Idea to increase spool and boost on primary
 

OK, so I'm think I know of a way to increase boost/efficiency on the primary and I'm wondering why no one does it(or maybe they do). Perhaps someone here will find the flaw in my idea but I haven't in the 30 minutes that I've been thinking about it.

OK, now for the idea

Before transition, the charge relief valve blows off boost from the secondary turbo back into the air box to avoid excess boost from building in front of the charge control valve. This pressurized air is basically going to waste. Why not route the air to the inlet of the primary turbo? Dumping that pressurized air in front of the intake of the primary turbo should help the primary build better boost and do so quicker. This would be easy to do for anyone running hardpipes. Just drill a 1" hole on the side of the primary intake pipe and weld a nipple for the charge relief hose to attach to. I think superchargers do the same thing on their wastegates/blowoff valve. They take the excess boost and dump it back in front of the charger.

I don't see the benefit. The vavle only starts shunting air to the secondary turbo to pre-spool it after 3000 rpm. Therefore, the only time you could possibly be assisting the primary turbo is in the 3000-4500 rpm range. Since the primary turbo already pretty much spools instantly in that range and can also hit whatever boost level you set it to by then, I can't see how there could be any benefit.

Issues I see with that:
By the time exhaust gas starts to get vented to the secondary turbo, the primary has hit its 10 psi peak a LOOONG time ago, so spool time would be unnaffected by it.

This would not allow the primary to make more boost (though I think it's capable of like 17psi, just not for long ;) ). The air coming from the secondary at that time would be much less pressurized than the air coming from the primary. So if anything, that 1" hole would probably vent boost from the primary back into the secondary turbo.

hmm, I'm no expert but I would think that this would just cause blow back throught he second turbo. It would be like haveing no blow off valve. The pressureized air from the first turbo would go the path of least resistance (kinda like that anyway) and instead of going to the rotors it would just go through the compressor of the second turbo, because the second turbo is not seeing enough energy from the exhaust to push the air the correct way. Follow me, I think I put it correctly....

No, he's saying put the hose from the discharge of the second turbo to the INLET of the first turbo, but whenever the second turbo actually starts boosting, a good portion of that will be going to the inlet (path of least resistance) of the first turbo instead of to the IC.

Correct, you plumb it to the atmospheric side of the turbo. So instead of the turbo using air from the airbox which is at atmospheric, you use pressurized air.

Hey Tyler,
I talked to my friend at work, and he said on his race FD, the primary runs out of breath before transition. He does hit 15PSI but it dies down to 13 or so before transition. This might help that out. I can't see how it could hurt.

Hoody

The air is already getting dumped back into the airbox which is where both turbos draw their air supply. So the stock system is already dumping the secondary's charged air back into the inlet. The system as stock already does all this.

So, you want to route the hose from the CRV back into the inlet of the primary turbo?

-I believe that's how it is plumbed in stock form. The CRV is just routed back into the clean air side of the airbox from what I remember.

-Also, my Apexi intake is plumbed the way you describe already.

So, in effect, it is a good idea,and it does in fact make a slight difference. I had the CRV vented to atmosphere for a bit a while back and noticed it spooled a bit quicker when it was again plumbed correctly.

yes and no. It does dump it back into the airbox, but the airbox is so open to the environment, the charged air would just disappear. If you dump it into the head of the primary where there's vacuum, then it would help free up the turbo.

It doesn't work that way. The charged air is sent back to the airbox which is already the source for the primary's intake air. The returned air will take the path of least resistance once inside the box. If the primary turbo wants that air, it will take it. If the primary has no use for it the box will pressurize (which is also a good thing) or escape out the airbox inlet. The only reason the air will ever completely exit the inlet is if the intake system is "backed up" with all it can hold and has no need for more.

There is no sense in plumbing the charge relief directly into the primary turbo inlet; dumping it into the airbox does the exact same thing. The entire airbox is under vaccum anytime the engine is running since the turbos are pulling air.

Again, you're both right and wrong. Pressure does take the path of least resistance like you said. But, since the airbox is so open(both to the atmosphere and it's own volume), you loose a lot of the pressure that's dumped into it. Basically, the vacuum produced at the inlet of the primary turbo is greater than the vacuum inside the airbox. This is because of the reduction of intake open of the turbo versus the airbox, the restriction from the air filter, and also any resistance in flow due to plumbing from the box to the turbo. I'd take out my old Fluid dynamics book and do the math but it's not worth the trouble. The best way to get that pressure into the primary turbo inlet is to dump it back in as close to the turbo inlet as possible. Even angling it so it blows the boost directly into the turbo would be a benefit.

