Mechanical Overrev

madjak · Postby **madjak** » Tue Oct 17, 2017 4:18 pm

I had an awesome weekend down at Collie. I bested my PB by 0.25secs on Saturday and then another 0.1 on Sunday to end up 6th fastest outright and first closed car. The fastest time set in an open wheeler was only few seconds faster than mine... Not bad for a U2L NA MX5!

but... on the main straight during my 3rd run of the day I had a brain fade and hit 3rd instead of 5th. I caught the buzz earlyish and managed to not fully release the clutch but looking at the logs that night I nearly hit 10,800rpm. It could have been worse but that must be some kind of record for a BP to hit and not be gibbed into 1000 chunks.

Thankfully I don't seem to have damaged anything in the process which is extremely lucky. I'm still deciding if I should pull down the engine or just keep running it.

rascal · Postby **rascal** » Tue Oct 17, 2017 4:38 pm

Where was that Arduino controlled pneumatic sequential shifter when you needed it.....

madjak · Postby **madjak** » Tue Oct 17, 2017 4:43 pm

Still on my work bench. I keep pulling my car to bits to install it encountering an issue and then having no time to fix it before the next event so I put my 6 speed back in. I think I've spent more time under the car switching boxes than I have debugging the pneumatic sequential.

greenMachine · Postby **greenMachine** » Tue Oct 17, 2017 6:24 pm

Did you do any more runs after the big rev? If so, and obviously no massive engine failure, that is probably a good sign, but ...

Forged rods I guess? I'd do a compression test to see if any of the valves are not seating properly following a kiss with a piston top.

Not forged, I'd strip the motor and replace the rods. I'd do it yesterday, and after that big an over-rev I would not start the motor again until it had the new (forged) rods. In this case you have been lucky so far, don't push it too far, stock rods are totally unpredictable after an over-rev. I'm guessing you already know this, but stated for the record.

That's a great result - well done!

:mrgreen:

ETA: If you have one of those usb inspection cameras, or can borrow one, while the plugs are out have a look at the piston tops for signs of impact with the valves.

Postby **NitroDann** » Tue Oct 17, 2017 7:46 pm

I would be blown away if stock rods would take that even once. but madjack has all the goodies including in the head so I am not that surprised it was fine. Probably clearanced your bearings and worked your springs a bit. If you have a great oil system and stiff enough valve springs it might be fine. They rev a bunch of hondas and 4ages that high all the time.

madjak · Postby **madjak** » Tue Oct 17, 2017 7:51 pm

Yeah I did 30 or so laps at full pace after the overrev... the entire time I was waiting the hear the clatter of metal or the whine of a bearing but the sounds never came. I happened to drop more time the second day so the valves are still sealing and the engine is putting out good power.

I'll certainly do a compression test. I do it after most events anyway. I need to fix a leaky valve seal so I might pull the head off so I can manage the valve removal a bit easier.

The engine has Crower rods with ARP 2000 rod bolts, Wiseco pistons, ACL race bearings. I think if anything was going to fail it would be the rod bolts or bearings. I've built the head to rev to 9000 but I've only taken it there a few times as I'm trying to keep rebuild periods to something reasonable... It's such a free reving motor with the super light flywheel, tiny alternator, half harmonic damper and EWP it's asking to be push higher. Plus it sounds so good up above 8500rpm

I'll post a goPro video of one of my sessions tonight. I had the camera set wrong so it is overbright. I also need to remove the waterproof case so I can get some nice sound of the car.

madjak · Postby **madjak** » Tue Oct 17, 2017 7:55 pm

NitroDann wrote:I would be blown away if stock rods would take that even once. but madjack has all the goodies including in the head so I am not that surprised it was fine. Probably clearanced your bearings and worked your springs a bit. If you have a great oil system and stiff enough valve springs it might be fine. They rev a bunch of hondas and 4ages that high all the time.

Maybe the engine is a bit more looser now. It's also due an oil change so I'll see if I get lots of sparkles in the oil.

One of the clubmans in the series runs a built Toyota BEAMs motor that revs past 11,000rpm with a sequential. I try to beat him but at 2/3rds the car weight he's always 0.5 second quicker. That thing sounds awesome though.

greenMachine · Postby **greenMachine** » Tue Oct 17, 2017 8:50 pm

I have had three big over-revs on stock rods. Two in the racecar at the same event, one of them a doozey :roll:

, no breakage, but I did follow my own advice and immediately get some forged rods.

The other was on a road/race engine at PI on the main straight - in those days I didn't know any better and just went on with things, engine survived for several years after that, but was never over-revved again. That was just luck, I shudder now when I think of it, after hearing the stories of rod failures.

:mrgreen:

Postby **NitroDann** » Tue Oct 17, 2017 8:57 pm

Woah !

How many revs?

greenMachine · Postby **greenMachine** » Tue Oct 17, 2017 9:34 pm

The racecar? Dunno, no logs - I was on the limiter, got third instead of fifth, was slow to catch it. It wouldn't surprise me if it got to 10 or thereabouts, but have no clue really. The second I got quickly, it was mild by comparison.

The PI one was a big one, but that was from a 7500 limit from memory, same thing getting third instead of fifth.

:mrgreen:

madjak · Postby **madjak** » Tue Oct 17, 2017 11:18 pm

Mine would have hit 11500rpm if I let the clutch out all the way. I'm pretty sure something would have let go with another 1000rpm.

It gives me more confidence I could run past 9000 now

madjak · Postby **madjak** » Tue Oct 17, 2017 11:42 pm

Not the over rev but my fastest time was the second run. The video quality is pretty bad sorry. It should be there in a couple of hours thanks to my awesome DSL upload speed.

RS2000 · Postby **RS2000** » Wed Oct 18, 2017 10:37 pm

greenMachine wrote:The racecar? Dunno, no logs - I was on the limiter, got third instead of fifth, was slow to catch it. It wouldn't surprise me if it got to 10 or thereabouts, but have no clue really. The second I got quickly, it was mild by comparison.

I presume you were in 4th gear. The car speed wouldn't have slowed much in that short time, so engine peak revs would be very close to:
Speed limiter revs x 3rd gear ratio / 4th gear ratio