Another dog-box question

I've read for years how a dog-box transmission can be shifted much faster than a synchro-type tranny, but I'm finding that's not really true, at least not on my Honda K24 drivetrain.

I speed up in first gear and want to shift to second, and being a dog-box, the clutch is not touched. The gas pedal is lifted slightly and the shifter lever is moved out of first into neutral. At that moment, the engine, input gear cluster, and output gear cluster are all spinning at, say, 6000 rpm. At this point there are two ways to go. The first way is to wait for the engine to spin down to perhaps 4000 rpm in order to match second gear's speed, and the the shifter will then cleanly drop into second gear with no grinding.

The second way is to force things, slamming the shifter into second gear without delay. The dog-box mechanically allows this but it isn't pretty because it instantaneously forces the engine to go from 6000 rpm to 4000 rpm. In about 0.01 seconds, engine speed is forced to drop 2000 rpm, which is like throwing a wrench onto the front pulley bolt. It's going to go "BANG", and if nothing breaks, the tires will likely chirp. After seeing people do it on-line I tried it exactly one time and won't do it again. Yeah I'm sure it saves X seconds per lap, but I'm the one who has to pay for fixing things... Also, breaking the rear tires loose has got to be a bad idea during a race.

I can't see any way to smoothly shift from 1->2 without waiting it out. The longest wait will always be the 1->2 shift because the gear ratios have the largest percentage difference. The other shifts take less and less time the closer together the ratios are. After realizing that I had to wait just the right about of time, I could shift with no ugly noises at all.

This brings up another question, waiting for the engine to spin down. I'm wondering if the ECU is artificially keeping engine speed high momentarily after the shift. Perhaps for emissions, perhaps to make shifting the stock box easier? I thought I remembered seeing something about "gear-hang" in KPro but maybe I'm confused, but if there is such a function I'd sure like to turn it off.

As for justification for not using the clutch is this from Hewland:

Manual. Best method: With no assistance from the engine management, the driver must lift off the throttle sufficiently to allow the dog ring to be pulled out of engagement. He should then stay off the throttle long enough to allow the dog ring to engage with the next gear. In practice, the driver can move the gear lever faster than he can move his foot off and back on to the throttle. Therefore the effective method is to apply load to the gear lever with your hand and then lift the throttle foot off and back on to the pedal as fast as physically possible. In lifting your foot, the loaded gear lever will almost involuntarily flick to the next gear before the foot is re-applied to the throttle. Another method is to load the gear lever with your hand, stay flat on the throttle and dab the clutch to release the dog ring. The overall effect on the gear shift is similar to the above method, but clutch wear may become a big issue. The worst method (most destructive and definitely slowest) is to attempt to change gear in a `passenger car / synchromesh` way, i.e. lifting off the throttle, dipping the clutch, moving the gear lever, letting the clutch up and re-instating the throttle. The method causes unnecessary clutch wear, does absolutely nothing to help come out of gear and usually causes dog wear whilst engaging the next gear. This wear is due to several reasons. Firstly, it is impossible for a driver to co-ordinate the complicated sequence of all five physical movements accurately. Consequently the engagement dogs often find themselves engaging whilst the throttle is applied. The lever is usually pulled more slowly as it was not pre-loaded, lengthening the `danger zone`.

My first couple of thoughts are, Is it a factory ECU that likes to "hang" the rpms for emissions purposes? And/Or is there a heavy flywheel in it? Part of that system is a lightweight flywheel which will spin down quite a bit faster than a 15-20lbs OEM flywheel.

Does your ECU have the capability for "shift without lift"? I know motec ECUs support it, but I'm not sure on others.

If so, the function can be triggered by instrumenting the shifter with strain gages and an amplifier.

Basically once you apply enough preload to the shifter the ECU cuts the engine for a predetermined time (milliseconds) to unload the driveline and allow the shift to take place.

Its an automated way to accomplish what you are doing manually by lifting your foot. If the ECU is smart enough to allow different timing depending on the gear you should be able to get the speeds to match up enough to remove a majority of the harshness.

