Weird pulsing in 5th

[tha881]

Reaching out for a bit of help here. I've got a strange pulsing in 5th gear between 3.5 and 4k revs. I'm certain its not chassis related as there is no vibration felt through the car or steering wheel. Also, it was always very smooth on the motorway before, I could mash the throttle at higher revs and it would just sound perfect. It's perfectly fine either side of the rev range where the problem is. It's a sort of pulsing or droning that is felt in the powerband. If I dip the clutch it goes away. I'm guessing that it presents itself more in 5th due to the longer gearing, when I go through a tunnel in 3rd its noticable.

I was adamant it was the exhaust as the baffle had rusted loose in my scorpion back box, but this has since been repaired along with a new centre section made up and it's still doing it. However, the surface of the backbox is very warped where they welded it back up. I took the backbox off today and went for a little blast up the motorway, but it was too loud to really know if it had cured it.

The noise definitely presents itself through the exhaust as far as I can tell. I've worked through loads of stuff including

Compression test - 160psi on every cylinder
Valve clearances
Air leaks
Ht lead resistance - just under 6k ohms
Tps tested - switching perfectly
Spark plugs nice and tanned
Fuel pump is working, although havnt done pressure test.

It basically leaves me with buying a new backbox but i dont want to throw 250 away. Does anyone have any experience of anything like this? My only other thoughts are the gearbox or diff.

[ChrisHC]

Just a thought - make sure all the windows are completely closed.

[tha881]

Thanks Chris, it's definitely nothing to do with the windows.

I'm 99% certain its exhaust resonance, I've ordered a cheapie back box just to test to see if it makes any improvement. I've also been onto the guys that fabricated the new center box and theyve kindly offered to have a look at it.

I've read that the standard centre boxes have a crossover pipe to help with scavenging. I didnt know this before and they didnt seem to know of one was in there. They still have my old centre box so we will have a look and see what's going on.

It may be that I end up with a straight through to back box with X or H section installed where the centre box was. I'm not too bothered about how loud it is I just want the resonance gone, failing that I will just order a standard centre section. Lesson learned!

[paultv]

Don't forget to check the prop, it's rotational speed is mph related regardless of the selected gear.

You need to check the centre bearing, pre load and then the universal joints for any sign of "notching" or slap. Once one UJ starts to wear it puts extra pressure
on the other UJ's and centre bearing making for rapid degredation. It also, of course upsets the overal balance of the drive system.

Rear drive shafts could also be suspect as could a wheel bearing - I did mine recently after I started to hear a rumble and then could feel it through the body.

No idea if this helps - if your issue started after problems with the exhaust then thats quite likely where the problem lies.

Paul

[tha881]

Thanks for the advice Paul, wheel bearings are most certainly still the originals and are on the list of jobs to get done. The guibo and centre bearing were replaced around 4 years / 13k miles ago so I didnt think it would be that, certainly worth a visual check though. It's the fact theres no vibration that makes me think it's not chassis related.

I'm hoping that putting an x or h pipe back in the exhaust system will equalise the pressure between the two pipes and remove the resonance.