Weber size
 

I have a twin plug 3.0L with 40mm weber's making hap at the crank. I have an opportunity to increase the venturi's to 43mm. Does anyone know if this will make any appreciable hp increase??

Basic rule of thumb...larger venturi=more HP at top end.
Smaller venturi=more torque at lower RPM.
Depending on your requirements...you can modify your carbs for use.
If you go too big on the size...you will end up with a dog off the line...and have to buzz the engine to very high RPMs to get going.
Also...smaller sizes tend to be a little better with gas mileage.
Bob

How will you install 43mm venturis in a 40mm carb?

What size is your intake port?

I believe the housing is larger than 40mm...so perhaps just the venturi is the part he is replacing.
I have worked with Webers up to 58mm (huge beasts) and all the internal parts were the limiting or size numbers that determine the overall number you end up with.
Bob

The basic barrel is 46mm dia as is the OD of the venturi.

If you bore the carb to 43mm dia you may break though into the bore - in the vicinity of the three progression holes under the brass plug.

It is worth making some detailed measurement before you bore them or you may have to make a plug to re-instate this area of the carb.

Performance Oriented in Calif is going to work on my webers. Paul is first going to modify my "IDT" webers back to "IDA" with some bleed hole changes. That will get rid of the low rpm popping and smooth out the transistion from the idle jets to the main jets. He is able to increase the venturis size and I kind of decided the go to 43mm. 46mm is to much for a street/occasional track car. I get 21mpg with the 40mm's.

What I'm wondering is since I'm making 245hp at the crank would there be any significant hp increase? If it is in the neighborhood of 5 hp I'm not sure it would be worth it as I probably wouldn't notice it.

Kind of talking myself out of the 43mm's.

My handy-dandy Weber manual shows about a 42 or 43 mm venturi in the graph. The standard formula shows 38mm. I'd run the 38mm because the 43mm venturi will not work very well as a venturi (where you want to accelerate the air to higher velocity thereby causing low pressure to suck in the fuel from the mains) inside a body that is only 3mm larger (at best).

You are not looking to maximize horsepower! Wrong MOE! You want to maximize the area under the HP curve between your shift RPM's. If you are revving to 7300, then 38mm is where I'd start.

First of all it would help if I learned and used the proper terminology. I was referring to opening up the throttle bore to allow a larger venturi.

I have 40mm weber's with 36mm venturis. So I was thinking of opening it up to 43mm and then using something like a 38mm venturi.

Well for a mostly street 4-5 times a year track car it probably wouldn't make an appreciable difference. Then with the cam I have GT2-102 which has a low torgue peak I might be giving some of that low end grunt away as well. The car only weighs 2200lbs dry so I should be happy with 9:1 power to weigh.

It's a vicious circle when you don't have anything to spend your money on and add to the car. Maybe I should just drive it, drive it and drive it.

OK Kent... now I see! I'd leave it as is. The venturi size really needs to match the cam.

At the track, work on your braking zones and smoothness. It helps to have a somewhat bulletproof engine. In the long run, you'll be better off.

Install a "real" dual master adjustable brake bias system with a dial on the dash. And you can always play with suspension and dampers. Play with the rake and experiment with how the handling changes.

Per our detailed phone conversation earlier today,....that's a capital idea,...:) :)