Camshafts and ignition timing.

The ignition timing would not change, but you might be able to make more power by changing the ignition timing as the modified cam probably opens and closes the valves at different times to improve cylinder filling. You would change both the initial timing and the advance curve to optimise the timing of the spark in response to different rates of cylinder filling as a result of the camshaft changes.

M

 

With all the knowledge foating around this site, I think you might find that your question (which I don't fully understand) will open a can of worms!

The first thing I would throw in to the pot would be to say that "explosion" is probably the wrong word. Petrol vapour "burns" and it takes time to burn. As engine speed increases the time allowed for the burn decreases, so you need to start the burn slightly earlier to make sure that all (or most) of the fuel has done its job before the exhaust valve opens.

There are, however a large number of variables, such as the volume of fuel mixture being burned and the speed with which it can get into and out of the cylinder, which can obviously be affected by cam profiles and the condition and design of inlet and exhaust manifolds.

Compression ratios have an effect on fuel density which will change the burn pattern and as an engine runs faster atomisation of the fuel gets more efficient and the fuel burn gets faster as a result.

No doubt others with much better knowledge will add even more confusion (or enlightenment) to this very wide topic!

 

I believe the optimal pressure from ignition to power ratio is 5 degrees ATC. So if the engine is timed to idle at 8 degrees BTDC, the idea is that optimal power from the burn is achieved at 5 degrees ATDC. This figure does not change throughout the rev range yet obviously the timing require to meet that 5 ATDC changes throughout the range.

I'm a tiny bit blurry on this stuff, but I can give an idea or what you're looking for.

Changing cams can have an effect on the dynamic compression ratio as often times the new cam can cause the intake valve to close later on the power stroke thus causing a loss in dynamic compression. Closing earlier raises this pressure. The later the intake valve closes, the higher in the rev range peak power occurs as the equivalent cylinder pressures are reached (equivalent to closing the valve earlier I mean) at a higher rpm.

How this relates to ignition timing possibilities is essentially the same as the idea is to have peak ignition timing (say 35 degrees) at peak pressure which is where peak torque is made. Raising static compression raises the dynamic curve as a whole meaning peak pressure could be made sooner thus requiring a ignition timing change or a cam timing change to make peak timing pressure made later in the rev range.


For example:

engine A has 10:1 static compression and 8:1 dynamic compression with the intake cam closing at 50 degrees past BDC. total timing is 35 degrees @ 6000rpm

engine B is the same only the intake cam closes as 55 degrees past BDC causing a dynamic ratio of 7.5:1. total timing is still 35 degrees but at 6500rpm due to a difference when peak pressure is made

engine C has a static ratio of 11:1 yet uses the same cam as engine B and C yet closes the intake valve like engine A at 50 degrees past BDC making for a dynamic ratio of 9:1. peak pressure is made at 6000rpm but total ignition timing can only be 33 degrees due to the subtantial amount of additional pressure.


Street compression ratios are usually recommended to be no more than 10.5-11:1. A higher ratio could be used if the cam had a very retarded intake valve closing to reduce dynamic compression, but peak ignition timing to optimize the pressure could not be used and low end driveability would be poor due to this combination of both. Best case for this is using race fuel and closing the intake valve at a reasonable cam recommended time frame and setting up for optimal timing for the static ratio in question combined with the combustion efficiency of the engine.


Things like cylinder filling from optimized ports and lift changes things a bit as well but now we're getting complicated.




Anyone feel free to clarify but I believe I have the short of it somewhat covered.

 

Notice that ignition advance is "all in" by 3000 RPMs, and then basically holds constant for the rest of the rev range. Above 3000 RPMs a new phenomonom occurs and that is the phenom of turbulance. Turbulance allows a mixture to burn faster So that after about 3000 RPMs, the rising rate of burn from turbulance pretty much counteracts the less time available to burn, and thus more advance is not needed.

Depends on how modified the cam is. Add 4-5 degrees on both ends fo both lobes and you can probably still get by on the old advance curve. Add 10+ on both ends to each and you probably would not like the advance curve. Notice I said curve and not advance. The total advance might be rather close for 3000-8000 RPMs, but you would want more advance just above idle through the difficult 2500 RPM band to make up for the loss in static compression from the longer cams.

 

Turbulance counts for old carbureted engines and engines with poor injection systems but has little relevance for modern engine designs with good quench ratios and combustion chamber efficiency.

