View Single Post
jluetjen jluetjen is offline
Registered User
jluetjen's Avatar
Join Date: Oct 2001
Location: Westford, MA USA
Posts: 8,595
Can anyone tell me about what is available for EFI
There have been numerous threads on this and there are a lot of people out there who have more creative solutions to this problem then I do.

and individual throttle bodies for this engine and what the advantages/increases might be?
1) ITB's will allow the use of cams with overlap.
2) If chosen correctly, they will also prove to be less of a restriction to the intake air then the CIS air-meter. This alone should result in some additional peak HP.
3) ITB's also have less volume downstream of the throttles. What this means is that there is less distance (read time) between when you increase the throttle position and when the additional mixture reaches the cylinders. Just a standard carb set-up may have 2x the capacity of an individual cylinder downstream of the throttles, so it will take roughly 2 revolutions before the mixture gets to the cylinder. The total volume of the intakes and plenum in a shared plenum system has a larger volume then this, so I would expect the throttle reaction to take longer. Either way you're talking fractions of a second, but ITB's should have a faster throttle reaction.
4) ITB's presume the use of a mapped injection system. When you tip the throttle in with a CIS system or something like an L-Jetronic system, I believe the engine may go lean briefly which causes a momentary (fraction of a second) delay in the throttle response. I believe that this is for two reason
a) The inertia of the measuring plate.
b) The reaction time between when the throttle allows more air and when that signal moves the measuring plate resulting in the additional fuel being injected into the cylinders.

With a mapped ITB set-up (either electronic or MFI), as soon as you move the throttle, the system moves to the new point on the map and starts pumping the additional fuel. Virtually instant throttle response!

What is going to happen to my torque if I change cams and open the ports.
It depends. Some thoughts...
1) I assume that your engine has 49 mm intake valves. With a 3.4 liter capacity you're pretty well maxed out through the valves at 6300 RPM. The situation is somewhat similar to a NA 944 (2V). You can increase the capacity but chances are you won't make much more HP. You can go more radical with the cam but the power band will be getting narrower and only move up the rev range a little. And if you don't increase the rev's (keeping the capacity the same) you won't increase the HP much. Going to a larger capacity will just reduce the peak HP engine speed and result in roughly the same HP, but at a slower engine speed. So to really make it work you'll need to increase the valve side and do some port work.

2) If you go to a cam with more overlap, you can get a couple of benefits:
a) You can fit more duration in without having the intake closing be so late that it compromises the dynamic CR.
b) The increased overlap (with a properly tuned intake and exhaust system) will result in an engine with a stronger "on cam" affect at higher rev's. So you may lose a little at lower rev's (not a big deal with your big engine), but you'll gain significantly more above 3500 RPM.
You don't need to go overboard, but just going to overlap comparable to an E or Solex cam will make a noticeable difference and still provide fine streetability. Keep in mind that you'll most likely be well beyond passing any sort of emissions test.

3) Alternatively, going to twin-plugs and increasing the CR will help out across the board. Keeping the same duration cams, but narrowing the lobe angle will increase your dynamic CR and increase the "on-cam" affect per point 2b above. Curiously, I would expect that increasing the dynamic CR will increase your peak HP engine speed slightly and as a result increase your peak HP. The reason is that the extra CR can "prop-up" the cylinder pressures even though the intakes and/or valves are restricting the intake charge. To really make this work will require a mapped ignition system and some dyno time.

Finally, a moment of lateral thinking...
"If you can't raise the bridge, why not lower the river???"

If you're maxed out with the cheap HP from the engine, it's time to go back to putting the car on a diet. Losing car weight will make the throttle response feel faster AND make the car act like it has more HP. A lot can be done on this while spending relatively little money.
'69 911E

"It's a poor craftsman who blames their tools" -- Unknown
"Any suspension -- no matter how poorly designed -- can be made to work reasonably well if you just stop it from moving." -- Colin Chapman

Last edited by jluetjen; 11-03-2004 at 07:13 AM..
Old 11-03-2004, 07:06 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #7 (permalink)