Ignition advance at cruise rpm?

[ptuomov]

Lots of variables to consider, but probably in the neighborhood of 10-15 degrees more than the mechanical is providing at that RPM. If you are all in at 35 degrees at 2500, then you could prolly be at 50 degrees or so when cruising

Makes sense. Now, consider the experiment of dropping the compression ratio by a point (but the car of course taking the same amount of power to cruise and this consuming a little more fuel and air a little less efficiently). How would you expect to change the cruise timing?

[turbo camino]

If you keep getting unsatisfactory answers, maybe you need to ask better questions.

[ptuomov]

If you keep getting unsatisfactory answers, maybe you need to ask better questions.

Maybe... Is it that I am not communicating effectively what my question is or that I am communicating the question effectively but the question is simply not interesting?

[hoodeng]

Back in the day of mechanical advance distributors with vacuum advance pots you could have a same size and build engine used in a number of applications, light two door, family wagon with all the fruit, light commercial pickup etc.

They would have the same distributors with different mechanical advance characteristics to suit full load applications, they also had different curve and total adv vacuum pots to suit part throttle applications that would transition over a wide rpm and MAP range, i recall a number of different paint dot coding's on these parts. The amount of R and D testing was legendary to come up with curve combinations for each vehicle that would be safe under all conditions.

I dont think there is a fixed number in this application that fits all.

I still recall guys disconnecting the vacuum advance in belief that there was some power advantage when this was the only ignition in use. When doing Auto elec post trade the instructors [ who some were hotrod freaks] spent a lot of time dispelling this belief, and showed the advantage of vacuum ignition control and its relevance in street application when appropriately curved to the engines needs.

Cheers.

[rustbucket79]

I just don’t see how you could blanket determine if a combo needs “x” then a half point lower needs “y”. Assuming optimized air fuel ratio, complete combustion is complete combustion. I don’t see a degree or two making a difference in a cruise condition. Maybe I’m out to lunch.

[ptuomov]

I just don’t see how you could blanket determine if a combo needs “x” then a half point lower needs “y”. Assuming optimized air fuel ratio, complete combustion is complete combustion. I don’t see a degree or two making a difference in a cruise condition. Maybe I’m out to lunch.

I am not looking for a simple universally applicable formula that gives you a final answer. I’m looking for a rule of thumb that I can use to get it into the ballpark. I guess I am just indirectly asking the question how does the combustion speed change with compression ratio in part throttle conditions. For simplicity, let’s say the ingested charge mass remains the same: how much faster will the higher compression ratio engine burn the charge?

[ptuomov]

I agree that the factories spend a lot of R&D time on these kinds of questions. That’s why I was asking about a quick short cut. Given a high quality factory cruise ignition map calibration, how much ballpark more part-throttle cruise ignition advance should I add if I drop the compression by one point and how much should I retard it if I add a point of compression?

Back in the day of mechanical advance distributors with vacuum advance pots you could have a same size and build engine used in a number of applications, light two door, family wagon with all the fruit, light commercial pickup etc.

They would have the same distributors with different mechanical advance characteristics to suit full load applications, they also had different curve and total adv vacuum pots to suit part throttle applications that would transition over a wide rpm and MAP range, i recall a number of different paint dot coding's on these parts. The amount of R and D testing was legendary to come up with curve combinations for each vehicle that would be safe under all conditions.

I dont think there is a fixed number in this application that fits all.

I still recall guys disconnecting the vacuum advance in belief that there was some power advantage when this was the only ignition in use. When doing Auto elec post trade the instructors [ who some were hotrod freaks] spent a lot of time dispelling this belief, and showed the advantage of vacuum ignition control and its relevance in street application when appropriately curved to the engines needs.

Cheers.

[Firedome8]

On a chassis dyno at say 40mph set the timming for the highest vacuum at that speed and then road test. Just thinking as the efficiency of the burn reaches optimum you will have less throttle position for the same speed. Your Thoughts ?

[hoodeng]

The reason more advance is used in part throttle high vac situations is because it takes longer to initiate combustion for a given rpm point, think of it as the fuel particles being further apart in a cylinder that is going through the same pressure volume cycle when combustion occurs between these particles as it would under a full throttle stoichiometric mixture.

Hence more ignition lead is required.

An engine only makes enough power to overcome the load placed upon it, regardless of its total power capability.

As an aside, i remember being told how much horsepower is required for a locomotive engine to idle when i was an apprentice, i dont recall the figure but remembered a 2,000 hp engine used a staggering amount just to be ready to work.



