Fuel injection - ITBs or Plenum,
r1_pete - 7/9/15 at 08:22 PM
I've been planning and working on EFI for the jag for a couple of years now, initial plan was ITBs using a modded XJ6 inlet with Triumph Throttle
bodies:
EFI Trial Fit 7
ITB Progress 1
Then I did a bit more research and consensus on the web seems to be a Plenum setup is more suitable for road use, so I set about a plenum design;
Plenum finished
Time has come to tear things down and fit one of them, but I just can't decide which way to go, there is so much conflicting info.... ITBs can be
adjusted for air flow per cylinder, but take more initial setup, better top end but reduced mid range, then there's the argument longer trumpets
/ runners restore some mid range. Plenum is easier to set up, doesn't need balancing, supposedly better drivability...
Question really is, Is there that much difference?
I have both ready to go, the ITBs will fit better, with the plenum there is a lot of 'stuff' crammed in round the throttle body, TPS,
Linkage, HT Leads, and its at the point where the engine frame narrows, but plenty of sideways space for the itbs....
Appreciate any views on which way to go......
Cheers.
Pete.
mark chandler - 7/9/15 at 08:49 PM
ITB's, looks nicer, sounds better and goes faster.
As long as you make the linkages well there is no reason for them to lose balance, BMW have been fitting ITB's to straight sixes for years now!
coyoteboy - 7/9/15 at 09:00 PM
ITB's tend to be more rapid response and CAN make slightly more power but won't be much, if anything, over a well tuned intake manifold, and
it can't make use of variable runner lengths that some OEM engines have. That said, I'm fitting it to my V8 because it sounds awesome and
looks good (and I'll not miss some torque at the bottom end in a light car with a 300lbft V8 onboard).
Chris_Xtreme - 7/9/15 at 09:45 PM
I went with ITBs on my rover v8, well just for the heck of it. I got about 10 more hp than over the holley carb that was on it, tho the sound is far
more impressive (tho dampened alot by the air filter 
) but the mpg is massively improved, that is no doubt really just switching to efi. this is
great for range ( the cost of the switch to efi, well probably wipes out any fuel saving, but it was a project for me)
one thing to consider - which someone said to me, which is if you want to super charge, you will want a plenum not ITBs. -- you will want to verify
this thought!
jeffw - 8/9/15 at 07:59 AM
No reason why you can't use ITBs with forced induction....just as I have.
Ivan - 8/9/15 at 09:12 AM
The biggest advantage of ITB's is that it is easier to get even flow to each cylinder if you design the air filter box properly and if your
injectors are situated suitably you will get better fuel vaporisation at idle or high vacuum cruising because of the increased speed and turbulence
close to the throttle plate. The downside is cost, complexity and need for slightly bigger manifold runners to allow for the air resistance and
turbulence caused by the throttles.
I personally believe that the average driver will not notice any difference in responsiveness between the two systems.
The advantages of a plenum system is less complex and cheaper, throttle pedal feel should be lighter and easier to balance if you have more than one
throttle on a V engine. The down side is - not as cool to talk about in the pub, more difficult to get an even balance of air flow to each cylinder
and you might get worse fuel evaporation at low throttle settings and just maybe slightly slower response.
[Edited on 8/9/15 by Ivan]
MikeRJ - 8/9/15 at 10:57 AM
quote:
Originally posted by Ivan
The advantages of a plenum system is less complex and cheaper, throttle pedal feel should be lighter and easier to balance if you have more than one
throttle on a V engine. The down side is - not as cool to talk about in the pub, more difficult to get an even balance of air flow to each cylinder
and you might get worse fuel evaporation at low throttle settings and just maybe slightly slower response.
IMO the biggest advantage of a plenum based system is being able to use manifold pressure as a reliable indicator of engine load, which means easier
calibration and better driveability. Honda use a plenum on their VTEC engines which make very good power. The K20A makes 215bhp in standard form,
with 240-250bhp easily achievable through bolt on parts whilst retaining the plenum.
A single MAP sensor does not work very well with ITBs as the tiny volume between the valve and the throttle means there are massive fluctuations in
the pressure. You can make the best of a bad job by mixing these signals through various filters, but you still get very limited vacuum range.
Triumph now use one MAP sensor per cylinder and the ECU samples each one at the most appropriate point during the inlet stroke which overcomes this
problem.
old_timbo - 8/9/15 at 12:19 PM
One thing to watch with the plenum is the cam shafts being used. If you have cams with a high overlap, with a single plenum the engine will run poorly
(difficult to get a reliable idle) below about 1800 rpm. High lift with modest overlap works well. When I converted my Pinto from carb to injection I
had to change from a Piper 285 (ultimate road) to a 134 (fast road injection). Made a huge difference.
MikeRJ - 8/9/15 at 03:27 PM
quote:
Originally posted by old_timbo
One thing to watch with the plenum is the cam shafts being used. If you have cams with a high overlap, with a single plenum the engine will run poorly
(difficult to get a reliable idle) below about 1800 rpm. High lift with modest overlap works well. When I converted my Pinto from carb to injection I
had to change from a Piper 285 (ultimate road) to a 134 (fast road injection). Made a huge difference.
Which injection system did you use? The original Pinto injection system uses a flap type airflow meter which are extremely sensitive to reversion in
the inlet, and work badly with long duration cams.
old_timbo - 10/9/15 at 10:51 PM
I'm using Megasquirt and have junked everything Sierra other than the manifold, throttle body, TPS and fuel rail. I tried all sorts to get a good
idle i.e. speed/density, Alpha N, different manifold pressure take-off points, manifold pressure dampers. The cam change was the only thing that made
a difference. I'd read elsewhere that larger overlap than std performance cams were a problem with single plenums, but was dubious and thought I
would be able to tune it out. I couldn't.
