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Breaking an engine and clutch, because of shifting to wrong gear in downshift

Breaking an engine and clutch, because of shifting to wrong gear in downshift

Breaking an engine and clutch, because of shifting to wrong gear in downshift

(OP)
Hi,

Right now I'm getting the blame of breaking the cars engine and clutch, because an experienced mechanic was able to hear some noise in the engine, that was very alarming, and apparently indicates sudden boost to high revs, like shifting down to a wrong gear.

I'm wondering how damaging is it to a car engine, if you shift from 4th to 1st gear on a diesel car and then lifting the clutch enough so the revs suddenly hit high? Not sure, if it hit the red or not.

The car has driven 5000+ km, since I last drove it. If I really had damaged the engine and clutch 5000km back, then is it reasonable, that a damaged engine can last that long?

Best Regards,
Driver :)

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

Depending on the vehicle I would expect the clutch to fail before you damaged the engine.
OF course it is possible. I bent a valve once and it lasted 5km before it failed.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

You can OBLITERATE an engine by downshifting to the wrong gear. If you went from 4th to first and the clutch came out all the way, it is almost impossible that the engine didn't over-rev. Over rev high enough and you can break pretty much every reciprocating part- bent valves, wiped cam lobes, broken chains, spun bearings, bent or failed rods, cracked pistons.

The number of times I would disagree with Ed is very small, but I would expect the clutch to transmit enough torque to over-rev the engine in just about any conceivable drivetrain arrangement.

The clutch is designed to handle enough torque to quickly accelerate a relatively large mass- the entire vehicle. Run the system in reverse and you have a large mass with a lot of inertia (if you're cruising around in 4th gear) driving a relatively small mass. The engine will not win. Even if the clutch slips, it is still transmitting torque, and it doesn't take that much torque to drive an engine WAY past the RPM levels it is designed to handle.

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

LPS for jg... missed downshifts are a major cause of blown engines.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

All manner of things can happen when you miss a downshift but typically you find a weak link in the powertrain like a U-joint or FWD chain, or the engine overrevs and you get major piston-valve contact. Given its been 5k+ kms I would tend to doubt the engine was damaged, usually that damage would very quickly show itself.

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

Depends on the engine.. I've seen engines with every valve bent, broken teeth on timing gears, cracked pistons, cracked valve springs, etc etc that would still run. Not run well of course.. but you could drive the car.

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

A diesel is always an interference engine, if it's a pushrod engine it'll bend pushrods. OHC will do other things depending on how the valves work - break rockers, break cams, compress valves. They are low revving, even in a car, so easy to over rev on a downshift. What sort of damage was done ?

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

Disagree. Not all diesels are interference nor does that mean anything in this instance, even non-interference engines can be damaged if oversped as valves can float and contact pistons. Pushrod engines also very commonly mangle valves without damaging valvetrain otherwise. Had he damaged the engine I believe it would have been noticed by him or someone else in the past 5k kms, regardless if it ran or not. I completely agree btw that recip engines can still run with ungodly amounts of damage, the hardest part about working in engine development is deciding which mangled parts to keep as desk trophies!

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

CWB1 - What diesel engine(s) with poppet valves are you thinking of that are not "interference" engines?
Other than the extremely rare Buchi and the Brotherhood sleeve valve engines; and they are not car engines, I can't think off-hand of any non-interference diesels.

PJGD

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

PJGD, some yellow and red diesels aren't true interference engines in that improper crank-cam timing doesn't result in damage as it would with most interference gasoline engines, often valves recess within the piston bowl combustion chamber without issue due to deep bowls and large piston-head clearances. Granted, I'm speaking gear driven industrial engines but the points remain, not all are interference and even non-interference valves can float enough during overspeed to interfere. Another interesting phenomena during overspeed is reciprocating assembly stretch, a few years ago I had a large industrial diesel overspeed allowing a piston to contact and eject an injector within a few feet of me. Lesson learned, during extreme transient testing don't enter the test cell!

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

Pushrod whip is another interesting phenomenon that can result from an overspeed condition, (or because of badly spec'ed pushrods) where pushrod elastic buckling causes retardation of valve opening events at high RPM. This can mainfest in damage in extreme cases, or just very odd torque curve characteristics at high RPM.

Machines that reciprocate at thousands of RPM can do interesting and terrible things to themselves if left to their own devices or badly designed.