Ferrari started catching onto combustion chamber efficiency with the 308 4v motor design and grew from there. With combustion chamber efficiency less timing is needed due to additional turb already being created. The modern engine curves even in race engines Ferrari has does not really go 'all in' by 3500rpm like they used to with Daytona racing engines etc.

 

What's the total bhp at the wheels? Any idea regarding the spec of the cams? Neat motor

With the 308 2v engine, I would not go 100% all in at 3000rpm, but certainly adding a large chunk of your timing by then is acceptable practice. Say if your total timing is 35 degrees, you could go 30 degrees in at 3000rpm then reach peak timing at peak torque rpm, which in your case is probably around 6k rpm?

 

wow. those are definitely steel billet cams. expensive! nice setup there

 

Those cams are Frank Capo's cams that are in that engine, he lives in Australia, he has made cams for me in the past, has some great grinds that are proven and was one of the pioneers of 308 performance products, he has switched building allot of Subaru stuff for racing as far as I know that last time we talked. Great guy!

 

Something to remember when playing with early 308 ignition is that the advance curves quoted might run to 3,500 rpm, but that's distributor RPM, which is half the crank speed.

Regards,

Carl.

 

No this will not cause any change in ignition time. You can make it change under certain condition…..

 

Hi Aircon,

There may be some rules of thumb at play here, but in general, once you vary from a stock setup things are likely to change somewhat. The factory would have devoted a lot of time and effort to determine a reasonable compromise for a timing curve. Assumptions are made as to fuel quality, CR, cam timing, mixture strength, temperature, and use. At best, the OEM timing is a compromise. If you start changing things, the results of the whole development effort can be called into question, especially if you are after the perfect curve. Simply stated, if you're after the best curve for your engine, I think you have to be prepared to spend a lot of time on a dyno and you have to have an idea of what you want to accomplish. Is it max hp, torque, milage or driveablility you are after? Is it some particular combination of these assets you are after?

The OEM system is strictly rpm based. This is pretty good from a performance point of view by lacks the ability to alter advance based upon engine load. The engine may tolerate full acceleration mode nicely, but may not take advantage increased advance under cruise mode.

I have a fully programable xdi-2 ingnition with vacuum advance fitted to my 1978 308. I plan to explore this issue over time. I find the issue challanging and daunting at the same time.

Best regards,
Bill

 

Setting timing with MAP and load variations is as much art as it is science, but it is the only way to really iron out the flat spots with these things. I have a similar setup but obviously have not run it yet as my engine is not back together, but I look forward to the challenge. Where are you pulling MAP from? It's a bit tough with these things...

 

Hi Ferraripilot,

The vacuum advance is currently disabled while I sort out idle/off idle mixture adjustments. The advance is currently T-ed into the line that provides vacuum to the vacuum resevoir sphere. the connection is on the carb side of the the one way valve of this circuit so senses vacuum in the number 8 inlet tract. I probably don't get a valid signal until the vacuum sphere is sucked down to a workable value. This probably takes a minute or two after start up. I chose this way so I didn't have to disturbe the plumbing when using my manometer system to set carb balance.

In order to enable vacuum balancing, I removed the hard plumbing used to evacuate the charcoal canister and replaced it with hose barbs tapped into the resulting holes in each of the intake manifolds. This enabled me to hook up a bank of 8-manometers and go about balancing idle and off idle air flows as required. When balance is set, the charcoal canister circuit could be reestablished with the replacement silicone tube and T-fitting array.

Today, I don't plan to reestablish the vacuum canister purge function as its use affects the balance and mixture somewhat. So, if necessary, I can plumb the vacuum advance to any of the 8-available vacuum taps in any of the inlet tracts, or to any combination of them if I think I need a stronger load signal.

My understanding of the XDI-2 vacuum mapping does not place restrictions on the advance relative to the vacuum signal strength. This means, one can dial in almost any vacuum advance for a given vacuum value. Said another way, it's up to the programmer how much advance to assign to a given absolute pressure.

I hope this makes sense.

Bill

 

Yup. Makes perfect sense. I will probably run my MAP off the 5-8 bank normally emissions charcoal canister line off the manifold. We will see how that works out. I will arrange the ignition map once I know a bit more about how the engine will run at idle and various rpm etc. I suspect these will run at around 45kpa at idle like most everything else.