ignition timing help for newbe!

[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.

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.

I was originally confused by your responses, Pete, but you seem to have clarified the situation to support my methodology? Let me explain!

From your opinion, it's not acceptable to back of a few degrees and in the second extract it seems less acceptable to back off by a tad - 1/2 degree (say)?.

later in the thread is an enlightening explanation:

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.

Hence my improved loss of confusion - If peak torque and optimum VE occur around the point where pinking occurs, how come my well explained empirical method to get the engine slightly away from the point of maximum torque and where only trace detonation might occur (with its resulting minimum of piston damage) was so seemingly bad?

Throughout my explanation and advice I have emphasised the empirical approach helps to overcome the many variables in Chris' setup, bad CR, restricting air filter, confusing pulley timing marks, modified dissy advance curve, etc.

All this in response to Chris' request " -

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

Pete, I appreciate that for some reason we appear to have crossed swords on this issue previously and I don,t really expect a continuing debate because

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

But now, encouraged by your above explanations, I feel confident that my process, as explained, will bring the timing on Chris' car very close to optimum, to avoid pre-detonation,and achieve peak torque and optimum VE.

[kiwicar]

Hi
To find out the actual mechanical CR I am afraid you have to remove a head and measure the volume of the cylinder head, then the thickness of the gasket, how far the piston is down the bore the volume of the piston dish and work out the volume above the piston. Once you have done that devide the swept volume of the cylinder by this volume and add 1 that is your CR "static", your effective mechanical CR is that number times 2 devided by (1+ cos (inlet valve closing point [38 degrees])).
You say this engine has worked OK for 10 years. . was it in a much lighter car before? or has it always been in this car? Have you just changed to the current carb set up? to me it sounds like it is going very lean as you open the throttle and it is causing it to pink. . . it may have always done it but being in a lighter car it was able to rev through it without you noticing it as a problem
Sorry about the obvious questions, but this shouldn't just start on a previously working engine unless you have made a major change, and as I say the CR seems about right to me from your initial figures.
Best regards
Mike

[sidecar]

Have you just changed to the current carb set up?

Mike the chap has changed from a 2" filter to a 3" one. This in its self may change the AFR if the old filter was causing any sort of restriction but the change also has another knock on effect...

Basically lots of people run drop base filter bases and this can place the top of the filter very close to the mouth of the carb, sticking a 3" filter on then moves the lid away from teh carb and this can make a big change to the airflow. I don't know whether the OP is using a drop base, he could be!

[kiwicar]

Hi
Yes I saw the change in the filter and yes I recodnise that it can have a large effect on AFR, after all like many others I put K&N filters onto minis and motorcycles in my youth without apreciating this. . now I am a little older and wiser, what I was trying to establish is . . is this infact a new carb or one that has been on the car for many years? ie is it still on "out the box" jetting or has it been partially sorted? Because they are very different starting points for getting my head around what is going on and because with that much cam, and so little timming it should not be detonating even if the mixture is a mile out. Also the combination of cam and 10.5:1 pistons is a little unusual especially in what has aparently been a competition car, one that would have shown up any tendency to pink on an engine where it previously did not do so.
Best regards
Mike

[kiwicar]

I suspect, like you are implying that this previous filter lead to standing waves through the carb that lead to a very rich mixture, which in turn was leaned out to compensate, changing the filter would then eleminate the standing waves, and lead directly to a very lean mixture, and I agree with you this is possable, however without knowing the age of the carb, it is only a guess, and personally I am not happy that this is all that is going on.
A note on the previous comment about the plugs, you can see evidence of pinking on the plugs without chunks of ally melted onto the electrode in that the deposits take on a faint metalic luster quite early on and the deposits are far less powdery than is normal.
Best regards
Mike

[sidecar]

Hi Mike, I was not even going so far as thinking about standing wave or anything that clever!

I've come cross Edlebrock/weber 4 barrel carbs (like the one that the OP is using) where the filter lid is setup to be very close to the top of the carb, this restricts the air going into the carb. A giveaway to this issue is when the air lifter stud sticking out the top of the carb is short, this means that the lid is close to the top of the carb. Dropbase filters make the situation worse.

This all came to light when I was working on a friends 427 SBC, he was running an Eddy 850 carb and had to lean the mixture off a huge amount from the base settings before the AFR was anything like being right. I then noticed that the fliter stud was only about 20mm long and the cast ali filter lid that he was using was 10mm thick which left about 10mm of gap for the engine to breath through. Once the mess was sorted out we had to richen up the carb a massive amount, the increase in power actually put the willies up my mate! Whilst testing the AFR I asked him to boot the throttle hard so I could see what the mixture did on my LC1 gauge, we were doing 90 MPH at the time, the rear wheels lit up even though the road the road was dry!

