Cam to use with Webers question
 

I'm building a 428 and have a 48IDF Weber system for it set up by Jim Inglese. Originally I was going with a Holley 3310 so I bought a suitable solid roller cam for it. I contacted the cam vendor the other day and the guy said that my cam was probably not a good choice for an IR system as it has too much overlap which can cause intake reversion. Question is what are the effects of intake reversion? How much is too much overlap? Only cam specs I have are:

Cam: Herbert CGH2
Duration @ .050 240/240
Lift .640
Lobe sep 110
Lobe center 110

For some reason they don't publish the intake/exhaust open-close event specs so I don't know what the overlap is. I have a degree wheel on the engine now & will get those numbers this weekend. It will be rare that I will rev past 6000 (I say that now) but I suspect reversion is more of an issue at lower rpms. True? Just what happens during reversion?

I suppose I should mention that a smooth idle is not desired and engine vacuum is not needed.

Jim Inglese can hopefully advise you on a proper cam for your setup. A stock cam will probably do just fine. I am ignorant of the physics of reversion, but I had to do the same thing when I added Webers to CSX 2401. The hot cam wouldn't work properly, as Jim Inglese advised me, fortunately before I had the engine rebuilt and the Webers added.

Cam
 

Webers like a wide lobe seperation CL 114
I ran webers on my 428 with a Comp 282S which had a 110 cl
Motor ran ok but would of run better with a wider CL
Call Oregon Cams they know FE's or ask Brent he also ran webers on a 428
Jon

This is about as close as I can find in the Herbert catalogue:

CHH2 240/240 DUR@.050, 640/640, 110LC, 2500-6500 - HERBERT CAMS

280 at 110 would be inlet open 30 BTDC, close 70 ABDC, exhaust open 70 BBDC, close 30 ATDC. Overlap advertised would be 60 degrees.

240 at 110 would be inlet open 10 BTDC, exhaust close 10 ATDC. Overlap at .050 would be 20 degrees.

The issue with picking a cam for Webers is not just the LSA, because 110 would be fine if the cam was only say 210 @ .050.
Once you start getting into above 230 @ .050, the LSA becomes more critical.

Reversion
 

Reversion air flow happens at idle (low rpm) with higher valve overlap cams. The intake valve is opening, when the piston is still moving up. At higher rpm and power the exhaust gasses have enough mass and momentum to crate a vacuum and pull air through the intake valve. At idle this does not happen and the piston shoves exhaust gasses out the intake valve. This blows air and fuel backwards through the carburetor. A typical single carburetor manifold has long runners and a plenum that adds up to a lot of volume, compared to Webbers. The volume dampens the affect. The Webbers have very little volume, and the reversion flow causes a haze of fuel to float above the carburetors at an idle.

The reversion flow not only makes tuning the fuel air mixture difficult, that haze of fuel only needs a spark to make for a very bad day.

Let the experts guide you here. Red neck engineering does not mix well with Webbers.

Ok, I 'm confused.... who is the Redneck mechanic? The folks suggesting he call Jim Ingleses or ask Brent....one of the BEST FE engine builders or the guy who can't spell webers?
Jon

I read olddog's informative post more as a generalization rather than a pointed critique of any other post. JMHO.

You have to pick:

performance
smooth idle
easy start

...and you can pick only one!

The original post mentioned the cam was purchased with a single carb in mind. I got the impression that after being told the cam was wrong for webbers the fellow was hopping someone would tell him that he could get away with it, as often times you can get away with a lot of things that are less than ideal. I was trying to warn him not to take the word of just anyone telling him what he would love to hear. Now I could be totally wrong in that impression, in which case I apologize in advance. Just don't want to see a fellow enthusiast have to learn the hard way, when it can be avoided.

I have no experience with them but read a bunch out of interest. Webbers are not something you can just diddle with until they work. You have to do things right.

I get what your saying, I guess I have been fortunate to of been able to tune a set of Ida's on my 428 with the help of a few members and Jim Inglease. i've been to Jim's shop several times, Webers are not hard to set up...that beeing said this member is not using Ida carbs he is using IDF carbs which is a 1st on a FE motor.
Jon

The WEBER IDF is a great carb for the street, a bit better at part throttle driveability than the "race only" IDA.

