Hello All

Well i have owned my 98 GS400 for a month or so now, and since then i have done a few performance mods. i am an avid drag racer, so naturally the Lexus will see its fair share of track time

mods to date are:
drop in K&N filter
HKS axle back exhaust
removed resonators
Tein SS's
20" Maya DTV's with 285/30/20 Nexens in the back.

now i went to the track for the first time in the Lexus, on sunday, and needless to say i was a tad bit disapointed.

i had 4 runs and all 4 were between a 15.18-15.25 with mph's between 94.67-94.89 60's ranged from 2.38 to 2.52

i know that the large wheels and tires are slowing me down....but i thought for sure i was going to be laying down 14 second runs.

so now im back to the drawing board...i will be doing the usual maintenece items (plugs, fluid changes etc etc) to make sure the car is in top shape...and im getting my hands on an SAFC and installing my innovate wideband to make sure the car is running the Air/Fuel ratio it should be.

HOPEFULLY those things will get me into the 14's as i dont think the car is performing like it should be.

my REAL QUESTION, is what to do next?
TQ Converter?
Supra 3.76 rear end?
Headers?

whats going to be my best bang for the buck????

my goal is to get this boat down to a 14.5 CONSISTANTLY, big rims and all....

TIA..

 

Torque converter is the best bang for the buck hands down. You will see fourteens with that mod alone and probably get your 60 foots under 2 sec as well. That should get you into the mid fourteens.

 

Is the torque converter's only purpose is for launching? PI mentioned torque efficiency but some of us will get CEL using PI. I don't know how much torque effiency will translate to better time. 14's is what the car suppose to do anyways ...right?

 

yes but not if you are on heavy wheels... i think that has alot to do with it...

 

Get rid of the heavy wheels!

 

TC, Headers and an SAFC for tuning.

There's always the option of saving up and going with a supercharger as well.

 

thanks for the replies guys....YES i know the big wheels are slowing me down...thats why i am intersted in the Supra rear end...i need to lower the gear ratio because of these big wheels.

my goal is to run solid, mid 14's with the 20's. why you ask? well ive spent the last 15 years racing honda's and dragging slicks and skinnies with me to the track, and swapping them on and off the car just doesnt apeal to this old guy anymore :P

so im really leaning towards the TC or the rear end....tuning with the SAFC is mandatory since im a bit obsessed with tuning and making sure the engine is performing as it should.

i am seeking advice on getting it to the ground, since this is my first RWD, and first auto too boot lol

 

First RWD? Just to let you know... a litle slick on the road and with your wheels turned a bit... it'll go sideways. Keep the VSC on for a while. It's a heavy beast.

 

I know a great way to get below 15's...expensive but lots of fun

I'd get the headers for sure and the LSD.

 

Yes, the torque converter provides better launching but that's exactly what he needs. The higher stall speed will let the car launch harder which will help compensate for the additional weight of the wheels. A rule of thumb is that every .1 second you know off your 60 ft will be .2 seconds off your completed run. Most people gain roughly .3-.6 off their 60ft. This would put the OP squarely in the mid 14s. As far as a CEL he won't have to worry about it as he has a 400, 430's are generally the only ones affected by the TC CEL.

Trust me, do the TC, you won't be dissapointed.

 

...make sure you also get an additional tranny cooler like a B&M supercooler ($60) because the high stall TC really heats that fluid up!

Oh, and MANY MANY people do NOT realize that maintenance is KEY before modding (such as what the OP suggested earlier). That ALONE could help performance out a lot. and judging by you owning a GS400 it is obviously at least 9 years old or more with probably close to or more than 100K miles so if proper maintenance hasn't been done (ie fuel treatment, throttle body and MAF sensor cleaning, oil change, diff fluid change, spark plug & O2 sensor replacement) I would do it immediately.

Just an aside - I hate when I see or hear about "races" on the net against a GS400 and people (that are in a Nissan Altima/Maxima or Acura CL-S) are like those things are slow...look at this race; meanwhile the car obviously could be 10 years old with terrible maintenance; the car could have underlying issues going on...and then it's funny because some of the Lexus guys chime in and comment and say, "there is no way that was the V8, it must have been a GS300."

Sorry if I went off on a tangent...just showing the importance of maintaning ANY car. For all you know your heavy wheels could be playing a big part in your poor track times but I wouldn't be surprised if you found more underlying stuff going on. GL man and congrats on your GS4!

 

^^ thanks

and yes all the maintanence stuff will be done this weekend...and if i get enough time (and energy lol) i will attempt the SAFC/wideband install as well. im sure the plugs will thank me for the safc...this thing just feels like its pig rich in the upper rpm's...and from what ive read online, that suspiscion is probably true. every once in a great while i get the "rotten egg" smell from the cats, so that supports my really rich theory.

im hoping with all the maintnence done, and with the safc/tune i will be hitting the times i "should" be hitting...sounds like the TC will be an added bonus!

what are your guys's thoughts about "rebuilding" the converter with a higher stall? there is a guy locally to me that does this and everyone raves about his service....although i havent talked to him about a Lexus converter....

rebuild the current converter? or buy a PI Dragon converter?

