Ignition Timing & Compression Ratio's
Discussion
I've currently got a timing problem with a 2000 Griff 500, simply put it wouldn't stop pinking. This had been rectified by the dealer, but has mostly been achieved by winding the igntion advance right back. This has killed the performance of the car (quote from TVR mechanic, "this is the slowest Griff 500 I've ever driven").
First a simple question, just for my information, what way does the rotor arm in the dizzy rotate? In which case, which direction of rotation of the dizzy housing will advance the ignition?
Now something a little more technical, I've been told that this problem is more than likely caused by too high a compression ratio (although it is yet to be measured). What does this mean and what are the possible solutions to this?
As a footnote, with the timing set at 28 degrees at 4000rpm (no vacum advance) and running Super Plus (which I always use) + Millers octane booster, the car will still pink a bit, but performance is a lot better.
Neil
First a simple question, just for my information, what way does the rotor arm in the dizzy rotate? In which case, which direction of rotation of the dizzy housing will advance the ignition?
Now something a little more technical, I've been told that this problem is more than likely caused by too high a compression ratio (although it is yet to be measured). What does this mean and what are the possible solutions to this?
As a footnote, with the timing set at 28 degrees at 4000rpm (no vacum advance) and running Super Plus (which I always use) + Millers octane booster, the car will still pink a bit, but performance is a lot better.
Neil
Hi Neil:
On the Rover V8 I think the rotor arm spins clockwise as viewed from above. To verify, either pull off the cap and watch it as you crank the engine (not recommended with cats!) or turn the crank with a spanner, or whatever, but I'm sure that's correct.
To advance the ignition firing point, you rotate the distributor the opposite way, so the trigger on the distributor shaft reaches the pickup earlier, thus the piston is further down the bore on the compression stroke at the point the spark fires.
Compression ratio is the relationship between the maximum volume of the cylinder, with the piston at the bottom of its stroke, and the volume of the combustion chamber that the fuel/air is crammed into at top dead centre. The volume of this space includes any bowls in the piston crown, valve pockets etc., the actual chamber in the cylinder head and even the thickness of the head gasket.
So if, for instance, the cyl volume is 433cc and the chamber volume is 43cc, that's as near as dammit a 10:1 compression ratio.
It can be lowered by, for example, increasing the thickness of the head gasket, which increases the volume of the combustion chamber, or by a change of pistons (as I have on the 390) to a different crown shape.
Hope that helps...;-)
W.
On the Rover V8 I think the rotor arm spins clockwise as viewed from above. To verify, either pull off the cap and watch it as you crank the engine (not recommended with cats!) or turn the crank with a spanner, or whatever, but I'm sure that's correct.
To advance the ignition firing point, you rotate the distributor the opposite way, so the trigger on the distributor shaft reaches the pickup earlier, thus the piston is further down the bore on the compression stroke at the point the spark fires.
Compression ratio is the relationship between the maximum volume of the cylinder, with the piston at the bottom of its stroke, and the volume of the combustion chamber that the fuel/air is crammed into at top dead centre. The volume of this space includes any bowls in the piston crown, valve pockets etc., the actual chamber in the cylinder head and even the thickness of the head gasket.
So if, for instance, the cyl volume is 433cc and the chamber volume is 43cc, that's as near as dammit a 10:1 compression ratio.
It can be lowered by, for example, increasing the thickness of the head gasket, which increases the volume of the combustion chamber, or by a change of pistons (as I have on the 390) to a different crown shape.
Hope that helps...;-)
W.