Shame an IDF is limited to 48mm, unlike an IDA where 51.5mm is common and 58mm if you know where to look.

The IDFs sidedraught cousin (DCOE) is available up to 55mm. ;)

Hello - is this thing working? Posted a reply last night but it didn't show up :confused:

Is there something special I have to do to get a reply posted if it contains a couple of URLs? I've tried twice but no results. It's just simple text with a couple of complete URLs.

Something like this?:

https://www.google.com.au/search?q=f...ih=704#imgrc=_

Gary

The answer is #1 in this thread: http://www.clubcobra.com/forums/all-...bies-read.html

Aha - that explains it. Wonder when I graduate from "newbie"? Anyway, here is what I attempted to post with the URLs removed.

Hi guys,

First - thanks for all of your input. I finally got the cam measured. Here is what I have:

Intake opens 31 BTDC @ .050
Intake closes 32 ABDC @ .050
Exhaust opens 73.5 BBDC @ .050
Exhaust closes 7 BTDC @ .050
Lobe lift .368 (rocker ratio 1.76 = .648)
Overlap 24 degrees
Intake centerline 92 degrees
Exhaust centerline 130 degrees
Lobe Center: 110

It's a Herbert CGH2 -> link to catalog listing of cam

I've heard good things about this cam including it was one of Harvey Cranes favorites. I'm not sure why they don't publish cam specs. Maybe they want to keep it secret? Not really possible. Maybe they don't want to be held to a spec they can't keep? Hope that's not it!

Anyway, no olddog, I wasn't trying to get someone to tell me what I would love to hear. No redneck engineering here. Actually I am an engineer (electrical). What I was wondering was just what the physical affects are with intake reversion? I know it can result in a fuel fog above the intakes. Does it go away right after you open the throttle? A stumble off idle? Backfire risk out the carbs? I guess I was hoping to hear from someone who actually went through the condition.

I have no problem buying a different cam if necessary. Herbert suggested their CH6PJ which has a lobe center of 112. I haven't yet talked with Jim and I don't know Brent. Kinda new here. I just don't want to give away an obvious racy idle but if it's necessary so that the Webers run properly, I will. As for G-Pete's postulate - no brainer. I pick performance. My only prior experience with Webers was a 45DCOE on a Lotus many years ago (still have the carb).

Yes, saltshaker, these are 48IDF carbs using special adapters on a Blue Thunder manifold that Jim Inglese set up. As far as I know, this is the only such FE setup. Probably should have stayed with the Holley 3310 and Edelbrock F427 or PI manifolds I have but damn the Webers look cool!

Pic -> link to picture of my 48IDF FE Weber system with adapters

Hope this makes it...

You had sent me pictures of it when you bought them....Webers will smooth out the idle...I have been to Jims home and shop several times and would encourage you to talk to him. Brent is very approachable and a huge wealth of knowledge.I can send you the specs I used on my 428, not sure if the IDA and IDF carbs would be set up the same way...let me know if you want that info....if Jim set your carbs up follow his path! You can also contact Pierce Manifolds ....also very knowledgeable when it comes to Webers and FE motors
Jon

I would get your cam remeasured, those figures do not make any sense.

As far as I see, if you installed that cam with an intake centreline of 92 instead of say 106-110, it would negate the use of a cam in the first place.

My "educated guess" as above, would be closer to what you have.

The last IR multi webered engine I built for a friends car had a cam of 240 @ .050, 292 @ .006, and still idled as good as some EFI cars do. In fact, I wish I went wilder in the cam department. The engine pulled well from low speed, torque peaked at 5000, horsepower peaked at 7300.

The reversion doesn't really happen at idle, it starts happening when you have the throttle open under partial to full load.

The exhaust sees the "free path" through the intake tract at the overlap period, hence the amount of valve overlap is a critical factor, BUT the closing point of the intake valve allows part of the inducted cylinder volume to go back out the carburettor throat.
The fuel fog sits above the trumpets to be inducted in on the next stroke. So now the engine runs richer than intended, because of the multiple passes of airflow (forward, backward, then forward again) through the carb venturi.
It can be tuned out, that's the balancing act of emulsion tube, choke size, air corrector etc.

IR reversion or "stand off" has been around for years. It's just knowing how to contain it with a well designed "system" of intake components, cam profile, cylinder head, exhaust system etc.