Turbo Theory - Tell me why I'm wrong
 

Tell me why this would not work....

Get a 60-1 compressor wheel and make your target 14 PSI. Now use a tiny exhaust A/R like .48 or something. You would spool up very quickly with this. OK, now your turbo is going to be spinning like crazy so the wastegate opens...

you have anticipated this and you have a HUGE wastegate and a HUGE connection where it bolts onto the manifold. Just for the sake of argument say it's 80 mm in diameter and it has a dump pipe so that it has very little back pressure on it so it can flow a lot.

Let us also say that you have a perfectly controlled wastegate so there will be no wobbling of it open-close-open-close etc.

In theory you would spool up very fast and then the wastegate would divert almost all of the exhaust out the dump tube leaving only a small amount to pass through the turbine. So you would not be choked by the small exhaust housing right?

Hmmm wait a second, I guess that to make 14 PSI at the compressor at X CFM flow rate, you need Y amount of work done on the turbine wheel side... so I guess you WOULD be choking the system because even if the A/R is small, you would then need a LOT of airflow velocity or a lot of pressure in the exhaust to get the needed 'impact' on the turbine to get it speeded up to the needed levels.

I think I just answered my own question... oh well.

Brian

It would choke the motor - as well as surge like a bitch.

A "big" compressor and a "small" turbine will spool quickly - too quickly.

The turbo will be flowing more air than the motor can injest at a given RPM. The air will flow back towards the turbo and you'll get the chug-chug-chug sound. Also your turbo will suffer a quick death.

Unfortunately - there is no magic turbo. However N2O is the magic chemical. :)

I *heard* from someone that the Indy/F1 cars used to do this. Thats how they were amking crazy power out of those small motors. Huge compressor, small turbine, HUGE wastegate.

when I was in italy I got to see a oldy F1 turbo V8 at the Ferrari Galleria

I was surprised at the really small turbine housings and the big compressors that they had,

the inconell tubular manifolds looked to be about 1 3/4-2 in in diameter, but then there is this tiny turbo wheel. THey had moderatly sized internal wastegates. anouther had external, but they didnt look to big.

guess it works considering there making like 1400 hp out of a 98 ci v8

Exactly my point

No, not really.
There's pics of "Indy" set-ups in Corky Bell's Maximum Boost book - they run HUGE turbine A/R's. Why do they do that? Because they are concentrating on a very narrow RPM band, and everything is optimized for it - there is very little need for low-end or broad power band.

The problem with Wargasm's original post is that the turbine being tiny chokes the exhaust flow on the top end. With that very little exhaust flow over the tiny turbine section, you're getting a very limited amount of haft speed - this is pure physics. Exhaust flow = more shaft speed. Why waste all that exhaust gases bleeding through the wastegate when you can apply it to the turbo? I realize you're only wanting 14psi, so why not step down to a smaller turbine section (i.e. T3), and run a larger turbine A/R?

Why not use a more modern turbine wheel design and step up to a Garrett GT-series components? Ala GT35?


-Ted

Ted I think you and I agree completely. Once I had typed out my post, I came to the same conclusions as you have. I was considering going to a GT series turbo but I'm not entirely sure if I will really make any more power and if so, will the gains be worth the money.

Do you have any RX-7 graphs with GT compoonents?

Brian

Understood.

As I stated...I *heard* thats what indy/F1 used. At least it sounded like a good idea :)

OLD Turbo F1's use very large A/R's for the Exhaust turbine and wheel.

I have many specs for the most powerfull engines ever in F1 (BMW 4cyl) and they use same or very similar size turbos as what we are using on 2.616lt 13B's !

I have a classic photo of a old HART F1 4cyl engine (850bhp) as used by Ayrton Senna to terrorise many established drives when he made his debut with the Toleman team in 1983 and it a had a Garrett T51 turbo charger with 1.22 rear housing on it :D They had a power band from 8000rpm to 10200rpm.

The pictures many of you see of twin turbo 1.5lt cars simply look so small becasue they are running two turbines on a 1.5lt engine ! The single turbo cars 4cyl HART's and BMW's of which the later were the most powerfull ever produced (1350 odd BHP proven !) give a good indication of just how large a turbine section is needed for maximum efficiency and power.

As Ted points out its at the expence of BOOST at idle :D but these are racing cars.

Wargasm, sounds like you are describing my set-up - lol.

My experience is you can definitely keep it from making the power drop off in the powerband in the upper rpms by using a very free flowing turbo back exhaust and a large WG, but you cannot escape BOOST CREEP.

Its last incarnation (it has been off the car 5 times for boost creep fixes) was stock '89 TII divided manifold, stock '89 TII turbo housing (~T3 size) bored to fit T4 "O" trim exhaust wheel clipped 15 degrees, w/ scroll's apertures (slots) widened from 5mm to 10mm and a 60mm wastegate. T04B 60-1 Hi Fi compressor.

Boost creep still, it is currently getting bored for "P" trim exhaust wheel.

Much of the boost creep may be due to the less than optimum flow to the WG due to the turbo housing design, but I can attest that the power (nor boost creep) does not drop off up to 8,000rpm even in this restrictive housing.

-EDIT- I am running 17psi, boost creep is ~20psi when the pop-off valve opens.

https://www.rx7club.com/forum/attach...postid=2128309
https://www.rx7club.com/forum/attach...postid=2128216

If you want very good spool up and very good flow I would recommend using a large Variable Geometry Turbo (VGT).

I plan on working w/ the Garret GT VGT unit off the Ford Powerstroke 6.0. The vanes are hydraulically actuated so there is some complexity there, but you don't have to worry about fitting in a wastegate and WG tubing to your manifold since VGTs are WG-less.

Please, someone do this before me so I can learn from your experience :p:

Whoa, neat setup there... got any dynos?

Brian

What if you used 2 turbos.
Say turbo A - a very small turbine on a moderately sized compressor and turbo B - somewhat larger turbine sided turbo with maybe a larger compressor side.

You could take the excess exhaust gases from turbo A from the low RPM opening wastegate and the DP (at all times) to turbo B. You'd have to make sure that the turbine sizing on turbo B was such that it didn't create too much backpressure for fear of REALLY choking off the engine.....but could it work? Is this something that has been done before? It seems to me someone must have thought of this before. I guess it sort of functionns like the stock 3rd gen system and its flapper door but on a mechanical system instead of solenoids, pressure chambers, and a rats nest....

You'd need a pretty huge WG for turbo A...

The size of the turbine wheel is the main factor in backpressure control and the a/r mainly determines when boost "comes in " and should be matched with the motors power band , fit the largest wheel you can into the smallest a/ r housing that would take it and you'll have great response and spool with good power , I am quite familiar with the garden hose analogy , but volume is also very important (not only pressure and velosity) in the workings of a turbo and we all know how much exhaust volume our motors produce and how sensitive they are to backpressure . Getting this volume out is more important than putting it in .Why everyone wants to force 14psi of boost into their engines @ 2000rpm without letting it out is beyond me , work toward reducing the pressure in your exhaust manifold instead of increasing it in your intake manifold .My turbine wheel is larger than what most of you have on your compressors ,that being 71mm in a largeframe 1.22a/r housing (since I run a half bridge and my power band is higher up ) and I was bustin' diffs and breaking my PPF at only 10 psi / 7000rpm!!! , now that I have taken care of those problems my ACT extreme clutch cannot hold anymore , and this is still at 10psi of boost !!.