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Discussion Starter #1
I’ve read about this on other forums, so I thought it would be interesting to get into this discussion specifically relating to the Opel CIH and it’s unique in head cam.
I thought of putting this post in the “Cams, cams & cams” thread which is a really good read by the way but I thought this would get lost in there, this makes for easy reference for anyone else considering either of the two options.

I’ve been driving with a reground cam in the engine since 87’ when I got my first rebuild done, the shop owner convinced me to go that route, parts weren’t available locally back then, no internet and Opel parts were not easy to find. I did request a more aggressive cam.
The car has always been dependable and ran smoothly, but seemed to have a little more pep before the first rebuild. I’ve been reading some pretty good threads recently and discovering more, seeing how cam card numbers can be deceiving 🤔

The lift and duration numbers on the cam card (used on both cams) using the 1.5 rocker multiplier .405”/256 At the cam .270”/ 204 duration @ .050.
I degree‘d both the cam I’ve been driving on for 30 years & the one I’m now using now, for both cams all measurements were taken at .050” off base & the top of the lobe. They both came up close.
The one that I’m using now has .266” lift with 200° duration @.050 .
I measured the cam I drove around with for 30 years and it was .258” lift with 203° duration.
All of these numbers look as good or slightly better than the stock hydraulic grind I’ve always thought, right?

I’m going on an old RallyBob post here for the stock hydraulic cam 385”/205°duration @ .050”, I’m not sure what Bob used as the multiplier which segway's into the next paragraph. The next closest information I could find on the cam lift itself was .257” on the cam specs from Teknaar’s post. On a side note I recently read somewhere that advertised duration is .004” off the base. I have yet to check that someday to see how close that is.

The post that recently got my attention was again RallyBob’s post on the recent roller rockers thread (sure are sweet roller rockers btw). If you take Bob’s 1.44 to 1.48 to one multiplier, for simplicity let’s just use 1.45”. My actual lift on either cam goes down. More like .385“ & .374” Adding to that the base circle being reduced because they were both stock hydraulic cams.

I measured 1.344” base on my stock solid grind cam, Manta Rallier checked his stock & got 1.340” on his hydraulic for the base circle. On my old cam I measured 1.270” at the base circle.
We were discussing what the reduction of the base circle has to do with the change in geometry on the valve stroke, my concern was the positioning of the rocker arm on the valve tip during the stroke.

Unfortunately, since I wasn’t aware of any of this when I did my 2.0 conversion over a year ago I don’t know how much the base was reduced and the on the cam I’m driving now since I didn’t check it, I can only guess.
On the old cam about a .07” or 1.8 mm reduction occurred to the base circle. I don’t know the formula or how to calculate the geometry of how much this base reduction contributes to the rocker arm lift loss? If anyone has an idea I’d be curious to know. I’m also not sure if the duration gets affected by the reduced base circle?

I really think now that a new hydraulic stock cam would give the rebuilt 2.0 better torque & HP? It would raise the base circle again and wouldn’t that raise the rocker arm ratio too?

So here’s a hypothetical situation, if you want more lift on a reground stock cam, from say .257” to .290” the metal will come from somewhere, that’s by reducing the base circle more.
So my summation is especially with our CIH engines I won’t be trying to increase performance on the stock cam, next time I’ll get a new one.
 

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More to follow but realize that the radius reduction is what you care about, which is .070 / 2 = .035"

And from some info that Rally Bob recently posted some info on the rocker action, a longer valve results in the effective rocker ratio being closer to 1.5. The base circle being smaller lowers the lifters and changes the angle of the rocker just like a longer valve. So you may be better off rather than worse off with the reduced base circle. (Edit: See below thread...from TC..... looks like I remembered this backwards.)

Make sure that you understand that the base circle by itself does not directly effect the rocker ratio, but only via changes in the above way in which the rocker action changes.
 

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Discussion Starter #3
More to follow but realize that the radius reduction is what you care about, which is .070 / 2 = .035"

And from some info that Rally Bob recently posted some info on the rocker action, a longer valve results in the effective rocker ratio being closer to 1.5. The base circle being smaller lowers the lifters and changes the angle of the rocker just like a longer valve. So you may be better off rather than worse off with the reduced base circle.

Make sure that you udnerstand that the base circle by itself does not directly effect the rocker ratio, but only via changes in the above way in which the rocker action changes.
Mark, do I have this backwards? I’d like to think so, I like what you said better 🤗
Here is a copy and paste:
“A) Longer valves and/or a reground cam with a smaller base circle will reduce the ratio.
B) Shorter valves, full base circle cams, and longer lifters will increase the ratio”.

What I have is half of “A“ a smaller base circle, I’m still thinking reduced ratio?
 

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Isky will grind you a new cam. Pick the profile or profiles you want, type of cam and LSA. Brand new cam.
 

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Discussion Starter #5
Isky will grind you a new cam. Pick the profile or profiles you want, type of cam and LSA. Brand new cam.
That’s what I had in mind, they told me that I needed to talk to Ron who comes in in the afternoon. I’m anxious to try one of the RallyBob split profile cams I’ve seen on his posts & I’m wondering if they have any of those on file and or if he knows Mr. Legere.
 

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Mark, do I have this backwards? I’d like to think so, I like what you said better 🤗
Here is a copy and paste:
“A) Longer valves and/or a reground cam with a smaller base circle will reduce the ratio.
B) Shorter valves, full base circle cams, and longer lifters will increase the ratio”.

What I have is half of “A“ a smaller base circle, I’m still thinking reduced ratio?
LOL NO it looks like I have that backwards Tom.

If the above was what Bob was saying, then he has done the work to know. I could not find that recent post. If this is generic info from somewhere else, then it may or may not apply.

FWIW, this matter of geometry change and ratio changes versus valves, rocker, rocker fulcrum location, is shared with all rockers to some degree or another. For those running really high lifts (like 500"-700") then the variations in rocker motion can induce velocity changes that impact spring selection too. But there are probably just a very few folks here in that catagory.
 
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