(I try to avoid going out in his car if I can! )

[kiwicar]

Hi Mr Sidecar,
as I say lets see what comes back about the changes made since this was a working set up. I do hope it is as simple as leaning out as a result of the new filter it then is an easy fix with a logical explanation behind it.
Best regards
Mike

[sidecar]

Pete, I appreciate that for some reason we appear to have crossed swords on this issue previously.

But now, encouraged by your above explanations, I feel confident that my process, as explained, will bring the timing on Chris' car very close to optimum, to avoid pre-detonation,and achieve peak torque and optimum VE.

Hi Ramon,

I was not planning to have further debate over your method of setting up the ignition timing but for some strange reason you now think that I agree with your method! I do not wish to come across as rude but I don't agree with your method, it will only work when an engine has been built with too much static compression. You post up your method whenever someones asks about setting up ignition timing and you posted it up this time before knowing what the CR is. (Be it too high or OK).

Your method will result in a setup that does create the highest peak cylinder pressure that the engine will tolerate before it detonates but it does not take into account the crank angle at which this peak pressure occurs, creating a huge amount of pressure when the crank is at the wrong angle will produce less torque than a lower peak pressure at the right crank angle. As I have stated before the peak pressure needs to occur when the crank is 15-20 degrees ADTC. People that are more clever than you or I have worked out this angle though years of testing engines.

I'll explain why your method is not correct for a well thought out and well built engine using an imaginary engine that has been built with a reasonable CR for the intended fuel. Lets assume that someone has built a 4.6 lump with a CR of 10:1 and it is strapped to a dyno.

The engine is run with following timing...

24 degrees, the BHP recorded as 280
26 degrees, the BHP goes up to 295
28 degrees, the BHP goes up 300
30 degrees, the BHP stays at 300.
32 degrees, the BHP drops to 295
34 degrees, the BHP drops to 280
36 degrees and the engine starts to detonate.

With your method of advancing the timing until the point of detonation then retarding the ignition the above engine would end up being setup with 35 degrees advance when clearly 28 is a better figure to go for.

If we consider a similar engine built with a CR of 12:1 which is too high for the fuel being used it might pull 280 BHP at 24 degrees, 290 at 26 degrees then detonate with 28 degrees, clearly the best the we could hope for with such an engine is to run it at say 26-27 degrees but rest assured the engine would run better and have more torque lower down the rev range if the CR was reduced and the timing advance to the point where the power either plateaued or starts to fall then pulled back a degree. An engine with too high a CR never reaches a plateau, the power rises in line with an increase in ignition advance up to the point it runs into detonation.

I know that the above engines are 'made up' but the figures will not be far from what can happen in the real world.

Finally and I know this might not be that relevant with the sort of engines that we build but a true race engine will not even tolerate detonation for a few seconds before the pistons and big ends are destroyed.

Oh, one other thing that might come up in this thread...Modern production cars and how the 'black box' will advance the ignition to the point of knock then retard it a degree.

Well yes some cars to run like that but they are a different kettle of fish from our cars. Production cars spend 99% of the time with the throttle just cracked open, the result of this is that the VE is very poor and as a result the peak cylinder pressure will be low. A cheap and easy way to get back some of the lost performance and MPG is by pushing the static CR up higher than ideal in order to increase the peak cylinder pressure. If however the driver does floor the throttle the VE does go right up and now the peak pressure becomes too high, the engine knocks but within a fraction of a second the black box has pulled back the timing in order to save the engine. An engine setup like this is basically a trade off, the engine gives better performance when the throttle is just cracked open at the expense of the maximum BHP that the engine could produce. (I read all of this in a book, it's not my knowledge, I'm just passing it on).

Regards,

Pete

[chris-stockton]

You say this engine has worked OK for 10 years. . was it in a much lighter car before? or has it always been in this car? Have you just changed to the current carb set up? to me it sounds like it is going very lean as you open the throttle and it is causing it to pink. . . it may have always done it but being in a lighter car it was able to rev through it without you noticing it as a problem

no it was in the same weight car ~ 550kg all in.

[chris-stockton]

Have you just changed to the current carb set up?

Mike the chap has changed from a 2" filter to a 3" one. This in its self may change the AFR if the old filter was causing any sort of restriction but the change also has another knock on effect...

