Best Mods for the GS 400

[lexusben]

I have a 98 GS 400. I was wondering if there are any good Engine Mods for this vehicle. I am thinking about getting the SRT Intake for my car, but other that that I'm not sure what to do to increase performance. There aren't any headers available for the GS 400 right now so I'll have to wait for that if they ever become available again. Any suggestions are welcome.

 

[spf_lexus]

Try adding a mandrel bent 2.25" dual exhaust using the stock front "Y" pipe design with a set of S&S headers and you should open up 30rwhp or so.

 

[spf_lexus]

edit:

Use the stock front "Y" pipe that merge the front cats and modify from there.

 

[Thermactor]

I have a 98 GS 400. I was wondering if there are any good Engine Mods for this vehicle. I am thinking about getting the SRT Intake for my car, but other that that I'm not sure what to do to increase performance. There aren't any headers available for the GS 400 right now so I'll have to wait for that if they ever become available again. Any suggestions are welcome.

vortech supercharger of course!

 

[JBrady]

Unfortunately, S&S Headers went out of business

Fortunately PPE Engineering bought the jigs to build them from S&S liquidation sale.

Unfortunety PPE has not began production as they would like to change them and want/need an R&D car in the shop to do so.

I encourage anyone wanting this header option to contact PPE to show demand which could result in this header becoming available again.

www.ppeengineering.com

Now, in addition to headers the APEX NEO seems to be getting good reviews for dyno tuning this engine.

Maybe some increase in RWHP (due to weight reduction, rotating mass reduction) can be seen with a smaller torque converter and a very real performance improvement.

I would like to experiment and optimize the intake and exhaust flow with high velocity porting of the intake manifold, intake ports, valve job, exhaust ports all of course combined with headers and unrestricted exhaust system along with tuning on these engines. I am guessing with stock camshafts there is probably another 20rwhp on top of headers and tuning alone to be had. This should put potential in the 300rwhp range.

 

[spdrcr771]

Thats my goal 300rwhp. I think im gonna do cams & SAFCII next & see how close i get, b4 i think about doing any porting work. Only 48whp to go.

 

[JBrady]

Thats my goal 300rwhp. I think im gonna do cams & SAFCII next & see how close i get, b4 i think about doing any porting work. Only 48whp to go.

I think 300rwhp is doable on the stock cams.

Keep in mind that drivability and smoothness will likely decline with a cam change.

What I would LOVE to see is a way to increase the fuel cut and allow the engine to rev to 6500 or even 6700rpm with the higher numbers requiring or at least strongly suggested to have upgraded connecting rods.

Lextreme member RMS proved that even the first generation cams had a power peak at 6500rpm and was down less than 10hp from peak at 6800rpm.

RMS gained roughly 5% power from 5700 to 6500rpm with stock ports and stock valves with no porting. This should mean we could easily have 15-20hp or more in just turning the engine to a higher RPM with no other changes.

The first gen engines had peak power rated at 5700rpm where the VVTi engines are at 6000rpm and obviously should be at 6500+

With the better cam timing and profile of the VVTi engine it would make more sense to turn our engines to a higher RPM on the stock cams which would keep low rpm torque and drivability intact.

 

[spf_lexus]

What we really need is a dyno plot before/after for a stock 1uz lower manifold and then a straight runner manifold so we can see how a naturally aspirated motor responds to the longer/shorter runner lengths. My supercharged uz DEFINENTLY had more power at 6000-6500 but considering the FI, I dont think its an accurate scenario.

I think a VVT would respond very well to a raised fuel cutoff since it has a short runner design from the factory. Your right about the rods, but if their upgraded, the VVT could be a potent N/A. SAFC neo isnt very powerful but for this it can trim fuel and raise the cutoff for a decent price.

 

[spdrcr771]

when you say fuel cut, are you talking about the rev limiter? explain more please.

 

[JBrady]

when you say fuel cut, are you talking about the rev limiter? explain more please.

Yes, rev limiter.

I am unaware of anyone breaking into the factory ECU and changing anything on a VVTi engine. Maybe an interceptor device, IOW, from the RPM signal (crank trigger?) to the ECU place a device that will intercept the RPM signal and tell the ECU the RPMs accurately until say 6100rpm and keep sending 6100 even when actual RPM goes higher.

 

[spdrcr771]

That kinda sounds alot more complicated to do, then to just spend the money on a stand alone, right. If it was still sending 6100, then wouldnt the needle still be reading 6100, but the motor keeps going to Oops.

