Turbos? (4agebug)

[AirPower]

I didn't want to hijack 4agebug's project - so I'll post my questions here. I hope he'll see this.

1) I want to ask the A/R ratio of the turbo he's currently using.

2) Does anyone else have some turbo experience? What works/did work well with a motor and what cc?

3) Who can build a decent turbo exhaust header?

I've got a T3/T4 turbo on the way - hot side .70 A/R and turbine .48 A/R. I checked the turbo map for the compressor and for the expected CFM - it matches more or less - my worry is the big exhaust A/R but I took a "big" one so that it doesn't "choke" the motor at high RPM. 4AGE has gone through this whole process and I would appreciate some input from him, but anyone is welcome to comment.

Thanks a lot guys.

Mario

[Tony Z]

too big a A/R might starve the engine lower down in the power scale.
Get out of the thought that power is rpm dependant as it is in N/A engines, in a Turbo engine, rpm does play a role but its airflow that counts --- you can be cruising at 6000rpm running 0.4 vacuum or be on boost at 1.2bar at 4000.
Make sure you know at what speed you want your engine to perform best and design the turbo system to fit that. If drag racing is your goal, then high rpm boost is what you want - 5000rpm upwards. Street cruising, a very quick spool up from 2000 to 5000 is ideal cause thats where you spend most of your time, street racing, aim for a system that starts boosting from 3500 to 7000.
Dont forget to build the engine to handle the big power of boost or otherwise run low boost. Remember that a stock beetle engine redlines at 4200rpm and 7000rpm will kill the case and crank in minutes.
I'll be doing a lot more research into this in a few years......... By that stage I hope to have my own place and I'll def build my own exhaust and incorperate a few tricks I have learned.

[AirPower]

You're correct there Tony -a big A/R can cause a lot of lag, but the opposite is also true - to small an A/R (hot side) and boost builds very quickly but the exhaust back pressure robs the motor of power at higher rpm because the gasses can't get out fast enough.

I was hoping there was someone who already figured out the perfect combination - I'm being lazy - I know.

I'm also wondering if a big enough waste-gate can cure the above problem (back pressure)?

[Tony Z]

a waste gate diverts waste... i.e. it diverts exhaust gas away from the turbine when you are on full boost. I think you aim to divert the exh gas to limit your boost at a lower value than your dump valve operates at, which improves efficiency - what is the point of compressing air, adding heat and then dumping it.
You would like to have a remote dump and remote waste gate if possible, its just more efficient, quicker acting and you can put it somewhere to look good.

Perfect combination.... no such thing, if there was, guys like Porsche wouldnt be experimenting with their (now) computer controlled variable vane technology which was previously mechanically (exh flow and boost pressure) operated.

[eben]

In my Audi I have a recirculating (as opposed to atmospheric) wastegate - function - prevents the exhaust from fully going through the turbo ie. make sure you get the required amount of boost.

it also has a diverter/dump valve which basically diverts boost to the athmosphere at gearshifts or sudden closing of the throttle to prevent overboost or turbo stall (when you get a pressure buildup in the compressor housing.. can damage it) it also ensure that the compressor can keep spinning to provide boost for after the gearshift.

Then there is the N75/Wastegate Frequency Valve which is basically boost control via the Motoronic management system which can make the wastegate open sooner or later as conditions require.

Thank heavens it should be simpler on a flat four

[Bugger]

Depending on your CC you are going to build I can not then really comment on what you have orderd and what I have gatherd through your other posts it will be around 2.4

I first had a .69 single scroll on my buggy and was not happy with it. then changed it to .80 twinscroll and felt the differance your compressor side sounds a bit to small what comp wheel are you getting #25 hopefully at least my comp housing on my buggy is a .60 big mouth they flow more than the old .70 housing that I also had but when I am going to finish it again for racing the comp side are going to go much bigger remember the buggy was very littel used for street driving basically just for Drag racing Foot flat high RPM and boost

On my Rotary I first had a 1.00 exhaust but also changed it to a 1.32 ye I know it is huge Turbo lag is more but can rev it to 10000RPM now with boost and the comp housing and wheel is a Equivalent to a small T66

[4agedub]

Hi
This is my current setup.

78mm stroker crank, 94mm pistons, engle tcs20 cam and 8.4:1 compression. The turbo is a turbonetics t3/t4. t04 vtrim compressor and 0.48 stage 3 exhaust. I reach full boost at about 3800rpm (0.8bar).

My engine has not been sorted out fully yet, so I can't say if the turbo is 100% right.

[AirPower]

Thanks guys

That is one HUGE turbo you've got there Bugger - my problem is that I need to be able to drive this buggy on the street, so too big a turbo won't suite me that well - I don't want to rev the motor to max all the time - it won't last.

Thanks for your input 4age.

The turbo I've ordered is also a T3/T4 - it's marked as a T04E here's the compressor map:



Now according to my calculations (2400 cc motor and 85 efficiency) I should get about 25lb of airflow at 5500 rpm (I think that works out to about 198 CFM). So for 1 bar of boost, which is point 2 on the y-axis I'm pretty close to max efficiency of the compressor.

