I made the mistake of thinking this way a few years ago when first thinking about a setup like this. The pressure ratios of the two compressors does indeed multiply. You are correct that with a positive displacement blower it will move a fixed volume of air. The part you are missing is the fact that a turbo or supercharger does not increase the volume of air into an engine. They increase the mass of the air by increasing the density. The volume air flow into the engine does not increase. The volume of air into the compressor goes up but not out of the compressor. A compressor running a bar of boost might have 500cfm of air at 1bar into the inlet and 250cfm of air at 2bar at the outlet.

 

Thanks for the opinions guys and keep them coming . I ordered the S475 today and realized I had enough 4" ss pipe and bends from the DuraMax catback I replaced with the 2, 3" pipes, to make a 4" Down pipe to the duels. If I can get my torque converter deal figured out, I think this setup will work much better!

It's not that this isn't running good, but I think I can make it run MUCH better! It's so much fun to drive with the near instant boost. Here is a display of how quick it builds boost

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Andy, you are correct that the pressure ratios multiply, but they are not the same as they were before boost. After some anlaysis, I realized I short changed the TVS in my last post. The important thing to remember when doing any two stage system is that the second stage has a higher inlet pressure, and reading off a compressor map needs some correction. At a given boost pressure and inlet temperature, an engine will flow a known amount of air. You calculate the inlet flow at standard conditions, so say for this engine it's flowing 920 cfm at 15 psig and 140 inlet temp. The turbo is sized for that flow rate, but the pressure ratio is still unkown. It takes some iterative calculations to determine what the interstage pressure will be. The interstage pressure and temperature have a large effect on what the blower's pressure ratio will be. I have attached my analysis for this system.

This illustrates why you are having boost creep issues. 140 degree intake temp is probably pretty reasonable for your setup. As you can see in the attachment, the turbo is only putting out about 5 psig, the blower is doing the rest. It also looks like the T76 is way off into choke. Good luck with the bigger turbo and wastegate!

 

TwinturboC10, WOW! Thanks for taking the time to crunch these numbers. I wish you weren't clear over on the other side of NE, as I would love to have you over to pick your brain more about this, I'm in Wheatland WY. From the looks of things, I think I'm really on the right track with this new turbo. In it's current form, It's still a 75x100mm compressor wheel, which is still a little on the small side, but it is an easy and not too expensive upgrade to put a larger compressor wheel and housing on this turbo with out having to upgrade the entire turbo. Plus it has enough turbine size and flow to drive what ever compressor wheel i put on it with very good efficiently. Under most circumstances it is usually to much causing it have a good bit of lag but i think in this case it is well suited. I think with some playing around with bigger pulley sizes on the blower (to slow it down and make the turbo work more), I can really get the best of everything out of this setup. Also, am I not correct in thinking that being off to the right side of the compressor map isn't really choke, like being off the the left side, but merely less efficient use of the compressor. Like as in it needs a lot of flow, but not a lot of pressure, so it spending fast, but without a lot of load resulting in a less then ideal use of the compressor, but still not really hurting it that much in performance or creation of excess air temp. Kinda of like blowing with the wind? What do you think?

 

when I built a twin-charged VW Corrado, boost creep was huge when I tapped the wastegate signal from the turbo. I had to tap into the pressure after the supercharger to get the manifold pressure under control. In this case, it was a little m45 eaton being fed by a T3 on a 1.8L 8V motor with 8:1 compression. The eaton was providing maybe 5 psi peak on its own and the turbo got things up to about 17psi total. Peak power was about the same, but things happened a lot faster.

Hope this helps a bit .

 

That makes sense. Blowers have increasing boost with rpm. Boost is not going up because of the turbo increasing boost, it is the nature of the blower. More rpm=more boost.

 

Ya, I am referencing the WG off the manifold under the blower, its just everything is too small and can't flow enough air Hopefully parts will get here soon so I can get this thing working better

 

Got the S475 today and was able to mock up figure out what else I will need to big this big momma on the truck, lol.

 

I have found this thread SUPER interesting. How did this work out for you after the larger turbo?

 

I actually just took the blower off yesterday and am gonna run it turbo only again with a S475 and have a S480 1.10 A/R on the way. I took the truck to the track a few weeks ago and was only able to get with in about 2-3 tenths of my old times with a single Master Power T70. The MPT70 times where with 15 psi and these runs were only 13psi. The truck was VERY responsive on the street and jumped into boost with virtually no lag, but as another guy pointed out, that's a multi-thousand dollar intake manifold for just the better bottom end it gives me, lol. So I sold the blower and built a quick spool valve that should make the S475 spool much faster. Basically it should spool more like it has a T3, .66 a/r housing until it opens.

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I have been wanting to build something like that valve you have there but have not got the opportunity. I was just going to use a big wastegate to feed the one side off of the incoming exhaust. ?! I hope you find time to keep this thread going. I'm interested to see your results.

 

Now thats a sweet lil piece of engineering, as long as the butterfly plate don't burn up.

 

It shouldn't it's all made out of 304 stainless.