Jeep Garage  - Jeep Forum banner
1 - 14 of 14 Posts

·
Registered
Joined
·
34 Posts
Discussion Starter · #1 ·
hi,
I'm just curious about the launch control, why is it such a low rpm? If you look at GTR's or BMW's etc.. the launch control is at something like 4k or higher.. With the LSD and AWD system in the SRT, I would of thought you would get a heaps better take off in a higher rev range?

Of course its combined with some tyre smoke and rocket style G force into the back of your seat. ;)
 

·
Registered
Joined
·
426 Posts
I would asume it was plenty of trial and error to get the best times but some caution built in to increase the lifespan of the drivetrain.
 

·
The "REAL" JeepGarage Founder
Joined
·
1,379 Posts
The 6.4 makes plenty of torque and doesn't need to be screaming to launch. Im sure the SRT crew knows the sweet spot.
 

·
Registered
2012 WK2 SRT8
Joined
·
200 Posts
because the large majority of cars using launch control are Forced induction (mostly twin turbo) therefore to overcome turbo lag in the low RPM range they need to launch for higher up the range. Also with the amount of torque and the huge mass of the Jeep launching at anything higher will significantly decrease the life of the transmission.
 

·
Premium Member
Joined
·
1,544 Posts
My guess is the transmission is the reason.

Most of the other cars you used as an example have gearboxes that have actual clutches where as the SRT tranny is full auto planetary gear style and does not have a high rpm stall type torque convertor some some race cars that run a slush box.

This is just a guess but probably why it is like that.
 

·
Premium Member
2005 Jeep Grand Cherokee
Joined
·
2,807 Posts
Its 1 word, this one word has been known to be non existent in the import world since the dawn of imports, its what separates the men from the boys (or the men from the men with a lot of extra scratch to blow on a lesser product). This word I speak of is called TORQUE. 6.4l V8 has WAY more torque which is usable power below 5252 rpms, with this power down lower, naturally it needs less RPM to perform a perfect launch. Now the imports that rev to 8k rpm dont hit their max torque till after 5252 rpms which means to get the power to the wheels the rpms have to be higher.

This 5252 number is the magic number that dictates if the car has more torque than horsepower or visa-versa.

Even then with the gtr if you use this launch control your kissing your warranty good by on a 20k dollar transmission, so YES, a lot probably has to do with drivetrain longevity, taking the option and the argument for warranty repairs more out of the customers hands by being a little more gentle.
 

·
Registered
Joined
·
426 Posts
Its 1 word, this one word has been known to be non existent in the import world since the dawn of imports, its what separates the men from the boys (or the men from the men with a lot of extra scratch to blow on a lesser product). This word I speak of is called TORQUE. 6.4l V8 has WAY more torque which is usable power below 5252 rpms, with this power down lower, naturally it needs less RPM to perform a perfect launch. Now the imports that rev to 8k rpm dont hit their max torque till after 5252 rpms which means to get the power to the wheels the rpms have to be higher.

This 5252 number is the magic number that dictates if the car has more torque than horsepower or visa-versa.

Even then with the gtr if you use this launch control your kissing your warranty good by on a 20k dollar transmission, so YES, a lot probably has to do with drivetrain longevity, taking the option and the argument for warranty repairs more out of the customers hands by being a little more gentle.
5252 is just a number used to calculate HP. HP = Displacement, bore and stroke and cam profile dictate what rpm your peak TQ will come in at.
This is a quote I found online that sums this up,
"An engine that is tuned to have peak HP below 5252 rpm will always have more TQ than HP, engines with peak HP above 5252 rpm will always have more HP than TQ
 

·
Premium Member
2005 Jeep Grand Cherokee
Joined
·
2,807 Posts
How is that different from what I said? Not sure I am following.
 

·
Registered
Joined
·
426 Posts
How is that different from what I said? Not sure I am following.
I was lost when you said,
"This 5252 number is the magic number that dictates if the car has more torque than horsepower or visa-versa. "
Most won't know what the 5252 rpm refers to.
 

·
Premium Member
2005 Jeep Grand Cherokee
Joined
·
2,807 Posts
Got ya.

Sent from my SCH-I500
 

·
Premium Member
2005 Jeep Grand Cherokee
Joined
·
2,807 Posts
Ignition timing can be adjusted at ANY rpm, not sure I follow the logic.
 

·
Premium Member
2005 Jeep Grand Cherokee
Joined
·
2,807 Posts
Hey Joe, This is what I was speaking of... not what actually creates or dicates torque simply its direct correlation with this number. Using this calculation for HP:

hp = (torque * RPM) / 5250
You can see plugging numbers in that it backs what I said, (1000tq x 3500rpm)/5250 = is only 666hp. More tq below 5250 rpm means there will be less hp than torque. And visa-versa. Again no comment on how torque is generated simply stating that they have a direct relationship with the number 5250 which is just a constant.
 

·
Registered
Joined
·
2 Posts
What I wanted to say is when you power brake at1200 rpm, the variable cam timing and ignition timing is advanced to WOT creating big power. That is what causes the seat of the pants launch in the Jeep SRT8.
 
1 - 14 of 14 Posts
Top