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ITB's - Alpha-N Vs Speed Density characteristics

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I am running a set of Borla ITB's based on Alpha-N on a Ford v8, performing very well, however I am at the novice level of tuning so I rely heavily on the expertise here in this forum.

A near acquaintance has recently purchased and installed a similar set on a smaller Ford v8. He had all kinds of issues with the ECU so the manufacturer upgraded him from their cheaper model to the mid-level unit that is capable of running in Alpha-N. However, under the advice of one of their tech guys, he continues to try and tune it in Speed Density mode. The car is running but he clearly still has some issues with smooth operation.

Being a novice in ECU tuning, I don't know how to have an intelligent conversation with him to get him to switch to Alpha-N. What would one expect to see in engine performance issues with a street driven, 347ci V8, running ITB's based on Speed Density? How can I identify which issues are tuning issues and which are due to using the wrong mode? His car idles fine, cruises pretty good, has some transition issues.

The big hangup in changing modes is "they said so", referring to the tech support team on his ECU.


There are only three results from any change -- better, worse or the same. You can always return to where you were if the Alpha-N setup isn't better.

If you have the time, what's the harm in trying?

BTW -- His problem could be as simple as using MAP as the acceleration enrichment signal, when he needs to use TPS with ITBs. Or maybe the MAP signal is OK, and he just needs to tune the enrichment to improve the transition. Lots of log studying to determine what to change. Best if you can create repeatable situations, so you can make a change and tell if it's "better, worse or the same".

I like David's take on this. Also, depending on the ECU, there may be an option to use both.

For example, on a Link ECU I run an Alpha N (TPS vs. RPM) primary fuel table, with a MAP based correction on top of it (4d Table), to get the benefits of both methods.

The MAP based correction can initially be set up as a straight 1:1 correction of fuel compensation vs. intake manifold pressure, since fuel density will need to increase or decrease in proportion to air density, relative to standard pressure.

If you go this route, you won't have to use such a broad range of values in your TPS based primary fuel table since the MAP based correction is already accounting for much of what you're approximating with your TPS based table. This is a benefit on systems that only allow a relatively narrow range of values in your fuel table. The MAP correction also reduces tuning effort required and speeds up the tuning process.

I agree with both, however maybe I didn't make the situation clear.

The other guy's setup is ITB's he's trying to run using Speed Density, because the manufacturer's tech rep said that would work best. He's basing all his beliefs on this one statement. He's able to get the car to idle and works ok under load but he is having transition issues.

I think he should switch to Alpha-N but I'm not the manufacturer and my opinion is apparently carrying little weight. I was hoping to get some insight into the issues he is likely to be running into so I can make a sound arguement that he should really look into changing to Alpha-N


Like David said, the transition issues may be related to typical transient tuning i.e. accel enrich, but they may actually be related to the inherent inaccuracy you get at certain throttle angles if the calibration is SD based only, and the driver is simply transitioning through an area of the primary fuel table which doesn't reflect actual air mass.

Simply put, with ITBs, the throttle angle is too great a factor in engine air mass to ignore it, and a barebones SD approach doesn't account for throttle angle.

Thanks, Mike - the throttle angle comment makes it much clearer

You're welcome!

I'd go along with David's suggestion, of trying it.

Personally, I wouldn't mess around and just run it as TPS based with a fixed FP if NA.

If a sealed airbox is used, mass flow metering can be used at the entry to the airbox, with the FP either fixed or referenced to the airbox.

If forced induction, SD makes sense, with some messing around with transitions and FP regulation referenced to manifold volume.

This thread is great. I am finishing up on a buddies car this yea rand it will be ITB's, in a plenum, turbo charged. My plan is to go the route Mike described, AlphaN with MAP(SD) corrections. It is really good to hear that this is a powerful combo and I am not insane for thinking it will work, cos I am an idiot per my wife.

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