a MAF too small

[Dave Zug]

> Date:          Wed, 21 Jul 1999 16:33:32 -0400
> From:          Frederic Breitwieser <frederic at xephic.dynip.com>
> Organization:  Xephic Technology
> To:            gmecm at efi332.eng.ohio-state.edu
> Subject:       Re: a MAF too small
> Reply-to:      gmecm at efi332.eng.ohio-state.edu

> | I have an 89 5.7l Iroc, 165 ECM, ARAP (vette) binary. I have put a
> | supercharger on it and the computer has no idea what airflow is
> | coming in after about 3000 rpm cause the MAF reads 255 right about
> | there.
> 
> When you double the airflow into a MAF based system, the MAF will of
> course max out at about 1/2 of redline, so what you are experiencing is
> a reasonable "irritant" considering what you have described.  There are
> many ways to fix this, one is to recode the eeprom in the ECM, one is to
> play with fuel pressure, change mafs, and do other funky stuff, however
> there is an easier solution.  Someone either here or on another list
> mentioned this to me, I ignored and discounted it, but after testing it,
> success was achieved.
> 
> Create a bypass for the MAF.  Simply run a tube at a 30 degree angle off
> the pre-maf tube, connecting to the post-maf tube, also at a 30 degree
> tube.  By increasing the diameter of this tube, less air will go into
> the MAF, thus produce a lower reading.  I did this successfully by
> doubling the airflow, doubling the fuel injector size, and it worked
> well, except at idle.  Idle would rock between rich/lean/rich/lean and
> become annoying.  The solution to that was to have a spring loaded lever
> on a valve that reduced the airflow in the bypass tube during idle, and
> open the second you tap the gas.
> 
> Took a lot of tuning as far as hose diameter, playing iwth the valve,
> and the car in question still idles a little strange, however when the
> supercharger kicks in, its a blast to drive.  Also, there were no
> modifications to the ECM or the wiring harness, which makes it easier
> for people who don't have the ability to rewire or change code.
> 
> Hope that helps.  Before you change things around, you have to do some
> math... I just said "double the injector size" and that might be way
> off, it was merely to illustrate the concept more than anything :)
> 
> 
> -- 
> 
> Frederic Breitwieser
> Xephic Technology
> 769 Sylvan Ave #9
> Bridgeport CT 06606
> 
> Tele: (203) 372-2707
>  Fax: (603) 372-1147
> Web: http://xephic.dynip.com/
> 

Frederick this is EXACTLY what I came up with as the theory. I had 
not considered the injector part though.. hmmm. I have been kicking 
this idea around for a good bit, I may have mentioned it on EFI list 
a while ago I'm not sure.  As for doubling.. It makes sense that if 
you can bypass only what air you need to stop at a 253 reading.. 
thats all you should do cause that will help the idle the most.. keep 
the velocity UP thru the MAF. The "side tube" was next after proving 
the concept with holes. If the holes dont have the desired effect.. a 
piece of hose and a clamp to seal it and only a minor battle scar to 
show for the effort  ;-) THanks 
Dave Z. www.delanet.com/~tgp