Forum » General Tuning Discussion » IAT/Charge Temp & MAP/Boost: compensation tables...

IAT/Charge Temp & MAP/Boost: compensation tables...

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The attached density correction table's numbers are default settings in the SR20VE n/a basemap that came with the Nismotronic software that I started with for my Turbocharged SR20VE setup planning to run up to ~35 psi. The table is based on MAP load and RPM and contains factors that are the percentage of IAT & If I choose to disable this feature, IATs alone will be used in place of the density corrected IATs. I remember Andre mentioning this topic in a webinar and of discussion on the old forum, but I didn't find too much on it if anyone cares to link me...

I based the IAT trims table on 2.5% per 10* C (sensor located 12" before the TB), but I'm most curious about the IAT Density Correction numbers that from my understanding allow you to give air density interpretation bias toward ECTs allowing more accurate air density calculations of the air actually entering the combustion chamber based on engine speed. So is it wise to enable these corrections before I start dyno tuning or could they skew things while I'm trying to build accurate base maps? Do these numbers seem relatively close to effective in your experience and is it wise to still use these iat trim numbers with the density enabled if i use it? I could always start with the iat trims alone and get an accurate map first then try the iat density corrections later...

Attached Files

Using a charge temp approximation table (which is essentially what you have here) can give you a more accurate approximation of the charge temp and more stable AFR when tuned correctly. Tuning the table however is difficult as there isn't a very clear method of deciding how to approach the tuning. As I've just mentioned in another thread, this is on my agenda to develop a technique that can be applied to this table.

If you choose to use the charge approximation technique then you need it enabled during the tuning process as it will effect the table values otherwise. A more conventional IAT-only correction still works very well in most cases provided the IAT sensor is pre throttle (as you have mounted it). This will simplify your tuning and will probably still give you good control over the AFR. I will say though that if I'm using a simple IAT only correction table I like to have this set up as a 3D table with a load axis to allow me to reduce the power of the correction at low load and low airflow. This reduces the effect of heat soak at idle altering the AFR too much.

what are your thoughts on "boost compensation" (used in nismotronic) that applies a set factor dependent on MAP increments to the IPW on this millisecond based software with the option of 'raw' figures (1-255) which I believe megasquirt has as well. If I'm understanding this correctly, it would allow me to scale back the numbers in the fuel table (based on the chosen "SD factor") from the 1-100 range mimicking a VE based table allowing me to think of the fuel cells in terms of a 1-100 percentage while the boost comp numbers adds fuel dependent on the boost levels...if I can even manage to get it into a usable range that I can quickly quantify. It just seems like it could make things a bit vague for me in trying to build a very accurate fuel map, but I can certainly see it being helpful as a safe guard in high boost levels...

Many millisecond-based ECUs do basically the same thing in the background - Double MAP = double fuel.

It has the effect of flattening the fuel table and offering improved resolution. As you say this also has the effect of making the values in the fuel table feel a little like a VE based fuel model albeit unscaled.

I'm unfamiliar with the nismotronic ECU and I'm unsure how the raw figure works out in terms of scaling. My gut says this could be a little messy if you don't have a solid understanding of what the background correction is. Of course you can tune around it but the idea of the MAP comp is to make our lives easier so that the fuel table only needs to account for changes in VE rather than air density.

that puts it into perspective better for there a general rule of thumb of incremental changes upward/downward in the MAP values like the IATs?

Yes, the general rule is to double the injection pulsewidth when the manifold pressure is doubled. All things being equal this would mean that if you have the same number in your fuel table and you doubled the manifold pressure from let's say 100 kPa to 200 kPa, the measured AFR would remain constant - This does assume that VE remains constant over this manifold pressure change too.