ignition timing help for newbe!

[chris-stockton]

hello, looking to try and get my ignition timing set up on the westy, the car is quite heavily modified so difficult to know where to start.
basically the car was running well before i took the engine and box out, the only main differences i can think of upon putting it back in are that i got a 14 x3 filter instead of 14 x 2 and i took the dizzy out to prime the oil pump, i marked everything and put back.

Now the symptoms. between say 2-3.5k rpm if you put your foot down, the is a flat spot then it goes. now today i definately heard some pinking again same rpm if you put your foot down.

The guy who built it many years ago said 28 deg BTDC when fully in at 2500 rpm?

So heres the spec. rv8 4.3 stage 3 heads, dizzy has bespoke advance curve - no vac advance, webber 4 barrel, 1 off cam.

when ive used the strobe on it, it is very hard to see whats marked on the crank pully, there are 2 distinct white marks, 1 i think is tdc and the other the 28 btdc mark.

I have run it at 3000 rpm which is the point that it stops advancing and set it on the white line which i believe to be 28. getting pinking.

should i be looking to set my timing at 3000rpm or idle? (which is lumpy)

or does it sound like my timing is not my problem and something else?

[sidecar]

28 degrees seems to be around the norm for the larger RV8 lumps but I'm surprised that it detonates with this figure as it is still quite a low all in figure. How high is the CR?

Also I guess your mixture could be a little lean now as you have fitted a bigger filter, the bigger filter will have lifted the filter lid further away from the top of the carb, I have seen quite a few engines where the lid is way too close to the carb and this will kill the air flow.

If it was my engine and spend some time getting the timing marks spot on using a piston dead stop, after that you can be sure what your timing is.

My 4.6 lump runs 32 all in but I do use a 'clever' MSD setup. it actually runs something like 10 degrees from 0-500 RPM, this ramps up to 20 from 500 to 1000 RPM. It then goes to 30 degrees at 2700 RPM. Between 5000-5500 RPM it then goes to 32 degrees.

With a dizzy setup I'd be looking for something roughly like 14-16 at idle which ramps up to 28-30 degrees at 3000 RPM. I would not run a vac advance unless it can be setup to work well. (The Eddy 500 carb vac take off does not suit the Lucas dizzy vac canster very well at all). If your lump can not tolerate the above figures I'd suggest that you have an issue like maybe too much CR or a mixture that is way too lean.

[chris-stockton]

cheers mate -

ok some more info the builder tells me the CR is 10.5:1 - dont know if that is high for one of these?

the dizzy is apparently came from DJE? with a modified advance curve - no vac advance.

you could be right re the air filter -? i may order a 2" version and see what happens also.

But ill look at sorting out the markings on the crank wheel - or at least establisging whats what

[ramon alban]

Hello Chris, with you having so many variations/differences from a standard setup, my advice would be to set the ignition timing empirically to gain the maximum performance without any detonation - pinking - under full load.

To that end please have a butchers at this very brief extract from my essays on the basic RV8 electronic ignition system.

Empirical Timing is a dynamic process performed with the distributor clamp bolt loosened just enough for the unit to be rotated without undue force. Drive the car on a selected route where one can subject the engine to heavy uphill acceleration at low engine speed in a higher gear to listen for pinking, the high-pitched knocking noise, caused by premature detonation. If pinking is heard, stop the car and rotate the distributor very slightly to retard the ignition and test the car again under the same conditions. Use small adjustments each time until the pinking is eliminated.

My full 'Components and Testing' article is available on my web pages

http://www.vintagemodelairplane.com/pag ... ion05.html

Whereas the 'Description and Analysis' portion is at

http://www.vintagemodelairplane.com/pag ... ion04.html

I realise the empirical timing process (as written) is very simplistic, ignoring, as it does, the additional aspects of vacuum advance and modifying the timing curve using different mechanical advance weights, etc.

However both those subjects are fully covered in my two articles and because I doubt that there is a simple answer to your original question, studying the relevant parts of the articles may give you the opportunity to better understand the whole process and make changes to your set-up to achieve the best possible timing curve.

[sidecar]

Chris,

I think that 10.5:1 is pushing the limit for a RV8 lump when used with modern fuel. The old P6 ran this figure and people now have issues with these as 5 star fuel is on longer available. Now some people on the forum do run very high CR's but they also run very long duration camshafts too.
10:1 is the limit that I set when building a road going RV8.

