ignition advance

[disco-v8]

the normal ignition timing on my standard disco should be set at 5 degrees BTDC but most people who know a thing or 2 about these engines no they can be set to 12 degrees BTDC on static advance (on a 3.9)

ive been running mine at 10 degrees BTDC before i did a few more power up grades to it...... before i upgraded the engine even more it was running at 10.5 BAR CR, but now ive put the older tin head gaskets on, and afew other things.... to increase the CR, its gone to 15 BAR!!!!!! which i think is, super, smashing GREAT!!!!!!



but my question is...... will this effect my static advace of 10 - 12 degrees BTDC ?????? if its advance to high will pinking happen early now?????

thanks in advance

adam

[RoverP6B]

Hello,

Yes, with increased compression, your engine will be far more likely to ping under load, especially with the same level of ignition advance.

You may find that you will need to retard the ignition somewhat to prevent pinging.

Also the distributor may need regraphing to prevent too much advance occuring too quickly, hence also leading to pinging.

Best of luck!
4.6Rover P6B

[sidecar]

You can't just go adjusting the static advance without working out what it's doing at full advance. It's too much full advance that will blow your motor to pieces.

I've done alot of work on my 35DML8 dizzy to get it to something like right. At the moment I'm not running enough static but running the right amount of full advance for my 3.5 lump. (35 degrees, but only 8 static)

The 8 degrees that Rover specify is to allow for morons that load the engine up possibly with knackered weak advance springs!

You need to restrict the total amount of advance that the dizzy can give so that you can run more static without over advancing the full advance, I did this by making small sleeves that fit over one of the spring pegs, this reduces the total advance. I measured the dizzy advance with varrious size sleeves, remembering that what is measured will be doubled as far as the engine is concerned. (I did this with the dizzy off the engine).

I can now restrict my mechanical advance to 21, 23, 27 degrees. (8+27 gives me the 35 that I'm running). When the weather gets better I'll try the 23 degree sleeve with 12 degrees static.

The two dizzys that I've measured both had 32 degrees total advance (16 measured on the dizzy). This is too much to be any use! (They both had 16 stamped on them but I measured it anyway).

The standard springs are usually knackered, RealSteel do a kit.

I found that the one silver spring and one gun metal spring gives full advance at 3.5k, two silver springs at 3k, one copper and one silver at 2.5K. I run two silver springs.

A normally aspirated 3.9 lump needs to be running around 32-34 degrees unless the compression has been pushed too high in which case you will need to reduce the peak cylinder pressure by lighting up the mixture later in the compression stroke. You should note that an engine in this state will not perform as well as one with a lower compression ratio and the right amount of ignition advance.

I could be wrong here but I'm sure that I read somewhere that with todays fuels most engines will not tolerate a cranking pressure of more than 180-200 PSI, 15 bar is 217!


Hope that helps!

Pete

[CastleMGBV8]

Pete,
Thats some very useful information, and confirms what my own research produced.

The recommendations I got from very reliable sources was with a road tuned 3.5/3.9 that you need 10 degrees at 1000RPM and all in 34 degrees at 3500RPM Individual engines may vary of course and only rolling road time experimenting with various set ups would get you any benefit and probably not much for the time and cost involved.

Can I ask how you tested the dizzy to confirm the revised advance curve? I have got a spare 35DLM8 which I was going to send out to be recurved bur it appears I can do it myself using the information you've provided and the kit from Real Steel.

Thanks.

Kevin.

[CastleMGBV8]

Pete,

Having just printed off and re-read your post you mentioned the sleeves to limit total advanc, did you mean that the sleeves go over the advance weight stops rather than the spring pegs or do they serve both purposes?

Kevin.

[sidecar]

Pete,

Having just printed off and re-read your post you mentioned the sleeves to limit total advanc, did you mean that the sleeves go over the advance weight stops rather than the spring pegs or do they serve both purposes?

Kevin.

