flow

[two89w]

This a follow on from my last post(7000rpm/480hp)
Below is a flow chart of my heads,current carbs,and larger carbs.
From looking at the chart,how much hp(estimate)would I get from going to the 48's with 42 chokes?
Its hard to see but the redish/pink line is my current carbs
And the light blue is the 48 ida's with 42 chokes.
what chokes would I need to get close to matching my head flow?

[Kevin M]

You can’t size a carb based on flow numbers alone. The problem with flow numbers for both heads and carbs is that they are tested for steady state flow. (Where the air simply flows continuously in one direction). In a running engine, airflow is starting and stopping very quickly and it is vital that you keep the velocities as high as possible.

If the flow velocity drops, you loose the ram effect from the rapidly moving air column and can loose 10% or more in power just from that. That is why if you have two heads with the same flow numbers, the one with the smaller ports will usually make more power.

With a carb you have the issue that it has to meter fuel accurately and in such a way that the fuel is well mixed with the air. This again requires a pressure drop through the venturi. Too large a venturi will loose horsepower because of a poor mixture. (There is also the issue that you want good part-throttle throttle response and a decent idle).

If you go back to your last thread, I gave you the formula for figuring venturi size. It suggests that 44 mm to 45 mm would be ideal. Since Weber only offers 44 mm and 46 mm venturis (and the 46s don’t work well at lower speeds) you would be best to start with the 44s.

Finally, realize that all of this only serves to get you reasonably close. If you want to maximize the performance, you have to experiment with the engine. One of the great things about Webers is that you can change their size by changing venturis. Larger Venturis will (up to a point) give you more peak power, but will hurt low speed throttle response, etc. You have to test and find what works best for your particular engine in your car and for the way you drive it.

[Peruch]

KMCGOWEN,is right,have a 331 combo with Vc junior ported with 10.8 CR,custom roller cam designed by Buddy Rawls (www.wighat.com/fcr/3)and 48 IDA's bored to 51.5mm by Jack Sachette (www.jaycee-ent.com)with 44 choke,165 main/140air,bottom is steel crank etc...,made 540 HP/454 lbs torque!!!,Do you have flowed your head with manifold and carb or only head?
Bernie

[Kevin M]

Bernie,

That sounds like a great engine! What RPMs are you seeing Peak Power and Peak Torque at? Also, what are the lift and duration at .050 figures on that cam?

Kevin

[two89w]

The dark line above in the graph is the head only.
The other three are head,manifold,carb together,just different size carbs.

[Neal]

As a point of reference...

347 ci

My 48 IDAs are setup as follows...
38 mm chokes
65 idle
120 holder
160 air correction
145 main
55 bypass
F7 etube

Timing is set to 30 degrees total.
My compression is 10.25 to 1.
Cam specs are 284/294 and .544/ .573 or
238/248 @.050 and .340/.358 at the cam. 110 lobe center. Installed straight up.
1.6 rockers
Headers are 1 5/8" primaries.
TEA ported heads


456HP @6100 rpm
455TQ @4200 rpm

Something to compare your setup to.

[Peruch]

KMCGOWEN,
537@7000,428lb@5000,custom "thight lash roller cam:
260/260 @ 0.50
0.666/0.666
1.60 roller rocker ratio
Regards,
Bernie