What causes an engine to 'buck' at low speeds?
 

I've been pondering the idea of a FE powered Cobra; that's why I'm posting here.

One thing I've noticed is that some cars I've driven seem to have a tendency to buck or surge at lower speeds.

Driving very slowly at, say, 10mph in first gear. If I add just a little power, not really to accelerate but to maintain that speed, some cars I've driven seem to 'buck' (I don't know how else to describe it).

It's this sudden, unwanted, repeated brief application of a little more power, having nothing do with the position of the throttle pedal.

It makes the car surge forward very briefly, then not, then surge again.

To me it feels like the engine rocks in its mounts and something with the geometry of the throttle linkage causes a slight opening of the throttle, which accentuates that cycle.

I know I'm not explaining it well, but I also felt this tendency in a Porsche 993 Carrera I used to own. Creeping along in first, if I didn't apply the throttle very delicately, the car would do the same thing.

What causes this? Does it really have anything to do with the geometry of the throttle linkage? The highest powered, most highly modified Cobra I've driven so far didn't do this, but two others have, and I can't quite understand why.

It doesn't seem to be a result of the state of tune of the engine, at least that's how it appears to me.

Cam profile generally.

Can be engine capacity, cam, inlet manifold, carb/s etc as a combination.

One of my cars does exactly that, cruising in a traffic jam at 15km/hr 1st gear, can get quite violent at times and have to take foot away from the pedal.
The "bucking" is difficult to control when you can't stop the weight of your own foot.

One thing could be the state of tune. If a carb has a lean spot right off idle, it could cause a hesitation or buck.

Most likely, what you're experiencing are the characteristics of the cam. A lot of engines have a personality more for upper rpm running and they're very inefficient at low rpms. If you happen to be driving/cruising in that rpm range, you can get the "buck".

With the engines that we do, sometimes you can get away from it, sometimes you can't. But usually, we try to make sure the transmission is such that when you're cruising, the overdrive ratio allows the engine to run at an rpm where it's out of the buck zone.

As stated, high performance cams have low vacuum at lower rpms and have trouble maintaining a constant afr.

Just a thought-maybe to light of a flywheel.

IMO Their is no such thing as “too light a flywheel”. Especially in a street tired Cobra. The lighter the better.

I would say it’s too tall a rear gear pulling the RPM down below where the cam is happy. I had that in my SPF before I changed to 4.10s.

Jason

What you're describing could be either the state of tune of the engine as indicated above or a syndrome of driver induced oscillation similar to PIO (pilot induced oscillation) when flying.
More common in rotary wing aircraft than in those other things where the wings don't move. Sort of like a short coupled aircraft where the distance between the wing and the Hstab is short and small movements in pitch control have large results in pitch attitude change.
Probably more common in the big block cars than in the small blocks because of the torque put out by the BB's. Caused by your body moving just slightly fore and aft as a result of the car's acceleration and that movement being transmitted down your leg to the throttle.
Like most other things with our toys it can me moderated by concentrating on being smooth with throttle modulations at slow speeds. In some cases it could also be addressed by increasing the throw of the throttle linkage so very small movements of your foot on the pedal would have less effect on how much the throttle is opened.
DonC

To add to DonC's comments about the throttle. I was having trouble with this in an old car with a heavy clutch, and very light throttle peddle. I switched to a stiffer spring and it greatly reduced the oscillating at lower speeds.

I get the big block buck in my car in any kind of cruising below 1900-2000rpm. I can't go into 5th below 60mph. Keep the rpms up and she's happy.

Another contributing factor is drivetrain lash. Combine that with a lopey engine and driver-induced oscillation (right on, DonC!) and it's buck city.

Thanks, guys! It makes sense now. Never considered the cam profile as a contributor, or driveline lash, or PIO either.

And y'all are right, keeping the rpms up just a little helps a lot. The worse case I experienced was probably closer to idle, definitely below 10mph. And in both cases, only in first gear at low speeds with very little throttle opening.

It doesn't really bother me. But experiencing it again had me thinking there was something more seriously wrong that was causing the problem.

The Porsche could be pretty bad, actually, but in that case (come to think of it) it was induced by PIO via the throttle.

Many thanks!
Natalie
:-)

Natalie,

I fought this with my single 4-barrel on my 427 when I first put my car on the road, and never really round the culprit, though it had to have been carb tuning because the day I installed the 2x4 carbs its stopped doing it. Now, I can let the clutch out and the car will pull itself down the street without bucking in 1st gear at 850 idle rpms with no pedal input.

FWIW...

Jim

I can't comment on the root cause, but I would suggest that what could be a minor problem could be exacerbated by softer than ideal (ie old) engine and gearbox mounts, and too light a throttle spring.
Cheers,
Glen

Its call positive feedback (through the pedal).
With these high torque, low weight cars, when a little fuel is given, it surges forward forcing the driver back into their seat with the result of letting off the pedal which in turns causes the car to engine break which causes the driver forward and in turn giving it more fuel and surging forward, forcing the driver back into the seat and letting off the pedal, and so on and so on.

Same thing happens on a 4wd truck in low range (max torque).

Driver-induced oscillation, letting the clutch out a little too soon before the cars speed has caught up to the idle rpm with very light accelerator pedal pressure. Factor in the springs in the clutch disk oscillating and it's just action /reaction. Either drive it out of the oscillation or put the clutch back in and try a smoother transition.

I'm betting this is the cause 99.9% of the time.

Z.

With the correct tune and carb/cam set up the bucking can be avoided. It might take more experimentation than most owners will/can do. I get the sense most of our Cobra's don't have the right combo and bucking is pretty common. Mine is better when warm, but I push the clutch in and roll at low speed to avoid it. Like someone said "Cobras don't do slow very well".

I even had this on a racing 4cyl with a light flywheel as well as nearly every Race engine we ran on track.Often a shift to the next gear stoped it.Ther was always a certain RPM that it occured usually when barely touching the pedal.Violent pulses,Im sure not the best at driveability,but it was never a problem when I didnt have a big stick in ther.So any ideas on the off idel lean spot?A different power valve maybe?

It sounds like a part-throttle lean-surge which is a common problem when running larger cams in street applications. The idle circuit has a major influence on drivability at very small throttle openings such as slow cruise and light acceleration. Fuel mixture, at throttle openings from just off idle to the point where the boosters come in, is controlled by the idle feed restrictors and the idle air bleeds. Idle mixture is controlled by the idle mixture screws. Generally, cams that produce low vacuum need enrichment of the IFR/IAB to help a lean-surge condition.

It seems most on here assume that it must not be tuned correctly (of course all of ours are perfectly tuned and if we owned the car in question it would never buck). I completely disagree. We put barely streetable, high performance engines in a light-weight race car and then complain that it doesn't drive like mom's minivan (okay, that's a bit of an exaggeration to make my point). Fact is, low vacuum at low rpm equals low torque and bucking if load is applied too fast.

I just don't want Natalie to get the message that any cobra that bucks, must need a tune or have other problems.