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Stinger radar detector.
Electronic boost controller.
K&N free flow air filter, with heat shield.
PCV catch can, back to stock again.
ERL water injection.
Zener diode, against fuel cut.
Brembo disks, karbon brake pads with SS braided lines.
Selbys 22 mm. adjustable rear swaybar
G-Force ECU upgrade.
Better intercooler efficiency.
Balanced injectors.
Stock Turbo CT26, rebuild and upgraded with the Garrett T04E/50 mm trim compressorwheel.
New clutch and new synchro's at 116,628 miles.
Eleminated fuel pump relay
Fuel delivery.

Modificaties to do:

Laser shield. Probably not, because it is highly illegal in the Netherlands.
Blow Off Valve.
Upgraded fuelpump.
Adjustable fuelpressure regulator

Stinger radar detector.

Lucky for me, an speed control detector is legal in Holland. So this is the first "must have" before using big $$$ in modding the car.
It still sound strange to me when people put big $$$ in their car, without the use of an radar detector. Most of the installed money can never be used because the cops are "always" watching you.

If I should keep me to the speed limit, I could rip the last three gears out of my car.

The latest technology is the laser shield. This shield stops reflecting the laser to the cop, so he can not read your (too high) speed. Very important!!!

Elektronic Boost Controller.

First some basics:
If stock the ST 185 does have max. boost:
1 - 0.50 bar.
2 - 0.50 bar.
3 - 0.70 bar.
4 - 0.70 bar.
5 - 0.70 bar.
This 0.50 bar is the max. boost of the stock wastegate. In higher gears the ECU controls the boost and adjust it to 0.70 bar, by bleeding of some air (some think its works different, but I think this is what happens).
If the engine is cold, or too hot, or if the ECU "thinks" there is something wrong, you never get more than 0.50 bar.

Because I thought she was a little slow, I asked advise on the GT 4 mailing list.
HKS EVC IV They told me to install an Electronic Boost Controller. In my opinion it is the most easy and safest way to improve performance.
Mine is an HKS EVC 4. The only reason not to choose an EBC is they are not cheap!
Installing was a bit difficult because of the bad description, but she is working in my car from 1998 and is doing a good job.
Now I got: max. boost of 0.80 bar, in every gear. It is not possible to get more boost for me, because the ECU limits the boost at 0.83 bar. So my next step will be an G-Force upgrade. This will give me correct fuel- and ignition maps up to 1.0 bar.

Note: the ST 185 Carlos Sainz edition does have fuel- and ignition maps up to 1.0 - 1.1 bar, I heard. How to make an cheap switchable boost control device, visit Paul Rennison's GT 4 FAQ, and read how to make it, made by Ivan Safranek.
An EBC is also an option, but it is not the most cheap solution, but for me: I am very happy with it.

K&N, "free flow" air filter

Just because I want most things ready before I crank up the boost, I installed an FIPK from K&N. I noticed a little difference in throttle response and accelerating.
If you love nice sounds, this mod is an "must do". Under accelerating you can hear the air get sucked into the engine, shifting gears gives some kind of blow off noise. The "blow off" noise is coming from the boost pulse whats is coming back, via the airfilter, at the moment the throttle is closed under boost. This is really bad, I do need an Blow Off soon, what will take care of the high boost pulse, and will raise the life of my turbo.

Recently an Toyota Supra twin turbo what produced 368 rear wheel hp., changed his stock air filter box into an free flow air filter.
Result was an power gain of: 16 hp.
This means, the power gain was 4.35 %. And if we look from stock, it will be 6 %.
Not bad for just changing into an free flow air filter.

Next mod was an custom made air filter box, avoiding hot air being sucked into the engine, very important.
Underhood temps are going up to 60 degrees C., and higher. My intake temps now are 5 - 10 degrees C. higher as ambient.
Some strange thing I noticed is, my intake temps are 15 - 20 degrees C. higher with the headlights up???

PCV catch can.

PCV = Positive Crankcase Ventilation.
In stock setup the PCV is routed back into the intake. This is causing oil parts being blow into the intercooler.
I read a lot about making an catch can, this should be better, cleaner and gives more horse power.

After a while I installed my catch can, my car started to smoke. This looked bad, real, real bad.
It became as worse as I decided to rebuild the turbo, I thought this was the smoke problem. But when I got my turbo back, they told me, they did nothing, because my turbo was in excellent condition!

They advised me to relocate the PCV back to the intake again, and guess what, after the relocating: voil, no more smoke.

So, I don't know what went wrong, but she still runs fine now, so I keep it this way.

ERL water injection.

Before cranking up the boost and for safety reasons, I installed an ERL system 2 water injection kit.
Installing the kit was done by myself!!! I know, it's hard to believe, but it's definitely true.
It took me about 6 months installing the kit, but it runs fine now. I did get a lot of help from Richard Lamb. Richard is the person from ERL Aquamist. He always answered my E-mails very quickly. A lot of company's can take a look at ERL, about how you make customers happy.

