Discussion
Purely theoretical questions here. But what actually breaks if you over rev and why? If you get away with it once or twice is the effect cumulative?
If you revved one engine to a few hundred RPM or so over the red line and kept at those revs until it broke. Would it brake the same components as if you kept increasing the revs until it could take no more?
If you revved one engine to a few hundred RPM or so over the red line and kept at those revs until it broke. Would it brake the same components as if you kept increasing the revs until it could take no more?
I used to be a regular poster on s2ki which is the Honda S2000 forum when I was an owner, mechanical over revvs were discussed regularly, after an incident engine failure was regarded as being imminent.
Check out this prick https://www.youtube.com/watch?v=ZsGR6o4dnxU
Check out this prick https://www.youtube.com/watch?v=ZsGR6o4dnxU
Edited by Urban Sports on Sunday 15th June 18:27
There can be different ways an engine will break, pistons hitting the valves which don't get out of the way in time for one, but it usually seems to be (at least with BMWs anyway) things like the big end bolts stretch, and the bearing cap can become lose, and then you get knocking as it can rattle.
Crafty_ said:
Surely pistons and valves can only come in to contact if the timing changes ? Doesn't matter how hard an engine revs, the stroke won't change ??
I'd agree rod bolts are likely to give up first.
As I understand it, the valve springs can only control the valves up to a given point (rpm) after that, the valves can "bounce" open and this is when they contact the pistons.I'd agree rod bolts are likely to give up first.
Tim
I don't think it's something that happens too much these days unless big changes are made, but what you could get a situation where the valves don't get pulled out of the way to be closed by the valve springs quickly enough, and the piston can clip them, putting a bend in the stem for example.
Lighter valves and stronger valve springs would be the solution, but it's not normally something you would encounter on an engine around its normal red line.
Lighter valves and stronger valve springs would be the solution, but it's not normally something you would encounter on an engine around its normal red line.
Crafty_ said:
Surely pistons and valves can only come in to contact if the timing changes ? Doesn't matter how hard an engine revs, the stroke won't change ??
I'd agree rod bolts are likely to give up first.
No, if you over rev the valves can't close quick enough before the piston rises. Essentially the valve will be left behind as the cam lobe passes. The piston will then hit the valves. I'd agree rod bolts are likely to give up first.
Crafty_ said:
Surely pistons and valves can only come in to contact if the timing changes ? Doesn't matter how hard an engine revs, the stroke won't change ??
I'd agree rod bolts are likely to give up first.
On engines that use springs to close the valves, you can reach the point where the piston reaches the top of it's stroke before the spring is able to close the valve, resulting in the piston striking the valve. Search for 'valve bounce' if you want further info.I'd agree rod bolts are likely to give up first.
Urban Sports said:
Check out this prick https://www.youtube.com/watch?v=ZsGR6o4dnxU
What an absolute cock, he might get away with it but his vandalism could end up leaving the next person to rent the car stranded.For me the biggest worry would be the huge forces on the big end bolts.
been in a car where the driver went to drop it down a gear and went from 5th to 2nd - BOOM con rod let go through the block, completely terminal, that was a 1.3 ford cvh engine in the 1990s
in 2004 I did the same at serious speed in a brand new quattro A4, it made a horrific noise and almost put me through the windscreen with engine braking but I dipped the clutch quick and it went on to exceed 100,000 miles without any signs of damage.
SO it depends on many many factors - but the best bet is not to do it.
in 2004 I did the same at serious speed in a brand new quattro A4, it made a horrific noise and almost put me through the windscreen with engine braking but I dipped the clutch quick and it went on to exceed 100,000 miles without any signs of damage.
SO it depends on many many factors - but the best bet is not to do it.
Major cause of con-rod failure after high revs is the oil film between the big-end bearing & crank failing and allowing extensive metal to metal contact which can cause the bearing to instantly weld to the crank and with everything still turning at high speeds then the con-rod is what lets go.
Grayedout said:
Major cause of con-rod failure after high revs is the oil film between the big-end bearing & crank failing and allowing extensive metal to metal contact which can cause the bearing to instantly weld to the crank and with everything still turning at high speeds then the con-rod is what lets go.
Are you sure that happens? It usually just spins the bearings.Evoluzione said:
Grayedout said:
Major cause of con-rod failure after high revs is the oil film between the big-end bearing & crank failing and allowing extensive metal to metal contact which can cause the bearing to instantly weld to the crank and with everything still turning at high speeds then the con-rod is what lets go.
Are you sure that happens? It usually just spins the bearings.Gassing Station | General Gassing | Top of Page | What's New | My Stuff