VGT kit for 300 Tdi - group buy

[patch1]

Dont know if its been done but im sure its been spoken about and it would be a good supercharger for 1200 quid.

[Badger90]

allard sport have done it on a seires 2.25 diesel..

im not sure if they have done it on a 2.5Tdi lol.. would be interesting

cheers

sam

[mobyone]

a supercharger, without doubt would be better than a turbo upgrade. but you would probably not get a job specific well researched and tested kit for the same amount. you would have to pick a charger, find a gear wheel you want to spin it from, pick a pulley to select the revs it will spin at...

the list is long and by no means cheap. but it would be a very good mod that would easily outperform a turbo, even a vnt one.

but this thread is not really about that. i thought about mentioning this, but again, not really what the thread is about.

anyone interested in a supercharger should get a centrifugal type which is basically the compressor side of a turbo attatched to a shaft, pulley and driven by normally the camshaft as it is faster revving than the crank.

vortech have units which dont even need plumbing into oil lines as they are self contained. not sure on there price though.

don't go roots/eaton type, far more weight and far less efficient. also more maintainence.-- dont go roots --!

anyway, i hope you get your numbers up to get the discount, cheaper is better after all.

richard

[Aragorn]

I dont think it would.

Because Superchargers are geared from the crank, they tend to have low boost at low revs, and the boost climbs with the RPM reaching a peak at the red line (or before if the charger isnt big enough)...

This might be fine for a petrol engine, but on a diesel it would mean you lose all of the low end torque that people like dervs for. Dervs are torquey because the turbo comes in with a bar of boost at 2000rpm, which makes the engine perform like a 4L TDi rather than a 2L one.

[Michele]

One question I have on these whizzy turbos, how does variable vane system in the turbo actually work - is it purely mechanical with the vanes operated by the boost pressure (...)

It is

no electriccery

[Bowie69]

If you have a supercharger running at a speed relative to the engine, surely the boost is linear relative to the engine speed (ignoring losses), so the boost would be pretty fixed across the rev range?

A 2.5l N/A engine needs a certain amount of air at a certain number of revs, supercharge it and the charger speed is relative to engine speed, therefore putting in a relative extra amount of air into the cylinders on demand. So how/why can boost be higher at higher revs unless it is engineered in? 

You might be referring to engineered-in lower boost at low RPM to calm an erratic idle, but you don't mention this... The various types of charger will give different characteristics, maybe you are referring to just one type?

I also don't understand why the heck being geared from the crank makes any blind bit of difference to low speed boost....

[Aragorn]

Not really, because the volumetric efficiency and flow rates of a supercharger arent linear. Yes it will flow more at 7000rpm than it will at 3500, but not twice as much. It could be three times as much, it could be 1.5 times, depending on the design of the charger and its target application. There is also the relation between pressure and volume, again they are not directly linked, and depend on the design of the charger.

Plus a diesel engines VE drops pretty quickly as the revs climb, due to the camshaft profile, further complicating matters.

If you look out some dyno plots of various yank V8's (they're the easiest thing to find that use chargers) if you can find plots that include the boost pressure, you'll see how the pressure climbs with the revs, rather than staying constant.

If you think about it, you couldnt be generating a bar of pressure at idle...

[Aragorn]

Some examples:

http://www.scienceofspeed.com/products/engine_performance_products/NSX/ScienceofSpeed/Laminova_Intercooler/testing/pressure-graph.jpg

http://i237.photobucket.com/albums/ff47/SuperL98new/camaro-2.jpg

[Bowie69]

 

If you think about it, you couldnt be generating a bar of pressure at idle...

I have, and can't see the reasoning here....

[Aragorn]

How do you mean you cant see the reasoning....

A supercharged engine produces no positive pressure at idle. Thats a fact. The compressor simply isnt spinning fast enough to generate enough pressure.

I've just provided two graphs of random yank motors with superchargers fitted, and both show next to no boost at low revs, and a steady climb afterwards.

