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Opeler
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Discussion Starter #62
Just saw PJ's 2.4L head for sale on FB and he specs a cam with split specs of 246/242 duration at 0.050", lift of 0.459 intake and 0.435" exhaust, and LSA of 108 which is lower than stock Opel of 110. Opposite the 112 LSA recommended above. Why would this be?
 

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Opeler
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I assume this is for the higher low end torque. The following is RB's comment about this cam in one of his posts:

I like Cam Techniques CR1 cam (was F306/F290-8). Torque is awesome...my favorite Opel cam. Not a racing cam but healthy. Recommendations back in the day were: at least 10:1 compression ratio, 38 DGAS or bigger, ported intake, 2” exhaust with Sprint or header, big valve head. Requirements were a re-curved distributor, deleted vacuum advance. Generally 12-18 degrees timing at idle depending upon compression ratio. Max 34-36 degrees. I saw an 18 HP improvement in one of my engines using this cam, compared to a mild hydraulic.
 

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Opeler
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Discussion Starter #64
Yes, this would be for a 2.5/2.5L type engine. Probably a little less than 10:1 compression to allow for readily available pump gas. So my question was regarding the different recommendations on LSA?
 

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Just saw PJ's 2.4L head for sale on FB and he specs a cam with split specs of 246/242 duration at 0.050", lift of 0.459 intake and 0.435" exhaust, and LSA of 108 which is lower than stock Opel of 110. Opposite the 112 LSA recommended above. Why would this be?
That cam was my go-to hot street cam for a 1.9/2.0 engine. But it idled a bit rougher and needs higher compression to work well.

I actually used the same grind with 106 degree LSA for autocross, hillclimbing and rally use. Nastier idle but crazy mid-range pull.
 

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Keep in mind, that in addition to changing the LSA, you can also advance or regard the cam timing to put the power exactly where you need it.
 

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I did the Chevy offset bushing mod to my cam gear to establish 3 degrees of advance on my cam. Very simple and inexpensive mod.
 

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Did you document that anywhere?
This is what we (John Warga and myself) did for our cam gears (and Sterling Rempel's), photos are of my gear and engine. We are running the OGTS Combo Cam, (notionally 0.420 lift, hydraulic grind). Sterling and mine are set up with the 4 degree advance bushing (John's engine is a stroked 1.9 to 2.4, and still a "work in progress"). To the best of my cam degree'ing ability. we both ended up a touch under 3 degrees advanced (after some block and head surfacing)
 

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Opel Rallier since 1977
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There is one thing in the info above in post #65 that is usually misinterpreted about a narrow LSA cam: the lower RPM torque band is better. This is only true about the lower end of the mid to high RPM torque band, around 3000 to 6000 RPM in many of these engines. This is where the exhaust scavenging is working, and is the main torque band, where the torque flattens out to within 90-95% of its peak value.

What this statement about narrow LSA cams does NOT mean is that torque at the low RPM's is increased. This would be typically at or below 2500 RPM for many of these Opel engines. This misinterpretation that gets made is that somehow a narrower LSA makes low RPM torque better. The reality is the opposite: a narrow LSA makes low RPM torque worse. This goes hand in hand with the rougher idle of a narrow LSA cam.

The reason that a narrow LSA cam has LOWER torque at low RPM's is that the exhaust scavenging effect dies off once your operating RPM drops below mid range. When that happens, the added intake reversion of the narrow LSA eventually reduces torque as the RPM gets low enough.

The whole way to see how torque works in any engine of this general design is first see how well the cylinders get filled each cycle. Whatever helps cylinder filling increases torque, whatever hurts cylinder filling decreases torque. When the RPM is high enough, exhaust scavenging kicks in and cylinder filling approaches 100%; this is reflected in a number call volumetric efficiency (V.E.) and V.E. is seen in the dyno charts for newer dynos. You will see V.E. approach 100%, and sometimes exceed 100%, in the main torque band of an engine, when the exhaust scavenging is in full swing.

Once the RPMs get low enough however, exhaust scavenging dies off, and then the valve overlap that increases with a narrow LSA, and that is the source of exhaust scavenging, stops being our friend and becomes our enemy. Reversion into the intake increase as RPM's drop lower and lower, reducing cylinder filling (and thus V.E.) and eventually it gets so bad that we end up with a rough idle.

