compression ratio/vol efficiency vs ignition timing
 

One of the concepts one (me) should have before venturing into the art of building a track motor. CR/VE ( compression ratio/volumetric efficiency) and its relationship to ignition timing. If you have a collection of motors with the same static CR, but they vary in a number of ways primarily cam design and head flow ability, your dynamic CR will vary as well. As you make changes within the motor to increase your dynamic CR you then need to pull back your timing, my understanding. The greater your dynamic CR, the greater the density of the intake charge and the speed with which that took place. So why does that require a reduction in timing. I would appreciate any illumination into this black art. Bob

From chat gbt

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Less advanced is needed as the higher dynamic compression puts the charge closer to the point of autoignition - think of it as being much like a diesel; no need for spark plug after starting because the following charges autoignite at the critical point during the compression stroke.

You want the peak ‘push’ force of the piston down the bore to happen about 12-14 deg ATDC. Ignition timing BTDC designed for peak push ATDC. If you raise dynamic compression without changing to a slower burning fuel (higher octane), burn happens faster, less timing required.

Bob,
I think you are actually heading towards asking a question about combustion speed as this is the sole determinate of optimum ignition timing from my understanding.

Combustion speed is influenced by more than dynamic compression ratio and Volumetric Efficiency and anytime the speed of combustion changes due to engine modifications or some other influence, the timing needs to be adjusted to produce maximum brake torque where peak combustion pressure occurs just after TDC (somewhere between 11 and 17 degrees is what I've seen quoted).

Kevin Cameron, long time and much respected technical writer often writes on these topics. This one may be of interest:

https://www.cycleworld.com/story/blogs/ask-kevin/accelerating-combustion-increase-motorcycle-engine-power/

Thanks for sharing the Cameron article Peter. Reminds me of the great Gordon Jennings tech articles. Tech explains it all !

The excellent article points out that it's not the combustion "speed" that influences timing, but the combustion time. Speed influences the time, but so does combustion chamber size and turbulence and dynamic CR. The article discusses these factors too.

Cr/ve
 

14* I have heard is the accepted compromise. Without very sophisticated and expensive test equip, how is that determined. I have installed a J&S knock sensor system. Took it to the dyno yesterday. I think once I figure out the nuances of this piece it will be a good tuning aid. Started at 20* BTC and had 267 HP. Added 2 degrees and had 287HP. Two more degrees and had 291 HP. At the top of that run the low end of my LED bar flickered briefly. The LED bar has low, medium and high light intensity to indicate knock and timing retard. Will get to the track and see if I have made some progress with that demon ignition. I am fairly accomplished at some of this and amazingly ill informed on other areas. That I had never knowingly experienced ignition issues and stayed very busy on other aspects of the car, was caught a bit off guard. Very much appreciate the input provided. To quote Benny Hill, ''Alvays a lurrrning". Bob

Bob, IIRC, you have a twin plug engine, correct?

As you know, twin plugs decrease the timing required, because the plugs start ignition at two separated locations, so the flame(s) do not have to travel as far to get across the chamber, especially if you have a dome in the middle. Faster ignition requires less timing advance for the reasons discussed.

According to others here who run engines with twin plugs, somewhere in the mid-20's ignition advance (non-vacuum) is about optimum. The higher octane race fuel burns a little more slowly than street fuel, so requires a bit more advance. With street gas, you will end up in the mid-20's for ignition advance, and with race fuel, in the mid-upper 20's. That's about what Henry and the others here have said about their twin plug engines. Your experience on the dyno aligns with theirs.

Unfortunately for us, these old air-cooled engines were designed with hemispherical chambers and domed pistons, which do not allow much advantage for turbulence and squish, although they do enable fairly straight intake and exhaust ports, which improve breathing. Some different piston shapes have been designed to improve combustion, such as the Moritz-style slanted piston top, which concentrates the charge closer to the single spark plug, and the "peanut" style piston top developed by FAAPGAR (RIP), and others. Some, including Porsche, have modified the head to close off the sides of the open chamber and use a flat-top piston, to get a small concentrated chamber ("bathtub" shaped), flat squish zones and maximize turbulence. If I were designing the engine today, that's how I would go about it.

