RV8 Engine timing

[rev-erend]

I've been meaning to time my RV8 for quite some time ..

basically from the time I decided to replace the cap and rotor arm - ever since it's not run as well.

A local specialist suggested that the caps (old black, new blue) could be a few degrees different .. fair enought I thought.

So to do the job well - I market the flywheel up at 8 degrees and 28 degrees before TDC and the static engine marker with some white tipex.

What I found was that both old and new caps were both 8 degrees at tick over - so no difference but the new cap gave a mis fire .. the old one just sounded better.. (thanks Rimmer brothers - thats why I sold the TR6 cause the parts were rubbish).. anyway - I tried this several times and always prefered the old cap as it was not missing.. even though the posts and the coil contact button looked like they have had better days.

Anyway - to the point of my post.

With the vacuum pipe blocked (mole grips) the engine was set at 8 degrees at tickover and this equated to 28 degrees at 4000 rpm - so I was very happy with that .. but low and behold when I release the
vacuum pipe - the timing altered to around 14 degrees tickover and around 40 degrees (I guessing this figure) at higher revs.. to me this made no sence - and would be igniting the mixture to soon and possible causing detonation (and knock) ..

So - I followed my heart and V8Racings advice and blocked the Vacuum advance..

So - I'm happy and the car sounds great.

What do the experts thing of this tale ?

[GreenV8S]

The cap can't possible affect the timing.

I've used the blue caps without any problem but they do have a rather poor reputation. It's also possible that the new cap you have is actually the wrong spec for the rotor arm. I don't remember all the details but I remember reading that one of the early variants of the RV8 had a different cap and rotor arm with the contacts at different heights to all the later ones. I'm guessing, but perhaps if you mixed and matched these parts it might not work very well. I don't suppose you've done this, but worth bearing in mind that just because the bits look about right and seem to fit, doesn't mean they're exactly right.

8 deg static timing is about right, some people go up to 12 or so, it depends on the spec of the engine and the fuel used. Squashing the vac advance line isn't a reliable way of blocking it off because the vac capsule is completely sealed; if there is even the slightest leak the advance will eventually kick in. You really need to disconnect the tube from the vac advance capsule.

You'll see the advance increase with revs and also when the vac advance is activated. You need to remember that the vac advance only kicks in under part load and it has no effect under full load.

[Boosted Ls1]

Vacuum advance is for cruising conditions when you have a light throttle position, far far less fuel in the cylinders and high vacuum. In these circumstances you really need the spark earlier to ignite and burn all the molecules of fuel as they aren't exactly holding hands It's an economy tool to save fuel. Now if you drive flat out all the time then you don't really need it as you would have a wide open throttle and less vacuum.

Boosted.

>> Edited by Boosted Ls1 on Saturday 11th March 21:39

[leorest]

GreenV8S said:

Squashing the vac advance line isn't a reliable way of blocking it off

Hadn't thought about this before but makes sense especially if you think about the action of clamping the vacuum pipe is going to increase the pressure in the pipe between the clamp and the diaphragm.
It's a good thing to learn at least one thing every day.

[Boosted Ls1]

leorest said:

GreenV8S said: Squashing the vac advance line isn't a reliable way of blocking it off Hadn't thought about this before but makes sense especially if you think about the action of clamping the vacuum pipe is going to increase the pressure in the pipe between the clamp and the diaphragm. It's a good thing to learn at least one thing every day.

That's not what GV8 means. The pressure in the pipe isn't really an issue (there won't be any) but clamping it won't seal it properly. Plug it with a pencil or a small rod or bolt. This way it will be properly sealed and stop an air leak to the engine as that's what makes the revs increase. The cannister can be open to fresh air as all you're trying to do is stop manifold vacuum upsetting the static timing while you adjust it. Does this make sense?

Boosted.

[eliot]

Keep the vacuum advance, part throttle is much better. Has no effect at WOT as others have stated.

