Toymods Car Club
www.toymods.org.au
F.A.Q. F.A.Q.    Register Register    Login Login    Home Home
Members Members    Search Search
Toymods » Tech & Conversions » carb cfm to engine matching?

Show: Today's Posts  :: Show Polls 
Email to friend 
Return to the default flat view Create a new topic Submit Reply
AuthorTopic
Super Jamie
Regular


On Probation

Location:
North East NSW
Registered:
December 2002
 
Re: carb cfm to engine matching? Sun, 17 April 2005 11:12 Go to previous messageGo to previous message
basically, no engine will never reach 100% volumetric efficiency. the manifold and head and throttle butterflies and combustion and exhaust etc etc all take away from the full potential of the amount of air the engine can draw in. on a fairly normal street motor, you're likely to end up with max 85%VE (at your torque peak rpm) perhaps falling as low as 50%VE at idle. on a well modified motor, you can achieve 95%VE and higher. so multiply your 190cfm by 0.85, thats really about how much air your engine needs

yes, the formula is correct and real. it's mathematics

i think you're misunderstanding the formula, it calculates airflow requirements based on the efficiency of the engine, how you then deliver that air is a whole different kettle of fish. you can never OVER-AIR an engine, but you definately can SLOW THE AIR DOWN too much, by installing a carb with too-big throats. you can also restrict the air too much, by making the throats too small to flow the engine's max potential

if you put a big 750 double pumper on your 13T, it would be pathetic until you reached enough rpm to get the airflow up (likely 4000rpm or more), then it would really launch! if you put a nicely sized stock carb on there, sure it doesn't flow as much up top as the 750, but the air flows alot faster so your car idles and has response at 1500rpm and so on

the idea is to find a carb that gives good flow up high, without slowing down the air too much at low rpm, and hence reducing engine response and driveability. you may also want something that doesn't use alot of fuel if that's a concern to you

however, it goes without saying that a downdraft carb is likely to be less efficient at its air delivery than a sidedraft, as the air has to turn a whole 90 degree bend. so a 200cfm sidedraft is going to be more beneficial than a 200cfm downdraft

by the way, a dcoe sidedraft flows 180cfm PER THROAT, so one dcoe40 flows 360cfm. two flow 720cfm, etc

there are lots of different carb types to choose from. a downdraft is easy because it's what the engine came with, so you usually don't need to change much (manifolds, hoses, etc) and they are cheap and plentiful. a race downdraft like an IDA or IDF would go well, but requires a different manifold and will chew alot of fuel. a sidedraft DCOE or DHLA gives ideal air delivery, however they're expensive to aquire and tune properly

all these carbs can have the throats modifed (some in ways easier than others) to provide better airspeed at low rpm. fuel delivery is tuned by use of different size fuel jets, emulsion tubes (which mix air and fuel at a different rate as the vacuum changes) and sometimes air jets. some carbs atomise fuel better than others, so you'll get a better burn (and hence more power) for the same amount of air and fuel out of a better carb (say compare the mixture quality of a good DCOE to a crappy stock carb)

an SU is a variable-choke carb, so it always provides ideal airspeed, then you just tune fuel delivery by a needle and seat. often expensive to aquire, time consuming to setup but very simple when working
  Send a private message to this user    

SubjectPosterDate
Read Message   carb cfm to engine matching? MoonbreakerSat, 02 April 2005 05:11
Read Message   Re: carb cfm to engine matching? MoonbreakerMon, 04 April 2005 07:10
Read Message   Re: carb cfm to engine matching? MoonbreakerSat, 16 April 2005 12:27
Read Message   Re: carb cfm to engine matching? Super JamieSun, 17 April 2005 11:01
Read Message   Re: carb cfm to engine matching?  Super JamieSun, 17 April 2005 11:12
Read Message   Re: carb cfm to engine matching? MoonbreakerSun, 17 April 2005 13:41
Previous Topic:AFM Guts
Next Topic:w5x into ke10

Goto Forum:
-=] Back to Top [=-

Current Time: Thu Jan 23 06:13:05 UTC 2025

Total time taken to generate the page: 0.0071959495544434 seconds

Bandwidth utilization bar

.:: Contact :: Home ::.

Powered by: FUDforum 2.3.8
Copyright ©2001-2003 Advanced Internet Designs Inc.