Math: tiny chebby P/S truck motor + 4000 stall converter = yuck!

I learned something about tiny chebby P/S truck motors this weekend..........they don't have any phuckin torque!

Ever since my friend with the ugly Willys broke his ancient Lee Shepard 500" P/S motor and picked up a tiny (357-358?) P/S truck motor to replace it (replace big motor with tiny motor??? chebby guys.....go figure) there has been a back & forth argument going on. I wanted him to use the 7" dual disk clutch/5 speed G-Force trans combo that came with the PST engine. And he wanted to use the high dollar/high RPM TH400 trans & 6700 stall converter that was behind the 500"........I finally got tired arguing & gave in.

So like dumb asses during the last 2 weeks of rain & closed tracks we never thought to hit the brake & deck it in the shop. Arriving at the track we had no idea how high this 6500-6700 converter would stall behind a tiny 10,000+ rpm high winding engine with a sheetmetal tunnel ram & two doms..........the results were damn embarrassing.

This no torque "toy" of an engine could only muster about 4000 rpm stall out of this converter at only 5/8 throttle. Trying to deck it full throttle resulted in the rpm's falling back down below 4000 rpm where it finally stumbled, blubbered, & died. He tried a number of different ways of decking the engine but no deal. He decided to make a pass trying to hold the throttle at about the 5/8 point during the launch and trying to slowly squeeze to full throttle as he progressed down track.......it didn't work. When the trans brake released it fell flat on it's face chugging & bucking forward as it blubbered black smoked out to about maybe the 100-150ft mark where it finally started to clean up. Then as it must have finally reached the start of it's power band it just went phuckin ape $hit as the rpm's shot for the moon & it finally got up on the tire & tried to run.

Moral of the story: Race cars are all about the combo.....the "whole" combo. if someone else spent time/money ironing out a 10,000+ rpm engine/clutch combo, don't throw a tight 4000 stall converter at it because using an auto trans is hopefully "easier" than messing with/adjusting a tiny dual disk clutch.

Damn embarrassing weekend........but at least I guess I did win the argument in the end.

DILLIGASDAVE wrote:I learned something about tiny chebby P/S truck motors this weekend..........they don't have any phuckin torque!

Ever since my friend with the ugly Willys broke his ancient Lee Shepard 500" P/S motor and picked up a tiny (357-358?) P/S truck motor to replace it (replace big motor with tiny motor??? chebby guys.....go figure) there has been a back & forth argument going on. I wanted him to use the 7" dual disk clutch/5 speed G-Force trans combo that came with the PST engine. And he wanted to use the high dollar/high RPM TH400 trans & 6700 stall converter that was behind the 500"........I finally got tired arguing & gave in.

So like dumb asses during the last 2 weeks of rain & closed tracks we never thought to hit the brake & deck it in the shop. Arriving at the track we had no idea how high this 6500-6700 converter would stall behind a tiny 10,000+ rpm high winding engine with a sheetmetal tunnel ram & two doms..........the results were damn embarrassing.

This no torque "toy" of an engine could only muster about 4000 rpm stall out of this converter at only 5/8 throttle. Trying to deck it full throttle resulted in the rpm's falling back down below 4000 rpm where it finally stumbled, blubbered, & died. He tried a number of different ways of decking the engine but no deal. He decided to make a pass trying to hold the throttle at about the 5/8 point during the launch and trying to slowly squeeze to full throttle as he progressed down track.......it didn't work. When the trans brake released it fell flat on it's face chugging & bucking forward as it blubbered black smoked out to about maybe the 100-150ft mark where it finally started to clean up. Then as it must have finally reached the start of it's power band it just went phuckin ape $hit as the rpm's shot for the moon & it finally got up on the tire & tried to run.

Moral of the story: Race cars are all about the combo.....the "whole" combo. if someone else spent time/money ironing out a 10,000+ rpm engine/clutch combo, don't throw a tight 4000 stall converter at it because using an auto trans is hopefully "easier" than messing with/adjusting a tiny dual disk clutch.

Damn embarrassing weekend........but at least I guess I did win the argument in the end.

You're right, Dave, there's a reason they all ran 5 speeds behind those combos. At least you proved you were correct.

Nevs wrote:At least you proved you were correct.

Yea, but now I have to learn how to adjust one of these tiny clutches. I hear the adjustment window is so very small on them that you could easily miss the mark. Supposedly it doesn't take much adjustment to go from the clutch lugging the motor, to it slipping too much & glazing everything.

DILLIGASDAVE wrote:

Nevs wrote:At least you proved you were correct.

Yea, but now I have to learn how to adjust one of these tiny clutches. I hear the adjustment window is so very small on them that you could easily miss the mark. Supposedly it doesn't take much adjustment to go from the clutch lugging the motor, to it slipping too much & glazing everything.

Very true. We went from the 10" clutch behind the 815" engine to the 8" clutch. Much more narrow tuning window in that case. No experience with the 7" stuff, I can only imagine it doesn't get any better...If he doesn't have a clutch machine, he might want to start looking for a used one. We surfaced our discs' every pass. The good news is, once you find a baseline, it gets a little easier