Cam to use with Webers question

[Gaz64]

Originally Posted by Gaz64

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.

Originally Posted by ksquared

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...

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.