XJ6 & XJ12 Series I, II & III 1968-1992

Knock sensor question

 
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Old 02-26-2019, 04:42 PM
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Default Knock sensor question

Trying to understand the xj6 SIII 6 cylinder emissions system and possibly improving performance. When smog controls were just being phased in manufacturers de-tuned their engines by retarding the ignition timing among other things. So when you bought a late 60s or early 70s car the first thing you did was advance the timing back to where it was pre-emissions. Of course, you couldn't do any thing about reduced compression, etc.

As far as I can tell the emission contol system on my 1987 SIII controls the fuel injection system but the ignition timing is controlled by the distributor vacuum advance and the centrifugal weights inside the distributor. No computer control, no knock sensor, etc. Basically what was on my 1949 Chevrolet when I was a kid. Is this correct?

Can I advance the ignition timing a little bit at a time until I start to get pinging and then back it off a tad? I know there was a significant difference in horsepower between the US spec cars and the UK market. Other than compression, what can I do to get a little more get up and go?

The car in question is a 1987 and the heads have been off and as far as I can tell everything is in factory fresh condition. Thanks for your help.

Jeff
 
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Old 02-26-2019, 08:21 PM
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Originally Posted by Yellow series3 View Post

As far as I can tell the emission contol system on my 1987 SIII controls the fuel injection system
Well, sort of


but the ignition timing is controlled by the distributor vacuum advance and the centrifugal weights inside the distributor. No computer control, no knock sensor, etc. Basically what was on my 1949 Chevrolet when I was a kid. Is this correct?
Basically, yes.

But.....

There is a vacuum control device under the intake manifold that limits (or delays?) vacuum advance under certain conditions. I'll have to refresh my memory on the exact function and operation



Can I advance the ignition timing a little bit at a time until I start to get pinging and then back it off a tad?
Yes, you can. But the factory setting for your '87 is 17BTDC....which is already a fair amount of initial advance. If it's already at that setting I'm not sure there's more more to gain....but you can certainly experiment. And, make sure the vacuum advance is indeed working.

I know there was a significant difference in horsepower between the US spec cars and the UK market. Other than compression, what can I do to get a little more get up and go?
Not much, really. Er....not inexpensively, at least.

Ditching the cat converters might be worth a few horsepower (five, maybe?). The air injection pump might account for a couple horsepower. A Euro spec timing curve might help a little.

Low restriction exhaust and air intake might gain a few horsepower

Advancing the camshaft timing helps a bit. Not more total power, but more power at low RPM

The biggest problem on these cars is the 2.88 final drive ratio, 3-speed transmission, and 4200 pound curb weight. Not a good combination for lively performance, even with a bit more power


Cheers
DD
 
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Old 02-27-2019, 10:58 PM
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Thanks, Doug for your insights. One thing I'll do first is check to see that the vacuum advance is working. That would certainly account for sluggish performance. I think you probably hit on it with the weight and gearing.

I am in the fairly unique position of owning two SIII cars, one stock and one with a small block Chevy with 4-speed automatic. The later has a very mild and well used 305 v8 that had about 200 HP when new. About the same as the stock but is way more peppy. I guess that is due to a couple hundred pounds lighter and the 4-speed.

Another big difference between the two is that the v8 has very lithe steering with a beautiful light feel. I think Carl suggested that might be due to higher pressure from the GM power steering pump coupled with the Jaguar steering rack.

So the v8 car is a blast to drive and the stock one is stately and serene. If I had to pick between them I guess we know which one I'd choose.
 
 
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