Knocking sound when revved???
Turbo Mini Forums

Hi guys.
I've searched but can't find an answer.
I've timed my ignition to 6 degrees BTDC at 1000rpm vac disconnected (standard dizzy), with fuel pressure set at 3-4 psi and an Avonbar Hi boost needle. I'm running a standard turbo cam (just for the time being) and no intercooler just yet and Accralite +20'' 18cc pistons.
The problem I'm getting is when the engine is revved I can hear it knocking. When I rev it by hand I can feel this knock and it feels like it's within the inlet manifold.
I would love to go the megajolt root but I've not got mega money :). Over Christmas the engine is coming out to have the mg cam installed and a decent intercooler set up, but I don't fancy taking the head off as a new gasket is something else I'd have to buy (Not being tight, I have kids and it's Christmas).
Regarding ignition timing I'm slightly confused. Benross says (in other threads) the figures that I've set my engine too are what he recommends then other threads say things like 28 degrees advance can someone enlighten me?

Thats what mine did. reved the engine and it knocked its tits off.


That was a Center main bearing failure.

Oh poo!!!

The whole engine has only done around 200 miles. The bottom end has Turbo Crank, New Big end and Mains bearings, Centre Main Strap, High Tensile Main bolts (from USA, TurboDave I think), ARP rod bolts and polished stress relived con rods.
I think only a full strip down is gonna reveal the problem.
It does go really well though apart from this slight knocking.

Ah, slight knock, chances are its not the center main then.

I really do think its timing related. Well I hope it is

It would make sense to have someone check it out who is able to diagnose the noise. And dont be revving it until this has been done, in case it is something sinister.

9.85 @ 145mph
202mph standing mile
speed didn't kill me, but taxation probably will

Do you have roller rockers?

if so whip the rocker cover off and fire it up for a few seconds to see if the knock vanishes

Timing should be ok as set...plus you would never feel a knock from timing except under load unless it was waaaay off.

Take a look at what Joe C says....look for a shiny spot inside of the rocker cover for tappet clearance! Mine sounded like the pistons were about to fall out the firs time I heard this.

Also, just make sure your tappets are set correctly and make sure head studs and rocker pedestals are torqued properly...a few thousands out on tappets will make a ton of noise when revved. Should be about .016 to .018 when cold.

Eric
my pics.... http://www.fototime.com/inv/7AAF37CF3EFA956

It also make me think at a tappet/rocker noise, a well worn rocker shaft is also meant to sound like this.
never try 28/30° advance timing at full rev (5000ish) with a 9.4 std cr.
It's what's needed to build the last hp's, but it need to lower cr to 8.2-8.5 and is boost depending.

Have you dropped a washer in the cylinder?

I think it maybe a lean idle mixture,as it only seams to happen when it's ticking over and the throttle blipped, or when stuck in traffic and then slowly pulling away.

as above but .......... if you are running a dizzy, your ingnition timing it needs to be timed in with a strobe 6 deg before top dead centre at 1000 rpm with the vac pipe disconected for this purpose.

keep us informed

I still don't think that it is a fuel issue as a weak mixture would just make the car stumble. A knock would mainly happen under heavy load with fuel or timing. A quick check with a lambda probe would verify this though. I use the Innovate LM-1 on mine with a bung in the down pipe.

Keep looking for mechanical issues...not fuel or ignition in my opinion... :-

Eric
my pics.... http://www.fototime.com/inv/7AAF37CF3EFA956

Try as Joe suggests above...

My car sounded terminal until I sorted the rocker cover clearance.

In the 13's at last!.. Just

Right boys. I had a mess this afternoon.
Checked rockers...OK
Checked fluids...OK
Checked plugs and wiring...OK
Checked timing...OK
Checked for leaks...OK
Turned mixture one complete turn clockwise...Knocking stopped.
I then took it for a blast to see how it went (by this time it was nice and warm) and it flew! Had a fiddle with the idle speed. Pulled loads better from standing, driving at 'miss daisy' speeds and boosts up nicely when being hammered. So good in fact that I got some phenomenal clutch slip.
So all in all I'm gonna leave it set to that, engine will be coming out soon and I'll doubt I'll use the car again before Christmas. Happy days.

Edited by madcatminis on 7th Nov, 2010.

Glad to hear you got things sorted....I'd definitely be sure to get it set up correctly though (either this motor or a new one) before driving too much as you could have severely damaged things being so far off on your mixture. Just takes a few high power blasts down the road to make a real mess of brand new pistons with the wrong fueling.

Eric
my pics.... http://www.fototime.com/inv/7AAF37CF3EFA956

Well done chap!

glad it was nothing serious :)


Also to elaborate on the timing....

As fab says 29-30 deg is for full throttle high rpm,

At idle with the vac disconected ypou will see around 8 deg, however soon as you connect the vac pipe the advance will go to somthing like 15 deg. this may be the figure you have seen elsewhere

The vac is there to advance the ignition under light loads, ie idle cruise and deceleration.

Edited by Joe C on 7th Nov, 2010.

Cool JoeC, you've cleared the timing thing up for me.
I understand why I see stange figures that people use.

there you go