IPSC

Here is a completely theoretical question for those in the know...

I am not even sure how one would do this but, would it help speed up turbo spool if, the up pipe that feeds the turbo had a built in Velocity Stack or Venturi right before it went into the hot side housing?

Discuss...

IPSC

eniac

That's basically the concept behind the stock J-boot and why it has such a small turbo inlet compared to the size of the AFM side.

blown 944

He isn't talking about the cold side

I am actually looking to do this very shortly, but instead of a trumpet stlyle outlet I will use a tall narrow outlet.

I am going to try this on my .96 T4 housing. I just got done logging down all the info over this last week as it is. The next step is to change to my .58 and then log that. Once done I will pull it once again and add the pipe.

I don't think it would work in most other housings due to space but in this one ther is plenty of room.

eniac

Oh yeah I see he said "hot side" now. I am at work so only half read it in between calls, lol.

I do not think it would make much difference on the hotside as the air is already compressed and in the process of expanding. This theory works on the cold side since the air coming in is not yet compressed. I don't have time to explain the dynamics but you would want as large and as consistant size of pipe as possible from the engine to the turbo. The air can expand in the x-over pipe and help spool up time. If you have any transistions, such as a small turbo hotside inlet, it should be as smooth of a transistion as possible similar to a velocity stack. This was covered in other thread about porting and polishing the turbo hotside.

Now having one on the exhaust coming directly out of the turbo would make a difference but would be slightly deafening.

IPSC

My thought was this: The exhaust gas is hot, under pressure and moving at a specific speed through the crossover pipe. If it could speed up just before it hits the hotside would that not spool the turbo faster?

IPSC

Raceboy

No, because it's not the gas speed that turns the turbine, but heat energy.

IPSC

Oh ok, I thought it was the speed and pressure of the exhaust.

IPSC

blown 944

I would have to respectfully dis-agree with that. Considering I am still running at about 1550 deg just below spoolup 4500rpm. If that was correct the spoolup would already be done.

Having used a large turbine I can honestly say that the size and velocity is also plays a big factor in turbo spoolup and total shaft speed. After all, it is still similiar to a pinwheel if it is a smaller concentrated area of airflow the pinwheel spins faster.

I will give you some real hard numbers after my install of the .58 turbine tonight.

It is in essence a similiar experiment. .58 is almost 1/2 the size of my current .96. If it doesn't act any different I'll be the first to say it, but I'm pretty positive (based on other cars) that it won't stay the same, both in spoolup and total boost

eniac

The difference between an intake velocity stack and one inside the crossover pipe is comparing hot expanding air to cold compressed air. The air inside the crossover pipe is hot and in the process of expanding. This gives added pressure inside the pipe, and the turbo. By adding what would effectively be a restriction with a venturi would increase your EGT, increase crossover pipe pressure, and INCREASE spool up time. At this point in the process the gas simply needs room(a larger pipe) to expand. The larger the pipe the more it will be allowed to expand without slowing down and the lower EGT's you will see. Anything tapering to a smaller diameter pipe will do nothing but inhibit this process.

Now like I said, if it does have to taper into a smaller pipe then you will want it ported and polish like a velocity stack would be, but it's best not to have the smaller diameter restriction in the first place.

blown 944

I think we are speaking the same language herre but, I am just looking at a very specific area.

If you can visualize th exit of the up pipe and the inlet of the turbine housing for a moment (larger tubo). There is an area that is larger in size than the pipe and the snail shell (turbine wheel area). What I am speaking of is not necessarily a restriction but an extended previous restriction of the std up-pipe carried into the turbine housing. Thus eliminating the funnel area of the turbine housing allowing compressed exhaust gasses to remain compressed all the way into the turbine wheel where they could then realy expand with great velocity.

I am only speaking of a T4 size myself here. However I think a T3 could also work.

95ONE

I had ZERO room for this. There has to be some 90 bend right before the Turbine. I don't see how you can have any room for any working velocity after the 90!

But, I am more than curious on your findings. Seriously, is your up pipe stock from the collector? Did you get one larger (Or customize the one you have like I did?) Or are you going from the restriction, to a big pipe, back to a restriction again. If that's the case, then I don't see how it will make any difference. Once again. Anxious for true numbers back to back, to verify.

eniac

I see what you are saying. Im curious to see what happens, just be sure to keep a close eye on the EGT.

gt37vgt

well the velocity stack already exists in the the turbo hot side if you match the size to the in let of the hotside you've done all you can .
i have herd plenty of accounts of trying to maintain velocity of the gas working for turbo response so using smaller primaries and less big open spaces as you will find in our cross over . in doing you this you will tend to carry more heat to the turbo hence the confusion .I'm going to play around with putting plates in to make the primaries a shade longer and better directed and run longer secondaries .
And i really feel in any application below 400whp the cross over pipe should be more like 2".

eniac

With a small crossover pipe even if it did happen to decrease spool up time, you will see a huge fall off at higher boost. This goes back to the "long running and wish it would die" debate on if a k26/6 can hold 15psi to redline in 4th gear. With a small crosspipe it cannot, with a larger upgraded pipe and an open exhaust it is possible.

The stock form does reduce to a smaller size going into the turbo much like a velocity stack which as mentioned the only thing you can do with that is make it as smooth of a transition as possible. Extended the small diameter section would in effect be adding resistance.

blown 944

It would be after the 90. I am talking about the area between the mounting flanges. I took some pics last night of the turbing housing area I will post them later. If you can imagine extending the pipe right into the turbine housing.

I did leave the stock size crossover. I had asked your thoughts earlier about the pipe sizing. I felt I would leave it for now and maybe experiment later. I will be curious of your finding with one vs the other (small vs large).

At this point I am on hold. I broke the transaxle last night after taking it out for a few runs. I think it is the R&P. Not sure if I will be able to pull it tonight. I ended up walking about 5 miles at about midnight and had to get up early to get it home (very low on sleep).

It ran great using the .58 hotside I had great boost from about 3200 rpm and I can even rev it sitting still and get about 5 psi. It was like a whole different car it was very fun from a stop and held great boost .

Now I just have to swap out the trans and get to the dyno and see what it's doing.