree, but you can't get around the rule of area of a circle. Even here the s400 is 50% in size but 63% lighter.

I also wouldn't discredit the older turbos to much, sure they won't put out the max level that the newer stuff does, but most turbos designed for oe were done with efficiency in mind, not all out power.

 

Powerstroke turbine and shaft weighs 14.4oz. So a pair is 28.8. Roughly 18% lighter for a pair compared to the s400 turbine and shaft you have pictured.