have I bust my engine or exhaust?

[Craigybaby]

Typical, just as I'm telling people how well my cars running I go and do something stupid...was in 4th gear probably doing about 4500-5000rpm then accidentally changed to 3rd instead of 5th!!!! It was a slowish gear change but still, the engine was racing immediately so I quickly dipped clutch, although don't think it hit the limiter but could of done. Thought nothing of it really but about 10 mins later noticed engine feeling and sounding asthmatic and wouldn't go over about 3000rpm. Managed to get home and restarted about an hour later and it seemed to be revving better but didn't want to take it over 4000rpm (that was in neutral obviously) but there was a rattling coming from around the middle of the car which sounded exhausty....any ideas please as I'm not mechanically minded.....is it more likely to be an exhaust issue (& therefore cheaper to fix) than an engine issue?
Cheers, Craig.

 

[bmnelsc]

You won't know until you get in there and have a look. The rev limiter only works to cut off fuel when you attempt to exceed the limit under acceleration or when free revving the engine. The rev limiter cannot limit RPM when the transmission/drive wheels force the engine RPM such as what happens when you downshift at too high a speed. Shifting into 3rd and letting out the clutch allows the road wheel speed multiplied by the gear ratios to determine engine speed. As I understand it, the most likely damage is to the valve train...you may have bent some valves. If its running on all cylinders then you probably haven't holed a piston and the "exhaust y" noise you are hearing is air leaking past some valves which no longer seat properly. A compression test will tell you which cylinders are no longer sealing and therefore likely have a bent valve.

I don't know how likely it is that there may be connecting rod damage - perhaps someone with experience can comment further. If this is a possibility then you'll have to pull the oil pan to inspect the rods. Good luck!

 

[Richair]

I'm afraid to say that you should expect the worst; it sounds like you may have bent or broken a valve. Get it compression checked as your first port of call before resorting to lifting the head. There's no reason for anything to have gone a mis with the exhaust...

I lunched the motor in my pal's civic type-r race car through a mis-shift; I only momentarily came off the clutch but that was enough to completely shred a piston and break the rod. Valves out the back of the exhaust job. Not pretty and only a week before our first race in the car! Putting that one right was 'character building'... Easily done though.

If its still running it shouldn't be anything like the above though, hence why I say it's likely to just be the valves.

I hope luck is on your side!

 

[Craigybaby]

Thanks gents, the engine itself sounds ok, but from what you say and what I've read elsewhere it's going to be an expenaive mistake.

 

[Craigybaby]

Just had a very careful school drop off and all seemed ok at low revs albeit with a rattly exhaust somewhere in the middle by the sounds of it.....once the kids weren't in the car couldn't resist giving it a bit more on the way home and again it revved fine up to 4200 before changing into 3rd....so maybe/hopefully/please god it's not as catastrophic as feared....booked in tomorrow morning at my local garage so fingers x

 

[robwright]

Have to agree with the sentiments already made and say that I would not test your theories until you have had at least a compression check done. Hopefully that should tell you all you need to know.

 

[zcacogp]

Hmmm, I may be foolishly naive and optomistic but I'll say it may not be that bad. Assuming you have an S2, 4th is a ratio of 1.03:1 and 3rd is 1.4:1. This is a between-gear ratio of 1.35, so if you were doing 4500rpm at the point of shift then you would have hit 6116rpm, below the rev limiter of 6500rpm. If you were doing 5000rpm at the point of shift then you would have got it to just under 6800rpm, which is not a lot higher than the rev limiter (and you probably would have lost a few revs in the gear change, particularly if you fumbled it, so I doubt you would have had it as high as 5000rpm before you released the clutch).

Therefore if your numbers are correct then you may well not have done anything too nasty. However the proof of the pudding really is in the eating and a compression check is the logical next step.


Oli.