You could just keep retarding the ignition in the hope that the peak cylinder pressure is reduced enough to avoid detonation but an engine running in this configuration will not perform as well as one with a lower CR and a decent advance curve. Also in my humble the days of advancing the ignition until the engine knocks then backing it off a few degrees are long gone, this is a pi55 poor method of setting up the ignition. Some engines may infact never knock in which case this method would just leave you contintually advancing the ignition. The likes of Recardo, Vizard and others have worked out that the peak cylinder pressure always needs to be 15-20 degrees ATDC and really a dyno is needed to set this up. Luckily the RV8 has been around for so long as no many people have messed about with them that it is possible to just state a figure which will be close enough for your engine. (28 degrees plus maybe another 2 degrees on top if your CR was reduced to a better level)

I would not go back to a 2" filter as this could just be restricting you air flow in order to mask a problem. How much above the carb is your filter lid at the moment? The stud sticking out of the top of the carb gives a good indication of this figure. Also what type of lid have you got? The thin pressed steel ones that are slightly cone shaped give better under carb clearance than the thick cast ali Edelbrock ones.

To sum up in my humble I'd pull the CR down to 10:1 max, sort out the air fuel ratio and run a decent timing curve if it was my lump. The cheaper and not so good option would be to peg the timing back in the hope that the detonation goes away along with sorting out the air/fuel ratio which you should do anyway. (do you know what rods, primaries and secondaries are in your carb?)

[ramon alban]

Also in my humble the days of advancing the ignition until the engine knocks then backing it off a few degrees are long gone, this is a pi55 poor method of setting up the ignition.

A few degrees? I think that would deffo be pi55 poor!

I agree that it is an inferior substitute for pseudo scientific setup on a dyno or RR, but as a place to start it is a reasonable process.

Also the empitical process, as described, does not work by crudely "backing it off a few degrees" as you put it. It tries to be much more accurate that that!

In reality the backing off would be as small as 1/4 or 1/2 of a degree - NOT - a few degrees!!!!

That really is crude and invalidates the care that can be taken in performing the process as described.

When I used this process to adjust timing on my RV8 it came out less than 1 degree different to the 'nominal' setting that I chose, bearing in mind that the setting in the manual is not going to be correct for some of todays unleaded fuels.

Also, taking into account the obvious age related changes that will have taken place inside the engine regarding component wear, cam condition, plus choice of fuels and oils and choice of Efi settings/ECU then the empirical ignition timing process is a cost free and entirely valid starting point.

Even just changing fuel supplier can make a difference to the pre-detonation characteristics of ones engine - as explained in the following article:

http://www.vintagemodelairplane.com/pag ... ane01.html

Some engines may infact never knock in which case this method would just leave you contintually advancing the ignition.

No doubt, but by and large, the normal RV8 does exhibit the pre-detonation symptom and for the most part offers the owner enthusiast an opportunity to try the process instead of dismissing it as too coarse.

[sidecar]

Your whole method is based on the assumption that an engine makes the most power when the timing is just slightly backed off from the point of detonation and this simply is not the case. Some engines will tolerate a huge amount of ignition advance before detonating. What will happen in these engines is that the peak cylinder pressure will occur far too early in the power stroke, this causes a loss of power as the conrod and piston are not at the right angle to transmit the power to the crank. Also power is lost due to that fact that the piston is still rising on the compression stroke against this high pressure gas.
Your method does not deal with the timing at idle or the actual advance curve only the total advance.

We have had this discussion before, you have linked to the same green text before, I can not be bothered having the discussion again.

[sidecar]

Hi Chris,

If you want to read up on how YOUR dizzy would have been modified then check this out...

http://how-to-build-a-pilgrim-sumo.wiki ... by-members



Also the below might be useful with regards to your carb...

http://how-to-build-a-pilgrim-sumo.wiki ... ing-system


I know that it must be very temping for you to just back off your ignition, 99 out of 100 people would just do that but really your engine has a CR that is too high and this costs far more money to sort out than twisting the dizzy!

Regards,

Pete

[ramon alban]

Now the symptoms. between say 2-3.5k rpm if you put your foot down, the is a flat spot then it goes. now today i definately heard some pinking again same rpm if you put your foot down.

