48 Ida

[Scott Calabro]

Hello All !

Does anyone know the CFM rating of ONE of the 48 IDA carb's that is furnished with Redlines kit for the 302 SBF ? I'm confused..........

Thanks,
Scott

 

[Scott Calabro]

To clear things up a bit........ One 48 IDA equipped with 37mm chokes, flows what CFM @ 28" water ?



S

 

320 cfm each barrel

 

[Scott Calabro]

Hi Peruch,
Thanks for the info. Is the CFM you referenced from a Weber document or other technical publication ? I ask because I think for the 289 I plan to build, the total sum of 2560 CFM seems at the outset to me to be way to much carb. I think I want to see less than 100 CFM per barrel.

CFM = CID X RPM X VE divided by 3456.........so........

36.61 (1, + .030 over 289 cyl) x 7000 (RPM) x 1.15 (VE) divided by 3456 =
85.27 CFM multiplied by 8 gives only 682 CFM.

Does anyone have experience with a similar set-up ?


S

 

[Russ Noble]

Scott, a quick search would have turned up this recent thread of Ron's. It may lead you to the answers you require. If not do a further search, it's all been covered before.
http://www.gt40s.com/forum/tech-eng...k-power-choke-size-8-barrel-venturi-size.html

Cheers

 

[Scott Calabro]

Thanks so much Russ......I knew a veteran GT40 fan would chime in sooner or later ! Your reply helps more than you know.......
rockonsmile
Best regards,
Scott

 

[Dean Lampe]

Your math is correct, however if you are using a single plane intake with a carb that would allow 682 cfm, each cylinder could pull from the same carb. So 320 is about half of that. Probably not enough for each individual intake runner. That is a pretty simple example and probably flawed but the numbers for webers are different than a traditional 4 barrel.

 

[BenL]

I ask because I think for the 289 I plan to build, the total sum of 2560 CFM seems at the outset to me to be way to much carb.

If you put air filters on the carbs, would that bring it down to a more reasonable number?

 

[Dean Lampe]

You don't want to bring it down. It will be barely enough now. I have run the webers and the 4 barrels, this will be a good set up. What do you heads flow at max cam lift? I will bet that it is close to 300 cfm. My Yates heads flow almost 360 cfm at .600 lift.

 

[Scott Calabro]

Thanks Dean,

I plan on using AFR 185cc heads. Their specs say with a 4.060 bore @ .600" lift = 277 cfm int. and 199 exh. I still cant get my head around how when one cyl. theoretically only needs 85 cfm @ 7000 rpm, I will be supplying it with a venturi that can support 320 cfm. I figure it would result in a crappy vacuum signal at transient throttle positions and driveability issues. I guess everybody has good luck with them, and besides it wouldn't be fun to me if I didn't have to tinker with them and they look so cool !

Regards,
Scott

 

[Jac Mac]

Thanks Dean,

I plan on using AFR 185cc heads. Their specs say with a 4.060 bore @ .600" lift = 277 cfm int. and 199 exh. I still cant get my head around how when one cyl. theoretically only needs 85 cfm @ 7000 rpm, I will be supplying it with a venturi that can support 320 cfm. I figure it would result in a crappy vacuum signal at transient throttle positions and driveability issues. I guess everybody has good luck with them, and besides it wouldn't be fun to me if I didn't have to tinker with them and they look so cool !

Regards,
Scott

Scott,
I have just clicked as to why your having trouble with this. In the 720 deg of engine rotation for each cyl the intake valve is only open for approx 280/300 degrees of which approx 1/3 is not used, Therefore the carb must flow at approx 4 times your 85cfm rate to fill the cyl in the time provided. There are a whole heap of other factors in this too, but that should help you get the picture.

cheers
Jac Mac

 

[Scott Calabro]

Thanks JAC MAC,
I checked and you were correct without even knowing my cam specs ! I will be using a Crane solid roller, the intake valve is off its seat for 288 deg of crankshaft rotation. I guess the real benefit of the Weber IDA is that if the 37 mm venturi is to big or to small I can change it to suit my engines specific needs. Since the build will be dyno tested I'll just make sure I have extra parts on hand for tuning.
Thanks again for your help rockonsmile

Scott