Hi Kevin,

If you take a look at how the advance bob weights work you will see that one of the pegs that one of the springs is clipped around also acts as the stopper for the advance mechanism. (You can see the heat treated bit of the mechanism hitting the peg, at least it is on the 35dml8 anyway).

I actually built up weld on this heat treated bit then filed it to shape, with no sleeves I now get 13.5 degrees on the dizzy, I have made sleeves of different sizes that slip over the peg once the spring is removed, when the spring is replaced the sleeve can't fall off. The Realsteel springs are a right git to get off so be careful or you'll damage them.

The problem is that there is very little space to fit these sleeves, if they get too big they foul another part of the mechanism. You can file out some more clearance but its a bit tricky, just take your time and use a dremel or something similar.

There must be some clearance otherwise the thing could stick on full advance!!!

You can hold the dizzy in a vice and get a protractor that you can fit where the rotor arm goes, rig up a pointer to check the effect of your modifications. (Check it even before you do anything).

The sleeves need to be tall enough so that they can not ride up the peg.

None of this work is particually hard to do but you do need to take your time and double, tripple check your results. If you are into getting the best out of your motor but can't afford a mallory then doing this sort of stuff is at least cheap and must help.

I've just modded my mates dizzy, the springs on his were very slack so that some of the total advance would have been used up even at tickover, he pointed out that this could mean that we did not need fit a stopper, I pointed out that this was a bodge, he agreed!
We did not do any welding on his dizzy, we just used a slightly bigger sleeve.

One other thing that could save you 45 quid, if you knock out the roll pin for the gear drive so that you can pull the whole shaft out don't assume that the hole is drilled centrally, check the fit of the gear both ways round. If you just bung the gear on and bang the roll pin back through without checking the hole alinement you could bust the gear! (I did!)

Hope that helps!

Let us know how you get on,

Pete


P.S. I tested when the advance was all in using a very "scientific" method, I got the missus to slowly rev the engine whilst my head was under the bonnet with the strobe, when I could see that all the advance was in I waved my hands about, this either ment that she should note the rpm reading or that I was being sucked into the carb! (Remove the vac advance tube and bung the end up whilst doing this).

I have made some extra timing marks at 32, 34, and 36 degrees on my damper when I had the thing off. I used a junior hacksaw blade then ran some white paint into the cuts, infact I've done this on TDC, 2, 4, 6, 8, and 10

You should also check that your pointer points at TDC when the piston is at TDC otherwise everything single reading will be wrong!

[CastleMGBV8]

Pete

Thanks for the tip about the gear alignment, can you tell me how you remove the studs which the plastic cover sits on, mine seem to be firmly screwed in and i didn't want to distort the part the cover screws thread into by applying too much force with a screwdriver.

Did you check the curve up to 3500RPM before you limited the full advance ie did the diizzy give 35 degrees at 3500RPM just with the change of springs and would have advanced further if you had not modified max advance.
The ideal curve which is in fact a straight line from 1000 -3500RPM and advance should increase steadily between these two points in the rev band.

The 3500RPM figure seems to coincide with the peak torque RPM of an average road modified engine which may be a coincidence or not.

Kevin.

[sidecar]

Pete

Thanks for the tip about the gear alignment, can you tell me how you remove the studs which the plastic cover sits on, mine seem to be firmly screwed in and i didn't want to distort the part the cover screws thread into by applying too much force with a screwdriver.

Did you check the curve up to 3500RPM before you limited the full advance ie did the diizzy give 35 degrees at 3500RPM just with the change of springs and would have advanced further if you had not modified max advance.
The ideal curve which is in fact a straight line from 1000 -3500RPM and advance should increase steadily between these two points in the rev band.

The 3500RPM figure seems to coincide with the peak torque RPM of an average road modified engine which may be a coincidence or not.

Kevin.

