Terry,
Typically the pressure drop from the input side of the carb to the output
(engine side) will have a pressure differential of between 1.5 - 2.5 "Hg @
Wide Open Throttle (WOT).. The pressure differential largely depends on the
engine speed, bore/stroke ratio and camshaft profile.
Cheers
Feral Errol
----------
From: Terry <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Subject: RE: Airflow calculations
Date: Thursday, September 14, 2000 10:01
Craig & Errol,
Thanks for the info guys. If I have this correct now, the pressure
differential at wot on a hypo engine could be more than 1.5" hg? and
therefore the carbie gets more air/fuel through the same hole. This is
where
a camshaft to match the engine characteristics and fuel delivery can have
quite an impact, high lift vs duration and compression pressure. A lot of
engines in the mid eighties were very different in these areas and are very
difficult to tune out detonation with lower RON fuel (re post on the L24
recently). I guess if you're head is not flowing at an optimal rate then
you
could play with venturi sizes to get the best out of it but the camshaft
specs seem to have a huge effect on these engines.
I know from my Ford days that you can have basically identical engines,
same
600 or 780 carbies etc but the 351 with the 4V head and GTHO cam absolutely
craps on a 2V with the GT cam. Same thing with a Datto engine and this is
why a certain jetting & choke setup will not or not seem to work the same
on
2 similar spec engines.
What started me thinking about this a few days ago other than having a slow
day at work was why do the US boys love Hitachis so much, other than they
do
provide a very driveable engine that is easier to get good fuel consumption
out off too, but to get the performance level that even a single DCOE is
capable of that is anywhere near tuned right will kill the hitachi fed
engine.
terry
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Errol Smith
Sent: Thursday, 14 September 2000 6:30 AM
To: [EMAIL PROTECTED]
Subject: Re: Airflow calculations
Craig,
The cfm figures for carbs are calculated @ around 1.5" Hg as this is the
approximate pressure differential across the carb @ Wide Open Throttle?. As
you pointed out if this constant is used, it makes a convenient basis for
comparison of flow rates
Cheers
----------
From: Craig Overend <[EMAIL PROTECTED]>
To: [EMAIL PROTECTED]
Subject: Re: Airflow calculations
Date: Thursday, September 14, 2000 2:01
Terry,
My understanding is that for any fluid(air) to move from one point to
another there must be a force acting on it. This can be a pressure
difference or gravity for example.
To calculate volumetric flow rate you need to measure the pressure
difference across a known calibrated orifice.
To compare airflow CFM readings you need to know at what pressure
difference(airspeed) it was measured at. For example 100CFM@10"
Water(.3613963 PSI) is NOT the same as 100CFM@20" Water. If you were to
convert the 20"water to
10"water so you could then directly compare them and you'd get lower
than 100CFM after conversion.
Holley use a strange test pressure something like 1.5" of mercury, I
cant remember exactly.
So unless the pressure difference is known CFM ratings alone are
useless.
Craig.
> Terry & Heather wrote:
>
> Kevin,
>
> very interesting post.
>
> I have just one question - carbies are rated at a certain cfm eg
> Holley 780 cfm (from my Ford 351 days)
>
> How do you calculate the cfm rating of a carbie, sort of like a pipe I
> guess which is along the lines of the original question.
>
> What's the cfm rating of a DCOE45?
>
> Terry
>
> -----Original Message-----
> From: [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED]]On Behalf Of
> Kevin
> Sent: Thursday, 7 September 2000 10:33 AM
> To: [EMAIL PROTECTED]
> Subject: Airflow calculations
>
> This is the formula we use at work to work out air flow
> needs
>
> Air flow (CFM) = Displacement (cubic inch) X RPM X VE
> 1728 CF
>
> CYCLE FACTOR (CF)
> CF: 4 cycle diesel/petrol engine = 2
> CF: 2 " " " " = 1
>
> VOLUMETRIC EFFICIENCY (VE)
> Naturally aspirated diesel/petrol engine = 0.85
> Supercharged engine = 1.2
> Turbo engine (we use this on for diesels) = 1.4
>
> Eg Airflow = 109 x 6000 X 0.85
> 1728 2
>
> = 555900
> 3456
>
> = 160.85 cfm (this suits a 1800cc engine)
>
> Litres to CI
>
> 1 LT = 60.5 CI eg 1.8 lt X 60.5 = 108.9 CI
>
> >>>>
>
> <?fontfamily><?param
> Tahoma><?smaller>-----Original
> Message-----
> From:
> [EMAIL PROTECTED]
> [mailto:[EMAIL PROTECTED]]On
> Behalf Of brad
> Sent: Wednesday, 6 September 2000 12:57
> PM
> To: [EMAIL PROTECTED]
> Subject: airflow calculations
>
> <?/smaller><?/fontfamily><?fontfamily><?param
> Arial><?smaller>does any 1 know the
> formula for calculating the cfm of a
> pipe
> <?/smaller><?/fontfamily>
> <?fontfamily><?param
> Arial><?smaller>thanx in advance
> <?/smaller><?/fontfamily><?fontfamily><?param
> Arial><?smaller>Brad
> <?/smaller><?/fontfamily>
>
> ---------------------------------------------------
> Kevin Harlan
> WILDKAT SPARES
> 898 STUART HIGHWAY
> BERRIMAH NT 0828
> PO BOX 325
> BERRIMAH NT 0828
> Ph: 08 8932 5544
> Fax: 08 8932 5788
>
> ("`-''-/").___..--''"`-.____
> `-.(@ @ ).`-.__.`)``-..-'-"""
> (_Y_.)' ._ ) `._ `.
> _..`--'_..-_/ /--'_.' ,'
> (il).-'' (li).' ((!.-'
>
> EMAIL: [EMAIL PROTECTED]
> WEB: www.wildkatspares.com
>
>
--membersozdat-------------------------------------------------------
> OZDAT Mailing List Please Note:-
> Send (un)subscribe requests to [EMAIL PROTECTED]
> Send submissions to [EMAIL PROTECTED]
> No unauthorised redistribution of this email
> http://www.ozdat.com/ozdatonline/index.htm
> http://www.ozdat.com/ozdatonline/listindex.html
> http://www.mail-archive.com/[email protected]/
>
---------------------------------------------------------------------
>
> --membersozdat-------------------------------------------------------
> OZDAT Mailing List Please Note:-
> Send (un)subscribe requests to [EMAIL PROTECTED]
> Send submissions to [EMAIL PROTECTED]
> No unauthorised redistribution of this email
> http://www.ozdat.com/ozdatonline/index.htm
> http://www.ozdat.com/ozdatonline/listindex.html
> http://www.mail-archive.com/[email protected]/
> ---------------------------------------------------------------------
--membersozdat-------------------------------------------------------
OZDAT Mailing List Please Note:-
Send (un)subscribe requests to [EMAIL PROTECTED]
Send submissions to [EMAIL PROTECTED]
No unauthorised redistribution of this email
http://www.ozdat.com/ozdatonline/index.htm
http://www.ozdat.com/ozdatonline/listindex.html
http://www.mail-archive.com/[email protected]/
---------------------------------------------------------------------