Personally, based on my pretty good understanding of how transmissions work based on having actual coursework and repair work on autos and manuals under my belt, I've always felt that a 'dog box' had no real advantage for anything other balls to the wall timed driving events. Which is obvious, of course, but maybe what's less obvious is that i would never even consider one unless i had a big budget and was already in close enough contention in timed events that faster shifts would be worth the money, or could repeatedly (NOT anecdotally) break a stock trans in a way that a dog box would actually address...

The only thing they are good at is breaking less when you driving like you're trying to break it. They suck for every other kind of driving. You kind of have to get over your mechanical sympathy and embrace the urge to try to break your gearbox, to ever recognize the strong point of a dog box.

I don't know if you have ever ridden a sport bike, but it is basically the same method of clutchless upshifts.

Pull on the lever prior to lifting on the gas, when you lift it will release tension and the gear will fall out of first and should drop the revs enough to let second engage without too much drama.

The other question I have would be your flywheel. Is it keeping the revs up?

Thanks for the replies.

The flywheel is already light... 8 lbs.

The "need" for the dog-box is because Honda transmissions are well known to break when dealing with higher-than-stock power levels, which mine is. It's in a weekend car so there's no concern about living with it as a daily driver.

The ECU does have a "shift without lift" feature, though I want to understand what's going on with it as-is before going nuts. In fact, even if that's enabled, the engine still has to to spin down, so it's not necessarily a sure fix.

And yes, I'm looking into whether the Honda ECU is holding rpm abnormally high, and whether the KPro add-on board can turn that off.

former520 wrote: I don't know if you have ever ridden a sport bike, but it is basically the same method of clutchless upshifts. Pull on the lever prior to lifting on the gas, when you lift it will release tension and the gear will fall out of first and *should* drop the revs enough to let second engage without too much drama. The other question I have would be your flywheel. Is it keeping the revs up?

I drove a shifter kart exactly once and the shifter was awesome. On that kart, engine spin-down either happened so fast or the gears were close enough that there wasn't any lag. Fast effortless shifting, and I was left to wonder, why aren't all cars like this?

In the PPG gear set, first gear is 2.615 and second is 1.611, so that means that the engine speed most slow by that proportion in order to drop into gear without grinding.

For example, say you rev to 7000 rpm in first gear. After you take your foot off the gas, the engine must coast down to 7000 * 1.611 / 2.615, or 4312 rpm before it'll drop straight into second. That's the amount of time I have to wait, so it's far from the instant shifting I hear about. I realize that you CAN slam the gear shifter straight into second and it will go, but anyone with any caring about how their stuff is treated will be left thinking, "you have to be kidding, seriously?"

And no, I never rode with anyone using a dog-box, so it's possible that I just have to learn to not care about the tremendious "bang" that happens at each shift...

On all of the bikes I've ridden, there was always a distinct throttle modulation to allow a smooth clutchless shift, any revs, any load. The bike/quad- engined formula cars I've driven were definitely harder to get right, it may have as much to do with the driveline between the trans and tires as it does your technique- I was always under the impression this is part of the reason for "cush drives" on bikes.

So, I guess keep practicing- there should be a sweet spot where you can preload the shifter, lift, and reapply throttle to get a near instantaneous shift without feeling like something is breaking.

Say what?! My Honda drivetrains have all been pretty solid.

The "need" for the dog-box is because Honda transmissions are well known to break when dealing with higher-than-stock power levels, which mine is.

A dog box will not fix this, it only changes how the transmission is shifted. The failures are almost always an issue with the mainshaft and countershaft being forced apart. Even the S2000, which has the most fragile drivetrain I've come across in a Honda, doesn't break gears until the torque is enough to deflect the shafts.

In reply to sobe_death:

With the straight cut gears, usually comes dogs.

True there is some technique to it. The difference between a shifter kart and a K series transmission is the gearing though. My shifter's gearing is ridiculously close, which helps. Especially when I can move around grid at 3-4K(while slipping the wet clutch), yet rev it up to 12-13K to get into the rev band for launch.

I guess I should have asked this before but is it 8 pounds for the flywheel and then clutch on top of that? What size is the clutch? a 5.5" clutch has MUCH less inertia than a normal 10" clutch. When you get your MOI down, the transmission should be much easier to shift.