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Mon Nov 30, 2015 11:57 am

vw training video show that a vw bug engine makes most power when it fires a spark a the top of the stroke i.e. tdc so why are there some engines where the timing is set to fire atdc .How does this not result in less power?Thanks.

Mon Nov 30, 2015 12:02 pm

Please post he video. I'd like to see it.

Mon Nov 30, 2015 12:06 pm

jarmchairpilot wrote: vw training video show that a vw bug engine makes most power when it fires a spark a the top of the stroke i.e. tdc so why are there some engines where the timing is set to fire atdc .How does this not result in less power?Thanks.

Hi there!

It's true, that the most power could be produced if we start combustion at the top of the stroke. But here on planet earth we are subject to the laws of physics. And those laws say that gasoline takes time to burn. It doesn't take long, but it is not an instant explosion like we think.

VW engines were also subjected to emissions regulations, which is why different distributors are timed differently. If you retard the idle timing, you need more air to make up for the lax spark timing. This made a cleaner burn during idle. As soon as you step on the pedal with an ATDC-timed distributor, the timing jumps up ten degrees or so anyway. So there's no actual power loss here.

So even though the timing at idle could be anywhere from 5*ATDC to 12*BTDC, the timing during normal operation at different speeds and different throttle positions is mostly similar between all different distributors; the only difference is where it idles and how it advances.

We advance the spark, because remember gasoline takes time to burn, and eventually the engine will be going so fast that the burn takes longer than a piston stroke. So instead of firing the spark at TDC, we fire the spark before TDC and the most powerful part of the burn still occurs on piston downwards "power" stroke. It is important not to set the spark too advanced though, because then the most powerful part of the combustion burn could happen before the piston is ready to move downwards, then you get detonation, or "pinging/preignition" which can severely damage the engine, since the burn will actually be working against the engine's rotation.

Clear as mud? :P
Robbie

Mon Nov 30, 2015 12:10 pm

It does result in less power.

The problem is that you are operating under the assumption that the specifications for the engine are set with maximum power in mind and no other factors considered. In reality, there are many factors to consider including engine longevity, fuel economy, driveability, thermal management, emissions, etc.

Different model years had different timing specifications, largely due to differences in distributors over the years. My understanding is that those engines where ATDC timing is specified at idle have both a vacuum advance mechanism and a vacuum retard mechanism, the vacuum retard serving to pull the timing back to 5 degrees or whatever ATDC at idle, presumably for emissions control reasons.

If such an engine no longer had the original distributor, the original timing specifications would not necessarily still apply.

Mon Nov 30, 2015 12:33 pm

You guys make perfect sense of a tricky subject to me,many thanks,now gonna make my mouse motor sing on the road where it belongs!

[https://youtu.be/BbX-oC1WSbsyoutube]

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Mon Nov 30, 2015 1:39 pm

Think of it this way. Maximum power and efficiency can only be created when the spark is soon enough to get all the fuel burnt up by about 10 to 15 degrees after top dead center.

The actual amount of time it takes to burn it all up changes, primarily with changes in RPM and engine LOAD, but also with AFR's and engine temperatures. So the distributor in a car has a constantly varying ignition timing. More RPMS = more advance. Less RPMS = less advance. More LOAD = less advance. Less LOAD = more advance.

As spark ignited at the perfect time creates the most power from the fuel. This results in less fuel used because while cruising you have to let more off the accelerator pedal. Also since you use less fuel you tend to have a cooler engine.

Generally speaking a spark too soon can result in engine damage.

A spark too late results in less power, but can cause other effects. A later spark is less efficient so the engine runs hot. Also a late spark creates less NOx.

At idle all engines run much more retarded than perfect. This is mainly an effort to keep the engine warmer. A warmer engine in turn evaporates more fuel and creates lower HC emissions. It also allows the engine to idle leaner and produce less CO emissions.

As a comparison, perfect timing at idle would be around 15* BTDC. Perfect timing at cruise at 3,000RPM be around 45* BTDC. The engine never reaches those numbers because you'd get more emissions. Still, the wide open throttle curve is pretty close to max power timing. Usually wide open throttle emissions aren't taken into consideration because WOT situations are momentary.

Mon Nov 30, 2015 1:53 pm

I wouldn't say that that video is 100% correct. The throttle plate diagrams in relation with the vacuum advance and retard ports is incorrect. Also their explanation of detonation is based on the old myth that it's caused from two flame fronts colliding instead of the excess pressure and heat causing a spontaneous combustion.

Mon Nov 30, 2015 2:04 pm

So firing at TDC can, in theory, provide the maximum power, but it comes at a cost. In a car with a spark advance, the firing time can, in a TDC tuned motor, move to before TDC, resulting in pinging, or detonation, potentially causing engine damage. Firing at TDC also maximizes the thermal load on the engine, making it run hotter. Your ATDC tune depends a lot on your dizzy, and where its advance may lead the firing position to go based on engine load and RPM. A TDC tune might be fine for a stationary motor that will run at an absolutely steady RPM, but leads to backfiring, sputtering and overheating in a road engine- ask me how I know.

Mon Nov 30, 2015 2:54 pm

vectorsprint wrote: So firing at TDC can, in theory, provide maximum power.

Only if you had a very fast flame or a very slow engine. The VW air cooled engine has neither. At 800RPM (slowest speed) and wide open throttle (fastest flame) the engine will still get maximum power at around 5° to 10° advanced BTDC. All VW aircooled engines can tolerate that and many run at that advanced when you floor it. I've never known of one of our engines with DVDA or SVDA timing to blow up or ping because someone was putting along in first gear at idle speed and then floored it.

Now take into consideration that at idle you are not at full throttle. As of such, at idle the engine can tolerate much more advance, and benefit from it as well in terms of efficiency. Some guys connect the vacuum advance right to the intake manifold and idle at 15° to 20° advanced! The result is that you have to turn back the carb quite a bit to keep the idle speed down, which in turn results in a much cooler idling engine. Of course it also results in more HC emissions as a result.