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ITB Accel Enrichment

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I am tuning a big block Ford FE with a Borla EightStack system (ITB) The ECU is by Performance Electronics. All is going pretty good and I am at the fine tuning point.

I have a little bog in the acceleration compensation and curious if this is just an inherent issue to the ITB design - you get a LOT of air in very short order. I've attached the enrichment table with my current settings. No matter if I increase or decrease the enrichment, it still seems to work the same. Sitting at idle and q quick blip on the throttle and the engine bogs then takes off.

Am I missing something? Is this just a characteristic of the ITB's? Any suggestions on how to tune it out? Ideas on what to look for?



Attached Files

Acc'n compensation is normally required when a plenum, or other issue, slows the response of the fuelling to sudden throttle applications.

As a general rule of thumb, ITBs are much less affected by this qand I'd suggest disabling that in your map, to see how that works. I suspect the ignition timing may be contributing to the issue.

What is it like if you are diving at low rpm, in gear, and suddenly apply full throttle? Or if the throttle is applied more gradually?

If you have someone blip the throttle for you, and you look at the exhausts from outside, does it look clean or is there a puff of dark smoke?

Looks like injectors are further from the ports than a factory style manifold which may need a fair bit more fuel for wall wetting. You may need to see if the enrichment is or can be asynchronous rather than in cycle as the ITBS will allow charge in potentially after the ecu has calculated fuelling on the lower throttle angle/manifold pressure depending on what you are using for load and how everything is calculated in the ecu.

Another potential route, are you using ignition based idle control? I have had an engine that did something similar off idle and it turned out to be a TPS calibration issue where it wasn't coming out of idle state right away and the ignition control was compensating by pulling the max timing it was allowed to.

Oh, forgot - depending on your specific installation, there may not be enough vacuum off idle to accelerate the air columns initially, also giving a slight delay.

It's going to depend on your exact application, and build, but a small increase in idle rpm may help?

As you suggest, though, it'll probably turn out to be something that can be reduced but, ultimately, lived with.

Slightly off topic, a few days back I came across an all alloy FE build with over 500 CID and well over 600hp - if interested I'll see if I can find it again with magazine name and issue details?


If you happen to find the link, I'd be interested in reading it. My build is a Shelby aluminum block, 468" (4.25 x 4.125). The engine hasn't been on a dyno however my builder, an experienced FE guy says it would do over 500hp easily.


Ah, Hotrod Magazine, April 2021 edition - hard copy, don't know if they have it on-line.

It also uses the Shelby block - bore 4.370 and stroke 4.250 to give 511 CID. Survivor Motorsports Felony cylinder heads, etc, for 627hp on your 91 octane.

Cool, those look a lot like Jenvey Heritage bodies?They triggered a thought - while you will have balanced the bodies at idle, using the air bleeds, did you check the linkage was operating exactly equally on initial cracking of the throttle?

I'm a big advocate for running filters on the street, but those have been shown to cost between 50 and 100hp, depending on the engine - they're VERY restrictive!

The bodies are Borla EightStack, a copy of the old Weber 48IDA.

yes, balancing is a trick. I have them set to balance at idle using the air bleeds combined with some cylinder compensation below 3% TPS. The cross rods then maintain balance above the 3% mark matching my air flow meter readouts and Lambda from side to side. I've got the flow to match and side to side lambda to within a couple percentage points and the engine seems happy. Kind of tricky to get it to this point. Many things to adjust and they all inter-act with one another.

Since it is a street car primarily, I choose the filters over the silica issues you get if you run without them. Wish there was an attractive filter that would fit the weber look.



There should be plenty of filtration options, but finding something that meets your needs cosmetically, and that still clears the hood, may be a problem. You may get some food for thought here - https://www.demon-tweeks.com/nz/motorsport/engine/air-filters/ - but there are other suppliers, too.

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