I was just looking at some old scans, and I did do a search to read up on some posts about folks using HM spark advance. Currently I am running 8* intial advance, 42* max spark advance(stock) and about 10* HM SA, as long as all the parameters are met for HM. Looking at my scans, I was in the 36 deg area of my main SA table, and when the LV8 was low enough, my scan tool read about 45*, which indicated about 10* advance for HM. But with the max SA set to 42, I was confused, then I read that this is the MAX Advance that the ECM can ADD to the timing. So if you allready got 8deg in the dist, then you can get more than 42 advance.

I see no Knock counts at all. I just wonder what the benefits are, if any. I see lots of folks ZERO out all the spark adders, and just use the main spark advance table. Tends to make things more simpler.

 

I don't use it with engines that have modern combustion chambers. Older heads get highway spark but only 2-4 degrees. I also find that I can get away with leaner AFRs with newer heads. It's logical considering the newer fastburn designs can be leaned out at WOT... so why no under lower lood too . And lastly, I get out of highway fuel/spark with even the lightest amount of increased TPS. When adding timing you should watch the oil temps. Pistons get hotter with advanced timing!

 

Tuning by polling.....
Sorry, but your poll is meaningless.
You tune to make the car happy, ie give it what it wants. That means experimentation, what works for one guy and his driving style/ car's mods may or maynot work for another, that's why universal one size fits all chips just don't work as well as a chip tuned for you and your car.

 

My Caprice came with it enabled from the factory. According to TC, it adds 2.1° @ 50kPa (and lower probably). There's also an enleanment that goes along with it, but I have no idea what the target AFR is...
I've gone back and forth over the years, turning on, swearing I saw a MPG increase, then turning off, feeling better response and no mileage decrease. Then I'd turn it back on and claim to see an increase...

I dunno, I guess if I can't tell it's better with it off. I think swirl port heads tolerate it ok.

 

I've got it enabled on the 86 TPI, and max advance is at 48.16°. HM AFR is only at 15.4, and max HM advance is 9.84°. That said, this is a 305, and I only log about 500-600 miles/year, so while I have no problems with it, I can't really state that it is saving any fuel.

 

I know 8D has a max amount of SA setting.
So no matter what all the tables add up to. You can only get that amount sent to the dist. You have to look at the correct value from ram to see what the actual sa to the dist is. Not all data streams send this I believe.
A little research in the ram area using the hac for your bin will tell you. I can't recall the name of the value now... too little sleep again.

Typically you'll see people say it feels better with 40+ degrees.
Don't recall many proving it. The one I do recall had 9.5 cr and a XE284 cam and heads that were too big for the cid.

 

Wouldn't the timing be (90 - 48.16) = 41.84 degrees because the spark is going to go to the previous cylinder. The way I see it is that you can't have more than 45 degrees. That is why there is the max. spark advance term that Z69 mentioned.

 

The 42° limit (8cyl, small cap) that is set in the calibration tables is a distributor limit. Placing the base timing at 6° BTDC allows up to 48° of 'at the crank' timing. A base timing of 10° BTDC allows up to 52° of 'at the crank' timing.

Recall that the inital (base) timing is subtracted off in the code. Then the SA is checked against the max value.

RBob.

 

Good point. I forgot about the subtraction done in the code.

 

So what would be the absolute max spark advance you should be limiting to(SAFELY)? 50deg? The max spark advance in the constants is set to 41.84*, that is the max that the ECM can ADD to the already advanced (mechanically) distributer advance. Since I have base timing of 8.09, I could get 49.93* total max spark advance.

Looking at some scan data, with HM spark in, I have seen 45 deg advance as reported.

 

It all depends upon what the engine likes. I tie the highway SA to highway lean cruise. So that the only time SA is added is when the AFR is commended lean (say 16.1:1). A lean mixture burns slower, so need more advance to get the peak pressure at the correct place in the cycle.

RBob.

 

Ditto. When I was first experimenting with HM, I began using the spark advance after I completed my testing with the HM AFR. I found the lean mixture made my throttle response decrease and the engine felt "spongy".

So I started increasing the spark advance in HMSA and found my throttle response increased and the spongy effect was lessened. I did have a functioning EGR on my car at the time.

When I dropped out of HM or my EGR began to turn-off (such as when climbing long grades), I turned off the HMSA in gradual steps which matched the HM AFR. Once I was back to 14.7:1, HMSA was completely turned off.

I got my best gas mileage with my HM AFR at 17.4:1 with 47* total spark advance.

 

Yea I generally try to have HMSA working while I use the HM fuel as well. But it may be a little trickey getting them to work togethor. I just checked my parameters, and while I have them come on at the same temp., i have different load values to disable. Got to change that! Plus one is based on speed, and the other rpm, so gonna have to be smart bout those values. I think my AFR in HM works it way up to 17.0 at the lowest load, and I add 10* of advance. Got to be carefull on some of this stuff!
Good thing for my LM1 WB, or else wouldnt even mess with it!

 

I feel the same lack of throttle responce but measured no gain going from 40 to 44 degrees of SA but did notice a slight increase in throttle response AND oil temps . All I cared about while cruising was keeping the oil temps low and mpg... I got that with 40 and a 16 AFR (wideband, 16.5 commanded), leaning it out more and adding timing didn't improve my mpg. Best recorded with the small tires and 3.73's was 24mpg which I feel is the best I can do turning 2800-3000rpm. There is probably a correlation with the RPM at which we're cruising and the amount of SA (and AFR).