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oldtech

Quick timing question MK2F GT - car picks up quicker after cambelt change

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oldtech

MK2F GT:

 

I'm trying to understand TDC vs ignition and how a belt tooth either way can affect performance:

 

Which direction on crank pulley timing notch will advance the ignition, if that's the right term for this - clockwise when facing crank/cam sprockets toward the forward edge of timing gap, or counterclockwise toward rear of gap?

 

I timed the crank pulley slightly clockwise lined up with forward edge of the gap which ended up bang in the middle after a couple of rotations. I think it was further back last change at the other end of the gap which it's been running like for past 2 1/2 years.

 

I don't know if it's the current weather - thought hot & dry weather was slower than cold & condensation though? But the engine seems to have more torque now.

 

Engine seems more 'alive' compared to before but I thought having the pistons fire after TDC further down on their travel (clockwise at gap) would decrease power as more volume? Surely more power = more anticlockwise back at TDC or have I got any of this wrong?

 

Want to learn - please help me understand 🙂

 

 

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kiran_182

Tdc-1 is usually the one for adjisting older engines to new fuel, it can indeed perk the car up

 

You need to rotate the dizzy too so your sparking at the right time 

 

Hot air is lese dense than cold so the car will feel sluggish compared to a cold day. Take it out on a hot day and then again in the evening when its colder and see the difference

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steveo3002

belt a tooth out = cam timing altered

 

dont confuse this with ign timing which should be done with a strobe lamp 

 

both will have an indepedant effect on the way the engine runs

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caretakerplus

As the distributor is driven from the camshaft, any errors in VALVE timing will also affect ignition timing.

 

The idea of having the spark before top dead centre (BTDC) is that this allows for the combustion process to reach its peak at the optimum time.

 

Many people seem to think that when the spark takes place, there is an 'explosion' in the cylinder - this is NOT the case, what you should get is a very rapid burning process.

If you DO have an explosion in there, it is possibly 'pre-ignition' you will hear it as detonation, commonly referred to as 'pinking' - this does NOT necessarily mean that the timing is wrong, it could also be due to carbon build up in the combustion space - a weak mixture will also give similar results., as can overheating.

 

In situations where you get 'running on' (engine continues to run - often roughly or even backwards!) after the ignition is switched 'off', this is a 'dieseling' effect, cause by severe overheating or carbon build up, but definitely nothing to do with ignition timing (except that retarded ignition timing can cause the overheating).

 

One tooth difference in valve timing can make a world of difference to the running of the engine, so it is possible that the timing was 'out' when the old belt was fitted, but you have now overcome that problem and improved the running.

With the above in mind, if the old belt timing was 'out' and someone has attempted to improve things by adjusting the ignition timing, it would now be as well to check the ignition timing with a strobe light and adjust if required.

NOTE this should ONLY be done once that you are quite sure the VALVE timing is correct.

 

Regards

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dvderlm

Agree. And the burn process is at the same speed, no matter how fast the engine spins, so spark has occur earlier before TDC the faster it spins up to a maximum that is mostly influenced by combustion chamber shape or fuel type.

http://www.firstfives.org/faq/timing/timing_for_modified_engines.PDF

suggests maximum 37 degrees total advance for bowl in piston cross flow.

 

Old belt might have made spark timing a bit delayed too as old cambelt stretches minutely  from each combustion downstroke before transmitting the rotation to the camshaft, but I don't quite believe that the camshaft itself distorts while opening valves against valve springs turned by cam pulley so distributor at far end is inaccurate , and yet some manufacturers prefer crank timing sensor (Ford EDIS) to  cam sensor (VW,Saab).  

 

Spark needs to fire before TDC give time for flame front to build. Setup for 5degrees static BTDC @idle to 2000rpm with vac advance disconnected and plugged on most VW Polo engines.

Spec Is 4 to 6 degrees, and I imagine this is to allow for valve to piston timing differences, such as skimmed head, worn belt, valve clearance or different fuel octanes.

 

I found this article useful when I was mapping my ignition timing

https://innovatemotorsports.com/resources/myths.php 

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oldtech

Wow lots of tolerance stacking too with aging ECU caps and sensors, worn cam lobes, original block & inlet manifold port casting tolerances etc all on top of valve and ignition timing.

 

I thought spark fired after TDC - shows how much I know!

 

Well I guess that makes sense then - if before the crank was too anticlockwise the spark would be firing too early - too far away from TDC. Now being more clockwise the piston has travelled further up the bore at the point of ignition which I guess explains the better running because of better flame-front.

 

Also, I always use 99RON - would moving the crank clockwise like this be closer to TDC-1 for modern fuel as explained above?

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dvderlm

Yes, it would match closer. Think GT was designed for "bleifrei 95", which matches its 10:1 compression, so you'd not be getting the most from 99 without adjustment. You should be able to advance even more if you are sure you will are sure you will only tank 99RON.

http://www.euroevil.de/vwtown/html/polo_mkb.html

Too much and you risk knock though.There's a knock sensor to retard on the G40 PY engine.

 

Edited by dvderlm

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