Ignition advance at cruise rpm?

[ptuomov]

I've got a question about ignition advance at part throttle cruise (not WOT). I'm interested in how much would the "smoothest, best drivability" ignition advance change in the part throttle 2000-3000 rpm cruise change if I either increase the compression or reduce compression one point?

[mt-engines]

I've got a question about ignition advance at part throttle cruise (not WOT). I'm interested in how much would the "smoothest, best drivability" ignition advance change in the part throttle 2000-3000 rpm cruise change if I either increase the compression or reduce compression one point?

there is no set number, i was able to run up to 60 degrees with egr and lean cruise. was worth 6mpg on my 92 buick roadmaster until i lost a ring. depends on load, throttle position, octane and AFR.

[ptuomov]

Ok take that 9.5:1 engine running pump gas 93 and therefore probably not that knock limited at cruise 2000-3000 rpm. Drop compression to 8.5:1. How much more ignition timing can you give it now at cruise, same rpm and road speed?

[rustbucket79]

IMHO those are both relatively low compression ratios, and I believe it would be application specific more so than an absolute based solely on static compression.

[peejay]

You may find that you can't get enough timing. You'd lose reliable ignition before you hit best timing.

A lot of engines had higher compression than ideal for WOT performance, so they could be more efficient under light load.

[ptuomov]

IMHO those are both relatively low compression ratios, and I believe it would be application specific more so than an absolute based solely on static compression.

I’m just looking for a general guidance. How much more air does cruise require with one less point of compression and how much slower is the burn given the lower compression? And the. When you put the more air effect and the direct slower burn effect together, what’s the net effect?

[novafornow]

I target 50 - 55. At light throttle, you can use a lot of timing.

[ptuomov]

I target 50 - 55. At light throttle, you can use a lot of timing.

Does that depend on whether it’s a low compression or a high compression engine?

[mag2555]

Here's a very common timing curve from a mid 60s GM 350 motor with 10.25 comp and 180 psi of cranking compression.

[ptuomov]

Here's a very common timing curve from a mid 60s GM 350 motor with 10.25 comp and 180 psi of cranking compression.

Thanks! How would you modify those values if the engine had a full point lower compression at 9.25:1 keeping everything else constant?

[mag2555]

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

[ptuomov]

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?

[naukkis79]

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?

You measure your effective compression ratio wrongly. At part throttle engine is throttled so air consumed per stroke is very low meaning that actual effective compression ratio is also very low. Like 2:1 and so on.

There's no rule how to calculate part throttle ignition timing, just increase it as long as power does as well without misfires.

[ptuomov]

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?

You measure your effective compression ratio wrongly. At part throttle engine is throttled so air consumed per stroke is very low meaning that actual effective compression ratio is also very low. Like 2:1 and so on.

There's no rule how to calculate part throttle ignition timing, just increase it as long as power does as well without misfires.

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

[GRTfast]

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