Engine timing consists of the following: Initial timing, the number of degrees of advance, determined by the position of the distributor when it's installed....centrifugal timing, determined by weights,springs inside the distributor. This can be checked on a distributor machine, and changed also, should you want to. Add the two together, and you have total advance...10 degrees initial + 25 centrifugal = 35 total degrees. You stated 28 degrees@4000 rpm..is this total or distributor ? For ECU controlled ignitions, the ECU has a built in timing graph, and initial can be checked at idle, total can be checked by raising engine speed till total advance is seen on timing indicator, just like a regular distributor. If you have a knock sensor, when "ping" is encountered, the ECU automatically retards the timing, till ping has dissappeared. Yours could be bad, not permitting spark retard, thus causing ping....the whole object of ignition timing is to "try" to match the burning rate of the fuel, and the rpm of the engine at that instant....not trying to complicate the subject, but here are a few conditions that affect timing. RPM,octane rating,vacuum,air/fuel mixture,temperature ( ! ),engine load,combustion chamber shape & spark plug location,and cylinder turbulance/flame propagation across the piston, and the "condition" of the ignition itself. Could be a faulty coil, possibly, or possibly a mechanical ( !!!!!) fault, allowing oil/water to enter/dilute fuel. Have fun, don't dispair, sounds like Greek, but is really simple.
quote:
I've currently got a timing problem with a 2000 Griff 500, simply put it wouldn't stop pinking. This had been rectified by the dealer, but has mostly been achieved by winding the igntion advance right back. This has killed the performance of the car (quote from TVR mechanic, "this is the slowest Griff 500 I've ever driven").
First a simple question, just for my information, what way does the rotor arm in the dizzy rotate? In which case, which direction of rotation of the dizzy housing will advance the ignition?
Now something a little more technical, I've been told that this problem is more than likely caused by too high a compression ratio (although it is yet to be measured). What does this mean and what are the possible solutions to this?
As a footnote, with the timing set at 28 degrees at 4000rpm (no vacum advance) and running Super Plus (which I always use) + Millers octane booster, the car will still pink a bit, but performance is a lot better.
Neil
You could have worked this out yourself by taking the dizzy cap off, but to save you the trouble it goes clockwise. So rotating the housing anticlockwise advances the ignition. If the car has been like this from new it could be the wrong cr, but could also be badly coked up or running lean, or just running hot. Don't put *too* much faith in the timing marks btw.
Cheers,
Peter Humphries (and a green V8S)
Thanks for the replies, I now have all the information I need.
For those how don't read the TVRCC Email List on yahoo, here is another good reply from a respected source:
There's lots of things that can cause pinking - retarding the timing isn't the best way to go to cure it though on the griff since you can only alter the total timing across the whole rev range, so you'll keep from pinking at whatever revs this is a problem, but lose power everywhere that pinking isn't a problem if you see what I mean. you could lower the compression ratio, but as the griff breathing is compromised to begin with if you can't get much mixture in the chambers you have to squeeze it hard to get the power back at high revs. assuming the engine is in good condition with no faults / heads not skimmed too far / etc then try reducing the intake temperature by lagging the exhaust manifolds and intake pipes, fit a lower operating thermostat and a "water-wetter" to reduce engine temp (don't use a high antifreeze concentration as this reduces the water's ability to transfer heat) - after that you're probably better off going for a mapped engine ecu rather tha major engine surgery so you can slightly overfuel to cool the chambers under load whilst still running a weak cruise mixture, and only retard the timing in the pinking-critical range, so getting your power back.
joolz.
Cheers,
Neil V
For those how don't read the TVRCC Email List on yahoo, here is another good reply from a respected source:
There's lots of things that can cause pinking - retarding the timing isn't the best way to go to cure it though on the griff since you can only alter the total timing across the whole rev range, so you'll keep from pinking at whatever revs this is a problem, but lose power everywhere that pinking isn't a problem if you see what I mean. you could lower the compression ratio, but as the griff breathing is compromised to begin with if you can't get much mixture in the chambers you have to squeeze it hard to get the power back at high revs. assuming the engine is in good condition with no faults / heads not skimmed too far / etc then try reducing the intake temperature by lagging the exhaust manifolds and intake pipes, fit a lower operating thermostat and a "water-wetter" to reduce engine temp (don't use a high antifreeze concentration as this reduces the water's ability to transfer heat) - after that you're probably better off going for a mapped engine ecu rather tha major engine surgery so you can slightly overfuel to cool the chambers under load whilst still running a weak cruise mixture, and only retard the timing in the pinking-critical range, so getting your power back.
joolz.
Cheers,
Neil V
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