Modifying the Air Intake System

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One of the most common questions we SHO people hear is "What can I do to my intake system to gain more flow?" This is a good question, as on many cars substantial HP gains can be had by adding a higher-flow intake system, or modifying the current one.

The sad truth is, the SHO comes with actually a pretty darn good intake system from the factory, as this missive I will transcribe below reveals. I am informed a man named Jim Kempf is the author, and it's waaaay back from 1999, but the wisdom (and math) hasn't changed any since then. But fear not! Some power can still be gained, if you are adventurous and/or rich (isn't that always the case?).

Subject: Interesting Air Induction System Modifications OPINIONS
From: Jim Kempf
Date: 13DEC99

Before I say anything, let me first say that even though I am a Visteon employee (we're still Ford a bit longer)[not! -ed.] what I say in the following must be taken as only MY personal opinion, as that is precisely what it is. If you find this information interesting, I would encourage you to verify my findings on your own, as any modifications you make to your car are at YOUR own risk and not mine, Ford's, Visteon's, my family's, etc.

On to some OPINIONS from testing I have recently had done. At long last, I have put on a flow bench a 1992 SHO air induction system. During the testing I have formed MANY valuable OPINIONS for myself and I think you will find them interesting too. My interest in this was from a posting from a person on this list a few weeks [make that decades -ed.] back. If I recall right, they pulled their saxocone from their induction system and swore torque dropped (a pretty sensitive butt dyno, I might add). I found it interesting in the 10 years have been around that nobody (published, anyway) has flowed the air induction system to determine what is going on, so that is precisely what I did.

One more thought before I start onto the opinionated testing results. Many think that torque and power are trade-offs in air induction system design. That is true in Some cases, where system lengths, diameters, and volumes are assisting/hurting the intake manifold tuning at certain RPMs more than a base design. However, air induction system restriction will always affect volumetric efficiency at all RPMs, so it is of particular interest.

As a rule of thumb, 3.3" H2O restriction on the flow stand is approximately equivalent to 1% power loss. Secondly, I've assumed that the SHO engine pulls ~300SCFM in lightly modified form @ 7000 RPM. If you've got more than just a cat-back and an 80mm MAF, flow rates will be a little bit higher.

On with the SHO air induction; these values are taken at 300-303SCFM:

Induction System Set-up Inches of Water Pulled
Base '92 system (as sold in '92) w/ new paper filter 10.0"
Base '92 system w/ K&N panel filter (new) 9.4"
Base '92 system w/ paper filter & MAF screen removed 8.7"
Base '92 system w/ paper filter, MAF screen & cone removed 10.2"
Base '92 system w/ paper filter, MAF screen & fender box removed 7.4"
Custom system 1, w/ Cobra K&N filter, Stock MAF w/o screen, and stock clean air tube w/ bellmouth for MAF 5.7"
Same as above, but no K&N or bellmouth on MAF 5.4"
Same as above, but no MAF, only air tube 2.5"
Custom system 2, w/ Cobra K&N filter, Cobra filter adapter plate, & Mark VII 80MM MAF 3.4"
Same as above, but with a bridgeless MAF from '99 Excursion 2.6"
Same as above, but with Ford paper conical filter 3.1"
Custom system 3, w/ 87mm Pro-flow [now Pro-M -ed.] MAF, K&N conical filter, and Cobra filter adapter plate 2.6"

Opinions I formed from this testing

(in order of importance to ME:)

1. The stock system is a 3% loss system, losing only 6-7hp compared to a bare throttle body. That is the limitation of the gain associated with doing anything to the induction system. Proclamations of gains up to this number and above should be regarded with the same caution of a fruitcake this Christmas [some things never change -ed.]. As another opinion, this is a pretty darn good base system.

2. Geez... the kid was right... pulling the saxocone will drop power! To understand the significance, you should compare iterations 3 and 4. Pulling the cone will drop power by up to 1hp and may have other effects on torque as previously noted. It makes more noise, and thus maybe sounds better, but it does drop power as well as torque. Many of you out there have made only this modification; not all is lost. Spend another 30 minutes and pull the whole box (expansion chamber) out of the fender. You'll pick up an immediate 2hp if you've already got the cone pulled, and you'll gain 1hp otherwise.

3. K&N... hmm... I've seen mixed results with K&N filters and this is a good case. The flow path through the stock box doesn't make full use of the filter; the MAF sits over the media, so the K&N filter in a stock box really doesn't help much... I figure maybe .5hp max.

4. MAF screen. OK, I've got to say it again. Pulling this out will affect your transfer function, and it will have the potential of screwing up a lot of stuff. I wouldn't recommend for anyone to make this modification, even though it is more beneficial than a K&N filter in a stock box. Enough said.

5. Custom system (row 6): For a lowly $50, I put this system together and ran it on my car. Buy a Cobra K&N. Shove your current MAF-to-airbox rubber seal into it. Use a good helping of "Seal All" or other sealant to make this an airtight seal. Connect it to the MAF, drill a hole in the air tube, and shove an ACT sensor in it (seal that, too). Buy a bit of foam/rubber for the filter to rest on (to support/isolate it from banging on the inner fender). The flow results show this should pick up almost 3hp. Even this minor modification might screw up the transfer function of the MAF, but I have had no problems. Again, do this at your OWN risk.

6. If you go with a custom setup like #10 (third from bottom), you should be able to pick up nearly 5hp on a stock SHO. How would I do this? Get a new MAF from one of the '99-vintage trucks, send it to Pro-Flow to have it recalibrated (they quoted me $75 + shipping), and get a new filter. Again, I've not done this but I may if I can keep my SHO around long enough. Their address is [irrelevant, since they no longer exist as Pro-Flow, and are now Pro-M, operating out of who knows where. I probably wouldn't bother looking up that calibration service, either. -ed]. The benefit of this modification is, no LPM necessary. [just buy a TwEECer and be done with it. -2010]

The rest of the note is contact information for the original author, which - as it's 11 years old - is likely useless, and he probably doesn't have a SHO any more anyway. Some advances in kits (CAIs, etc.) have been made since then, but the lessons remain the same: K&Ns are useless and serve only to let more dirt and dust into your engines (I have oil reports that prove this), and pulling only the saxocone is also pointless; just pull the whole outer fender box and be done with it.

Hope this helps.

--hawkeye18 20:38, 11 December 2010 (MST)