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MAP and Ignition Timing

Babaganoosh

Babaganoosh

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I'm pretty sure I know the answer to this but want to confirm and not really getting a yes on google. If I advance timing, will the KPA of the MAP sensor go up? I'm thinking yes because in my old carburetor days hooking a vacuum gauge and turning the distributor would create more vacuum. Correct me if I'm wrong but isn't KPA just a conversion of inches of vacuum?

If this doesn't make sense let me know I can try and reword it.
 
if the timing advance raises the idle rpm your manifold vacuum will rise. Since the IAC will compensate the vacuum might not change. Maybe a slight minimum air adjustment.
 
Babaganoosh said:
If I advance timing, will the KPA of the MAP sensor go up?
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Well, the MAP reading will change, yes, but the direction is unknown without more information. As stated above, it's because the idle speed is changing. On carbs, we were told to maximize vacuum, which only works because you're getting your desired idle speed with the throttle as closed as possible (engine is pumping the same air, but the restriction is increased).

Yes, the MAP sensor number is basically your vacuum gauge.
 
Blue85 said:
Well, the MAP reading will change, yes, but the direction is unknown without more information. As stated above, it's because the idle speed is changing. On carbs, we were told to maximize vacuum, which only works because you're getting your desired idle speed with the throttle as closed as possible (engine is pumping the same air, but the restriction is increased).

Yes, the MAP sensor number is basically your vacuum gauge.
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Why doesn't this work on injection? I'm assuming sense the IAC is basically a vacuum leak?

What I'm trying to understand is how do you know how much timing to give a engine? Watch the knock sensors and dial it back until pinging stops?
 
The IAC is a 2nd throttle and may be the only thing controlling idle speed, so it might be easier to do than setting an idle screw. If you have automatic idle speed enabled, just tweak timing up or down and see what happens to vacuum. If you don't, you'll have to manually tweak IAC steps to get idle back where it was for a fair comparison.

While running, there are various methods to set timing. More isn't always better, even if it's not pinging. The best is to get on a dyno and find the timing that makes max torque at every RPM. Apart from that, most will start with a table similar to what was stock for the engine and go from there making general changes to see what the engine likes. Most software packages will calculate things like torque and MPG if you can get all of the input parameters right. Then you can estimate what timing increase/decrease is doing in a particular area.
 
Blue85 said:
The IAC is a 2nd throttle and may be the only thing controlling idle speed, so it might be easier to do than setting an idle screw. If you have automatic idle speed enabled, just tweak timing up or down and see what happens to vacuum. If you don't, you'll have to manually tweak IAC steps to get idle back where it was for a fair comparison.

While running, there are various methods to set timing. More isn't always better, even if it's not pinging. The best is to get on a dyno and find the timing that makes max torque at every RPM. Apart from that, most will start with a table similar to what was stock for the engine and go from there making general changes to see what the engine likes. Most software packages will calculate things like torque and MPG if you can get all of the input parameters right. Then you can estimate what timing increase/decrease is doing in a particular area.
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I see what you are saying but how do you know if it’s what the “engine likes” a calibrated ear isn’t as close as reading the data. So what data do you look at? It has to be related to vacuum some how.
 
It sounds like you're asking the basic question "how do you tune an engine?" That's beyond a couple of forum posts, but there resources out there, like the HPTuners forum. Here's a basic guide describing the interaction of some of the factors https://www.megamanual.com/begintuning.htm#advance (If it looks like a menu, just keep scrolling down.)
 
Blue85 said:
It sounds like you're asking the basic question "how do you tune an engine?" That's beyond a couple of forum posts, but there resources out there, like the HPTuners forum. Here's a basic guide describing the interaction of some of the factors https://www.megamanual.com/begintuning.htm#advance (If it looks like a menu, just keep scrolling down.)
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I understand the basics, I just read it’s what the engine likes all the time.

Well how do you know if it likes it? What data tells you it likes it? It’s fuel injection you should be able to have data that tells you what it’s doing. The dyno will spit out Numbers of hp and torque at the set AFR, increase timing until the knock sensor pings and back off?

What I have done on multiple vehicles is go back to non injected vehicles, visual tail pipe color, read plugs and hear the engine and increase until it pings then back off.

I do realize I’m answering my own questions. :haha:
 
Idle is an easy example. How smooth does the engine feel? The idle quality usually changes a lot in response to AFR. To dig that out of your datalog, you'd have to calculate deltas of crankspeed and run a histogram or something complicated. How stable is the speed and does it return nicely to that setpoint when disturbed?

A lot of what people call "tuning" is setting the spark and fuel tables. This is easily done via science as you say. Hold it at an operating point and measure the outputs. The problem is that engines are running under transient conditions so much of the time. "Throttle response" is a widely used term that is not 1st-level measurable in a datalog. An O2 sensor is usually too far away to measure exactly what's happening and that's too slow, so you end up adjusting the response to changing TPS and MAP/MAF (which are fast) preemptively to get the response to feel right.

Running everything 2 degrees from pinging is not really the way to do it and you'd have trouble finding knock across all operating points anyway. Plus, the tendency to knock also depends on AFR. You usually deal in generalities, smoothing a curve between some key operating points. It's really hard to get the engine into all operating points anyway. For example, how to you run 500RPM with 0 vacuum or how do you get 6000RPM with full vacuum and 0 TPS? You can usually get a pretty good shape for that curve by knowing your VE and/or looking at a curve for a similar engine, then adjusting sections up and down by trend, more than absolute numbers in each cell.
 
Some sections of the table hardly matter because you hardly ever run there. It just has to feel OK and not stall or smoke, so you can get a lot of that from interpolation. Most of your focus will be on idle, the WOT/near-WOT top part of the table and the cells where you are cruising down the highway. If you have all of that, you can almost just extract out from there to the rest of the table and it will run fine.
 
I see what you are saying, at the end of the day a lot of it is feeling.

I think what I’m trying to say is what sensors do you look at to determine if your adding to much timing or not? Wouldn’t it be easier to read a data log then pulling plugs?
 
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