You can make your own timing tape if you know the diameter of your damper. Circumfrence=pi*diamter, divide that by 360, you get MM per degree. Draw that on a bit of paper (I used a computer drawing package) and stick it to the damper.

Big photo of it on the car:
www.mez.co.uk/dcam/DSCF0329.JPG

>> Edited by eliot on Sunday 12th March 10:11

[rev-erend]

Thanks for your suggestions..

[steve_D]

I have a splendid article on vacuum and timing but it is rather long.
Will post if anyone wants to see it.

It tells about how important vac advance is for a road car.

Steve

[leorest]

I'm interested.
thanx

[steve_D]

Based primarily on the Chevy V8.



Timing Mech vs Vac advance Good Read
This is a reprint from another board author unknown.

As many of you are aware, timing and vacuum advance is one of my favourite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimise all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative.

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimising engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburettor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late" to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.
_________________

[Boosted Ls1]

Well said that man.

Boosted.

[leorest]

WOW!
I'll sit down and read that tonight.
Ta

[rev-erend]

steve_D said:

Based primarily on the Chevy V8. The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation. At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph). _________________

Many thanks - that was just what I was after.

I was seeing just that and it was not what I was expecting - so I presumed perhaps my dizzy was on the way out or something (vac advance part anyway..).

That was really interesting.

[v8 racing]

Okay i suppose i had better explain why i tell people to diconnect the vac adavance on the rover engine! what was mentioned earlier is very correct, however all engines dont need the same advance curve etc.. vacum advance is used to give good fuel econemy and a smooth engine under cruise aplications, the problem becomes if you start modifying your engine and put a big cam in it and port the heads at the said cruise rpm the air speed is much slower than a standard engine and the air fuel mix can be very slow bad, fuel drop out etc, by running a lot of advance this makes things worse, more advance at cruise means more ftlb of torque so less throttle opening is required to maintain say 60 mph very slow air speed, shunting will apear! if you disconect the vac advance you will need to apply more throttle to make the same torque as before but the air speed will be much higher so better combustion less fuel drop out and a nicer running engine, although fuel consumption will be down a bit, this is definetly one of those questions that every engine builder will have a different answer too, alli can say is in my experience with ONLY THE ROVER ENGINE they do run a heck of a lot nicer in modified form with no vac advance

[rev-erend]

That's great Rob - must say the car runs brilliantly with it removed and now I understand why..

[morrisman]

steve_D said:

Based primarily on the Chevy V8. Timing Mech vs Vac advance Good Read This is a reprint from another board author unknown. As many of you are aware, timing and vacuum advance is one of my favourite subjects, as I was involved in the development of some of those systems in my GM days and I understand it. Many people don't, as there has been very little written about it anywhere that makes sense, and as a result, a lot of folks are under the misunderstanding that vacuum advance somehow compromises performance. Nothing could be further from the truth. I finally sat down the other day and wrote up a primer on the subject, with the objective of helping more folks to understand vacuum advance and how it works together with initial timing and centrifugal advance to optimise all-around operation and performance. I have this as a Word document if anyone wants it sent to them - I've cut-and-pasted it here; it's long, but hopefully it's also informative. TIMING AND VACUUM ADVANCE 101 ............ _________________

Damn, I tried to argue that point on a forum a couple of years ago, and made a fool of myself for not being able to get it across properly, even though I knew the basic facts. I'm going to copy this down and save it for future reference.

[morrisman]

One other point I'm surprised nobody has mentioned, is to make sure that the marked TDC is actually the REAL TDC.

You can do that by making a simple stop to screw in the plug hole on number 1 cylinder, and use it to stop the engine turning so the #1 piston stops just about a half inch before the top. I used a drilled out spark plug with a bolt and a couple of nuts through the middle to make my stop.

Turn the motor in one direction by hand (gently) until it stops against the stop, and mark the position on the dampener. Do the same in the opposite direction until it stops, then mark that point too. Split the marks exactly with a ruler or dividers and you have the exact and real TDC to set your timing to.