Twin Turbos - No tail vs Duck vs Turbo - Airflow/Downforce?

[RarlyL8]

Boost is full on at 3000rpm. Not sure at what point it begins to build. It is interesting to watch the boost gauge move up to 0.1 bar whan I climb a mountain at a sustained speed of 2500rpm. No throttle input is needed, the turbo adds the extra power. Cool.

What I would like is to have full boost at 2500rpm. Is this possible given what I have to work with? Wouldn't ANY header for a single turbo have runners that are too long? Twins are the only option for the absolute minimum runner length.
The shock that the car receives when boost hits is violent. I figure the earlier it comes in the better I can control the "hit".

Ducktail with my intercooler is definitely doable. Ducktail with a custom full bay intercooler is also doable. The unit just has to be more shallow than all the monsters we usually see. This is not a track car so that's not a problem.

No body mods will be done. I've got way too much time sweat and money in the paint now.

I know Mike the Mechanic Amac and Natchamp have twins. What is your boost threshold? How much does EFI play into this?

[efhughes3]

I just pulled a Carrera tail off my car and went to a duck. The stock tail weighed 41lbs, the FG duck weighs 11. I'm sure that staying with a FG tail, whatever shape you need to fit the intercooler will be much lighter that the stock steel stuff.

[beepbeep]

It's a multifaceted problem.

First of all, it's hard to make turbo-charged motor act as N/A one. There are certain tricks (ALS and such) but it's WRC/dragracing stuff that is hard to use on street.

Second, perception of "lag" can be divided into two components:
1. Shaft intertia = how long time does it take to spin the turbo when you floor it at, say 4500 RPM's.
2. Boost treshold = when does it start to produce positive manifold pressure if you keep your accellerator floored from 1000 RPM.

My understanding of your problem is that your turbocharger makes transition from no boost to lot's of boost in short rev range (around 3000 RPM), right?

Well, that's something that's connected to actual turbochargers compressor-map.

-- snipped text based on misscalculation, se below for correct calculations --

Also, there is lot to be won by stripping out heat and wrapping headers with thermal blanket. Using headers for heat is directly bad for turbo...turbo want's its exhaust gases as hot as possible. It lives on gas flow, and gases expand when hot -> more flow.

If your K7200 kicks hard and has steep boost buildup then I guess that left surge line on it's compressor map is quite steep. Unfortunately, there are very few compressor maps for bigger K-27 units so there is lot's of costly mumbo-jumbo and shops that sell "customized" things without any documentation. That's why I like Garrett. It's all there and there are no smoke screens.


Personal observation:

I have driven cars with small turbochargers giving lightning-quick boost from 1700 RPM and upwards and i didn't like it. Boost was ther in like, 0.2 seconds. Spoolup was so quick you couldn't notice car was turbocharged but you could still tell there was a small lag. It felt like driving big N/A engine with sticking throttle

I guess I'm just old-school and like decent turbo-kick :-)

[RarlyL8]

Turbo map? I thought that was a dragonfly's wing.

Just kidding. Great info. Maybe I should just be happy with what I have. The motor spins relentlessly to 7000rpm. Real easy to over rev in 1st and 2nd. I don't like to rely on the rev limit switch to react quicker than I can lift.
I can manually eleminate "lag" by not stabbing the throttle off boost. The motor will follow a quick foot feed from idle to 3000rpm and then a stomp. It just happens so quickly that I have trouble getting it right every time. Violent boost bursts break parts spin tires and can cause loss of control. Maybe I should look to the transaxle for help. A very tight posi might help things out.
I have less than 400 HP. How on earth do high output turbo folks deal with these issues? And how can Juan NOT spin his tires? That's nuts!

[efhughes3]

Originally posted by RarlyL8
And how can Juan NOT spin his tires? That's nuts!

Easy-he just breaks his axles!

[juan ruiz]

How on earth do high output turbo folks deal with these issues? And how can Juan NOT spin his tires? That's nuts!

Well my findings...................

