Vacuum Advance at idle or not?

Hi Guys,

I've read loads of contradicting information on this subject and I just wanted to double check I've got it right.....

Engine is a 3.9 V8 EFI.
Vacuum advance unit definatly works, if i suck on the pipe timing is advanced.
Bob weights definatly works, if i rev the engine (vacuum advance disconnected) timing is advanced.

At idle should there be any vacuum advance or not?

Thanks

Dave

In a word NO

Good, I'm glad you said that.

Ok so now I want to check / set the timing. So far I've read that it should be set anything from 16 Deg BTDC to 6 Dec ATDC Also one book says 3 Deg BTDC & another says 6 Deg BTDC..... WTF?!?!

Remove vacuum advance pipe. Get engine upto temp. Set tickover to 800 RPM. Set timing to (I'm guessing) 6 Deg BTDC.

Many thanks

Dave

:hysterically_laughiin general chat forum under HAYNES i put some data sheats ,have a look(they were a big hit)

By the sound of it your trying to do it manually?
For what they cost your better off buying a strobe or borrowing one.

They were the 2 books I was refering to. One says 3 Deg BTDC and the other says 6 Deg BTDC...

Just to add more confusion it also depends on what v8 you've got.
The MOD lumps are low compression so the timing will be different

I'm sure its out of a 95 / 96 disco. Will there be anything written on the block to determine if its high or low compression?

OrangeBobTail said:

I'm sure its out of a 95 / 96 disco. Will there be anything written on the block to determine if its high or low compression?

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It's getting on for 30 yrs since i last worked on the MOD stuff so not too sure but if i remember correctly the engine No will be totally different to a civvy one.
Somebody else might be along with more info but the only other thing i can suggest is just set the timing to one or the other figures you've got using a strobe as it will be obvious which ones correct.

try and get all numbers off your engine numder, then you can ID your motor
see pic(dont know how to get it big)
This is my mates engine cant get to mine so cant ID it ,realy buging me

Only one 3.9 with the low comp engine that's the 37L 8.13:1 manual.
The 35L manual and 36L auto are 9.35:1

freight rover ones are 8.13:1 and p6 and sd1 engines car either 10.5:1 or 9.75:1 respectivly if its had another engine fitted like a 3.5

its normally stamped by the engine number what the CR is.

if your running lpg you want it as far advanced as it will go anyway

Forgot how many different cr's there were but nice one lads the op should be able to sort it from the info

dellboy1959 said:

Forgot how many different cr's there were but nice one lads the op should be able to sort it from the info

Click to expand...

I agree the point if this info in an LR V8 forum IS? "freight rover ones are 8.13:1 and p6 and sd1 engines"

Im gonna put sd1 lump in mine- who knows what who put in his before he had it so worth checking-on the block like I said then he will know. I admit there are less options for a 3.9 but I said that. got pms again discool?

Only replying to the OPs question "Will there be anything written on the block to determine if its high or low compression" ...give a bit of credit he does know that his engine is a 3.9.
Also "I admit there are less options for a 3.9 but I said that" err.. did u say that ?

if you can see the engine number it should be stamped CR right by it

After a bit more research (and a few phone calls) the engine in question is a 36D. 3.9L v8 EFI 9.35:1 (high) compression ratio oringinally from an automatic Disco.

Been doing a bit of thinking this aft and seeing as there are so many different variations on ignition timing, play in cam/distributor & the possibility that the timing marks may not be bang on. I've decided to put 2 knock sensors on it, build a knock detector with a couple of led's and possibly a buzzer. Then basically advance it as much as I can before it knocks. Theroritical maximum power with no dead engine....

Big thanks for everyone's input!

Dave

Edit: I might then try running on 98 ron petrol and see how much further I can push it......

some of the p6 blokes struggle even on optimax at 10.5:1 and have to use octane booster too, you should be ok on 9.35:1 just to adjust it till it pinks then back it off a bit

I find advancing them till they pink then retard till it stops pinking is about the best option!