Urgent timing question

[daxtojeiro]

Hi all,
well I finally got the 500 engine running in the cobra today, but I was amazed that it needs the timing at idle set to around 20degrees BTDC for it to sound as if it was healthy. Around 8-10 it was firing out of the exhaust stroke, 20-25 it purrs, is that normal??? If so is that down to the cam? Ive double checked everything, even the TDC marks with cylinder 1 and a bit of wire in the plug hole. Im now curious as to the max advance it will take with idle at 20degrees,
Phil

[GreenV8S]

I know you checked, but surely the timing reference must be out? My 4.6 runs at 12 deg static and that seems to be at the top end of the normal range.

[Mutant Rat]

I believe the 'standard' advance at idle is 8 degrees.

Should be 28-34 degrees, depending on the precise engine spec, at 4000rpm with vacuum advance disconnected, but precise setting should be done dynamically based on exhaust emissions.

There are legends of a set of incorrectly marked front pulleys in circulation, but if you've checked TDC on cylinder 1 with a dial gauge, then presumably you have ruled this out?


>> Edited by Mutant Rat on Thursday 4th August 23:23

[bluebottle]

When i had to reset mine due to having to take the dizzy out to reprime the oil pump, i phoned TVR Power and their recomendation was 28Deg. @ 3k rpm vacuum pipe off as a starting point.
Apparently there are variances between engines....could be worth phoning TVR with the engine number for a definate answer

[daxtojeiro]

Well Ive checked the marks as thats what I thought was out, but they are spot on, 10BTDC and it sounds awfull, 20 and its as sweet as a nut. I played with mixture and it was defiently the timing, idleing at around 13.5 AFR, so that appears ok,
Phil

[Mutant Rat]

...cam will potentially make a big difference, though - obviously the valve overlap significantly effects how much air/fuel mixture the engine is breathing in at low revs.

What is the spec. of the engine?

[daxtojeiro]

no idea of engine spec, i bought it from a scrap car, I removed it from the car, so I know the mileage and thats about it,
Phil

[GreenV8S]

If it's the cam then I would expect the effects to taper off quickly as the revs go up, by 2-3000 rpm it should be clear of the big egr region. Take it further up to around 4000 and you should also have the mechanical advance maxed out so what you see is what it runs under load. If it really is running that much advance, surely it would pink like mad under load.

[daxtojeiro]

Thanks for all that,
well this is definetly looking unusual then, I must confess it looked wrong to me, but I wasnt sure. I have a mate coming round tomorrow so I can do some better tests as its hard on your own doing it and keeping it running. Dont forget this advance curve isnt governed by the distributor, its a mappable system Im running, so I can allow for these odd settings without having too much total advance. Ive got total at around 28deg. Will be interesting to see how much advance it will take without detonation,
Phil

[daxtojeiro]

Hi all,
well I figured it out, it was my fault, I forgot there was a knob on the strobe light, someone must have twiddled it and it threw the timing light out by a mile or so!!! How embarising, took me and my mate 3hrs to sus it out too!!! Silly me, all is running great at 12, have max at 28 at Wide Open Throttle, will soon be datalogging and seeing what it likes to run at, thanks for the help,
Phil

[old64er]

always check the true TDC, when i dialed the cam in mine was 3 degrees advanced according to the front pulley.
My static timing is at 12 deg any more and she does not want to tick over very nice !!