Ignition Advance vs Jetting

[Zach Gomulka]

I was always taught, the more timing you can safely run, the better. Makes the motor snappier, more responsive. So when tuning my GTV, I worked my way up to 35 degrees total advance on the 010, and that's where it's been for quite some time now. After that is when I started jetting the Zenith down for max performance/mileage.
On the way back from Vegas, I was talking to my buddy, and he has his own theory... He suggests knocking the timing back to 28 degrees total. This he said would allow me to use a smaller main fuel jet, and the end result would be more power and/or better mileage.
Any thoughts?

[Martin]

Ignition has no bearing on how much fuel the motor needs.

on the dyno i have never seen any benifit of running more that 32 deg's total advance.


Martin

[Jon]

If the engine needs a lot of advance to produce the power, the heads is suffering from "wetspots" in the combustion, to much fuel on one place, this comes from a less than favorable shape of the combustion chamber, and is then much harder to ignite. When the atomizing is good, the ignition will settle for less advance. 

[John Maher]

The less efficient your combustion chamber design, the sooner you need to start the fire.
Combustion isn't instantaneous - it takes time for the flame front to ignite the entire intake mixture.
The aim is to complete combustion so that the maximum 'push' on the piston occurs around 14° ATDC to achieve  maximum torque at the flywheel.
The slower (less efficient) the burn, the sooner you have to start the fire.

Spark plug location and chamber design play a significant role.
A modern engine design e.g. 4 valves per cylinder with centrally located spark plug might only need 20° max advance because full combustion takes place quickly and efficiently.
Gene Berg acknowledged as much with his semi-hemi chamber mod - in one of his newsletters he advised running an extra 2 to 4 degrees of advance (e.g. 34° to 36°) when running semi-hemi cut heads. My interpretation of this is the flame front takes longer to complete full combustion due to the greater surface area created as a result of the semi-hemi cut chamber. Plus the entire squish area has effectively been removed, resulting in a less efficient burn compared to a more compact bath-tub style chamber with large squish areas top and bottom i.e. it takes longer to get the job done so you have to start sooner in order to reach completion at the same point.

The wasserboxer (in standard form) is another example of inefficient chamber design. The chamber in the head is very shallow with much of the combustion taking place in a large dish machined into the crown of the piston. At certain points in the engine's rpm range ignition advance can be as high as 50° 

Compression ratio, deck height, bore diameter and type of fuel are also factors.

For the majority of Type 1 aircooled VW head designs, 32° total works out close to optimum but isn't necessarily the 'right' figure for everyone. It depends on all the above.

The best advance figure for a given rpm is the one that delivers maximum torque at that rpm. Having optimised advance for say 4500rpm, doesn't automatically mean that's the best figure for 6000rpm. The optimum advance setting usually occurs just before the onset of detonation. Peak power rpm (say 6000) might like more advance but can't be dialled in because that would result in too much advance (detonation) at peak torque. With a mechanical ignition setup you have to compromise.

Many aircooled VW owners run max advance in the sub 30° range. That's often because they're trying to compensate for something less than optimum about their setup e.g. too much CR, not enough octane etc but if that's what it takes to deliver best performance without the risk of detonation, that's what you have to run.
Alternatively, they may have a super efficient chamber design, run a tight deck, have plenty of squish area and relocated the spark plug(s).

[Zach Gomulka]

Much appreciated gentlemen!

The advise my friend gave to me was based on his experience with the 1915cc engine powering his '57, the same car we were in at the time. He built the motor close to 15 years ago, and as he says, "I was reading too much Gene Berg at the time." It is set up with low compression (7.something), I don't believe the heads are hemi cut, but it is running a fair amount of deck to get that figure. Cam is something in the neighborhood of an Engle 120, carbs are 45 Dells. It has been driven to the Bug-In, The Classic, and now Las Vegas this year, so who am I to argue?!
My engine is high mileage, original and completely stock, except for the Zenith 32NDIX, 010/Pertronix, and header with glasspack. That will soon change, but I'd like to see if I can get a little more out of it for the time being, and learn something at the same time. Any advise for my current set up?

[j-dub]

My comments are based on his positive results with 35 deg of timing. Typically when you see a engine that responds favorably to this much timing it is because the engine is running super rich. This super rich mixture takes longer to ignite/burn and that is how he is getting away with it.

That is why I suggested to him to lower his max timing and likely go smaller on his main jet, of course jet towards 12.5 AFR at WOT.