In the recent AutoWeek, it was stated that the top speed (150mph)of the GXP is limited by cooling and tires.

Does anyone know what exactly is the issue - capacity, airflow, etc?

Also, what is the "cool-it" mod for the GT and would it apply?

Thanks!

It's likely inaccurate. GT is electronically limited to 137MPH, where as an unlimited one will reach 165-170MPH before running out of headroom.

An owner here has had his GT up to 155MPH....

What's inaccurate? It's limited to 150mph by the factory (electronics obviously) due to cooling and tires.

What's inaccurate? It's limited to 150mph by the factory (electronics obviously) due to cooling and tires.

I've never heard of any car being limited due to cooling. I have heard about tires getting too hot and requiring the top speed to be limited. If cooling is an issue at high speeds that is new to me. If so please post up where you found this.

I've never heard of any car being limited due to cooling. I have heard about tires getting too hot and requiring the top speed to be limited. If cooling is an issue at high speeds that is new to me. If so please post up where you found this.

referenced here.
http://www.autoweek.com/article/20081121/FREE/811219997

I read that but I took it to mean the automatic transmission. Maybe I'm wrong....
Just never heard of cooling being a limiter.

Tire rating could be what they mean?

Tire rating could be what they mean?

That's what I think. But I'm probably wrong.

That's what I think. But I'm probably wrong.

The article refers to tires, and cooling the motor and trans "A Track Pack that will add coolers for the transmission and engine oil is coming but no date has been released for when".

It could really be that GM 'jumped the shark' with the good performance of the G8GT, and the GXP is really not so much of an upgrade. Already seeing testing that hints at handling performace equal or less than the G8GT.

If the trans fluid and/or engine oil gets hot enough to actually force stopping a top end run then there are serious issues. Adding a trans and/or cooler is simple but I can't see that being a limitation on speed, just doesn't make sense.

It could really be that GM 'jumped the shark' with the good performance of the G8GT, and the GXP is really not so much of an upgrade. Already seeing testing that hints at handling performace equal or less than the G8GT.

I'm guessing (hoping) that the GXP will be a very high 12 second car out of the box. Even if it is a 13.0 car it's still faster than the GT. The fastest stock GT time I've seen was ~13.5. In the quarter going well over 100 mph that is a decent difference.

Also, I'm expecting the GXP with the LS3 to put out around 340-350 rwhp out of the box. The Corvette's with the LS3 are putting down around 360-390 rwhp out of the box. The Corvette's drive train layout is a little more efficient at getting power to the ground so it's going to have a higher dyno numbers.

The GT's put out around 295 rwhp and IF the GXP puts down 340 rwhp then I think the performance gain is worth it.

I'm guessing (hoping) that the GXP will be a very high 12 second car out of the box. Even if it is a 13.0 car it's still faster than the GT. The fastest stock GT time I've seen was ~13.5. In the quarter going well over 100 mph that is a decent difference.
Also, I'm expecting the GXP with the LS3 to put out around 340-350 rwhp out of the box. The Corvette's with the LS3 are putting down around 360-390 rwhp out of the box. The Corvette's drive train layout is a little more efficient at getting power to the ground so it's going to have a higher dyno numbers.
The GT's put out around 295 rwhp and IF the GXP puts down 340 rwhp then I think the performance gain is worth it.

Understood, but straight line performance is only one performance criteria. If paying somewhat big bucks for an upgrade model, I expect better everything - HP, Handling, Braking, Suspension, Interior, Exterior. It all comes down to a value prop. Not sure I see a huge bang for the buck in the GXP, meaning I have a fast car for $40k, but need to upgrade the rims, tires, suspension, tune to remove limiter, etc. etc.... before I even remove the paper tags. All of that gets implied in the cost of the car......

I guess we look at it differently. I'm buying the GXP WITH the intent of modifying it. I like the thought of having the bar a little higher from the start. I guess I don't expect as much for $5-7K difference.
I'm just happy that there is a car with this performance level right out of the box that I can afford.

If the trans fluid and/or engine oil gets hot enough to actually force stopping a top end run then there are serious issues. Adding a trans and/or cooler is simple but I can't see that being a limitation on speed, just doesn't make sense.

Not true. The cooling system(s) of a vehicle are designed per the maximum acceptable heat rejection requirements of the components being cooled (engine, transmission, power steering, fuel, etc.). This is supported by the ECU and TCM programming. For instance, the radiator may have been designed and sized for a max engine coolant inlet temperature of 220F. This may have been equivalent to the engine RPMs, max ambient temp, and airflow across the radiator at 150mph. If you exceed those conditions, the ECU is programmed to derate power by fuel, spark, timing, RPMs, or whatever to protect the engine.

However, I don't believe Autoweek's statement. The engine cooling system would have been designed with safety factors and to meet elevated ambient temps (110+F) at max engine conditions. How would Autoweek even have the specific engineering knowledge of the development of this LS3 G8 cooling system to make that statement?

Worst case; increase the RPM and MPH limits in the ECU with HPTuners and run the car on a cooler day. Tires, aerodynamics, gearing, and GVW are another story though.

I'm well aware of the ECM pulling timing under some conditions, that's really not what I said at all.

However, I don't believe Autoweek's statement.

AutoWeek is one of the last auto magazines to make an analysis of the cooling dynamics of a car. The information came from Pontiac.

Another example of Chevrolet stating that if the boost is turned up in the new ZR-1, the cooling system will need to be upgraded.

Cooling systems can be maxed out at high speeds. There very well be underhood airflow OR other issues. Time will tell.

An owner here has had his GT up to 155MPH....



:)




...

