Quad IDFs to suit 5 litre v8

[Roy]

Hi, I have another question for ya'll. I'm looking at using 4x 40mm IDF Weber carbs on my 307 chevy (I can refer to it as a 5 litre if you don't like that word? lol). I've heard mixed opionions on whether they are to small or big enough.

I calculated the area of barrel size on my 600 Edelbrock vacuum secondary carb to be around 1580 square mm. 8 webers with 28mm venturis (from what I've read this sounds stock for 40mm idf) works out to 1560 square mm of barrel area. I'm buying some that already have 32mm venturis in them (works out to 2050mm for comparison). I'm still learning about these carbs and need to buy some books. So I really appreciate your advice.

I am upgrading the cam to a hydraulic flat tappet 235in,242ex @ 0.050", 110 LSA, .500ish lift from memory. Also upgrading the heads to World 305 heads which will be ported and give me around 9.5:1 on my 307. Gearbox will be getting upgraded to a 5 speed manual and my diff has 4.56 gears. I'd say car weighs around 3300-3500lb.

So... Could I please have your opinions on what parts to use as an initial setup with these? I'll be ordering my rebuild kits from overseas and would be good to get as much as possible (of what I need) in same order. Please remember, I am on a budget! I will be making my own alloy intake and rocker covers and really want to make my car more personal than the current (although very good) Edelbrock RPM Airgap and performer carb.

I'm also stuck on choice for filters they all look so nice. What do you like on quad down draft setups? I think the K&Ns look great, but are pricey. Ramflos look pretty trick too (only slightly cheaper) and would be more likely to fit under bonnet, but they don't seem to be used on quad down drafts that I've seen (any one have a pic of that?).

I hope I make some sense lol

 

[Andrew Robertson]

Hi Roy, your comparison is apples and oranges. You are using a dual plane manifold where half the carb supplies half the engine. Each pot on your engine is currently drawing through two barrels of the edelbrock each time it goes through the induction stroke. To have the same breathing in the upper rpm range you need a single throat that will flow 300cfm - from memory that equates to about a 48.

Your 307 will likely be dead by 5 grand with the 40 IDFs, and that's a mismatch with the cam you are intending to run. In my own experience I put quad 47mm OER sidedraughts (DCOE clones) on a 383SBC and it was pretty much over by 5 grand whereas the 750DP and edelbrock performer I had on there beforehand ran to 5600 on an otherwise identical engine.

Hope this helps.

Cheers, Andrew Robertson, Wellington

 

[Jim Rosenthal]

Since you can change the venturis etc on Webers, you are better off with a larger carb and experimenting with various venturis etc. Email Inglese or Redline or Pierce (or all three) and get their advice. They are likely to tell you that 40IDFs are small. Remember that 289s in GT40s ran 48IDAs even on some street cars.

 

[kiwi ka]

Roy
I am running 4 x 40mm twin choke Del Orto`s on my Rover 3500 and that is considered borderline, I have got more than enough get up and go but most of all just the most invigorating sounds just 300mm from my ears.
Cheers

 

[Roy]

Thanks for your advice people. Yeah I thought they were sized differently, but wasn't sure why. Thanks for the explaination. I guess I have to put this idea on hold for now then, the cost of using 48s is a lot more than I can afford. I'd really like to do it though. I can have a pretty healthy big block in it for the price of 4x 48 webers too.

I've got a spare block and heads sitting here ready for mock up so maybe I'll do a fabricated 4 barrel manifold instead.

Thanks for your help guys. Unfortunately this is something that's going to have to wait until I'm a bit more financial.

 

[Cliff Beer]

Roy, there's nothing wrong with putting the 40s on. Do the 40s you're thinking about using have venturis installed? They work best with a venturi (better atomization of the fuel) but you could use a minimal (large) venturi. It's not like the car will run lean and you'll burn up your pistons and valves - you can flow more gas by monkeying with jets and etubes.

If you've ever seen an old Lincoln with about a 400ci on it and a single carb about the size of a baseball then you know that a small carb doesn't necessarily produce poor running. Frankly, a small carb tends to get better atomization of fuel which improves driveability. Why? Because a smaller carb will generate a) greater pressure drop between upstream of the venture/butterfly and downstream, and b) the intake speed will be higher.

A small carb just tends to limit power output. It's really just a trade off - a carb too large tends to get good performance but at the expense of driveability (poor atomization). In some respects, less is more, including carburetors.

The suggestion of a single plane manifold is likely a good one.

 

[Randy V]

A small carb just tends to limit power output. It's really just a trade off - a carb too large tends to get good performance but at the expense of driveability (poor atomization). In some respects, less is more, including carburetors.

X-2 on this... It will run just fine and actually have a more crisp throttle response than with the larger chokes.. :thumbsup:

Carburetor Main jetting has far more to do with the ability to flow air than the displacement of the engine. You can only bring in "X" amount of air, and mixing in even greater amounts of fuel per that air flow will yield nothing more than a fuel mixture that is too rich..

 

[Andrew Robertson]

X-2 on this... It will run just fine and actually have a more crisp throttle response than with the larger chokes.. :thumbsup:

Carburetor Main jetting has far more to do with the ability to flow air than the displacement of the engine. You can only bring in "X" amount of air, and mixing in even greater amounts of fuel per that air flow will yield nothing more than a fuel mixture that is too rich..

....but guys I assume you're not suggesting he takes off a highly functional set-up like a Edelbrock performer RPM airgap intake and 600 carb and replaces it with 4 x 40 IDFs, especially given the cam and head upgrades being installed that are oriented towards top end improvement?

Cheers, Andrew

 

[Cliff Beer]

Andrew, you're right - not suggesting that a 40 IDF setup here will necessarily run any better than an Edelbrock/RPM with those heads and cam - might actually decrease total performance and driveability, particularly if proper tuning is problematic. I'm just saying that the 40 IDFs will work OK if tuned decently. In other words, if Roy is in the mood to monkey with jetting and etubes and chokes and such then he can have some fun with his existing 40 IDFs and probably have a decent runner in the end.

The webers do look the part, and can be fun to play with....so long as you're not depending on it getting you to work on Monday morning.....

 

[Randy V]

Andrew I believe Cliff and I are on the same page here..
If the cam profile has a lot of overlap, you can probably make it run _ok_ with a good single 4 barrel carburetor. At least good enough to be able to drive it on the road.
Swap that carb and intake for a set of Webers and it will run like crap anywhere but on the highway or racetrack where the RPMs are consistently high..

Now - if you were bent on running Webers with this camshaft from hell, I *think* you'd actually have a better running combination with the smaller chokes. It will still run like crap, but less so with the smaller chokes than the larger ones.

 

[Dave]

....but guys I assume you're not suggesting he takes off a highly functional set-up like a Edelbrock performer RPM airgap intake and 600 carb and replaces it with 4 x 40 IDFs, especially given the cam and head upgrades being installed that are oriented towards top end improvement

I had quad 44idf webers on 5ltr windsor in a FFR coupe. I took them off and fitted a Edlebrook performer manifold and a brand new 600 mech secondary Holley. Oh what a mistake that was. Engine felt like it lost all it's mid range. The webers made the motor so much sharper. throttle response on-off-on was awesome. The 5ltr ran better at idle with the holley but other than that the motor felt like I had retarded the timing 10deg.

I'm not saying drop the 4 barrel just that if I had my time again I would of stuck at getting the weber idle spitting problem I had sorted.