This statement verifies what I said. The higher vacuum in the primary is going to pull all the air it wants from the airbox; the charge relief air isn't going to make it's way out of the box because the primary will ingest it first. If the primary doesn't ingest it first it's because the primary is at capacity.

Sure there is some resistance from the air filter and turbo plumbing, but if you want to get rid of that you need to chuck all your inlet piping, the filter and the box. Not recommended.

You're trying to build a more complex system that will have no gains.

just do it. . . :D

First of all,
if you dump it into the airbox, you loose a lot of pressure just because of a matter of volume, on top of what might get lost out of the intake box opening. If you open a small can of pressurized air in a small room, you will see a significant pressure change in the room. Open that same can in a much larger room, the change in room pressure is considerably less. Also, if you shoot it right at the primary turbo, it might even help spin it up.

Second,
the resistance of the piping does matter if you take into account the fact that you're dumping the CRV boost upstream, before all that stuff, which means you loose a lot of the benefit of the boost. You can't use an "all or nothing" mentality here. Just because piping, filter, and a box creates resistance doesn't mean you either have to use it or not at all. It's just a matter of bypassing that stuff when you dump the CRV boost in, since it's been filtered once already.

But what do I know, I'm only a mechanical engineer. I have no idea what a "pressure head" is, let alone know how to calculate it.

Here's a simple drawing of an M2 style airbox. Which CRV inlet do you think would be more effecient in delivering the pressurized air to the primary turbo?
https://www.rx7club.com//attachment....postid=2824733

Knock yourself out :) You're splitting very fine hairs. A few degrees difference in intake temp would make a bigger difference.


...and the stock system already dumps the charge relief air in behind the filter and in front of the inlets, so it does in fact enter the system at point "A" in your diagram. The air does not pass through the filter a second time because it's dumped into the top of the stock box which is already past the filter. I can assure you that a small volume of charge relief air is not going to choose to go back through the filter and out the inlet duct when there is a much greater volume of airflow in exactly the opposite direction.

...don't confuse the compressor for the turbine. Everything behind the compressor is pressurized ("boosted") so if you tried to dump air in there it wouldn't work because you'd essentially be creating a leak on the compressor side. The compressor would in fact pump air out of that connection and so not send as much to the intake manifold.

Good intent. Less vacuum at primary inlet to allow boost to be made at lower pressure ratio. But 3K min for relief to open really is a show stopper. At that point the relief flow is too low to influence vacuum at the primary inlet.

Primary response should not be an issue. If pushing the seq twin limits by 7K, then primary limit is at about 3.5K, and need early transition vs stock 4500, to avoid primary boost taper.

Will there be enough intake flow volume created by CRV vent to make a significant difference?? Turbo spool is created via Turbines being spun by exhaust flow. Thus, spool rpm will be controlled by exhaust flow velocity and volume. I don't think "turborizing a turbo" by forcing an excess amount of intake air-flow into the Compressor, will create a faster spooling turbo.

Actually, the stock system looks more like this so points A and B are different.

https://www.rx7club.com//attachment....postid=2825184
and I was talking about putting it on the compressed side of the turbo, as you can see on the diagram.

actually, it's common practice on Deisel engines to use a turbocharger and supercharger in series. They use one to precharge the intake of the other, effectively increasing the boosting capability of the system. I think top fuel dragsters do the same thing, or at least they used to.

Yeah, I think on a stock system, there might not be a whole lot to gain unless you're running out of boost before transition because the primary can't keep up the boost at the volume needed.

I'm doing this because my BNRs just don't have enough boost on the low end.

WHAT? Post a link, because I can't think of a single automotive diesel engine that runs a turbocharger and a supercharger in series.

I think you need to just solve the problem with the BNRs. Either something is wrong with your turbo control system or their is something wrong with the BNRs (which would make it a clean sweep -- all four sets of Stage II and Stage III BNRs I've seen were bad).

Actually top fuel dragsters do not have turbochargers, only superchargers. I am not aware of any TF dragsters that ever used both. There may have been a few experiments, but the commonly used method was only supercharged.

Yep, I said it in the other thread. Kwikrx7 had the same problem and his turbos needed to be rebuilt by BNR. After the rebuild, they were fine.

they mention it here

under SAE Diesel Engine Technology SEMINARS
Combination supercharger/turbocharger


also here
I emailed Bryan to see what he has to say, but after talking with Rick, it sounds like it's just the characterstics of the BNRs.