Engine Build

[BEEK]

the rules makers made us run 500cfm holleys on our cars for a couple of years. wow was that a challange. best way to make tha holley work for us was to use a power valve plug, eleminating that circut of the carb . then it was somewhat tunable. but nothing beats the tunability of a weber

[zachmac]

Good eye. It is a 350 Holley. My husband loves his Holleys because they are less complicated than a Weber and he's always used them in his racing....so, the Fiat got this race ready Holley. The problem with it was it ran extremely rich at idle and leaned out at higher rpms so I built a set of Weber IDFs and learned how to balance and tune them. I don't know what cfm's would be best for an 1800, but I had read somewhere where the cfms for the Weber dual idfs were around 100 cfms per throat so that equates to more than the Holley. I wish I understood why a little ole 350 is sufficient for a big 350 but a little ole 1608 can handle a carb per runner. The Holley setup is now sitting on my 1608 engine in my Lemons racecar because of the awesome torque.

Madam Burchette,

Not sure where your weber cfm per throat number came from. The beauty of Webers is that they are infinitely tunable ; the agony of webers is that they are infinitely tunable!

The CFM for a given weber is determined by the choke size in the barrel, NOT the body. That is why you might see 48 DCOE carbs running on a 427 CI big block monster one day and the identical carbs running on a 289 small block the next. The CFM is changed by changing the choke in the throat, effectively reducing (or enlarging) the cross-sectional area for flow. I have some links somewhere for all kinds of calculators for figuring out WOT flow requirements and for sizing webers from when I was setting up my XKE for trip side drafts. I'll try to find and post the latter later.

Here is one for CFM requirements: http://www.4secondsflat.com/Carb_CFM_Calculator.html

As you can see you first need to know how high you'll be revving the engine and what your approximate volumetric efficiency is (how well does it pump air)?

In general a built (good flowing head) 2 liter engine should have a volumetric efficiency of about 90 to 95% versus a stock
motor of about 80%.and therefore should require about 235 cfm at WOT.

As far as the individual runners versus one hole (the Holley) its all in the VE. If the air to each cylinder is segregated and you don't have exhaust waves pulsing back into a common plenum (race engines tend to have large valve overlap) then it becomes much easier to get the air into the cylinder and out the exhaust: volumetric efficiency is improved! As VE goes up the total CFM we can flow, by definition, goes up and the larger carbs (chokes in the webers) we can use. In other words, individual runners flow better than a common plenum as the air pressure and pulses from the individual cylinders aren't fighting one another. Hope that helps.

BTW, the Jaguar with the triple side drafts was a banshee! I just couldn't ever get it to idle and transition they way I wanted for the street. But WOT was a hoot!

[zachmac]

I wish I understood why a little ole 350 is sufficient for a big 350.

Not quite. A 350 CI motor turning 6000 rpm with a VE of 90% (mild street tune) requires about 600 cfm at WOT. That is why most Chevy 350s on the planet had either a 600 or 650 Holley. A 350 cfm carb would be for a very NON performance 350 ci engine that only truned very low rpm.

[BEEK]

this is getting kinda off topic, but on my nascar late model stock / pro late model, we run 350 ci, with a 350 2bbl, makes 450hp. realise this is a serious race motor,flat tappet cam, dry sump. non ported big valve steel head" vortec" the small carb is being used as a "restrictor plate", as when we put a 600cfm 4bbl, the motor makes over 600hp

[mdrburchette]

All of you have more knowledge about this topic than I ever will. I only know what I read or witness so thanks for the extra reading material!

[BEEK]

i would not recomend installing a holley 350 on a fiat, webers are way better carburators in any cfm. the emulsion tube system, is very good for fuel atomization. fiats and thier owners have not been concerned in gas milage.

[narfire]

I'll second that Denise. This is a wonderfull source of information about these great little cars

[Zmatt]

i would not recomend installing a holley 350 on a fiat, webers are way better carburators in any cfm. the emulsion tube system, is very good for fuel atomization. fiats and thier owners have not been concerned in gas milage.

I guess that explains why webers are more expensive than Holleys. I wonder why muscle car guys tend to run Holleys if webers are so much better.

[BEEK]

ever seen a 750 cfm 4bbl weber?

[Zmatt]

In person no, but I know of them. Is a quadrajet really better than dual 2 barrel?

[ChrisSRT]

Maybe that's why GM ran 3 2 barrels on their highest hp street motors back in the day, but their highest hp race motors ran single 850cfm holleys. They all had the merits depending on what you were using it for.

Anybody else have some combinations they want to share that have worked for them? Anybody running the Autocross cams (42-82 82-42), from IAP or anybody else. The recommendations (thanks Bernie and Denise) so far have been the performance cams (40-80 80-40), which is what I was probably going to go with to be safe. I've read somewhere that running the slightly hotter cam on the intake side would help spread the torque curve while still giving more power, anybody running a combo like that, how's drivability?

[courtenay]

And I thought I was doing well changing my timing belt and water pump. You guys are playing in a league with which I am not familiar My head hurts!!

[mdrburchette]

I'm running the 42-82/82-42 cams in two of my cars. One has dual Webers and the other has that 350 2bbl Holley. Both are in my choice of engines: the 1608. Everyone says the 42-82/82-42 cams will cause rough idle and no torque on the bottom end, but I can prove both of these wrong.