 

[XR8tt]

Little revy engine in heavy car..
I would change final drive...To keep rpm in sweet spot..
Fit good exhaust, esp headers.. To sustain rpm ...
Heavier valve springs... To sustain higher rpm...

 

[spf_lexus]

Little revy engine in heavy car..
I would change final drive...To keep rpm in sweet spot..

Since you have a VVT sc400 spdrcr, you can swap a 1992-1997 differential to bump your 3.60 final to a 3.90 final drive.

 

[JBrady]

Since you have a VVT sc400 spdrcr, you can swap a 1992-1997 differential to bump your 3.60 final to a 3.90 final drive.

The 98-00 SC400, LS400 and GS400 all share a 3.266 final drive. The GS400 uses the vehicle speed sensors for the speedometer reading so you can swap gears without changing speedo reading. Not sure about the LS or SC400 speedos. On the GS400 this reduces top speed from 149mph to roughly 130mph due to the ECU looking at RPM in top gear for its speed governor.

This definitely increases rear wheel torque in each gear but causes the transmission to shift earlier vs vehicle speed. For specific tests such as 0-60 or 1/4 mile the performance may actually drop due to the need for the extra shift to reach each number.

 

[spf_lexus]

For specific tests such as 0-60 or 1/4 mile the performance may actually drop due to the need for the extra shift to reach each number.

I respectfully disagree. Spdrcr has an LSD I believe and coupled with the enormous off the line tq, his car should be able to hold that power with minimal wheel spin. I am pretty sure you can unplug one of the shift solenoids for the electrically shifted 5-speeder and it will default to a max out line pressure, which will raise all shift points. This would counteract the "possible" added shift by pushing its threshold above 60mph. Only downside would be a CEL, but it would disappear once plugged back in.

 

[JBrady]

I certainly hope my post did not sound disrespectful.

The problem is the factory rev limiter at 6300rpm. Would love to get around this but I have yet to see it done.

Stock gearing alows for 67mph in 2nd gear. Going from the stock 3.266 ratio to the Supra ratio of 3.769 or the 1st gen SC400 at 3.91 requires a shift to 3rd gear to reach 60mph. Same thing in the 1/4 mile. Stock alows for 105mph in 3rd gear which is good for most cars without FI or nitrous. Switching to 3.769 lowers this to about 91mph, of course even lower with the 3.91 which will require a shift to 4th gear for 1/4 mile passes.

That said a gear change will make the car FEEL faster and it will be faster in each gear but each gear is lowered in its MPH range. If you could rev the motor higher that is another story but with the stock limiter...

 

[XR8tt]

Yes but its not how fast it is in each gear...
It's the multiplication factor the engine has in each gear..
Over time, MPH is not quite as important...

 

[JBrady]

Correct. Power cannot be multiplied but force (torque) obviously can be.

The highest multiplication is in first gear. In theory it would be great to run the entire measured event (to speed or to distance) in 1st gear. Unfortunately to 60mph or 1/4 events that would take an insane redline and a non degrading torque curve.

With the fixed redline of the factory ECU we have limits. The point I made was relative to this limit.

 

[XR8tt]

Put a 2.1 final drive and you'll have jet .. Then !!!
Sheesh every car manu I know of that have performance optioned vehicles have YES a more rev able engine.. BUT at the same time it has closer gear ratio's OR shorter gearing...

 

[spdrcr771]

I certainly hope my post did not sound disrespectful.

The problem is the factory rev limiter at 6300rpm. Would love to get around this but I have yet to see it done.

Stock gearing alows for 67mph in 2nd gear. Going from the stock 3.266 ratio to the Supra ratio of 3.769 or the 1st gen SC400 at 3.91 requires a shift to 3rd gear to reach 60mph. Same thing in the 1/4 mile. Stock alows for 105mph in 3rd gear which is good for most cars without FI or nitrous. Switching to 3.769 lowers this to about 91mph, of course even lower with the 3.91 which will require a shift to 4th gear for 1/4 mile passes.

That said a gear change will make the car FEEL faster and it will be faster in each gear but each gear is lowered in its MPH range. If you could rev the motor higher that is another story but with the stock limiter...

This info here is pretty close to exact. 3rd gear is like 88-89. I think the real answer is, to make about 100 more hp & go back to the 3.266 w/lsd & that might get the 0-60 done in 2nd the quickest. The SC's speed is also sent from the ABS unit, just like the GS.