Ah, and the culprit chimes in! :wink2:


But yes, I can't see how the GXP's limit is anything but artificial. Any car can reach it's top speed due to drag limitations, but of course, you shouldn't be driving at that limit anyway right??

GXP is limited to 145-146 >>> Tires are the only factor...... Offically observed at the GM proving Grds in Chandler, Az.... The GT will top out at 160 un-governed.. Also the performance stats being released by the major 4 performance mags is over stated to help sell the car... relistic 0-60 times are about 4.9 sec. for the M6 and 5.2 auto... auto is heavier by 110 lbs and does not have the same excellent launching abilities of the 3.73's of the M6 ...the quarter mile figures are close: 13.0 M6 to 13.3 auto at 108-109 mph about the same as the lighter 2007 CTS-v with the same 3.73 gears.... ANything saying 0-60 in less then 4.8 sec is not accurate... The car is simply to heavy to accelerate all that quickly.. The GXP tops 175 un-governed with the M6 and 162 auto.... With DR's the M6 car ran a 0-60 in 4.4 and a 12.86 at 111 stock and with a simple re-tune (adding 30 bhp taking the total to 445 bhp) ran 0-60 in 4.4 on street tires and the 1\4 in 12.74 @ 113 ... more on par with the vette... This same re-tune M6 car with the DR's ran 4.0 sec 0-60 and 12.62 @ 115... This baby will be a beast with a simple re-tune for the tq mgt, add a CAI for another 12 bhp and headers for yet 15 more bhp and shazzam!!! over 470 bhp and 465 btq for little coin...


Also of note: The 09' G8 GT's we're testing out here at the proving grnds are running 0-60 in 5.5 to 5.6 sec and the 1\4 in 14.1 @ 100-101... Pontiacs 5.3 sec 0-60 sprint in their ads is missleading.

Hey There,

Hopefully the limiter being talked about is there due to the tires, but I would think they would have put ultra performance summer tires on the thing, z rated. Heck, I have had my 99 gtp up to 147mph before I let off the pedal... and that is with 15 psi boost :), and a few other goodies, It didn't break the 190f mark.

Maybe there is more of a plastic cowl or in the way of the radiator?
Oh well, nothing a bottle or two of water wetter couldn't fix.

Have a good one.

//\\//\\ikeD73USA

In the recent AutoWeek, it was stated that the top speed (150mph)of the GXP is limited by cooling and tires.

Does anyone know what exactly is the issue - capacity, airflow, etc?

Also, what is the "cool-it" mod for the GT and would it apply?

Thanks!

Tires are 'W' rated or 168 mph. I think the issue here, as is the case with the automatic CTS-v, is the tranny. The CTS-v manual isn't governed on top end, but the automatic is at 175 to, as they say, protect the transmission.

I had my ungoverned GT to 171 after about a 7 or 8 mile run up and the engine temperature was fine. The tires are rated to 168 so it's either the tranny or differential that has heat limitations.

It's likely inaccurate. GT is electronically limited to 137MPH, where as an unlimited one will reach 165-170MPH before running out of headroom.

An owner here has had his GT up to 155MPH....


Wow.. I recently had mine to almost 150 last week.. I was gonna ask what the limiter was on the GT's... Not heating going on here.. Although, the suspension could be tighter if those speeds will be common.. Whew, but it was fun..

The CTS-V's automatic is limited to keep the output shaft's rpm down or something like that.

Maybe the car isn't capable of running at above that speed for an extended period of time?

Anyone catch in the reviews them saying the Automatics are faster? Apparently the Automatics launch better :)

Anyone catch in the reviews them saying the Automatics are faster? Apparently the Automatics launch better :)

That's a standard truth. In drag racing automatics are pretty much always faster.

The CTS-V's automatic is limited to keep the output shaft's rpm down or something like that.



that's correct and I had forgotten

That's a standard truth. In drag racing automatics are pretty much always faster.

Torque converters FTW.

Torque converters FTW.

That certainly helps but mostly I think its a matter of consistency. Autos always (at least most) shift at the same rpm with the same characteristics. VERY few drivers can shift a manual at anything close to the speed at which an auto shifts.

Stock for stock, same car, automatics tend to have lower final drive ratios. A manual can at least be powershifted. Once you equalize the factors involved, with modern automatics, the powertrain loss becomes even more insignificant.

Tires are 'W' rated or 168 mph. I think the issue here, as is the case with the automatic CTS-v, is the tranny. The CTS-v manual isn't governed on top end, but the automatic is at 175 to, as they say, protect the transmission.

I had my ungoverned GT to 171 after about a 7 or 8 mile run up and the engine temperature was fine. The tires are rated to 168 so it's either the tranny or differential that has heat limitations.

150mph is VERY low considering it's essentially the same transmission as in the C6 and CTS-V.

99% likely that the limiter is set because the car can maintain 150mph indefinitely, think autobahn. While a run up to 175 or so is probably easily achievable, it probably cannot be maintained without more cooling power. Remember that GM has to cater to the lowest common denominator and ensure the car's longevity.

99% likely that the limiter is set because the car can maintain 150mph indefinitely, think autobahn. While a run up to 175 or so is probably easily achievable, it probably cannot be maintained without more cooling power. Remember that GM has to cater to the lowest common denominator and ensure the car's longevity.

Not sure if the 150MPH limiter is true or not, but if it is, that is not a good signal to the car enthusiast. Part of the reason cars like the GXP and SRT8 sell at a premium is that ALL AREAS of the car are upgraded to handle the rigors of performance car life (upgrade motor, transmission, cooling, brakes, suspension, etc.). Not that many of us drive over 150MPH regularly, but it is nice to know that your components are beefy enough to take the abuse, if required.