I use the 0.7 mm jet, mounted in the exit side of the intercooler. The intercooler had to be prepared correctly before installing the jet, because the material of the intercooler is too thin.
The pump and tank are mounted in the back of the car, so I needed 32 Amp wire to be sure the pump does get enough power to supply water to the front of the car.
The pump is capable for 11 bar, but mine is set at 7 bar, what can be checked in the cockpit.
7 Bar of water pressure is giving me an injection pressure of 6 bar, under boost conditions. I calibrated the system at 6 and 7 bar, so I know exactly, how much, and at what Rpm, the system is injecting.

About the amount of water:
The amount of water must be 15 % of the fuel what is used at the specific moment.
I dynoed my car, so I know exactly what hp. she delivers, and at what rpm.
The ST 185 has 430 cc/min. injectors, totally capable for 275 hp.
4 x 430 cc = 1,720 cc/min., but the max. duty cycle of the injectors is 90 %.
So, 1,720 x 90 % = 1,548 cc/min. fuel is needed with 275 hp.

This means: the amount of water: 1,548 x 15 % = 232 cc/min. for 275 hp.
My car delivers:
208 hp. at 6000 rpm.
232 cc 275 hp. x 208 hp. = 175 cc/min.
190 hp. at 5000 rpm.
232 cc 275 hp. x 190 hp. = 160 cc/min.
143 hp. at 4000 rpm.
232 cc 275 hp. x 143 hp. = 120 cc/min.
110 hp. at 3000 rpm.
232 cc 275 hp. x 110 hp. = 93 cc/min.

This is the figure for the (inter)cooling effect of water, if you use the water for detonation control, the amount of water should be taken from torque specs, instead of hp. specs.

N.B. Too much water may result in over cooling and cause power loss.

Results from the dyno:

Power gain with water injection: 11,6 hp en 37,6 Nm, not bad with a little water!
Dyno without water: 183.3 hp/5,970 Rpm. - 239.8 Nm/4,500 Rpm.
Dyno with water     : 194.9 pk/5,920 Rpm. - 277.4 Nm/3,990 Rpm.

Intercooler efficiency.

About the efficiency of our air/air intercoolers.
I don't have enough numbers about the exact (stock) air/air intercooling efficiency, to take a decision about what to do.
Some people did install an intercooler fan (example Dennis Heath: intercooler fan), but I am afraid the fan will block the flow of the air with high speed driving.
So the fan will give better efficiency under low speed conditions, but what under high speed?

An water/air intercooler is an option, but it is more complex, and untill I don't have exact efficiency numbers, I don't know if the results are better.

You probably wouldn't believe it, but I made an little intercooler fan. First it made an very irritating noise, so I made it switchable. The switch is mounted near to the gear stick, very cool ........:-)
personally I think it is one of the most ugliest mods available, but hey, it probably works .... and if not, I rip it of the car and throw it away.

Zener diode.

I have tested my Celica once (or twice, or more .....), with the zener diode.
First, a little bit theory:
Stock, the ECU cuts fuel as soon as the ECU receives an signal from the Turbo Pressure Sensor what is too high.
The Turbo Pressure Sensor does send an signal between 0 and 5 volt to the ECU. As soon as the Turbo Pressure Sensor detects boost above 0.83 bar, she does send an signal to the ECU with 4.3 volt and higher. But with 4.3 volt the ECU is activating fuel cut.
So, if we install an 4.3 volt zener diode between the wiring of the TPS, the ECU never sees more than 4.3 volt (or less). At this point we DO NOT have fuel cut anymore!!!
This means if you install the zener permanent, you will always drive around without fuel cut. And this scares me.
Still I believe the stock ECU of the air/air ST 185 has no correct fuel- and ignition maps above 0.83 bar. Testing once would not blow up the engine but, I think, it's not the way to go.
So, after the test I removed the zener, and made everything stock again.

Because the tolerance of the zener can be 10 %, I checked the values of the Turbo Pressure Sensor.
  stock zener 4.3 v zener 4.7 v
vacuum with 500 mmHg 1.33 volt 1.33 volt 1.33 volt
0.00 bar turbo pressure 2.55 volt 2.55 volt 2.55 volt
0.55 bar turbo pressure 3.65 volt 3.65 volt 3.65 volt
0.72 bar turbo pressure 4.00 volt 3.93 volt 4.00 volt
1.00 bar turbo pressure max. 4.3 volt = fuelcut 3.93 volt max. 4.3 volt = fuelcut

This is showing us the tolerance of this 4.3 zener:
4.3 -/- 3.93 = 0.37 / 0.043 = 8.6 %.
Actually, I must get an zener with max. 4.25 volt, or 1.16 % telerance :-), or try 10 x 4.3 zeners and keep the best one, this way the ECU is getting not too bad readings.

For an total schema of the stock GT4 Turbo Pressure Sensor: TPS MR2 site

Acceleration 100 - 160 km/h in fourth gear went 1 sec. faster with my boostcontroller set at 0.85, in stead of 0.75 bar. With my boostcontroller set at 0.85 bar, there is an flash of 0.90 bar, but no fuelcut.