In my current single-plug street engine using 91-92 octane gas, I'm using the Mahle Sport pistons (nominal 10:1) which look almost identical to the Porsche 3.2 piston with domes off to one side. These concentrate the charge closer to the spark plug. I left left out the base gaskets to bring the piston dome closer to the inside of the head to try to get more squish and turbulence (not to increase CR, although that did get increased a little). I got a minimum piston dome to head clearance of about .030". I am using 30* of advance, all in around 3500 RPM. Seat-of-the-pants, that seems to develop more low end torque, but without back-to-back dyno testing, I can't be sure. I know that I can floor the pedal from just off idle and it will generate good torque without stumbling all the way to 6500 (I am using CIS and William's M1 cam).

Always a trade-off.

Yes twin plug. I have talked to a number of people about squish and its benefits. Consensus was that there was benefit on more flat shaped chambers where you had a flat shelf/roof configuration. Not as beneficial on dome type chambers. I assume it has to do with the better more controlled mechanical push you get from the flatter piston/head squish area into a smaller pocket accelerating combustion. And depending on valve arrangement, no pockets to navigate. Obviously less distance to travel if you don't have to traverse a hill. A layman's assessment. Bob

Bob,
I think you will find that squish is common feature of nearly all high performance 2 valve heads as it is proven to increase combustion speed and assists in gaining reasonable compression ratios without resorting to turbulence killing, combustion slowing and heat wasting highly crowned pistons.

However low squish open chamber heads were a common feature in 70's American engines for emission reasons to lower compression ratios (to reduced NOX's) and reduce the release of unburnt hydrocarbons (squish areas allows unburnt fuel to hide.)

Henry published some great photos of 70's 911 racing heads that had bathtub chambers because the racing regs didn't permit twin plugging at the time. I'll try to find a link.

Found it:

https://forums.pelicanparts.com/911-engine-rebuilding-forum/434823-old-school-twist-3.html

also

https://forums.pelicanparts.com/911-engine-rebuilding-forum/619651-bathtube-combustion-chambers.html

I wanted to throw a couple of things out there. One is this high speed combustion video, which I posted awhile back. The dark boundaries are shock fronts, where the flame front has become sonic. Combustion proceeds by a laminar flame speed, and then turbulent entrainment, when the shock waves tear through the remaining mixture. It's kind of hard to explain. The temperature and pressure of the mixture do influence the combustion speed, but the main influence is the chamber shape (confining the mixture near the ignition source).

https://youtu.be/ZKYbw0brHPo

This is a comparison of the various Chrysler Hemi chambers over the years. We never found the quench area to be very significant with twin plugs. We also wanted to make sure the mixture was almost completely burned. Eventually they adopted the "un-hemi" with the chamber sides filled in. I think that was mainly to get the compression ratio up, not sure though.

The classic american wedge chamber on the other hand always experienced a big benefit from a quench area because the area was larger, and opposite from the single spark plug.

The other thing I wanted to mention is that a higher compression ratio makes more power because a higher compression ratio means a higher expansion ratio. You get more power out of any given RPM, ignition timing and combustion speed.

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Remembering Chryler's attempts at making hemi heads more practical, 25 years ago I made some heads utilizing the same principle.
I called them the "peanut" chamber. We were able to change a 2.7 MFI combustion chamber from just at 70cc to right around 51cc. That allowed us to run an 8.0 (advertised @ 8.5:1) 2.7 RS piston on a 66mm crank and generate just over 11:4. Big valves 47 mm intake and 41 mm exhaust
We were able to run this configuration on 93 octane @ 26 degrees of advance without detonation. The obvious benefits were lighter piston and better volumetric efficiency.
The 2.5 ran 8500 RPM with titanium rods and custom built slide valves, without issue.
We had some concerns about valve shrouding but Bill @ Extreme flowed them with good results.

Chryler head

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Supertec Porsche head

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