Clearly you could make that ramp occur a bit faster with different gearing

[Pete Attryde]

Surely if you select the pulley sizes correctly then it should be possible to spin the charger fast enough at idle to generate boost.

I accept that it will over speed if the engine speed increases much, but it should still be able to boost at tickover.

Pete

[TD5 power]

hi

mobyone can you explain how in a 4 stroke engine the camshaft rotates faster than the crankshaft?

aragorn i have very little knowledge of superchargers but my understanding is they are basically a pump that will move a certain amount of air for every revolution providing pressures stay the same. so bowie69 is right in that the output is linear ignoring the losses, pressure will increase only with the use of a charger that changes its output relative to RPM.

Why cant it generate 1 bar pressure at idle? though not sure it would be a good thing if it did

Matt

[Ex Member]

There are different types of superchargers. It is important to note which type you are discussing before making statements as they behave completely differently.

[Aragorn]

Compressors dont generate air in a linear manner, especially when the device your feeding consumes air in a very non linear fashion.

heres a worked example using the compressor map for a vortech supercharger:

http://www.capa.com.au/pics/library_vortech_testing_map.gif

If you look at the lines marked with RPM's you can see the superchargers range of operation at a given RPM.

You'll also notice that boost pressure (Y axis) is DIRECTLY related to shaft speed. If you want a PR of 2.0 (ie 1 bar) you need 45000rpm. Problem is the charger develops a minimum of 40cfm at 45000rpm, so if your engine isnt capable of swallowing 40cfm at 1 bar of pressure, "bad things" happen.

If your engine ingests say 10cfm at idle (its likely less than this), the maximum shaft speed you could use is 25000rpm, but the charger can only develop a PR of 1.3 at 2500rpm, so you can only get 0.3bar from it.

I hope that makes it easier to understand?

[errol209]

... ignoring the losses ...

Why cant it generate 1 bar pressure at idle? though not sure it would be a good thing if it did

There's the rub. At low revs, a supercharger (also called a blower for very good reason) will have some losses (pressurised air leaking back through the vane system) because it is optimised to deliver good volumes at lowish pressures (a few bar) and these losses will nearly equal the effect of the blower. At high revs the losses account for a much smaller fraction, giving you some pressure boost left over.

You could make a blower (more like a compressor) that worked well at low revs, but you'd have to dump insane volumes of air at 10 times idle revs because it would be a straight revs / pressure relationship - 10 bar boost ?

On a lighter side note, you could sort it all out by having an auto or CVT transmission on the supercharger: no lag and spot on boost all the time?

[Bowie69]

With correct gearing, you must be able to attain 1 bar at idle, so to say you "couldn't" is... wrong? misleading? poor choice of words? That's what I mean...

Do you mean 'car manufacturers that make supercharged engines design them so they have zero boost at idle'?

You've shown me graphs of a couple of V8 with chargers, but... I don't think that would cover the entire theory/options of charging TBH, just what a couple of people have done with their very high output, high revving motors....

Looking at one of those graphs there is a 4000rpm engine speed window where the boost line is pretty damned flat(4-8K), so to apply this to the 200TDI with appropriate gearing to give the flat boost line from 1000-5000rpm and you're sorted, no? I don't know what the 200TDi red line is, but this is just for example.

edit: Sorry to stray off topic so far... If mods see fit to split this then I understand why

[Aragorn]

Bowie: ye, you could gear it up assuming you dont run past the end of the allowable range for the charger, but you still hit the issue where your flowing more air than the motor can ingest at a particular pressure...