I have run it at 3000 rpm which is the point that it stops advancing and set it on the white line which i believe to be 28. getting pinking.

should i be looking to set my timing at 3000rpm or idle? (which is lumpy) or does it sound like my timing is not my problem and something else?

Chris. in your original questions above you mention pinking twice and if that symptom is bothering you, then my advice as written will help you find a spot where pinking is not present at full load.

From there you'll have the opportunity to address the other issues contributing to your setup problems.

Personally I dislike pinking in my bog standard 3.5, because generally the perceived wisdom is that is can cause engine damage.

However with so many variations to the engine in your Westy I think you are on a very interesting journey of discovery! pinking or not!

[chris-stockton]

hello - some reading up for me there :+) i will have to digest all that.

With regards the cam - heres a pic of the spec that may be of interest.
http://www.pistonheads.com/gassing/topi ... mid=306144

it was originally built for hill climbing and was run for several years in this state of tune. its then had another 7 years like this with the guy i bought it from. So its a pretty much tried and tested formula.

Its an eddlebrock air filter holder - quite flat, ill have to measure the height off the carb.

What i suppose im looking for is that simple fix as its ran for 10+ years safely - im presuming its either somethign ive done or something that has worn very recently causing this

[kiwicar]

Hi
10.5:1 CR for that cam should , if it is all installed correctly be fine. Closing at 38 after bottom dead centre is quite late, it should be an equivilent CR to a 3.9 efi cam and 9.6:1 cr it shouldn't be pinking . . . Are you absolutly sure it is detonating? not just another rattle from somewhere? Have you checked the plugs to confirm this, if you are absolutly sure it is pinking then you need to find out why, what is the CR, not told to you, check it by measuring it, is it full of carbon, is the cam timmed in correctly has someone done somthing odd to the shape of the combustion chambers? That CR is close to right for that cam, I would think actually you could go to 11:1 with that much duration. It has very little timming for a cam of that duration, though as said above that should be set up on a dyno, not just guessed at.
Best regards
Mike

[chris-stockton]

hello, yeah defo sounds like pinking, comes on worse if you floor it lowish revs high gear. also that combined with the small hesitation when you plan the throttle (sometimes).

how would i go about measuring actual cr?

i will pull the plugs nxt weekend for a look, they are all fairly new - but again a bit of a guess at what spec and gap!

heres a copy of the spec

High compression forged pistons (CR 10.5:1) DJE rods and crank lightened and balanced, Crower full race cam. Steel timing gears and competition valve springs, Stage 3 heads with 5 angle valve seats, shortened tapered guides, DJE valves, ported and flowed, reduced combustion chamber to suit composite head gaskets

[texpis]

Excuse my ignorance but how do yo see if its pinking by looking at the plugs?

Mick

[sidecar]

its either something ive done or something that has worn very recently causing this

I think that the "something you've done" is that the new filter as improved the VE and leaned off the mixture. The reason that it is pinking at 3000 RPM is because that will be around peak torque and peak torque is where you get a peak VE, that leads to a higher peak cylinder pressure.

I know that Kiwicar thinks that the CR is OK, I have a lot of respect for him but I still think that it is a bit high even with a long-ish duration cam. The chamber design of the RV8 is not great and the squish band does not work unless the block has been decked in a big way (1mm off them). Actually the chamber is not too bad but the dish in the pistons means that the squish band even when working is very narrow.

Knocking the timing back will lower the peak pressure and probably stop the detonation but it is a sticking plaster for the underlying cause.

This is assuming that the timing really is at 28 degrees and the CR is 10.5:1.

I personally don't trust anyone these days and therefore end up measuring everything when I build an engine, it takes ages doing it this way but that's just me. (My mates have to suffer the same process when I help them! )

[sidecar]

Excuse my ignorance but how do you see if its pinking by looking at the plugs?

Mick

If its bad enough and is allowed to happen for long enough the aluminum that gets blasted off the piston crowns ends up getting stuck to the central electrode of the plugs.

Trace detonation is harder to detect and you won't hear it which is another reason for not pushing the timing up to the point of detonation then retarding it half a degree. With the heads removed you can sometimes see that trace detonation has occurred by the loose 'dusty nature' of the carbon on the piston crowns.