21 replies to this topic

[Oldskoolbaby]

Im currenty undertaking a new engine build for my new car which consists of a modified 998 and a T3 :nugget: I know that there is going to be loads of lag and that there is the option of changing to a t2 to reduce it but is there anyway of modifying the engine or turbo to reduce lag. Ive heard that the compressor blade can be machined down to reduce it but will this not effect the boost? I thought this would create a space between the blade and housing, reducing the boost.
Also will head mods reduce the lag as ill be fitting a 1300 head on to the block wich ill port and open up before its fitted.

Any help will be ace, cheers

[dbmini turbo]

will a turbo cam not help things quite alot. they bring the power in lower down, not actually dealing with the problem of lag but has got to make a difference, im runnin a t3 but i have a race kent cam, so much lag its unreal. not sure if this helps mate, just an idea

[Oldskoolbaby]

well im only going to fit a Kent 276 in it so the power will be pretty much spread out after 2000rpm. You running on a 1300 though?

[dbmini turbo]

1348, a tiny bit bigger than the 1300, this will prob make mine lag more aswell then maybe

[Oldskoolbaby]

No, im pretty sure the smaller the capacity the more lag you get. Ive really got to stick to 1000cc or under as the car im getting has always raced in the 1 litre class. If I increase it, itll loose sme of its heritage I think. Its a Mk1 so its got a far bit.

[Wil_h]

Firstly i'll do the pedantic bit and tell you that you're using the word 'lag' incorrectly. What you are talking about is 'boost threshold'

Lag is the time between you putting your foot on the accelerator and the car reacting to it. All cars suffer from lag. When you are off boost (low revs) on a turbo motor and you put your foot down, the car still reacts to this it's just slow to accelerate until you reach the boost threshold.

When you are on boost (or above the boost threshold) there is a longer time delay (compared to a normally aspirated car) between putting your foot down and the car accelerating. This is owing to the turbo having to spool up. This is lag.

So, how to lower the boost threshold?

Well in my experiance you won't see any decent boost till 4500 rpm with an MG metro T3 on a 998. And if you fit a 276 cam I would expect this to rise further, and the transision between off and on boost to be more dramatic.

I've tried a few cams and found the KC 500 (MG Metro cam) to be the best for power and early boost.

I am currently in the process of getting a T2 (R5 turbo) for my 998 turbo, and if you want low-down boost this is a good idea.

Also fitting a fully mapable ignition system helps off boost performance and the boost threshold.

I'm not sure about porting the head. I have run a totally standard (port-wise) 12G940 head on mine and have somewhere around 140 BHP. I am going to do some work on the ports pretty soon though.

Wil

[Ade]

Two methods that are pretty tried and tested for this are

A. Run antilag (I'm not sure if you can do this with the engine you have) I had my old impreza set up for a mild form of antilag and my god it didn't half make a difference!! Plus if you get some chavvy muppet in a nova welded to your butt a quick blip of throttle and switch on the antilag on and hey Presto they back away quite quickly :grin:

B. You could install a nitrous kit. You can get kits that are designed to come in progressivley fairly low in the rev range and then phase out as the turbo comes in. It won't be a cheap option as the control electronics are quite complex but it works very well indeed!

Ade

[Oldskoolbaby]

This is all great stuff, but can I modify the turbo to decrease the threash hold?

[Wil_h]

Making sure the exhaust is nice and free flowing (including the turbine elbow) is probably as much as you can do in your garage.

I think turning down the compressor blades will not help. In fact it'll probably make things worse.

Smaller turbo is the only real way.

[mighty_mini]

if 'lag' is your biggest concern, why not Supercharge it?

[Oldskoolbaby]

In terms of boost, I dont think that ill be able to acheive the same as with a turbo. I really want to be looking at something like wil h's mini with 140 brake. To get this Im sure im gunna be looking for some pretty high psi figures. I will be doing head mods on top of the typical conversion to acheive as much bhp as possible so I dont have to rely on the turbo quite as much.

Also, a few people say not to bother with head mods in terms of porting cause Ill see enough power with the turbo alone but surely if the motor breaths and exhaust deeper, itll bring the turbo on song quicker?

Sorry if I ask alot of questions but im pretty new to turbocharging and I dont just want to jump in to it. :smartass:

[Sprocket]

Its not realy the size of the turbo that matters when talking about when the turbo spools but the A/R ratio(the Area 'A' of the inlet to the Radius 'R' of the inlet)of the turbine housing.

http://freespace.virgin.net/colin.milne/tturbo1[1].gif

I bought an IHI RHB5 turbo with a .38 A/R turbine housing.



The guy I bought it off was using one on a GSXR1300 drag bike and aparently spools from about 2500rpm (due to the small A/R). This may be a bit small, but its gonna be one of those suck it and see jobs, but for £120 for a brand new turbo I'll give it a shot, might have to change to the .41 A/R housing. Choosing the right size turbo is a very complex science indeed. The turbine wheel is the same size as is the compressor on all RHB5 turbos, what differs is the A/R housing. There is more than one Garrett T2 as there are IHI RHB5( same Compressor/Turbine) with different A/R housings for different size engines. Hybrid turbos are a mix of either larger turbine or larger compressor wheel to acheive specific tuning characteristics and again with the various A/R housings

Not only is this complex, what makes it twice as bad is the size of the compressor. You have to select the right size compressor to supply enough air at desired boost for the expected power without it running out of puff( too small, can't achieve desired boost and may over spool, compressor surge) or providing too much( to big, excessive boost, wastegate opens early) this is where you have to look at the compressor maps and specifications.

http://freespace.virgin.net/colin.milne/IHI-chart6[1].gif

http://freespace.virgin.net/colin.milne/ihi_specs[1].gif

[Bungle]

i know dans (The beans ) lifeboat has turbos and superchargers :grin:

the supercharger helps out with the low RPM power before the turbo spins up :smartass:

is this posible or will it take up to much space :sad:

lifeboat engines are two stroke diesels so this may make a difference :erm:

[Oldskoolbaby]

I know that the old Lancia Group B rally cars used Turbos and Superchargers or the same reason. Good idea I think.

Thats a huge help Mini sprocket. When you say the size of the inlet, do you mean the ports or inlet manifold size?

[ryan69]

Or fit another turbo that spools up sooner like that new 5 series BMW (you'll have 2 turbos :grin:) I dunno if this is possible in a small engine bay but being that there a 998 under the bonnet it gives you a little more room.