Cruise VE vs spark question?

**Kitch** · 03-31-2019

I just added some extra spark at cruise and now the BLM's are slightly rich, so I'd normally reduce the VE in that area but I was wondering what's better: higher VE at cruise with less spark or lower VE with more spark.
Increased volumetric efficiency sounds good but it now looks like I need less fuel with the increased spark which also sounds good?

**WASyL** · 03-31-2019

you like to have as much spark as possible with lowest VE possible, that makes most MPG. Also that generates a bit more heat but if fans are able to keep temp. in place then You are good to go. Just avoid knock situations.

best regards

**Fast355** · 03-31-2019

Honestly I am not sure why people think lean automatically generates more heat. It does not.

**Kitch** · 03-31-2019

Just looking over a couple of logs, I have one with spark around 30° degrees at cruise and another with it changed to 35°, if I look though the logs until I find points where the MPH, RPM, KPA and throttle position all match then the one with 30° spark gets about the same MPG and has a slightly higher VE requirement.
30° at 59 MPH cruise and 50 KPA on paper doesn't sound like a lot of spark advance though?

**Fast355** · 04-01-2019

Originally Posted by Kitch

Just looking over a couple of logs, I have one with spark around 30° degrees at cruise and another with it changed to 35°, if I look though the logs until I find points where the MPH, RPM, KPA and throttle position all match then the one with 30° spark gets about the same MPG and has a slightly higher VE requirement.
30° at 59 MPH cruise and 50 KPA on paper doesn't sound like a lot of spark advance though?

Its not! My L31 with the marine cam liked about 38-40* at that KPA and 2,200 rpm @ 70. At 2,800 rpm running 70 with aluminum heads I run 48 at cruise now. I cruise at about 40-45 kpa now.