Just curious if anyone else's HO/sixspeed truck seems extremely sluggish in second gear? Mine doesn't feel like there's any boost in second. In third and fourth it really opens up, though. I figured that this was just due to there not being enough of a load to spool the turbo with that low second gear, but I raced a buddy's '02 ETH/DEE (bone stock) vs. my '01.5 ETH/DEE (stock except for straight pipe), and he creamed me.
I got the jump on him, but he passed me in the first twenty feet - after shifting to third we seemed about equal, though. We also seemed pretty evenly matched from a roll. Both trucks are quad cab shortbed 4x4's, so there's no real weight difference, either... Ideas?
Chris

 

I'm 99% sure we have the same axle ratios, too.
Chris

 

BINGO !!! Absoultely. I HATE my truck in second gear. Extrememly sLuggish is an understatement. You are 100% right. IT sux bad. I despise it.

My current thinking is that the timing advance curve is not aggressive enough to advance the timing quick enough to keep up with quick RPM changes. I dont think its a boost issue. What I did to convince myself that its timing is this: Try cruising along at say 40 MPH or so in 5th. Then get on it hard and get some boost up, but before you pick up much speed, downshift fast to third and floor it. If its like mine, you will find it picks up RPM painfully slow, alomst feels to decelerate for second, yet there is still a decent amount of boost up from the inital punch in fitfh.

IT seems like it is a rate of RPM increase issue, not any particular gear or boost pressure. You just notice it in second gear because that gear causes the quickest rate of RPM change (well first does, but that gear is useless for anything other than being stuck in bumper to bumper trafic or manuevering a trailer).

The other though I had is that perhaps the rev-limter/de-fueling feature which comes in at about 3,000 RPM has an anticipitory (derivative gain if you are versed in PID controllers) feature that is looking at the rate at which your RPM's are increasing and if it thinks they are increasing to fast such that you will soon overrev, it starts to defuel much earlier than 3,000.

I am desperately searching for a solution to this problem. After $800 worth of TST eletronic stuff, I am begining to think that the answer is to go old school..... Mechanical injection pump .. P7100 with some good old fashioned mechanical tweaking.....

Good Post. Please others chime in about your 400HP, yet its still a slug from a stoplight truck.....


Zino

 

AAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHH!!! Control theory!









Chris

 

The thing is, my buddy's '02 was much better than mine - was there a change in the programming for the '02 model year? I know there was a post a little while back about a "Mad ECM" that was supposed to help low-end driveability and acceleration a ton. I'll dig it up and post the link - maybe that would add some to this conversation.

Oh, and I double checked with my friend, and we do both have the same gearing - 3.54's.

Chris

 

Interesting, maybe an 02' is better. Incidentally, mine is an 01.5 also and I have 3.54 gears. Try to find that MAD ecm post, that sounds interesting...

 

A lot of it is due to the high rotating inertia of the engine, flywheel and clutch assembly. Due to the low ratio of 2nd gear, the engine would normally rev to the redline very quickly, but rotational inertia prevents that. It has more time to rev in the higher gears; thus, rotational inertia saps much less performance in 5th or 6th than in 2nd.

Rusty

 

That still doesn't explain why the 02 is faster than the 01.5.

 

Can't comment on the 02 versus the 01.5 - I've never driven an 01.5. I just know my 02 won't outrun a Hyundai in 2nd, but let me get in 4th or above (especially with the 4.10's).....

Rusty

 

Rusty, I have thought of this as well. The rotational inertia is clearly high on these heavy crank and rod engines, but it just seems way too profound in second. I understand completely what you are talking about though ( I have a 41 Lbs crankshaft and 390 gram pistons in my drag car ). But if it the problem was from rotational inertia along, one would expect the reving in neutral to be kinda lazy, I dont know about others, but mine is not!!! This thing snaps like crazy in neutral, quick as a wip, and rocks the whole truck side to side.... but in second, its a DOG.

I never rode in a P7100 pump truck, but my guess is its like a rocket in second.... I just cant help but think this is an electronic thing Chrysler put in there......


Zino

 

What is the point of running a truck up to redline in 2nd gear? I can understand if you are trying to pull a trailer, or drag a sled, but if you are simply trying to race, why not just start in 3rd?

 

start in second and dump the clutch, with the engine reved a little. itll take off purty darn quick. at least mine does, but its an 02

 

Quick as a whip? REALLY? I don't see how ANY engine with the weighty internals of ours could be "quick".

I'm with Rusty. The recipricating/rotating assembly takes a certain amount of time to move from 1300rpm to 3300rpm. If there's no significant load on the engine, then this rate will hamper acceleration. In the higher gears, there's a much higher load on the engine, and this load is what limits the rate of RPM rise far more than the intertia of the internals.

Ask yourself WHY you used such lightweight running gear in your drag car! ACCELERATION. Think Cummins is primarily concerned with acceleration? Nope-- they want longevity, efficiency, durability. Those massive pistons with Chrome rings (who uses Chrome rings in drag racing?- no one) are built for the long haul.

So because the engine is built with acceleration as a secondary or even tertiary concern, the internals are very heavy. The only acceleration they worry about is under load-- towing. And when towing, the heavy intertia of the crank/rods/pistons is NOT the limiting factor to acceleration like it is when empty.


As for your timing "example" based on a sudden downshift from 5th to 3rd:
The truck will pickup RPM "painfully" slow downshifting from 5th to 3rd whether you have boost or not, to a point. If you're bellowing out smoke, then yes, boost is relevant. But you have to remember that you're trying to accelerate that heavy engine from low rpm to high rpm in an instant, and that's not going to happen quickly.

Note that this is NOT a problem that auto trans trucks have. That's because the TC and tall first gear (2.45 like all the 727s) ensure there's always enough load on the engine to where the reciprocating weight isn't the main source of drag on the engine. I'm talking about aftermarket auto trannies.

The OEM automatics can often feel sluggish as the loose TC not only doesn't get power to the ground, it doesn't load the engine enough to get the performance out of it.

justin

 

You guys are right - rotating weight is a concern for acceleration. But I don't think that's the concern here - why is it slower from 2200 rpm to 3200 rpm in second gear than the same rpm window in third or fourth? Shouldn't that rotating inertia of the crank/rods/pistons/flywheel/clutch/inputtrannyshaft remain constant through all gears, if you're running the same rpm window?

Here's my thinking: From the engine point of view, you are accelerating the exact same amount of rotating weight, and with more mechanical advantage to spin the rest of the downstream drivetrain. The only things I can see that would make it faster in the higher gears are (a) programming / how fuel is delivered and (b) increased load may help the turbo spool, whereas it wouldn't in a lower gear.

rjordan, thanks for the feedback on the '02. I sidestepped the clutch at about 1600 rpm in our race, and my tires chirped and she seemed to move out good for the first ten feet or so but the power just wasn't there thru the rest of second gear...

Dropping 100 lbs of rotating mass wouldn't be as big of a deal in a 20k lb truck as dropping that rotating mass in a 7k lb truck. As a percentage, it saps less power when you're accelerating the higher weight, right?

Great conversation, guys - thanks for all the input!! Let's keep this going...



Chris

 

Mine sure doesnt. Its a pittiful dog.


Like I tried to say before, believe me I truly understand all aspects of rotational interia, trust me. This is something more, I know it.......


ITS A CONSPIRACY

Seriously, Chris is right, there is something strange about this, and I notice no one except 2001.5's are complaining about it..... THats interesting....