Interesting SDS EFI results, Stock 1.8 vs. a 2.0 with a Scat C-25 cam Options

[Mark Henry]

I sold the 2.0 junkyard dog (a 2.0 I made from scrap parts) it was stock except for the C-25 Scat cam and installed my bone stock 1.8
Here’s what’s happening.

I haven’t even adjusted my values and I still have an OK reading on my narrow-band (Haltec AF 30) meter. I have a wide-band on the way from SDS, so I’ll have a direct comparison in about 1-1/2 weeks.

My duty cycles are down about 10% across the board, I get no more than 40% (WOT) duty cycle. At highway speed I’m no more than 20% DC. I’m still using the 2.0 injectors…I think I’ll swap them out for 1.7’s and see what happens.

The main interesting point is the 2.0 always lurched a bit at 1500 to 2500rpm, it was a bit of a PITA in stop and go traffic, never at normal rpms. My manifold pressure was always all over the place. I have since installed this engine in a bug with 44mm Webers and an A-1 header and it now has none of the lurching.

The stock 1.8 has no lurching at all and the manifold pressure is now rock solid! Power is down from the 2.0, but that was expected. It is just as smooth as the old L-jet except for start up, as I have no cold enrichment.

Cam choice is very important!

When I get the 2.6 done (looks like late summer now) it will have a special MassIVe cam so we’ll see how it compares in the drivability department.

Jake and I will be comparing notes to figure out the best combo’s for his and my customers.

[lapuwali]

QUOTE (914efi @ May 17 2005, 09:49 AM)

This is the main problem with the SDS system. It can't tune for cams with high idle MP. What is your MP at idle? If it is high (-5" to -10") and you tune that MP spot to idle well, it won't run well when it hits that MP value at a different engine condition. I notice exactly what you describe, under very low load you get a bucking(too rich). I have tuned it out pretty well by making compromises, but it will never be perfect without a 2D matrix like MS has. I have mine on a 911 2.2E. It will work better with stock cams that have a low idle MP and that never hit that value again during driving conditions, except maybe when you are off throttle. You can get the idle TP set so that it will idle without looking at MP, but as soon as you move off that TP value, it will jump to whatever the MP is telling it to do and you will have another jumpy type of situation. I think this is why you see a lot of SDS systems on cars with turbos, they have a more predictable MP vs fuel curve.

This really isn't a problem with SDS specifically, but with any speed-density system. Megasquirt, Haltech, even D-Jet all fall into the same boat. Most aftermarket systems are speed-density, in fact. If you're running a wild cam, you really have to use alpha-N (TPS instead of MAP as the load sensor), which SDS will do just fine (as will most other aftermarket systems).

With some systems (Link will do this, as will Electromotive), you can use both manifold pressure and TPS as load sensors, using TPS at low engine speeds where MAP tends to fluctuate unpredictably; and using MAP at higher engine speeds, where MAP tends to be fairly stable even with really hot cams. As far as I know, SDS doesn't do this, nor does MS (though there's some work in progress to change that).

In practice, alpha-N alone works OK on the street, if the MAP sensor is used for barometric correction. Bosch MFI is basically an alpha-N system with baro correction.