DIY_EFI Digest             Thursday, 25 July 1996       Volume 01 : Number 210

In this issue:

	Re: Throttle Body Sizing
	Re: Filtering supply lines
	RE: Throttle Body Sizing

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From: "Michael D. Porter" <mdporter@xxx.com>
Date: Wed, 24 Jul 1996 20:15:50 -0700
Subject: Re: Throttle Body Sizing

Orin Harding wrote:
> 
> While trying to decide on the proper size throttle body for my MGB EFI
> project, I found the following formula for determining carburetor air flow requirements in "Auto Math Handbook" by John Lawlor:
> 
> CFM = (RPM x displacement) / (2 x 1728)
> 
>
> What gives here guys?
>As you've guessed, this is a simplistic formula which does not take into 
account gas speed, volumetric efficiency or pressure drops. 
 
> To further this thought, shouldn't the intake valves duration somehow be included in this?
>Duration is only one aspect of what you need for the answer--since the 
valves open progressively, the rate of flow changes across the 
duration--and the absolute flow changes with manifold pressure.  If a 
carburetor choke (or a throttle body) were sized for average flow, it 
would present an undue restriction on peak flow.  Second, overlap must be 
figured into the equation, since the gas going down the pipe is doing 
useful work (if the tuning is correct) creating pressure waves, even 
though it has absolutely nothing to do with displacement.  Both of those 
factors change the CFM calculated at absolute pressure.  

As far as the best means of determining correct throttle body size, there 
are probably too many variables for the ordinary plug-in formula to 
consider, many of which would have to be empirically derived.  But the 
best means of making assumptions is to begin with the torque peak.  By 
definition, that is the point of maximum volumetric efficiency for any 
engine.  Unless your torque curve is flat up to 6000 rpm (if it is, tell 
us how to do it-naturally aspirated, of course!), the volumetric 
efficiency drops progressively at higher rpm, and you can make estimates 
from that with your revised formula.  Finally, since you're already 
limited to throttle bodies in the marketplace, the first rough step might 
be to make sensible estimates of horsepower, and then look around at 
throttle bodies on engines of comparable displacement and power 
production and pick one that fits.

Unfortunately, I've just been going through both of Philip Smith's tuning 
books looking for that handy plug-in formula, and it just isn't there.  
Much of this can be calculated _if_ you have accurate pressure data; no 
wonder access to the flow bench is so valued....   
> Hopefully one of you will have an answer to this.Don't know if it did, but at least it tells you were right not to size 
according to the simple formula. <g>
Cheers.

------------------------------

From: "Michael D. Porter" <mdporter@xxx.com>
Date: Thu, 25 Jul 1996 01:07:52 -0700
Subject: Re: Filtering supply lines

Darrell Norquay wrote:
> 
> At 02:14 PM 7/24/96 +1100, Stuart Woolford wrote:
> 
> >>  The only way to guarantee you are at a true 0v ground is to run
> >> a wire back to the battery terminal.
> >
> >and what defines the battery terminal as 0v ground?
> --battery explanations snipped--
> 
> >really - the battery terminal can be a particularly noise prone
> >ground point depending on the charging system design.. (from too much
> >car sterio install experience..)
> 
> If this is the case, then the battery ground lead should be repaired or
> replaced.  Any voltage drop from the battery to the chassis will affect ALL
> devices attached to the chassis.  In my car I have separate 2Ga ground leads
> going to both the engine and the frame directly from the battery.  This
> alleviated a lot of noise problems I was having with the stereo.
>We experience horrendous problems with electronic equipment on the 
full-size buses we produce, and have been literally forced by some OEM 
manufacturers using electronic engine/transmission controls to provide 
direct or near direct battery grounds for those controls.  It's the only 
way in a big vehicle with long paths to everywhere to minimize the 
transients and the noise.

> running directly back to the battery, then any noise or voltage drop
> generated on any given ground lead would have no effect on any other lead.
> Unfortunately, this is not the case, and auto manufacturers rely on the
> assumption that the body itself is a single homogenous conductor, which
> unfortunately it is not.
>If every weld is great, if every ground contact point is perfect, if 
every nut on every ground stud gets clamped down properly, then things 
work _reasonably_ well.  Corrosion and vibration take care of that in 
about a week. <g>  We try to minimize surface grounds wherever possible 
simply because of the difficulties in maintaining them.
>   Grounding and
> ground noise is a whole black art unto itself.
>Amen.

------------------------------

From: Paul Shackleton <p.m.shackleton@xxx.uk>
Date: Thu, 25 Jul 96 08:55:00 BST
Subject: RE: Throttle Body Sizing

>While trying to decide on the proper size throttle body for my MGB EFI

>snip<

>Here is what I think sizing should be based on.  I've used my MGB's cam 
data
>in this example:

>1) Peak RPM: 6,000
>2) Engine CI Displacement: 110
>3) Intake valve duration (Deg): 252
>4) Time for 1 crank revolution: 0.01 sec. (60 sec / RPM)
>5) Time for 1 deg. of crank rotation: 2.381E-05 (#4 / 360deg)
>6) Intake time per 4 cycle period: .006 seconds (#5 x #3)
>7) Intake time per minute: 21 seconds (#1 x #6)
>8) Engine CFM requirement @xxx.8 CFM
>    ((#1 x #2) / 2) / 1728)
>9) Estimated Throttle Body steady state flow requirement: 636 CFM
>    (60 sec./#7) x #8

>snip<

What throttle bodies are you thinking of using? Are you making your own and 
if so, how? Or are you using proprietary ones? I am just about to start 
making some for the 2.0l 16v Vauxhall engine in my Westfield, but have yet 
to figure out how. Weber, Lumenition and Microdynamics throttle bodies seem 
to be very expensive. ie more than the equivalent sized carb without any of 
the complicated carb plumbing!

Any ideas on DIY throttle bodies would be gratefully received?

Also, in calculation #7, shouldn't #1 x #6 be 36 seconds? However 21 seconds 
is the answer if you say in two engine rotations (4 strokes) the inlet is 
open 252 degrees out of 720. Multiply this up to a minute (x60) and you get 
21 seconds. I'm not sure whether the answer should be 21 or 36 though.

------------------------------

End of DIY_EFI Digest V1 #210
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