At an Toyota junk yard I bought an extra TPS plug with some wiring for $ 12 and the zener was $ 0.5 (close up zener). Between the wiring I installed the zener, and with this extra plug I can easily connect the zener if I want it to.
You see, this is easy and cheap, but I doubt if it is the good way.

Here an picture showing the boost sensor connector, the wiring colour coding, and the correct connection for the resistor/zener pair.

Here: you see the "zener diode" mod of David Greene on his MR2 turbo.


Brembo rotors and KVR pads are on the car, with SS brake lines. The brake feeling is much better now.
No vibrations under braking
No further results of fading yet.
Really scary is the fact that not long ago, an other GT4 owner with the same SS brake lines, broke one of them ???!!Gulp

Whiteline handling kit.

15-03-2000 Whiteline handling kit and adjustable rear sway bar. This is an big improvement especially at high speed.

Toe in0,00,0
Camber- 0,5- 0,5

Modification worth every penny.

Downside is that the polyurethane bushings are making an very irritating sqeaking noise ........ maybe I put back rubber ones!
After greasing them real good, the sqeaking noise is gone ........ pfffoei.


15-03-2000 G-Force ECU upgrade.
Car is running well, driveability is great.

Topspeed on speedo, stock rims and tires was 255 km/h, en is now 262 km/h. This is with 0.8 bar, and in the near future I hope to run 1.1 bar.

2e gear
3e gear
4 gear
Before G-Force upgrade
with 0.75 bar
164 kW/5.680313,5 Nm/4.200161 kW/5.890308 Nm/3.640176 kW/6.200325 Nm/4.090
After G-Force upgrade
with 0.75 bar
177 kW/5.800343 Nm/4.200181 kW/5.890 348 Nm/3.840187 kW/6.020353 Nm/3.845

Results with Streetdyno seem to be: 200 kW/6,000 and 400 Nm/3,400 at 1.1 bar. Exact numbers not available yet.

Streetdyno results are no real horse power but it is an usefull device to check if an modification did work. For real hp output look here.
Info about Streetdyno, is here available.

Checked Air/Fuel at the oxygen sensor, results:
0.85 bar945 millivolt
1.00 bar900 millivolt
1.10 bar880 millivolt

This values were measured at 6,200 Rpm in 5th gear, this way I ask the most of the fuel system. Topspeed was 265 km/h. (nice).
As shown, the A/F ratio is getting leaner at 1.1 bar, but nothing to worry about. G-Force did a good job.

New Streetdyno results soon.

Site from G-Force


24-07-2000 Injectors were cleaned and balanced. Now they are all four exact equal, for sure.
This way I am almost positive that every cilinder is getting the same amount of fuel.

*Test was made with cleaningfluid, what is more thick than fuel.*
Frequency 15 HZDuty 50 %Test pressure 2.5 KG/CM
Testing time 20 sec.Test temperature 20C 
 VolumeLeakage test
Injector 165 ccok
Injector 265 ccok
Injector 365 ccok
Injector 465 ccok

Turbo upgrade:

24-07-2000 Stock (tired) CT26 went to the States for an rebuild and upgrade. Monday afternoon Fedex picked up my package and the next monday at 9 am they delivered it back at my desk, as new, very nice.
Upgrade is made by Performance Techniques for 450 USD.

No results available, coming soon.
I am curious too about the oil consumption of the car with the rebuild turbo.
After we installed the turbo back, we noticed that the waste-gate was not opening untill 1.0 bar ......iieeek. Most of the time people are looking for more boost, but mine wasn't able to get her low??? After we bend the rod from actuator to waste-gate, she operated normally again. Maybe Performance Techniques does change the distance between turbine and compressor, due to the bigger Garrett compressor wheel. I will check this out with PT.

New clutch and synchro's:

At 07-2000 with 186,606 km. we installed an new (first time) clutch and rebuild the gearbox with new synchro's and bearings etc.
Because she is our daily driver we decided to install an stock Toyota clutch. At the same time we replaced all vacuum hoses and water lines because they were really bad. Unfortunately after we tried the engine in the car, we discovered another leak between intake and head :((( ........ bummer.
Injectors were cleaned and balanced, original gear lever was overhauled and made to an short shift system, fuel delivey pipe was removed and replaced bij straight through fittings, fuel pump relay mod was done and an fuel pressure gauge installed.
So, now she is ready for: better fuelpump, APEXi AFC, adjustable FPR.

02-01-2001; Unfortunately the headgasket of the Celica is gone :(
At the moment we are fitting an TTE 1.4 mm headgasket, together with an new timing belt, waterpump, 8.7 mm lift ST205 intake camshaft, and some other stuff.

Results will follow.

I hope this is of use for somebody, if not, just let me know.
If you use some of this info, remember: it is at your own risk.


Lots of info about the GT4

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