7 replies to this topic

[Purple Tom]

Hey,

I bought a VW Polo GT back in September as a daily driver/plaything. The engine is a 1272cc multi-point injected transverse 4-pot. It wasn't running great at all when I got it; overfuelling, missing, cutting out, generally farting around, and returning around 25mpg.

I did a general service straight away (plugs, leads, oil, filters), and then slowly started to replace and adjust parts on the car, there's a few sensors that need looking after (namely the lambda, air flow meter and throttle potentiometer), so I tested and replaced as necessary. I also replaced the coolant temperature sensor, and after testing the ECU replaced that as well (its the Bosch Digifant system). I also did mechanical work and replaced the rad, water pump, timing belt, thermostat and expansion tank. Things improved dramatically after that, running much nicer and it gave about 38mpg on the journey down to Newquay in November. I still wasn't happy with it as it was still not quite how I remembered my old GT, then I had a thought - the timing.

Went straight outside and checked it (don't know why I didn't before!), and it was roughly 25 degrees retarded! Turned out the previous owner had fitted a replacement dizzy (luckily the right one as the GT/G40 dizzy's are different to the 1043 and 1272 SPi dizzy's), but hadn't a clue how to time it in, so had just guessed - nice! Anyway, sorted the timing and the difference was amazing, loads more torque, more power, 50mpg!, and generally much nicer to drive.

What I want to know is could the timing being so badly wrong have affected the engine mechanically? I imagine if the sparkles were sparkling so badly before TDC it can't have been doing much good to piston tops/valve heads/combustion chamber and cylinder walls etc? Does anyone have any information on this? I reckon the engine has done around 10000 miles in this condition, and ever since I've owned it has had a slight rattle between 2k and 3k rpm (which I think is tappets as they're hydraulic and the same age as the engine).

I've asked similar questions on VW forums but you just end up going round in circles with no definitive answer - so can anyone shed any light? Or am I just worrying too much?

Ta!

Tom

[vasi]

Hi Tom,
Wear will be accelerated, but the engine isn't stressed enough to be an issue.

You're werriting

[Jammy]

As above, it would put more stress on mainly (I would have thought) the pistons, con-rods and crank, since if its firing as the piston is rising, the explosion will try to push the piston down, working against the general motion of the crank. (Worded very badly but I'm sure you know what I mean! ). I can imagine the situation being worse the higher the compression ratio, however I can't think of anyway of inspecting any damage other than stripping down the engine and having certain parts pressure/crack tested.

[Bill USN-1]

BTDC-Before piston is at TDC
ATDC- After piston passes TDC

Normal timing is BTDC to allow time for the fuel to burn as the piston comes across the top.
Too much and you get ping and knock as the gasses expand while the piston is going up.
ATDC is retarded meaning you lose power because the piston is already travelling down when the gas is expanded.

Less power more deposits from unburned fuel.
Cats get clogged pooor emmisions.

Change oil regularly and run some injector deposit cleaner.

[Leonard]

Hell Tom, if the ignition is retarded like you say, the mixture is being ignited when the piston is already on it way down the bore. ideally, the explosion would be pushing the piston down all the way from the top. as its going so fast, you have to ignite the mixture just before the piston goes of the top centre.
In your case, the mixture was exploding into an already expanding chamber, so alot of it is wasted, ie inefficient.

[fikus01]

happens to minis too, peeps fir a 998 dizy to a 1275 or vice versa and wonder why theres less power!! having timing too advanced will cause pinking and this does damage the engine!! having it too retarded is less damaging to the engine but does not help emmissions or economy!!

[Sprocket]

The folowing relating to optimum ignition timing where peak combustion pressure happens at optimum crank angle.

Retarded means closer to TDC (may even happen ATDC)and peak combustion pressure is past the optimum crank angle and you will have C**p performance and C**p MPG. No damage will result unless its stupidly retarded.

Advanced is further away from TDC (BTDC) where the peak combustion pressure happens before optimum crank angle. This depending on how severe, is BAD. The flame front moves towards the piston which is moving towards the flame. If peak combustion pressure is reached BTDC, detonation occurs. how severe depends on whether damage is done or not. Light det' is known as knock.

So which way was your ignition, advanced or retarded

[Purple Tom]

Mine was retarded, by about 25 degrees - C**p performance and C**p economy. My mistake in thinking that was resulting in detonation, obviously in my case it was the lesser of two evils.

Thanks for advice everyone!!