Buying bits and Tuning tips for GTti : eBay Guides

PLEASE BE AWARE THE HEAD GASKET KITS BEING OFFERED ON EBAY SEEM INFERIOR QUALITY.

I HAVE HEARD OF ONE GASKET ONLY LASTING A MATTER OF WEEKS.

After a few recent GTti items i have spotted being sold on Ebay, some fetching silly over the top money, I thought it's worth a quick guide to help you decide what hot and what's not when it comes to Tuning up GTti's.

These cars are easy and cheap to tune-up, but because knowledge is often fairly limited and performance parts are scarce, people tend to get carried away when a unusual item appears on Ebay...often claimng big power increases.

First of all,

Throttle body. Recently there was one being sold which had been mirror polished up inside, and it claimed a power increase of 3-6bhp!  The only way you will gain worthwhile power increase is if the throttle body is bored out bigger (39mm is possible keeping the original neck), and a new butterfly is made to fit. Just polishing alone would hardly do anything and certainly you wont notice any real gain.

Modifying the standard throttle body further, you can go to a max of about 42/43mm internal bore, but this involves cutting the original neck off, boring hole out to 45mm, making a new 45mm neck (outer diameter) to fit, and also make a new larger butterfly flap.

You may have heard of some people using a throttle body from another car (Toyota MR2 Mk1 is similar looking but much bigger). This wont work very well using the standard ECU, the airflow into the engine will be so different to standard. But its something to consider doing if you change the ECU to a programmable one (or a piggyback such as E-manage).

DRS Ecu's.... (Daihatsu Racing Service, official JDM Denso/Daihatsu factory made in plug-in ECU)

These appear every so often, and I sold mine last year for a fantastic price. Be aware that every DRS i know of (apart from the one i had) is a 1 bar / 15psi  DRS, In other words it is mapped to run about 15 psi boost pressure, many will still have a safety fuel cut but set at just over 15 psi instead of the standard ecu's 12.5psi.

These offer about 30-40bhp gain over a standard GTti, and they simply plug in just like standard ECU. However you won't really be able to re-tune it easily for even more power as there is no software or computer hook-up cable available to remap them. The chips inside could be removed and read by a chip EPROM reader/Writer, but this means you cannot tune the DRS whilst car is running on a dyno. So it would be a longwinded process.

Also, you can get just the same amount of power for less money by raising the boost up to the same 14/15psi as the DRS runs, but instead using a Fuel Cut Defender (FCD) connected to the standard ECU which will remove the standard safety boost cutout. FCD's are often hated as its seen as a bit of a bodge...but in reality if the fuelling is checked that it's not going lean, then it should be perfectly OK in terms of reliability, but only if you can resist not raising boost any higher than 15psi ! This is about the limit without needing to start retarding the ignition timing to suit more boost. 

DAD ecu's (Dawson Auto Developments)

These are a UK made ECU that is housed in a original ECU case, but totally different inside, still plugs straight in to ECU connector. Used on the official UK rally GTti's,  More is being learned about these ECU's and they seem quite powerful units considering their age (approx 1989/1990). To get car running they need to be used with a different MAP sensor to the original (to read higher boost), and possibly bigger injectors. I have one which is being looked into.

The Greddy e-Manage piggyback (blue version) is a great easy way for proper tuning, and once installed will have far greater scope for tuning than a DRS or DAD ecu, and for much less money. 

You can adjust fuelling and ignition timing via a laptop and installation is really easy, about 8-10 wires T's into the standard ECU harness and its ready to go! The car will start and run identical to standard until you start to play about with the setting of the e-manage, so there is no problem driving the car to a tuners/rolling road to be setup. 

Air filters / Induction kits...for max power the golden rule with a turbo engine is to fit the biggest free flowing air filter you can manage to fit under the bonnet! There are so many brands / shapes and sizes out there.

The latest Stainless Steel Mesh type flow the best,  although filtration quality is not as good as cotton or standard paper,

K&N or other brand cotton style filters are good all rounders and filter and flow OK.

Replacing the original filter in the airbox with a performance type "panel" filter is fine for cars running moderate power, I would though recommend replacing the soft rubber inlet hose from airbox to turbo with some stronger silicone hose, or better still a solid metal pipe. At higher than standard boost levels, the turbo sucks in the rubber hose and it actually collapses down and restricts flow. Not good!

Boost Controllers  - Conventional bleed valves work OK to raise boost, but best to use a manual boost controller (MBC) aka Dawes Device / Ball and Spring controller. These simple little manual valves stay closed until you reach your desired boost level, this makes the turbo spin up sooner and harder because the wastegate actuator stays shut for longer. Electronic controllers are a nice addition so you can adjust the boost pressure whilst driving, and they are better at holding a steady boost curve. But to be honest, from the testing i have done in the past a good ball and spring type manual controller are surprisingly effective considering they wil only set you back £20-30 compared to £150+ for a electronic controller

Turbo's

Another popular modification is the hybrid turbo. Whilst these offer great top end power, remember that the mid-range power will suffer considerably. They only tend to get boosting properly at 4000 rpm onwards.

So if you love the way your GTti drives and accelerates on standard turbo, dont swap to a hybrid!  Consider sticking with the standard turbo but just a bit more boost!   

