boost launch in D or OD ???

[jbone]

I recently read a forum where someone asked how to boost launch. When it was explained the person said to launch in drive not overdrive. Does it make a difference, and if so then how? Do you have to shift into OD in the 1/4 mile????

 

[Jer]

Re: boost launch in D or OD ???

Use Drive to launch. Shift into OD if/when you need it to hit 4th.

 

[Zewerr]

Re: boost launch in D or OD ???

Transmission produces more pump pressure in Drive. Therefore, less slippage in the tranny. Better on the tranny, better on the shifts.

 

[Syclone#1992]

Re: boost launch in D or OD ???

Moved to drivetrain tech.

 

[SY2932]

Re: boost launch in D or OD ???

If you're not aware, you don't have to depress the button on the top of the shift lever to shift from "D" to "OD". Just give the shifter a quick "bump" forward. Works great at the track or on the street.

 

[GMCSyclone1]

Re: boost launch in D or OD ???

Here is anothre thread on boost launching: http://www.syty.net/forums//showthread.php?t=29049

 

[Tooky]

Re: boost launch in D or OD ???

I recently read a forum where someone asked how to boost launch. When it was explained the person said to launch in drive not overdrive. Does it make a difference, and if so then how? Do you have to shift into OD in the 1/4 mile????

Another difference I didn't see mentioned yet, is the fact that most SyTys I've seen, from the factory if the trans is left in OD it will engage 4th gear way too early, around 80MPH and 3800 RPM (just off the top of my head, might be a little off). I suspect it has something to do with the uncommon arrangement the tranny has to allow a 3-4 WOT shift (something the majority of 700R4s can't do).

The tranny shifting into 4th gear too early will keep the engine from operating at Peak HP (RPM) going through the top end of the quarter mile.

Typically the shift to OD in the 1/4 Mi happens around 95-100 MPH depending how high you want to shift your truck. I would recommend 4500-4600 RPM. I would say most stock-motor trucks don't absolutely "NEED" 4th gear until they get into the mid-low 12s. At that point you begin losing a few tenths/MPH if you're stuck in 3rd gear.

 

[jwaller]

Re: boost launch in D or OD ???

leaving it in 3rd will also engauge the overrun clutches in 1st 2nd and 3rd to help hold the forward sprag and keep it from exploding.

 

[dgoodhue]

Re: boost launch in D or OD ???

Another difference I didn't see mentioned yet, is the fact that most SyTys I've seen, from the factory if the trans is left in OD it will engage 4th gear way too early, around 80MPH and 3800 RPM (just off the top of my head, might be a little off).

On the two SyTy I have had neither with do a 3-4 WOT unless I do it myself, if I leave it in OD it doesn't do it.

 

[sy#2921]

Re: boost launch in D or OD ???

i have just left mine in OD and it shifts fine in my opinion. The 3-4 is at about 42-4300. i have tried doing the D-OD shift myself but the truck just ran all the way to 4900 and wouldnt shift into OD untill i let off for a sec

 

[Tooky]

Re: boost launch in D or OD ???

Sy 2921 if your truck is BG's old one with the Roadmaster trans, I know it failed when KO had it so I am wondering who built this one? Because they must have done a great job, that's the first I heard of a proper automatic 3-4 shift. Remarkable!:thumbsup:

A buddy of mine had a Roadmaster trans that failed 2 or 3 times (he had them repair it), and it would exhibit classic rebuilt SyTy tranny symptoms... 3-4 WOT always too early (leaving in D and manual shift solves that). And it would only make the shift about 50% of the time. It would tend to just hang in D forever when he really needed 4th, like the dragstrip. :roll: :tdown:

 

[sy#2921]

Re: boost launch in D or OD ???

yes this is BG's old one and he told me that it failed on him once and then also on KO once maybe even twice. You arent too far away from me so we should gtg one of these days so you can help me finish going through the truck

 

[Tooky]

Re: boost launch in D or OD ???

I wouldn't mind taking a look at it. Maybe when I get mine together we should both test and tune at GLD. I hope mine is running in a month or two. I got jerked around big time, by two different tranny builders. :roll:

 

[nightdiver]

Re: boost launch in D or OD ???

I wouldn't mind taking a look at it. Maybe when I get mine together we should both test and tune at GLD. I hope mine is running in a month or two. I got jerked around big time, by two different tranny builders. :roll:

How come you didnt go with a syty transmission builder ?

 

[Crimson93Ty]

Re: boost launch in D or OD ???

Best to race in 3rd if you can. Pressure boost as mentioned by JWaller and 2 overrun clutches get applied. If you are in 3rd you have the forwards, the overruns and the 3-4 clutch pack holding. If it shifts into 4th, the only thing propelling you forward is the 3-4 clutches. The forwards are holding but not moving anything as they are in overrun kinda mode. That's why the 4th clutches burn up so often in these units. If you have to shift into 4th, then by all means do so but if you are doing the 1/4, it's probably best to leave it in D. D is a more stable gear anyway being a 1:1 ratio.
The difference in the shift point that was noticed is because of the slight decrease in boost pressure when in OD. Lower TV boost pressure means the governor doesn't have to spin as high of an rpm to create the pressure to overcome the TV pressure to allow the shift. Just my opinion.