I recently made some more changes and we set the low boost to 22 pnds at that level even with the drag radials it will spin them so we start to work on that and at 17 psi on the low side she performs great and no tire spin, we now have the high boost set up to 30 psi.

we will run depending street /track/ either 17-22 and the high boost is at 30 psi period, we are running 1 st and 2 nd gear on low boost regardless of street/track we will on 3 er gear with a remote control .switch to the high boost ,the car will run fine with that set up, we will increase the high boost to 40 psi and the low will stay the same, our power band is all mid range to top range which helps a lot with the traction a problem everyone seems to have.

we are 27-0 on the underground world and thats with cars that run low 10 sec at the track, the closest race that we had so far we won by 4 cars, back to square one traction and the ability to deliver power from mid range to top.

And yes we are breaking axles out of the hole, and yes we have found the solution, and yes we will continue our quest is just matter of time and $.

Good Luck with your twins, but you can make a LOT of power going single, and without a proper intercooler tt will do very little for you.

Of course thats just my 0.02

[RarlyL8]

40psi boost? And I thought I had issues.
Apparently that locked differential you have must work miracles. This may be the area I need to explore. I'm not familiar with the lock up ratios offered by Quafe and others but soon will be.
Again, I'm not after big HP numbers but increased driveability on/off boost.
So, given all the input by you fellas, is it reasonable to assume that I cannot achieve a significant decrease in boost lag by going twin turbos vs short wrapped headers and a single turbo?
Is it also reasonable to assume that no turbo exists that would pull full boost 1.0bar from 2000-7000rpm and support 400hp?

Duck tail weighs 11pounds? Where the heck to I order? What do these things typically cost? I'm checking Pelican as soon as I post this note.

[efhughes3]

I'm not sure if Pelican sells the ducks. I got mine from The Racers Group, $379. They have a CF version for $100 more.

[Lukesportsman]

Rennspeed is likely the least expensive.

[beepbeep]

-- snipped text based on misscalculation --

[edited, see my post below]

[juan ruiz]

(well, not much more than mumbo-jumbo KKK unit with Porsche-tax added)

[beepbeep]

Bollocks!

I somehow entered 6-strokes in my whiz-bang Excel spreadsheet so numbers are all screwed!

I'll get back with new numbers later this evening!

[beepbeep]

Here we go again:


3300cc 930 engine will gulp 34,8lbs/min of air at 5500RPM boosting 0.7 bar and will produce 302HP with 90% VE, 29% efficency, 40deg. C intake air (I used this to "calibrate" my Excel sheet as this is typical for 930 engine)

If we look extreme case and calculate for 6000 RPM and 1 bar of boost, same engine will use 43,0lbs/min and produce 381hp, corrected for 42 deg C higher intake temp and slighly less VE of 89%.

This means that you will be far outside of GT28RS map so I beg you all not to use any of calculations I made higher up!

It will be spot-on with GT30R though.

If twin turbos are used this means that 21.5lbs/min mass flow per turbo is needed, and best matching turbo would still be GT15 (altough little more extended into less efficient region).

So answer is:

Single turbo: GT30R
Dual turbos: GT15

Calculating boost treshold gives:

It will use 14,2 lbs/min at 2000 RPM and 0.7 bar boost assuming 100% VE (which happends at low revs) and 32 deg. C intake air temp.

That's well inside GT30R surge line and my guess is that you would see boost buildup around 1500 RPM if using one of those.

Doing the same thing for GT15 gives 7.1lbs/min per turbo which is on the endge but will still work.

So final decision:


Dual GT15's will give boost slightly later than single GT30R (but still 1000 revs before your current KKK unit) and will pump air with 71% efficiency at top end. They will have shorter spool-up time at higher revs though, due to low inertia.

Single GT30R will spool up slightly earlier using identical headers as GT15 (but probably the same assuming long headers used for single turbo) and will pump air with 72% efficiency, allowing slighly more power (but it will probably get nilled by using long headers).