 

What is your tracks altitude in Idaho? The higher the altitude the lower the air pressure and that is exactly the same as turning down the "boost".

That said, your 95mph trap speeds indicate a 14 second car already.

Your 2.4-2.5 60fts are your problem.

Yes, the heavy wheels slow you down but again... your trap speed is already there.

What was your launch technique? With the throttle by wire in our cars you can not spin the tires much. I would load the car up to 2000rpm against the brakes (do this VERY little and for the least time possible as it generates a bunch of tranny heat)
On the last yellow I release the brakes and roll into the throttle. The faster the roll the better up to the point of spinning. This is of course with the traction control OFF so if you start spinning you will probably kill that run.

Other prep includes unloading the car of any unnecessary weight which includes running only 1/4 tank of gas.

I was able to get 2.2 60 fts on 225/60-16 stock tires. You should practice your launch. Bigger heavier wheels/tires theoretically should give better traction even though they will take more power to turn.

Off the line the gear multiplication of 1st gear should make your tire weight diff minimal (percentage of weight change vs available torque). Most people don't realize that most dyno's recalc torque to engine speed. TRUE rwtq is MUCH higher and is highest in 1st gear. Take whatever the dyno says and multiply by the diff ratio (3.266 stock) and then by 1st gear ratio (3.357 stock) for a total multiplier of 10.964. So, if the car dynos 250rwtq the ACTUAL number at the tires in 1st gear is (250 x 10.964) = 2,741 pounds of torque. Even at 2000rpm you probably still have 2000 pounds of torque at the tires in 1st gear. So, if you add 50 pounds of rear tire/wheel weight the change is small relative to the apx 2000 pounds of torque available for LAUNCH in 1st gear. As RPMs and gears climb the weight of all 4 wheels becomes more and more important and YES you will turn quicker times with lighter wheel/tires BUT your 60ft times are primarily traction dependent. Practice, practice practice. The better you get the better everything else will work including a higher stall converter.

A torque converter will give you more torque at launch but unless you can hook that force it will just spin. The smaller lighter Precision Industries unit will make the car respond quicker to throttle imput and may allow slightly more power to reach the tires even above stall speed.

Changing the diff gears does not seem to make much difference in 1/4 mile times but the car will "feel" faster. This is due to the fact that it changes gears sooner. The longer you can stay in a lower gear the more torque multiplication you have. So, you gain in each gear but each gear is lost sooner. You will need 4th gear in the 1/4 instead of just 3rd. My car on stock tires will go 106mph in 3rd (at 6200rpm) with 3.769 gears top speed in 3rd drops to 92mph. You will also loose top speed capability from the stock 149mph down to under 130mph due to the factory limiter. Probably the biggest advantage would be limited slip. Careful here as many Supra 3.769 diffs are NOT limited slip.

The SRT intake and other intakes add some power with some claiming less and in some cases a drop in low speed and rpm power and response. With proper ECU tuning real power is available at higher RPMs.

Good headers will make more power and torque at all RPMs and will make the car faster and quicker in the 1/4 mile.

 

^^ Great post JBrady, thank you!

i had the same thoughts about the 3.76 rear end...as it is now, im going through the traps at about 5700rpm in 3rd...i didnt do the math, but was very concerned that i would be hitting 4th right at the stripe...not good. thanks for confirming that.

and yes i know the 60' is killing me....this is my first time launching a RWD and an AUTO, so this is going to take some getting used to :P

all 4 times i tried to launch the car differently...the first time i loaded it up to about 1500-ish and was really lazy getting off the brakes....that was the worst 60' of the day. (2.52 or something)

2nd time i got it up on the converter to the point where i THINK the tires were starting to spin while i was waiting for the light...and i was a tad lazy getting off the brakes again this time (too used to trying to slip a clutch off the line lol)....i mashed the gas and i knew the tires were spining well up until 4500 rpms or so. this 60' was a mid 2.4

3rd time i decided to just punch it...not getting on the converter, nothing...just let off the brakes and punch it. suprisingly this was the best 60' so far of the day. (like a 2.40 iirc)

4th time i got real aggressive on the converter, slipped off the brakes real well, and rolled into the throttle to keep the tires from spining...i think they spun a bit but it resulted in the best 60' of the day. like a 2.38

every time i spun the tires my buddy (the track photographer) said only the right rear wheel was spining (as confirmed by all the rubber on my passenger side HKS muffler lol) so i think with the TC traction is deffinetely going to be a premium....will a stock supra LSD bolt up to the gs400 ring gear?

 

oh and btw, track altitude is 2700ft....from my experiences there is a .4-.5 difference between here and seattle (sea level) i knew i had some things working against me from getting the fabbled 14.2 times the gs400 does stock (big wheels and elevation) i was hoping for a 14.7 or so....IMO thats what this car SHOULD be running...and the 95mph trap backs that up...so like i said before, i just need to get the power down to the ground!