 

[Craigybaby]

Cheers oli, that makes sense and thanks for your optimism, could have done with some before my sleepless night last night...had a look in the handbook (yes, got an s2) and it says when shifting from 4th to 3rd revs should not exceed 4700...if I was a betting man I would say I wasn't exceeding that...what sort of speed would 4700 be in 4th? Very much doubt I was exceeding 80, more like 75 but can't be sure. I didn't fumble the gear change but it was reasonably slow as is my usual style...
but like everyone says, see what the garage say tomorrow.

 

[simkin911]

Rattling around the middle of car ? Torque tube and may be totally unrelated to the gear change.

Good luck - hope you haven't broken my old car

 

[GlennS]

If you were doing 5000rpm at the point of shift then you would have got it to just under 6800rpm, which is not a lot higher than the rev limiter (and you probably would have lost a few revs in the gear change, particularly if you fumbled it, so I doubt you would have had it as high as 5000rpm before you released the clutch).

The factory "red line" on the (quite similar) 944 S engine is 6,840rpm and the Promax superchip raises the S2's rev limit to ~6,800rpm (which mine has hit on a few occasions) so assuming Oli's arithmetic is good, reaching that wouldn't concern me as such.

However, being suddenly "forced" to high rpm as a result of an incorrect gear change is going to put a considerable "shunt" through the whole transmission and engine.

 

[Craigybaby]

Hi Stevey, hopefully unrelated but seems a big coincidence...had a look on google maps and the distance between the f'd up gear change and me noticing a problem was only about 3 miles....

 

[Craigybaby]

Just had the call and it was "compression check was fine, all between 150 &170 but it's the middle section of the exhaust that's gone and you'll have to get one made up as we don't do them" .
so i think overall that's good news...

 

[zcacogp]

Yes, that fits the story. But if you damaged the exhaust with a fumbled gearshift then it would have been on it's last legs anyway, so you have probably just bought forward a bill for a new exhaust by 6 months or so. Certainly no biggie.

(An aside but you can't beat an enthusiasts web forum for a load of doom-mongering and "it's probably broken"-itis!) []


Oli.

 

[Craigybaby]

Just picked up, compression was 150,170,170,170 so I think that's pretty good.

 

[simkin911]

Might be worthwhile forking out for a custom system with a OE looking tail pipe.

ps - and put the cat / other bits of the system that are ok in your shed.

 

[Craigybaby]

Funnily enough had a brief chat with a company called tony banks exhausts in Leeds earlier who do custom exhausts, think he said about £350 for a full system or £100 for centre section....going to see them on Monday for a chat, but those prices are about the same as a porsche system aren't they? What do you think about the compression test? The 150 on the 1st cylinder is a bit low isn't it? anything of concern?

 

[robwright]

Did they not do anything more I.e. squirt some oil into the cylinder and see if it improved? That is pretty standard. 150 is about right for a turbo compression ratio. This is NA right? Anyway given the other 3 are identical I would say yes. Worried then I would probably say no. Could just be rings. Depends how far you want to go. Busted valve probably would have given much poorer results than that. My advice now would be get the exhaust done and see how she performs. Try not to fudge them gear changes though ??

 

[Richair]

ETA: ^what Rob said.

Sounds like very good news then!! See how it drives with the new exhaust section; the lower comp reading on no.1 could be completely unrelated (could be caused by one of several things). Although 20psi difference does seem like quite a gap to me...

I feel bad now for my scaremongering!!

 

[Craigybaby]

Gents thanks for all the advice so far, it's obvious it was given with the best intentions so I'm very grateful and I've learned quite a bit in the last couple of days...not sure on the quality or extent of the compression test, I don't think it's something they do on a regular basis and they don't get involved in engine work but I'm assuming there must be some accuracy in it.....150 isn't within the recommended 10% of the other 3 but I'm struggling to understand the significance of that...anyway like you say, get the exhaust sorted first and foremost....one last question, will the knackered exhaust explain the loss of revs/power?
cheers

 

[Richair]

Physically, no it won't. But with all that extra racket going on it might make you think there's more of a problem than there is (especially after scaring yourself after reading all the above!!). But if it's still sick after the exhaust repair, then the low comp reading would then point back to the worst case scenario of bent valves I'm afraid.