Change in RPM Indexes

[creizlein]

I would like to know what is the benefit of having rpm indexes as close as 500/600/700/800/1000 , on the low speed tables, as I have seen that is what most basemaps have, instead of just use (just for example) 500/800/1000 and save a few extra indexes for the places where you might need them, for example 2750 or 3750 (barely, i know, but still in better use than in 600-700rpm index).

I know that if they are there it should be for a valid reason but as a newbie I am not sure why.

I have changed mine to the following table, and I know it wont make a lot of a difference since its all interpolated as needed but I am just curious of what would be the benefit of the first Vs. the second?

Code: Select all

500		->	500           
600		->	800           
700		->	1000          
800		->	1250          
1000		->	1500        
1250		->	1750        
1500		->	2000        
1750		->	2250        
2000		->	2500        
2250		->	2750        
2500		->	3000        
3000		->	3250        
3200		->	3500        
3500		->	3750        
4000		->	4000        
4500		->	4500        
5000		->	5000        
6000		->	6000        
7000		->	7000        
8000		->	8000        

[crluver123]

closer together at the idle area allows better resolution for fine tuning idle afr's. Some engines require some peaks and valleys in the fuel curve around the idle area to run well throughout that entire rpm area.

Same goes for further up in the rpm range. different engines will require peaks and valleys in the fuel curve at various rpms. Ideally you want rpm points exactly at the peak high or valley low or anywhere the fuel curve changes angle.

For example, if you have the following rpm points (left) and fuel values (right):
2800 - 110
3000 - 120
3200 - 120
3400 - 120
3600 - 140
3800 - 125
4000 - 135
4200 - 145

...you need all of those rpm values except the 3200rpm and 4000rpm rows. Because all the others are points in a curve. But the 3200 and 4000 values are in the center of a flat line if you were to view them as a line graph, so the ecu would automatically use that value anyway if it was deleted due to how it interpolates between rpm values. Basically meaning that the below values would function exactly the same as the above values:

2800 - 110
3000 - 120
3400 - 120
3600 - 140
3800 - 125
4200 - 145

[creizlein]

crluver123, thanks for your reply, it is very helpfull, what you mention does make a lot of sense.
I guess its just a matter of preference and analyze of each particular engine/tune to determine the best overall indexes, and I also assume that the same could be applied for Ignition Advance not just for fuel table, so we might need to take both in consideration.

What was just interesting to me to understand was the 500/600/700/800 index range when honestly the care barely uses that area, and at least in my tune they are very lineal, so having just 500-800 seems to be more than fine, and all the base maps i saw are almost lineal on those 4 indexes but still the indexes are present, so I was trying to know whats the benefit or in the other hand whats the negative side of just using 500-800-1000 instead of 500-600-700-800-1000. (3 indexes vs 5 indexes)

[crluver123]

If it's linear, it's fine to remove some of those indexes around idle area. I removed a few in my tune quite a while back.

Regarding the ignition maps; you'll find that ignition requirements are proportional to fuel requirements, while also including rpm as a factor. Meaning straight fuel curves typically require straight ignition curves, or a slight curve in the ignition. And when there's a sudden rise in fuel, there should be a sudden fall in timing, and vice versa. Think of the fuel table as a VE table, because that's essentially what it is. And higher VE requires lower timing, lower VE requires higher timing. And add to that higher rpm requires higher timing and lower rpm requires lower timing.

[creizlein]

Very good talking/feedback, much appreciated.
I guess we'll never found out exactly why those indexes are exactly the way they are in first place but believe that if they are there its coz someone decided it was best to do, hehehe
But really, very good reading crluver123, thanks for the input.