How to find the perfect ignition setting?

[petnatcar]

Hi Guys,

Since rebuilding my engine (in 2000) I must use racing fuel in order to keep it from "pinging" under acceleration.

I made the mistake of running some 91 non ethanol with Lucas octane booster and was surprised how poorly the engine ran. It pinged so bad I was afraid to drive the car even after retarding the ignition.

During the rebuild the block was decked and the head was milled thus raising the compression ratio to the point where the engine runs best on a mixture of high test fuel.

The actual ratio for the fuel is 5 gallons of 110 oct to 7 gallons of 93 oct.

Although this is expensive the engine runs perfectly and sounds like a race engine.

My question is; do the stock distributor advance settings still apply for a high compression engine?

I monitor the condition of the plugs (Champion N12Y) which are burning cleanly but I haven't been able to accurately set the distributor with my timing light. I've gone more by 'sight and sound' to arrive at this setting but I can't help thinking I can get a little more power from the engine with the ignition set exactly where it should be.

I am using a Crane XR700 Ignition System which also works perfectly, especially in the upper rev range.

As a side note:
I did a compression test in 2008 and came up with average readings of 192 psi dry and 201 psi wet!
Original spec is 175 psi.

Any suggestions on finding the perfect ignition setting?

Thanks,
Peter

 

[Michael Oritt]

You don't say how much advance you are running now? Try playing around between 30 to 35 degrees and see how that works--report back.

 

[EV2239]

Camshafts dictate ignition timing and BJ8s need about 12 degrees static, but 10 at a low tickover with a strobe works well. Mine has 10 to 1 compression and 10 degrees at tickover and almost never pinks.

if you use one of the more sporting Camshafts, you'll probably find you need more advance at tickover BUT no more at high revs. It's normal to restrict total amount of advance.

As a general rule, no one has a hope of timing by ear and you're being cruel to you engine if you try, because, invariably you'll run with too much advance and pink a lot.

Brit sports car engines have peak torque at 3000 rpm, so when you have the timing right, they should pull cleanly from about 1-1200rpm, power should increase at just under 2000 and by 3000, the engine should be smooth and pulling well to past 5000. Most driving should be easy at between 1,500 and 3000 without pinking or hesitation.

Using standard pump fuel and assuming a BJ8 cam, I'd set the timing at 10 degrees with as low a tick over as it will run at and see what the engine runs like.

It's perfectly normal to skim the top of the block and the head to ensure a good seal, but it won't be by enough to affect compression ratio meaningfully. Total skim might be 0.020-0.030 and you'd need 0.120" to raise compression ratio and possibly be in trouble.

I think you need to know exactly what timing you're running and do some tests.

 

[vette]

Peter, the ideal timing, in rate of advance and final total advance, is vary much a variable based on the parameters that the engine was built with. Compression being one of those parameters. From the onset I will say that if you have a really high compression engine relative to what is practical for a street motor, you can reduce that compression by using different head gaskets. That is the only way that I know off to reduce your compression without doing more machining or changing parts such as the head, pistons or cam.
Your pinging situation may be able to be reduced or eliminated by making changes to the timing, (rate of advance as well as total all in). You can play around with it and do it by ear and you may solve it. But I would try to be more accurate. What I would do it to mark you dampener with a tape to know exactly where the degrees are. Then using a timing light, (any timing light) see what the total timing is right now. As Michael said, around 30 to 35 would be acceptable. If that is where total is then the rate and at what point in the rpm range the timing comes in is where you should try some changes. First know that the vac advance is adding timing to the initial right away. If the high compression can't handle that much quick advance then disconnect the vac advance tube at the distributor and plug the tubing so there is no leak to the intake manifold. This in itself may solve your problem. You also can buy adjustable vacuum units or you can buy different vacuum units that have different rates of advance. A typical vacuum unit will add anywhere from 6 to 12 degrees of advance. That can be too much sometimes. I would keep the vacuum advance off for awhile and tune the timing with the weights and the springs. Without a distributor machine it is near impossible to actually see where and how much the centrifical advance is coming in at, but lighter springs will bring it in faster and stronger springs will keep it from coming in sometimes almost at all. Weights the same way. So you can try different combinations. At each change use your timing light to see when and how much the pointer is changing on the harmonic dampener. If you are very observant you can see when the timing picks up and then when it stops advancing with the light. A remote electric tack placed on the fender will help you to watch the rpms and the dampener at the same time. One one technique that racers use with weights and springs is that they will use two different springs strengths in the distributor and sometimes they will use a stiffer spring but oblong the attaching loops on the pegs so that the attachment of the spring is loose thereby allowing the weights to fly out quickly but then come to a much quicker halt to keep the last 8 or so degrees of advance from exceeding the total that they have deemed the engine wants. Typically total advance is wanted to be achieved between 2000 and 2500 rpms on even hot rodded street engines. Then it will hold that total up to your max rpm limit.

 

[petnatcar]

Thanks guys. You've given me food for thought, now I gotta go work on the car.

Regards,
Peter

 

[Keoke]

I Just find me a hill and adjust the timing until the ping goes away.-:highly_amused:

 

[Jerry]

I have a head that has been skimmed close to 0.24, and the compression with this head is 245psi. It ran well with no pings with 3 gal of 100 octane to about 7 of 91. No pinging. I would guess that you have too much advance, but look at the total advance as suggested above. I did replace the springs in the distributor, but don't remember the details. I finally took the head off the car and put a mostly stock back on because it was too hard to find the high octane gas. The car was fast, but then how much HP do you need in a Healey if you are not racing?
Jerry