EFI guys, hotest cam/Comp ratio you can use with Speed Density?

[Mukilteo911]

I've read quite a bit of info that seems to say Speed Density EFi can't use long duration cams, if you want it to idle anyhow. I understand why and all that, but wonder how it relates to our 911's cam variations. So does anyone have experience using speed density EFi with an E or S cam? Work or not, what cam is the limit for great drivability?

Now I imagine compression ratio is not an issue for speed density. Am I correct?

[petevb]

Originally Posted by Mukilteo911

I've read quite a bit of info that seems to say Speed Density EFi can't use long duration cams, if you want it to idle anyhow. I understand why and all that, but wonder how it relates to our 911's cam variations. So does anyone have experience using speed density EFi with an E or S cam? Work or not, what cam is the limit for great drivability?

Now I imagine compression ratio is not an issue for speed density. Am I correct?

I can't answer your question directly, but what system are you using? As I understand it (no direct experience) a number of the better systems can be switched between Speed Density (using the manifold vacuum sensor) and pure N Alpha (using only engine speed and throttle angle) operation. You can then idle purely on the table and then switch to speed density once the revs climb. That seems the best of both worlds, as N alpha will tolerate any cam. Do you know if your system can switch modes or ignore the manifold vacuum sensor?

[jpnovak]

The problem will be the induction system and its tolerance of reversion. There are systems that can perform better than others. I am running a DC40-108 (mod_S cam) on my project with TWM throttle bodies. I am running Megasquirt. I use a blend of Alpha-N and Speed Density with approximately 60% SD. The issue with pure SD is that you reach a point above , for example, 4K rpm where your WOT MAP reading is at atmospheric. There is no way to gain acceleration enrichment with 100% SD. It is at this point where the TPS readings (alpha-N) will permit enrichment.

Megasquirt will allow you to set rpm ranges where you switch between SD and alpha-N. For example you can go 100% alpha-N through 1500 for idle control and off-idle tuning, then switch to SD during mid range rpm for drivability and then switch back to alpha-N for high rpm, WOT applications. It is quite versatile.

My car idles at 750 rpm. Its a little lopey but that is to be expected with these cams. I do run a large vacuum accumulator to average out the intake pulses. Without this, the car will not idle with a straight MAP signal.

I am not aware of any compression ratio limitations for any EFI system.

[Mukilteo911]

Jamie, what vacuum accumulator do you use?