Hi Kevin,

I did not check the curve with just a change of springs because as long as the springs aren't massively too strong the dizzy will always advance until the stopper hits the peg. With stronger springs it will just happen further round the rev range. The problem with springs that are too strong is that you will not get enough advance lower down the rev range. You need it to be all in around 3-3.5k RPM. One silver and one dark spring will cause full advance to be at 3.5kRPM but you should check this on your car.

I believe that the advance needs to come in fairly early on a tuned lump as there is much less turbulence in the combustion chamber at low RPM therefore the flame speed is not very high, after about 3-3.5k RPM the turbulence increases at roughly the same rate as the increase in RPM therefore no more advance is required. I'm sure that a standard lump would still run better with this setup but Rover have to err on the side of caution.

The studs on my dizzy just unscrewed with a decent fitting screwdriver, mine were a bit chewed up by someone before I got hold of the car but the smaller screws still screwed in and out OK. You can clean the ends up and flatten them off where the plastic cover bares down on them by spinning them in a pillar drill whilst holding a file against them. Get each one a similar length so that the plastic over is not distorted. The thread that goes into the dizzy casting is quite small so they should not be over tightened. Maybe someone has threadlocked them? You could apply some heat but be careful not to burn the pickup!

As you you have a spare dizzy at least you can modify one whilst taking you time over it.

On my mates car the 8 pointed cam had totally rusted onto the shaft, we found out that its also very brittle, it broke! along with the circlip and O ring!

Also the tiny plastic cap that stops the top part of the shaft being pulled off (Where the rotor sits) had broke, when we tried to pull the rotor off the whole shaft fell to pieces! We have now fitted a small washer in its place and managed to lockwire it in place. This did make filing the required clearance very easy to do but it can still be done with a dremel when the two parts of the shaft are attached to each other.


Regards,

Pete

[Paul B]

You can't just go adjusting the static advance without working out what it's doing at full advance. It's too much full advance that will blow your motor to pieces.

I've done alot of work on my 35DML8 dizzy to get it to something like right. At the moment I'm not running enough static but running the right amount of full advance for my 3.5 lump. (35 degrees, but only 8 static)....

Mine was the other way round: 12 degrees static, but only about 26 degrees at 3500rpm. I did some cutting and messing, added RealSteel springs and I now get the full 36 advance with about or so 10 at idle. It is a while since I checked so the figures are vague.

I'm not sure what the problem was with mine, or if there was inded a fault, but there is an interesting article here that might explain it a bit: http://www.seight.com/ignition.html

Anyway, when I test run it I simply floor the throttle on a slight incline, load the motor to max, see if it pinks. If it does, back off the timing, if it don't pink, then tweak a bit more advance on

[sidecar]

You can't just go adjusting the static advance without working out what it's doing at full advance. It's too much full advance that will blow your motor to pieces.

I've done alot of work on my 35DML8 dizzy to get it to something like right. At the moment I'm not running enough static but running the right amount of full advance for my 3.5 lump. (35 degrees, but only 8 static)....

Mine was the other way round: 12 degrees static, but only about 26 degrees at 3500rpm. I did some cutting and messing, added RealSteel springs and I now get the full 36 advance with about or so 10 at idle. It is a while since I checked so the figures are vague.

I'm not sure what the problem was with mine, or if there was inded a fault, but there is an interesting article here that might explain it a bit: http://www.seight.com/ignition.html

Anyway, when I test run it I simply floor the throttle on a slight incline, load the motor to max, see if it pinks. If it does, back off the timing, if it don't pink, then tweak a bit more advance on

Hi Paul,

I think that over the years the dizzys that have been fitted to the RV8 have had different amounts of mechanical advance built in. I guess the only way to be sure what you've got is to measure it and then make the required changes. (That's why I would not believe any numbers that were stamped on the mechanical advance mechanism).

Regards,

Pete

[CastleMGBV8]

Disco-V8, Name?

Sorry if I hijacked your thread but it has turned into an interesting discussion which is still relative to your initial question.

I would think that at 15Bar you have too much compression for a road engine 11/1?, assuming that the reading you got was accurate and was taken from a number if not all cylinders.