[mobyone]

whew, i knew i should have left it. what a can of worms...lol

td5, your right. i meant it the other way around. doh! having said that. the supercharger usually runs off of the camshaft as it runs at half the speed of engine rotation.

aragorn is right for roots type chargers. they have a fixed volume which compresses by a fixed amount. the more revs does not change that fact. it is just using the said charged air at a faster rate. for centrifugal chargers however,they very much act like turbo's. only they do boost from 0 all the way up to max revs. and my arguement was based solely on the centrifugal type of chargers.

pete, yes you are quite correct. you pick your gearing according to the boost levels you want. so yes, you can develop 1 bar of boost from idle-- however the revs it would be spinning further up the rev range would negate the boost developed as the charge temp would go up and efficiency would go down quite a bit. the blow off valve would be open more than closed and i would suggest that the charger would need alot more servicing for its hard running.

i really think that supercharging is the way to go; you never see a turbocharged drag racer as they simply dont have the boost from low engine revs like a supercharger does. but if you look at the cost of some of the kits out there, they are

several times more than what the turbo kits this thread was posted far are. some are £4600!

like i said, it is not straightforward. it does take designing correctly. and that is why i said that the group buy was good as it was made and properly designed for the 300tdi.

all the best on your group buy. and damn the 17.5% in the new year.

richard

[rusty_wingnut]

bar of boost at idle, surely that means poor fuel economy

The benefit of the VNT is you can lift off and cruise without max boost on, therefore munching through less fuel

[TD5 power]

hi

i didn't realise quite how bad the losses were for some chargers at low revs.

So from what has been posted i reckon for our application we need a centrifugal charger capable of producing required boost at 1000rpm say, then with a variable speed pulley on the engine side so we could alter the gearing as the engine speed increases solving the excess flow problem i think. I'm not sure you would be able to mechanically control the pulley so electrics would probably have to be involved.

Matt

[Aragorn]

With a normal turbo charger you wont be sitting on full boost while crusing either...

Theres not enough exhaust gas flow to keep the turbo at full boost.

Sat on the motorway in our A4 the engines producing around 20hp on the flat at 70mph. At those power levels the turbo will be spinning but wont be producing any meaningful boost until you reach a hill and open up the throttle to maintain pace. As long as you're over the boost threshold, the change is seamless.

Mobyone: my example a few posts back was actually of a centrifugal blower, not a rootes type. Rootes blowers like the Eaton M62 et al are positive displacement, so the RPM actually represents an airflow like bowie was suggesting.

http://s115271005.onlinehome.us/images/M62map1.gif

Theres a compressor map for a rootes charger. As you can see, the rpm bands are vertical, unlike the other map.

However, they only start compressing when the blower is up to 4500rpm ish and the blowers max RPM is around 14000ish. So you would need to gear the blower such that the engine governor is at 14000rpm. A TDI governs around 4500rpm giving a gear ratio of around 3.1:1. This means at idle, with the engine doing 800rpm, the blower is only spinning at 2480rpm. The charger would reach 4500rpm somewhere around 1400rpm.

I guess the diesels restrictive rev range actually works as an advantage here, you can push the charger harder earlier because your not revving out to 7000rpm.

Maybe it wouldnt be as bad as i thaught... however i'm still unsure what would happen if you build airflow quicker than the engine can accept it.

[Bowie69]

So cap the revs at 4000 on the TDi, 3.5:1 ratio pulleys and that would bring *good* boost in at 1300rpm...

I was trying to demonstrate the difference between diesel and petrol charging, given the rev range that they operate in, but you have clarified it a little now

As for 'not what you want when cruising', I thought people wanted VGT for lower and more continuous grunt for off-road driving 

[mobyone]

i totally agree bowie69. you dont need to bang right up the top of rev range, easily sorted by gearing. and a well designed inlet, bracket for charger, cam (why not?). very interesting this.

dare i say it, lancia delta s4. supercharger with a centrifugal cluth that releases at 2k by which time the turbo is spooled up and you are win win.

..... so far off topic michele, sorry.

i would get the kit, however i don't need the extra low down torque, yet!

richard

[BogMonster]

I think somebody should split this thread into "300Tdi VGT Group buy" and "Waffle about theoretical cloud 9 blue sky thinking supercharger fitments"

[Ex Member]

Traditionally on diesels, a positive displacement supercharger was used to get low rpm boost and a turbo charger for high rpm boost as you ran past the efficiency points of each device. The VGT allows the turbo to work over a larger range and negates the need for the supercharger in most applications.