And my personal recomendation:

GT30R all the way. It's single, it's easier to install, there is less piping to fabricate, it will have lower boost treshold than twin GT15's and breathe as well at high end provided you don't boost more than 1 bar @ 6000 RPM.


Sorry if I messed things up with my earlier post, mistakes do happend. I changed "4" into "6" noth thinking it's number of strokes and not number of cylinders.

Cheers!

[beepbeep]

And here are some butterfly wings to match:

[RarlyL8]

Thanks for the info!

Now one question. How the heck do you read those funky graphs?
Can you explain to me how one big turbo is able to spin up quicker than two small ones? Not only are the impellers smaller but the headers are shorter.

[Rob Channell]

One possible reason is they are also getting half as much of the exhaust gases each.

Wayne doesn't sell it, but here's a good turbocharger book.

http://www.amazon.com/exec/obidos/tg/detail/-/0895861356/qid=1101783280/sr=8-2/ref=sr_8_xs_ap_i2_xgl14/102-5911360-6472956?v=glance&s=books&n=507846

I like it but some people reviewed it and called it "dated".
Other people liked the Corky Bell book, but I haven't read it yet.
http://www.amazon.com/exec/obidos/tg/detail/-/0837601606/ref=pd_bxgy_text_1/102-5911360-6472956?v=glance&s=books&st=*

[Lukesportsman]

Corky Bell's is one of the best reads you'll have. You won't regret the $$ spent on it and you'll feel in better command of a turbo package. Great read for both the virgin and the experienced.

[juan ruiz]

beepbeep

if you were to compare side by side the T series turbos vs the Gt series for example

T- 66 = GT 28 and so on, it be cool to have a side by side comparasion since you are doing such of good job

[beepbeep]

Originally posted by RarlyL8
Thanks for the info!

Now one question. How the heck do you read those funky graphs?

Easy. Vertical axle is pressure ratio. If it says 1.7 it means 0.7 bar of pressure difference = 0.7 bar of boost.

Horisontal axle is mass flow. It's how much air you move per minute. "the wings" is how good compressor is working at certain regime.

For example, if you use GT15 and try to achieve 1 bar of pressure into engine that will only accept 5 lbs/min it won't work. It will just stall like clogged vacuum-cleaner.

If your engine uses 15lbs/min at same boost, you'll achieve approx 74% efficiency, which means that 74% of energy used to spin the turbine will be compressing the air and rest will be heating it.

Originally posted by RarlyL8

Can you explain to me how one big turbo is able to spin up quicker than two small ones? Not only are the impellers smaller but the headers are shorter.

It's not just a matter of spinning up. It's also a matter of internal geometry, flow, shaft frictional-losses etc etc.
(I didn't take into account header heat losses as it's very hard to estimate).

Twin turbochargers will also get half of flow, so I started by calculating how much your engine would be using in certain range (0.7 bar @ 2000 RPM) and plotting it into graphs.

It's 7.1 lbs/min for GT15 and 14,2 for single GT30R.

For 1 bar @ 6000 RPM mass flow figures are 43 and 21.5 lbs/min respectivly.

GT30R will pump with 72% efficiency there and GT15 will be around 68%...For low boost, GT15 will pump with 65% efficiency and GT30R around 68%.

GT30R is single unit with just one ball bearing shaft. Dual GT15's have twin shafts with plain-bearings (=more frictional losses)

GT30R can be fitted on same place as KKK papperweight, minimizing the amount of plumbing, both lubrication-, intake- and exhaust-wise.

Dual GT15's will have shorter headers (=good), approximately same boost treshold, pump more hot air at top-end (=worse), use dual intake and exhaust plumbing (=worse), have twin plainbeared shafts that need to be spun (=worse).

To me, case is simple. Your engine size and desired power level makes twin turbocharging unnecessary and costly project, especially when using state-of-the-art single turbocharger. Even more so considering you still want to keep CIS.

It's up to you...

[RarlyL8]

I've read both those books and more. Good stuff.

Thanks again for all this info. Much more specific than what I've read. This all leads to an additional question:
Given all the above info, why did Porsche twin turbocharge the 993T ?