My worry is - how do I know if the turbo will be scrolled up fast enough to reach this boost pressure at the 3500rpm that I want it to kick in?

Can I use a waste gate to dump exhaust pressure to prevent the back pressure from going much higher than the boost pressure?

[AirPower]

O JA! almost forgot - who can build me a decent exhaust header? This is not something I can tackle on my own - my welding skills are simply not up to the task and the primary pipe lengths are critical (I presume they must be as short as possible) - so a lot of bends will be required, basically I need an expert for this.Plies help

[Tony Z]

when you did your calculation to get 25lb of air, was this with our without factoring in the 1bar boost? (i.e. double the air mass as when N/A). Dont forget to add heat into the equation, if you are not intercooling, heat will prob be around 80degC at full boost.

Now, remember that a turbine is very inefficient if it isnt working in its designed speed range, so if you are out of its effiency range, the efficiency drops off very quickly (hense turbo lag and then suddenly coming on boost).
I think you have made a oopsie here. You factor in 5500rpm at 1bar boost. But you are asking about boost at 3500. Thats 16lbs of air (if your calculations are correct) at 1 bar and 3500rpm. Your turbo is on the surge line and effiency is down big time. You want your max boost and max rev to be just outside the effiency curve, but to the right so that with lower airflow and high boost (where you want full boost, from 3500 to 5000 for eg), your majority of the rev curve is in the high effiency range.

Do you get what I mean?

I reckon you need a smaller turbo, a bigger engine or much more revs.

Then again, I could be talking out of my ass as I have not got any first hand experience, but thats the judgements I would draw is it was my engine

[AirPower]

My calculations were that at 5500 rpm the motor will have an airflow of 25lb/min, the turbo can supply up to 35psi of boost (which is more than double what I require), but the waste gate will maintain boost at 14.8psi (1bar).So at 1bar of boost and the motor at 5500 rpm, the motor can flow enough air not to push the turbo into a surge state.

Boost should build as rpm and exhaust pressure picks up - so as far as I know you can basically draw a line from the point at 25lb x 2 down to the 0.0 point (bottom left corner). Basically what I'm saying is that because there is very little boost at lower rpm the heat is not an issue, but at max boost the turbo is at max efficiency (least amount of heat).

I'm hoping to rev the motor up to 6000 rpm - that's what I'm aiming for - so that will allow more airflow and move the point on the map to the right (horizontally). This leaves some room for improvement for the future - the more airflow I can get from the motor, the more likely the turbo will stay in it's max efficiency range.

It just hit me - I think we're saying the same thing, just in different words.

[Tony Z]

You are hoping for it to start boosting at 3500rpm. Boost will be in the area of 1bar. Airflow will be less than at 5500rpm due basically to the lower revs. That moves your curve to the left. The surge line is to the left. Surge = unstable, turbo cannot operate or will destroy itself pretty quickly if it operates in that range.
Not many people have ever heard a turbo surge... ask any Marchant Navy engineer what a surging turbo sounds like... probably the scariest thing you can experience in your life especially when you dont expect it.

Dont forget, that graph you posted is plotted without a waste gate or dump valve. Which means it is capapble of flowing much more than you need. So your waste gate will bypass exh gas past the turbine which regulates your boost pressure to a fairly straight line and the dump valve will dump excess pressure if needed.

I maintain you are moving too close to the left of the graph

[Bugger]

What CR are you aiming for your 14.8PSI and what fuel then

For a street buggy I would say have around 8to1 CR and run little lower boost with a high low boost electronic switch so if you are cought in the moment you can have your 1bar boost or just less or even more depending on the fuel and at what boost AND what temperature detonation starts

The hotter the motor the quicker detonation as well
With the slight higher CR and lower boost also less turbo lagg

Like 4age has said he has checked the grafts and will only know if he is happy with what hes got after tuning and driving and still there are so many external factors that play increment parts

That is also why I have built a water charge cooler on my bug from the beginning where I can keep the air temp in a sertain renge by switching on or off the water circulating pump automatically through the Microtech EFI system

The reason for this I have found With the intercooler the buggy gets tuned on a cold day then I go to Tarlton and it is hot now you have to play with the mixture knob again the next time I go its cold again the same story .
Now my airtemp can stay at a constant 40Deg and I`m a happy chappie


It is all horses for coarses

[AirPower]

Tony - so we get to the same problem - is there any way to predict when boost will start building? As you say if boost builds up to 1 bar too quickly, I'm off the chart and into surge bad for all. I was hoping there's some way to predict this.

It seems I'll have to simply run it and see.

Bugger - planned CR is low, 7.5 is what I'm aiming for (16cc dished pistons). I fear detonation like I fear the new laws the gov comes up with everyday . Also in the pipeline is a anti-detonation device (to retard timing). I would probably have to use an intercooler. Is it a bad idea to pipe the intercooler to the front of the car? The long pipes will surely have some impact on the turbo?

O JA - fuel must be pump gas - nothing added. I don't want to mess around with all kinds of additives - in town you run out of meth and what then - push the buggy home?

[AirPower]

Thank heavens it should be simpler on a flat four