Throttle Response

[jsawduste]

IF it ever warms up I`ll be able to finish my winter upgrades.
One of those upgrades was to reduce the sensitivity of low speed throttle response. Running one of Jeff`s 63mm TB and the low end is simply too responsive. Even worse now with the 5.38 gears. Think of being stuck in traffic idling in 1st gear (manual transmission) or trying to smoothly apply throttle in a rock field. Instead of a nice smooth transition my stroker snaps right up. Surprised the engine is so responsive down low even with 12cwt. crank.

Thought I had read of this throttle response issue as a secondary topic to a different discussion.

Thoughts, comments, fixes or is that simply how they react ?

[Cheromaniac]

I think the reason why the stock TB is tapered narrower at the bottom is to prevent the kind of jerky response on a very light throttle that you've just described.
I have a 65mm TB so my Jeep probably lurches forwards in 1st even more than yours (my stroker also has a 12cwt crank).
I think the only proper solution is to go back to a stock TB. Otherwise just live with the quirks and enjoy the extra power.

[6TIME]

Instead of a nice smooth transition my stroker snaps right up. Surprised the engine is so responsive down low even with 12cwt. crank.

Same here.. Contrary to the talk out there regarding crankshaft weight... I've run the 4cw and 12cw cranks with the 5 speed in the exact same motor and I cannot notice a difference at all in relation to this inertia idea. The 12cw winds up just as fast as the 4cw did, but is smoother thru the rpms. I think the flywheel weight will have much more affect of this rotating inertia idea than anything. The cranks mass is just too close to the crank centerline to cause a noticeable affect. I initially thought when placing that 66lb 12cw crank (with 20lbs of extra meat over the 4cw) into the block, that the extra mass would have to be noticeable... Nope, It's just smoother. The crank weight can make a difference in race/strip applications where engines rev hard and quick but, in 99.9% of ours we will never notice any drawbacks with the heavier crank.

[atias]

same here with the 62mm throttle body.
i just got used to it . or you can add a spring to the throttle linkage to make the pedal more stiff

[SilverXJ]

I don't notice it on my XJ that much.. maybe i just got use to it. However, I can certainly see an issue when going over bumpy terrain.

Maybe experimenting with moving the throttle cable ball and a cam style lever could resolve the touchiness.

[Alexia]

I simply trained my foot to only use the first 0.25" of pedal movement. Otherwise it is burn out city everywhere with the light rear.

[jsawduste]

Thanks for the responses guys.

Looks like this not an uncommon issue and in fact is a by product of the larger TB.
Chris, perhaps with the automatic the convertor is masking some of the sensitivity. Either that or you have a heavy right foot.

6Time I enjoyed your comments on the 4 vs 12cwt crank. It is one of the few comments I`ve seen that are offering a solid comparable foundation.

Curious if the even larger TB`s help mask the effects. On the surface I would have to say no as you mention your 65mm Dino. Could there however, be a "tipping point" where the larger 70mm might reduce the sensitivity ?

The heavier spring would just loose tactual feel and make matters worse. Ideally a snail cam in the first few degrees would be the ultimate answer at this stage. Another thought is to richen the AFR at tip in but that would seem to be a band aid at best filed with pitfalls.

My daughters TJ will lug down and/or idle in gear and pull from the basement with utter smoothness.

In the mean time it looks like the solution is attacking things with the banzi approach rather then finesse.

[Cheromaniac]

Curious if the even larger TB`s help mask the effects. On the surface I would have to say no as you mention your 65mm Dino. Could there however, be a "tipping point" where the larger 70mm might reduce the sensitivity?

I very much doubt it John. The reason is that the larger TB has a bigger opening for air to pass through at tip in so it becomes more difficult to modulate the throttle position. Consequently it would require very fine movement of the pedal to prevent a jerky response and that's almost impossible if the road surface is uneven.

[sly-jeeper]

hmm i like the idea of modifying the the throttle lever on the throttle body...if that makes sense, to give a slower more controlled opening of the throttle body...then again Ive read that tuning is a big part of it as well, I've noticed that on stock 4.0s if you very slowly open the throttle like by hand under the hood the very first movement of it only temporally revs the engine then it drops back to idle and if you let it snap closed the idle drops to like 400rpm or if you continue to very slowly open it will maintain a 750rpm idle until it opens a considerable amount then jumps right up over a 1000rpm, i think the added tq from the stroker makes this much more noticeable and i think the line between idle and accelerate isn't very fine and this may be where tuning helps

[jsawduste]

Have not had to chance to make that observation but it does make sense.

I am going to play with a snail cam design a little bit and see it there is a way to make the low angle TB openings less linear. It may be as simple as a rotating cam on the throttle linkage.

[sly-jeeper]

its worth a shot...maybe even find a cheap spare throttle body to experiment with before you modify your bored tb

[6TIME]

The first 1/4 or so of the throttle shaft rotation does open rather quickly due to the location of where the cable is mounted. That could be your fix... Keep us posted. If that doesn't work, there are setups that I've seen on 2 stroke outboards that have a progressive cam with a roller. This design would allow you to taylor your throttle opening however you'd like it. It'd take some fiddling tho.

[SilverXJ]

The first 1/4 or so of the throttle shaft rotation does open rather quickly due to the location of where the cable is mounted.

I was thinking about this earlier today.

1) Either move the point on the throttle body in then use a cam of some sort to work against the cable to make it appear shorter when you go WOT. Other wise you won't get WOT.

2) Move the cable mount on the pedal lower. Then either use some sort of cam or lift bend the pedal off the floor more so you get WOT.

The only problem is that out accelerator cables on the TB end are shielded in plastic.