What mods have you done to the engine, have the head faces been machined by more than 20thou what gaskets are you using, tin or composite?

Pete or anyone else,

Do you know what max advance the vacuum can gives. obviously this does not effect full throttle timing but does have an effect on part throttle drivability and fuel economy + 3-4 MPG at cruise RPM, particularly important with a car thats driven on the road with the good stuff, Shell V Power being well over £1.00 per litre at the moment.

My engine is running approx 9.8/1 compression(20thou off the heads) and V Power makes it run a lot smoother.

The information in the links to John Eales (JE Developments) and the article by Dave Andrews were very informative. John Eales states that the larger engines need less total advance and that 34degrees is correct for a 3.5 but a 4.2 only needs 29degrees and a 4.6 28degrees. doesn't mention a 3.9/4.0 but you could probably average that out to arrive at a figure.

All in all a very usefull dicussion into what I've always considered to be the black arts. I now have a much better understanding of what it is we are trying to achieve, whether it can be fully accurately done without proper test equipment is debateable because you really need to map the advance from 1000RPM up to the point of total advance at 3500RPM.

I was intending sending my spare 35DLM8 to the Distributor Doctor in the west country, he does a full rebuild which includes stripping the dizzy down, a thorough clean to as new finish, rebushing and rebuilding to a curve of your choice for approx £150. and guarantees it will be 100% accurate, which seems reasonable given that new Mallory which will still need recurving or a 123 adjustable dizzy will cost a lot more so may send it off for peace of mind. Limiting the max advance is obviously still a good thing and will protect you engine from the BIg Bang so is cost effective.

Kevin.

[sidecar]

Disco-V8, Name?

Sorry if I hijacked your thread but it has turned into an interesting discussion which is still relative to your initial question.

I would think that at 15Bar you have too much compression for a road engine 11/1?, assuming that the reading you got was accurate and was taken from a number if not all cylinders.

What mods have you done to the engine, have the head faces been machined by more than 20thou what gaskets are you using, tin or composite?

Pete or anyone else,

Do you know what max advance the vacuum can gives. obviously this does not effect full throttle timing but does have an effect on part throttle drivability and fuel economy + 3-4 MPG at cruise RPM, particularly important with a car thats driven on the road with the good stuff, Shell V Power being well over £1.00 per litre at the moment.

My engine is running approx 9.8/1 compression(20thou off the heads) and V Power makes it run a lot smoother.

The information in the links to John Eales (JE Developments) and the article by Dave Andrews were very informative. John Eales states that the larger engines need less total advance and that 34degrees is correct for a 3.5 but a 4.2 only needs 29degrees and a 4.6 28degrees. doesn't mention a 3.9/4.0 but you could probably average that out to arrive at a figure.

All in all a very usefull dicussion into what I've always considered to be the black arts. I now have a much better understanding of what it is we are trying to achieve, whether it can be fully accurately done without proper test equipment is debateable because you really need to map the advance from 1000RPM up to the point of total advance at 3500RPM.

I was intending sending my spare 35DLM8 to the Distributor Doctor in the west country, he does a full rebuild which includes stripping the dizzy down, a thorough clean to as new finish, rebushing and rebuilding to a curve of your choice for approx £150. and guarantees it will be 100% accurate, which seems reasonable given that new Mallory which will still need recurving or a 123 adjustable dizzy will cost a lot more so may send it off for peace of mind. Limiting the max advance is obviously still a good thing and will protect you engine from the BIg Bang so is cost effective.

Kevin.

Hi Kevin,

I also agree that 15 bar is too high and infact did say this in my post to Adam. (Disco V8 )

As his post was asking about timing then I think that all the replies in this post are still relevant.

The timing for the RV8 seems to be fairly well established, it is quite possible to predict within a degree or two how much your engine will require, I guess this is because the engine has been round for years, like you say the larger lumps need less advance.