Cheers.

[ptuomov]

On a chassis dyno at say 40mph set the timming for the highest vacuum at that speed and then road test. Just thinking as the efficiency of the burn reaches optimum you will have less throttle position for the same speed. Your Thoughts ?

Something like that would surely make sense. I was thinking about adjustable throttle stop and load-holding eddy-current hub dyno. At each rpm and throttle stop position, try to tune it for minimum BSFC by adjusting the ignition timing.

The practical issue is that the car got just moved away from the dyno... The high load operation is silky smooth and when driven in anger it works great. We just didn’t think about part throttle cruise much at all when chasing the horses.

[ptuomov]

The reason more advance is used in part throttle high vac situations is because it takes longer to initiate combustion for a given rpm point, think of it as the fuel particles being further apart in a cylinder that is going through the same pressure volume cycle when combustion occurs between these particles as it would under a full throttle stoichiometric mixture.

Hence more ignition lead is required.

An engine only makes enough power to overcome the load placed upon it, regardless of its total power capability.

As an aside, i remember being told how much horsepower is required for a locomotive engine to idle when i was an apprentice, i dont recall the figure but remembered a 2,000 hp engine used a staggering amount just to be ready to work.

Here’s one research paper that is consistent with what you say: higher CR and higher MAP both reduce various burn time measures:

https://pdfs.semanticscholar.org/235b/4 ... 571e5a.pdf

I agree that in steady state, the engine will only deliver what it needs to deliver — otherwise it would be accelerating.

[naukkis79]

YOU’RE DOING IT WRONG. Thanks, captain obvious. Work on your reading comprehension.

Calculating dynamic compression to compare part-throttle ignition maps is good starting point, but valve timing isn't part of that calculation, engine load is. So if your engine 100% VE is 500mg/stroke and compression ratio is 10:1 and your part throttle load is 100mg/stroke your dynamic compression ratio is 2:1. Extremely low compression (~1:1) also find combustibility limits so very low load needs less timing to be able to fire at every stroke. Easy to tune with computer-controlled ignitions, not so with vacuum canisters.

With MAF-based EFI dynamic compression ratio is always calculated in ECU so both fueling and ignition timing is easy to tune.

[Walter R. Malik]

What's the motors actual cranking comp hot with the throttle wide open after 6 hits on the gauge?

I've never measured it for this engine. It's got a relatively small cam, four valve head, etc. I've got a cruise ignition map from the factor for an engine that has about 8:1 "effective" compression ratio computed from the engine geometry and valve events. This engine has a 6.7:1 "effective" compression ratio. How should I adjust the cruise ignition map, theoretically speaking? I mean I gobbled together a map with a little more timing and it drives fine, but what should I have actually done?

So many variables to contend with:
Use the vacuum advance and keep giving it more timing until you hear or feel it surge slightly, then back it off until that goes away.

[travis]

Just thinking out loud here...

How would you be changing the compression? Either with a head or piston change, wouldn’t you be completely changing the burn characteristics, requiring you to have to completely start over on your tuning?

Or, different way of looking at it. It takes X amount of power to maintain a given speed. Higher compression will generally make more power for a given throttle position. Throttle position will control how much mixture is in the cylinder...so with higher compression you would be compressing less mixture more, than a lower compression which would need a bit more throttle opening to make the required power to maintain a given speed...more mixture compressed less. IOW...since you’re looking at part throttle cruise and not WOT (max possible cylinder fill), wouldn’t the net effect be about the same? Requiring about the same CRUISE timing?

I probably confused everybody...putting thoughts into words isn’t one of my strong points

[ptuomov]

YOU’RE DOING IT WRONG. Thanks, captain obvious. Work on your reading comprehension.

Calculating dynamic compression to compare part-throttle ignition maps is good starting point, but valve timing isn't part of that calculation, engine load is. So if your engine 100% VE is 500mg/stroke and compression ratio is 10:1 and your part throttle load is 100mg/stroke your dynamic compression ratio is 2:1. Extremely low compression (~1:1) also find combustibility limits so very low load needs less timing to be able to fire at every stroke. Easy to tune with computer-controlled ignitions, not so with vacuum canisters.

With MAF-based EFI dynamic compression ratio is always calculated in ECU so both fueling and ignition timing is easy to tune.

I was responding to a question about cranking compression.

I can measure the mass air flow well with this engine and, yes, it’s rpm by load table where load is a measurement closely related to the charge mass trapped in the cylinder.