To spot a hybrid for sale, look into the compressor inlet (air filter side), the compressor wheel will fill the size of the inlet hole fully, on a standard GTti turbo the wheel is noticably smaller.

Hybrids can also be made quite easily, all you need is to get your hands on a old Daihatsu Fourtrak 2.8 or Isuzu trooper 3.1 diesel turbocharger. Unbolt and remove the compressor housing and compressor wheel (reverse thread on the compressor wheel nut). Simply swap these over onto the standard GTti turbo. Ideally you should then send the turbo off to be re-balanced by a turbo specialist (but ive not bothered before and it's been fine)

You wil also find these hybrids wont run at standard boost pressure, they will want to make 15psi as a minimum as the revs rise, so you wil hit the safety fuel cut on a standard car if you simply just fit a hybrid.

Fuel pump

Dont waste your money uprating the fuel pump for the sake of it unless your pump is faulty and needs replacing, then you may as well go ahead and fit a uprated one. The standard fuel pump in good working order can certanly handle 150-160bhp.

Injectors

You only really need to think about swapping injectors once you are about 150 bhp, and by then you are more than likely going to be using a Emanage piggyback to control things so swapping to larger injectors and tweaking the mapping to suit is quite straightforward for any tuner.

GTti injectors flow approx 300cc fuel at the standard 2.5 bar base idle fuel pressure, or 325cc if you raise fuel pressure up to 3 bar (wil affect fuel economy as its running richer at al times.)

They are High Impedance, so ensure any replacement ones are also high impedance. 

Fitting bigger injectors without doing any ECU mods means you should also lower the fuel pressure to try and retain a decent low emission idle and off-boost fuel economy driving (adjust pressure by fitting a adjustable fuel pressure regulator to replace the standard one). It is possible this way if the new injector is not too much bigger flow than standard.  For example, a 370cc injector (measured at the industry standard 3 bar fuel pressure) could be made to flow similar to the standard GTti injector if fuel pressure is dropped down to 2 bar at idle. This is about the lowest fuel pressure most injectors are rated to still spray OK at. This type of DIY fuel tuning will require a wideband Air Fuel ratio gauge so you can see exactly what the fuelling is doing whilst driving, and especially during max boost. If you dont realy understand tuning things best to leave this method well alone as a totally screwed up engine is possible if it leans out.

A word of WARNING for whichever method you use to increase fuelling...GTti's NEED TO RUN QUITE RICH ON BOOST, even moreso than other turbocharged engines. Many tuners will consider A/F ratio of about 12-12.5 to be OK during max boost, Really i would suggest the fuelling ratio set richer at around 10.5-11.5. Remember you are getting a incredible amount of power (which means heat) from just a 993cc engine, so it needs a good spray of overfuelling to help keep it cool enough during high boost!

Water injection is a effective way of keeping things safe without the need to go so rich on the fuelling, Its a interesting modification and i will be trying myself when my car is built.

Clutches

REMEMBER CLUTCHES HOLD TORQUE, NOT BHP. 

A good condition standard GTti clutch (factory spec Exedy) should hold 120lbs torque (14/15psi for example)

Aftermarket standard replacement NKK clutch kit are certainly a little bit stronger and stronger made, 120-130lb torque

The pink coloured Exedy uprated cover (often refered to as  the +50%) will hold a more again, 150-160lbs torque using the standard organic friction disc.

Kevlar friction discs can be custom made (by Kestral Transmissions) and are said to provide more torque than standard organic friction (once bedded in) One problem is that Kevlar friction takes about 500 miles of careful normal driving to bed in before it actually starts to grip properly. Initially it may slip really easily.

A paddle style clutch friction disc is the more common route when even more torque capacity is needed, These adds another 25-30% torque capacity to the above figures. So a paddle disc with a uprated clamp cover should hold pretty much all the power you can extract from the GTti. But expect some judder and harshness when moving off, shorter lifespan, and eventually a scored flywheel surface. Having said that, some of the modern cerametallic paddle clutches dont seem too bad to drive once bedded in, especially ones with 4 or 5 friction "pucks". 

My advice would be to just use the Exedy uprated pink cover with normal organic friction disc, only upgrade to a paddle if you are making enough torque to make the organic disc slip!

Dump Valves

These will not make any extra power or performance, they are only worth considering if you run high boost (15+ psi) and even then the debate is out as to whether they really help prolong turbo life.

Of course if you like the dumping air noise then fine, personal choice.

I much prefer the compressor wheel flutter noise (air passing back through the compressor wheel) you get when no dump valve is fitted (large air filter needed to hear it). Like mentioned above, the flutter noise is of no concern (despite what some may say) unless you start using high boost.

A final note.....

I get many emails from people expecting or planning on making 200bhp with their cars. This is unrealistic for everyday road use and you run into all sorts of problems, such as gearbox, clutch, engine mounts and of course engine reliability.  Whilst 200bhp is certainly possible i would personally aim for a healthy 150-170bhp with a wide spread of torque. The car will still be staggeringly quick (the GTti is all about power to weight ratio and low gearing) and you won't be rebuilding things every month if it's done properly! 

Most GTti's that have the common bolt on mods (air filter/exhaust/ bit of extra boost) make about 130bhp once on the rolling road. With a bit of extra effort so you can safely run higher boost (Greddy E-Manage really is the most cost effecive way to do this) you will hit 150-170bhp quite easily.

Contact me if you have any GTti questions