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

Back in the 1960s I saw a pushrod type diesel engine that had been damaged by a truck driver, he had put the engine in low gear, then decided to hold the clutch in going down a hill, a process sometimes referred to as "( insert your ethnic minority here ) overdrive." at the bottom of the hill he was going too fast for the gear he had selected, and when he released the clutch the engine over revved then bent all of the pushrods. Other than the bent pushrods and damaged valves the engine came out of the ordeal quite well , was repaired and put back into service. The driver? He was fired.
B.E.

You are judged not by what you know, but by what you can do.

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift


I read somewhere that a truck maker who sold trucks in India (I think it was) fitted the gearbox with the old-fashioned sliding mesh type of gearbox when the truck maker's normal practice was to fit the more modern constant-mesh non-synchromesh gearbox type. The idea was to prevent the probably inexperienced local driver from selecting a too-low gear and damaging the engine.

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

(OP)
Hi All,

Thank you for the replies.

I see, that it is not unreasonable, that my bad downshifting may have broken something.

The car is Honda F-RV the latest version. Tho now it has driven about 10000km without having problems, except that the current driver says that the noise is becoming more obvious.

Best Regards,
Driver :)

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

What year, and how many miles total ?

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

My first car was a Hillman Minx that I got for free. Worth every penny.
It handled well, but was slow, until I removed the thermostat to prevent the engine heat from percolating the fuel in the feed tube to the carburetor.
Then it was less slow.
The brakes, even when newly installed, were a joke, like most cars of the day.
Of course I drove it like an idiot anyway.
One day I was downshifting going into a serious chicane (stone mountain on one side, rock wall on the other), and grabbed second gear a little too soon.
Broke a rocker arm, clean in two.
Well, I thought it was a clean break, but I didn't examine it closely enough.
With the new rocker in place, oil pressure disappeared almost immediately.
The third piece of the original rocker arm was a little nugget, small enough to pass through the oil pump screen and big enough to take out a rod journal.
Luckily the crank could be reground to fit the smallest available rod bearing.

Mike Halloran
Pembroke Pines, FL, USA

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

I have never had a vehicle that would've easily let me under down shift. The syncho pressure would've have been mad and I'd have heard things winding up and felt odd vibrations in the stick matched with a long delay (seconds) before the shift could've been completed. I'm surprised the OP could do a 4th to 1st shift in a diesel.


Does the complainant explain what he's hearing is wrong?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

I accidentally managed to downshift a rental Mondeo in the UK around 2000, from 5th to 2nd, instead of 4th (unless it was 6th to 3rd, instead of 5th). Tranny didn't complain when I shifted, but there was a bit of a commotion when I let the clutch out, and I was sympathetically very aggrieved on behalf of the driver behind me. surprise

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

Quote (itsmoked)

I have never had a vehicle that would've easily let me under down shift. The syncho pressure would've have been mad and I'd have heard things winding up and felt odd vibrations in the stick matched with a long delay (seconds) before the shift could've been completed. I'm surprised the OP could do a 4th to 1st shift in a diesel.

I have the other impression- in any modern trans with synchros, if you're trying to hustle the car around and you push hard enough on the shifter, it WILL go into the wrong gear.

There's a well-known phenomenon on 90s era BMWs where wear in the linkage and bushings of the shifter assembly lead to very easy 3-2 shifts (when you were going for 3-4) under full power.

This is common enough that it has a name in the BMW enthusiast community- the so called Money Shift.

Because it if happens to you you're about to spend a lot of it.

RE: Breaking an engine and clutch, because of shifting to wrong gear in downshift

The OPs question is unanswerable. Yes, a 4-1 downshift could damage the engine. No, there'll never be any way to prove it did or did not. The choice of wording hints at spousal disagreement.

Getting WAAY off topic, but in 1969, whilst awaiting delivery of a then-new BMW 2002 TI, I was given a Peugeot 403 as a loaner. It was a wonderful,durable, comfortable stable-handling old sedan, the likes of which the French haven't built since. Anyway, it had a 4-speed column shift. More than once, trying to complete a pass, the engine wound to valve bounce in third gear, the column shift got second gear instead of fourth. The Peugeot didn't have a tachometer, but the overspeed would have been considerable. The wonderful old car survived that abuse and was not making any untoward noise when the BMW finally arrived.

jack vines

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