Basically lots of people run drop base filter bases and this can place the top of the filter very close to the mouth of the carb, sticking a 3" filter on then moves the lid away from teh carb and this can make a big change to the airflow. I don't know whether the OP is using a drop base, he could be!

heres a pic of the current one - i think this would be a drop base? also the second pic on the engine hoist is of the old base - which i think is also a drop base one.

[chris-stockton]

what I was trying to establish is . . is this infact a new carb or one that has been on the car for many years? ie is it still on "out the box" jetting or has it been partially sorted? Because they are very different starting points for getting my head around what is going on and because with that much cam, and so little timming it should not be detonating even if the mixture is a mile out. Also the combination of cam and 10.5:1 pistons is a little unusual especially in what has aparently been a competition car, one that would have shown up any tendency to pink on an engine where it previously did not do so.
Best regards
Mike

The carb that is on it was bought in 2001

[chris-stockton]

On the point of where the filter now sits - see before and after pics. this is not all down to the change from 2" to 3" i also added one of those phenolic spacers to the carb to help the hot starting ** but it ran fine after the spacer was fitted with the 14 x 2**

Ive ordered a new 14 x 2 as they are so cheap to do a comparison at the weekend

[sidecar]

Er yep that's a drop base! (With quite a large drop on it!)

I see that the filter lid is one of those pressed steel "Edelbrock" jobbies. They are much thinner than the cast ali ones and they have a slight cone shape to them which gives a bit more carb-to-lid clearance.

Looking at the filter stud sticking up out of the carb I'd say that your clearance is not too bad now, (assuming that the stud just pokes out of the lid by a short amount) I bet if you reduce this by an inch though it would be getting close. It would be worth trying to actually work this out just for reference.

Actually looking at the size of the drop on the base I'm now wondering how close the lid comes to the actual base when a 2" filter is fitted. If the drop is say 1+3/4" then the lid would only be 3/4" away from the base which won't give much of an area for the engine to breath through. (I'll leave you to work out the clearance if the base has a 2" drop!)

Anyway the other symptoms that you describe such as hesitation would also indicate that your carb is setup too lean.

Have you ever pulled one of these carbs apart? If not then they are not too bad and you will need to pull the top off it in order to work out what primary and secondary jets you have along with the metering rods. If you post up what you find I can bung the settings into a spreadsheet which will compare them against the stock settings.

The problems start to occur when you then want to decide what you need to change. I'm not really a fan of guessing, really you need to monitor the exhaust gas so you know exactly what's going on but we can cross that bridge when we come to it! You might think it would be easier to just get the car on a rolling road, the problem with that is that in my experience most of the bods that run them do not have a clue how these carbs work, they are also to vain to admit it! (I'm sure this is not the case for all of them!)

Have a read of the link that I posted on how these carbs work! Also the Eddy web site has a PDF which describes them quite well. (The charts for jet settings in the PDF have several errors in them!)

In my link I state that I think that most RV8 lumps can run the same jets and rods but I now do not think this is quite true! I should update the article!

Fitting a 2" filter may well cure the problem but if that is the case then it has only cured it by the fact that your engine can not breath properly, you will get better performance by allowing the engine to breath along with jetting the carb correctly.

The carb spacers work well but you can get carb icing in the winter! Also they come in two forms and it can make a difference, one type has four holes, the other type has one large hole. (By the way its good to see that you have an Eddy manifold, not one of those low rise dual port jobbies!)

I wonder if the fact that you filter is now sticking out in the breeze is having an effect? (This could be good or bad, I'm not sure which)

[chris-stockton]

yeah i fitted the one with the 4 holes in it.

can you explain more about the carb icing?

[sidecar]

yeah i fitted the one with the 4 holes in it.

can you explain more about the carb icing?

Just for your info I've added a bit more to my last post!

Carb icing...it happens on my car but I live with it because it only a small issue, it can be a big issue if it happens in a big way.

On my car if I start the engine and the air temperature is very cold but there is some moisture in the air then after about 1 minute running on fast idle the engine starts to cough and dies. What is happening is that ice is forming between the tiny gap between the carb body and the primary butterflies, this block the airflow, the engine runs rich and conks out. I then blip the throttle and restart the engine, the small amount of heat in the manifold now keeps the carb warm enough for it not to ice up so the engine runs fine.

Like I said my problem is only minor, carb icing could be a bigger issue in that it jams the carb open which would be quite alarming to say the least!