With the method of modding the dizzy that I have outlined it is true that your not actually mapping the advance say at every 500 RPM, your just getting the static and max advance right and at the right RPM.

I'm not sure how anyone could get a simple dizzy to be spot on at every single RPM up to max advance, at the end of the day I think that the bob weights force is probably squared as the speed is doubled, the spring force is only doubled when the extension of the spring is doubled. It may be possible to build in some adjustment by having one slack spring so that it only starts to act after a certain RPM.

Also who knows what timing is required every 500 rpm round the range, you would need a dyno and alot of time to find that out and you would probably only gain a couple of BHP here and there at low RPM. My motor is in a Cobra which is used for Sundays and high days, it spends more time above 3k than below it!

At the end of the day doing the above is going to be alot better than just bunging it all together with a set of clapped out springs and too much mechanical advance. I'm quite into Wallis and Gromit DIY fixes, mainly because I'm skint! There is certain sense of satisfaction in sorting stuff out yourself.

I have not worked out how much timing the vac system can add to the total, assuming that the vac diagram does pull the mechanism round to its stopper then you should be able to work it out by measuring with a protractor how much the pickup can be moved by the vac system.

It is certainly a good idea to keep the vac and there should be no danger of it over advancing the ignition as it is only working when the VE of the engine is very poor due to the throttle only being cracked open.

Let us know how you get on.

Regards,

Pete

[CastleMGBV8]

Pete,

Yes, I hate paying someone to do what I can do myself especially when you get the response to an enquiry and they say we know what were doing, leave it to us, but won't tell you what there going to do, only how much your going to pay! if that makes sense.

I have a suspicion that the dizzy in the car is far from perfect the vacuum can has definitely gone, so may do as you suggest with the spare dizzy and swap that into the car. I'm hoping to build a 4.3 engine using a pair of Buick 300 alloy heads I have and that will need less total advance than the 3.5 so could send the old one off for the total rebuild and recurve as and when required.

Just as an aside can you tell me how you incorporate text from a previous post, Iv'e tried it several times without sucess,

Thanks,

Kevin.

[sidecar]

Pete,

Yes, I hate paying someone to do what I can do myself especially when you get the response to an enquiry and they say we know what were doing, leave it to us, but won't tell you what there going to do, only how much your going to pay! if that makes sense.

I have a suspicion that the dizzy in the car is far from perfect the vacuum can has definitely gone, so may do as you suggest with the spare dizzy and swap that into the car. I'm hoping to build a 4.3 engine using a pair of Buick 300 alloy heads I have and that will need less total advance than the 3.5 so could send the old one off for the total rebuild and recurve as and when required.

Just as an aside can you tell me how you incorporate text from a previous post, Iv'e tried it several times without sucess,

Thanks,

Kevin.

Hi Kevin,

I guess that they won't tell you what they're doing because what they are doing is nothing out of the ordinary, you might specify the exact curve (or straight line) that you want but they will know that you have no way of testing the dizzy when they hand it back to you! (I'm a cynical old git!).
I suppose that they would show you a graph of what the dizzy is doing.


Personally I would not bother sending the dizzy off, I'd do it myself. If you need any bits then Jim of JRV8 can supply them at a good price.

Your 4.3 project sounds interesting, I guess that it will end up running around 30 degrees.

If you reply using the "quote" button then the text of the post that you are replying to ends up at the top of your reply.


Good luck,

Pete

[jefferybond]

Pete,
The recommendations I got from very reliable sources was with a road tuned 3.5/3.9 that you need 10 degrees at 1000RPM and all in 34 degrees at 3500RPM

Yes, these are pretty much the figures I use, but I'm using megasquirt to control things. (much easier that faffing with advance springs!).

Even though I run 10 deg @1000rpm, I actually advance the timing below 1000rpm (to 20BTDC @500rpm), so that I get a better idle and cooler exhaust manifolds. Something that obviously 'aint possible with clockwork timing!

Jeff