Interchangeable Parts - L16-L20B

[datzenmike]

9 hours ago, Stoffregen Motorsports said:

Theory vs practice

 

Have you ever re-curved a distributor?

 

The added timing brought in at low RPMs is where the benefits are felt, but if not limited at total advance, can be harmful.

 

Why don't engine makers do this?

 

Why is advance always delayed so much?

 

If advance is intended to start the fire sooner because there is less and less time for complete combustion as the engine revs up and if it's 'all in' by 2K then what about the remaining RPM to 6K or 7K where the time is 3 X shorter? It seems like only 1,000 to 2,000 gets the benefit from this.

 

I asked distributor guy but he missed the post. I'm sure there's an answer but I have never seen this spelled out in any article.

[Stoffregen Motorsports]

Mike, consider me a trained monkey in certain aspects of things.  There are many things I have learned over my long career in motorsports. Things that were taught to me, and things that I have figured out for myself. Things I don't question because they just work, and questioning them would be a distraction and a waste of time.

 

Nissan used to sell the parts to recurve your single or dual point distributor. Here is a roadster kit - http://www.datsunroadster.com/PIC_PAGES/NEW_PARTS_PIC_PAGES/221-10_24.htm

 

Every time you ask one of these questions - Why? - I get the feeling that you're doubting, and me not being able to thoroughly explain the reason why will somehow negate the point I was originally trying to make.

 

Though I cannot explain the theory behind recurving a distributor in an L motor, it doesn't mean that it doesn't make power.

 

Also, for reference, there are many articles out there, old and new, which explain how and why. If you really want to know, read one of these instead.

https://www.badasscars.com/index.cfm/page/ptype=product/product_id=447/category_id=13/mode=prod/prd447.htm

http://www.superchevy.com/how-to/distributor-recurving-power/

 

[datzenmike]

No, I have to believe the empirical knowledge on bringing the advance in sooner. Everyone agrees on this, everyone writes about doing this, many DO this. But in all my travels I have never seen it explained what's going on. It's conveniently left out of every article on re-curving a distributor. Which begs the questions:

 

 

3 hours ago, datzenmike said:

 

Why don't engine makers do this?

 

Why is advance always delayed so much?

 

If advance is intended to start the fire sooner because there is less and less time for complete combustion as the engine revs up and if it's 'all in' by 2K then what about the remaining RPM to 6K or 7K where the time is 3 X shorter? It seems like only 1,000 to 2,000 gets the benefit from this.

 

I asked distributor guy but he missed the post. I'm sure there's an answer but I have never seen this spelled out in any article.

 

Who really killed Kennedy?

Where is Jimmy Hoffa's body buried?

Where is flight 370

Why do car makers extend total mechanical advance to near 3,000 RPMs when lowering it by 30% improves power and torque?

 

When I find out I'll let you know.

 

[Stoffregen Motorsports]

Timing is just one part of the performance puzzle. Understand that engines are basically air pumps - the more air you can get in, the more power you can make.  When performance tuning, you need more fuel to go with more air to make a bigger boom, and the timing needs to be advanced to meet the slower burn caused by the added fuel. That's how I have always understood it.

 

Here is one article that outlines the basic theory - https://www.enginebuildermag.com/2017/09/understanding-ignition-timing-making-maximum-power-means-knowing-science/

 

That article is drag racing related, and only a small amount of the information is relevant to our discussion, but one set of general rules mentioned in the article rings true.

 

Quoted from the above article -

As RPM increases, more ignition timing is needed

For higher octane fuels, more timing is needed due to slower flame speed

For large combustion chambers, more ignition timing is needed

For emission controls, less timing is used to reduce smog compounds

For richer fuel mixtures, more timing is needed due to slower flame speed

[datzenmike]

4 hours ago, Stoffregen Motorsports said:

 

 

Quoted from the above article -

As RPM increases, more ignition timing is needed

For higher octane fuels, more timing is needed due to slower flame speed

For large combustion chambers, more ignition timing is needed

For emission controls, less timing is used to reduce smog compounds

For richer fuel mixtures, more timing is needed due to slower flame speed

 

1/ So again, if the distributor is re-curved to bring in all the centrifugal advance by 2k what's left for advance for the rest of the RPM range??

 

2/ I suppose. All I know about octane is that it is a substance that makes gasoline less sensitive to self ignition. You can compress it more without it pinging.

 

3/ Sure. The farther the flame has to travel across it the longer it takes so start the burn earlier to end up at the right ending point. Which the article says is 2 degrees ATDC.... I don't agree. At that point the piston/rod/crank are stacked and any cylinder pressure can't push the cylinder down. It't my understanding that 15-17 degrees ATDC (plus or minus) is more advantageous. More, and the expanding gasses are chasing the rapidly accelerating piston down an ever increasing cylinder volume. Less and the pressure is wasted pounding the piston/rod/bearing and crank.

 

4/ Yes, across the board, this reduces maximum cylinder pressures and temperatures (see 3/ above) that result in oxides of nitrogen formation. EGR is better for major reduction of oxides of nitrogen and is adjustable to meed engine load. An advantage is there is no EGR at idle and full throttle so it has little effect on performance where less timing affects everything.

 

5/ OK.