The way to fight this die-off of cylinder filling (and thus torque) at lower and lower RPM's is to to increase LSA and decrease duration. Both of these have the effect of reducing reversion, which allows better cylinder filling and torque at lower RPM's. This is why stock cams and cams for low RPM operations like truck use typically have larger LSA and certainly have smaller durations, than cams in more race or race-like uses.

And if you care about fuel economy, then the same applies: you make the LSA's wider and keep the durations lower.

Our Opel engines seem to fall 'in-between'. With small displacement, the engine needs to operate higher 'up in the main torque band', especially with 70 mph RPMs being in the 3000-ish range, which is mid RPM's and where the exhaust scavengin is starting to work. So LSA tends to be a bit of a compromise for these engines... hence the production LSA ends up at 110 degrees, as opposed to a 112 or 114 LSA for a larger displacement engine.
 

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Well I have always found it interesting that I can drive my car in 5th gear at about 1500 rpm on level road and it pulls great with a LSA of 108. 10:1 compression, premium gas.
 
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There is one thing in the info above in post #65 that is usually misinterpreted about a narrow LSA cam: the lower RPM torque band is better. This is only true about the lower end of the mid to high RPM torque band, around 3000 to 6000 RPM in many of these engines. This is where the exhaust scavenging is working, and is the main torque band, where the torque flattens out to within 90-95% of its peak value.

What this statement about narrow LSA cams does NOT mean is that torque at the low RPM's is increased. This would be typically at or below 2500 RPM for many of these Opel engines. This misinterpretation that gets made is that somehow a narrower LSA makes low RPM torque better. The reality is the opposite: a narrow LSA makes low RPM torque worse. This goes hand in hand with the rougher idle of a narrow LSA cam.

The reason that a narrow LSA cam has LOWER torque at low RPM's is that the exhaust scavenging effect dies off once your operating RPM drops below mid range. When that happens, the added intake reversion of the narrow LSA eventually reduces torque as the RPM gets low enough.

The whole way to see how torque works in any engine of this general design is first see how well the cylinders get filled each cycle. Whatever helps cylinder filling increases torque, whatever hurts cylinder filling decreases torque. When the RPM is high enough, exhaust scavenging kicks in and cylinder filling approaches 100%; this is reflected in a number call volumetric efficiency (V.E.) and V.E. is seen in the dyno charts for newer dynos. You will see V.E. approach 100%, and sometimes exceed 100%, in the main torque band of an engine, when the exhaust scavenging is in full swing.

Once the RPMs get low enough however, exhaust scavenging dies off, and then the valve overlap that increases with a narrow LSA, and that is the source of exhaust scavenging, stops being our friend and becomes our enemy. Reversion into the intake increase as RPM's drop lower and lower, reducing cylinder filling (and thus V.E.) and eventually it gets so bad that we end up with a rough idle.

The way to fight this die-off of cylinder filling (and thus torque) at lower and lower RPM's is to to increase LSA and decrease duration. Both of these have the effect of reducing reversion, which allows better cylinder filling and torque at lower RPM's. This is why stock cams and cams for low RPM operations like truck use typically have larger LSA and certainly have smaller durations, than cams in more race or race-like uses.

And if you care about fuel economy, then the same applies: you make the LSA's wider and keep the durations lower.

Our Opel engines seem to fall 'in-between'. With small displacement, the engine needs to operate higher 'up in the main torque band', especially with 70 mph RPMs being in the 3000-ish range, which is mid RPM's and where the exhaust scavengin is starting to work. So LSA tends to be a bit of a compromise for these engines... hence the production LSA ends up at 110 degrees, as opposed to a 112 or 114 LSA for a larger displacement engine.
Again, it really depends.

As an example, Ford used the same camshaft grind in the old roller cam EFI 5.0 liter HO Mustangs as the pickup truck version of the same engine. EXCEPT for the lobe separation angle. This was confirmed to me by my old cam grinder


They use 114 degree LSA in the HO engines, and they would rev to 6000 rpms or so. In the trucks, they use the same lift and duration but 107 degree LSA. They had trouble revving past 4800 rpms, had a choppier idle, and were better suited for towing than for racing.

I think the point is, LSA is one tool, as is duration, as is cam advance or retard. By mixing and matching specifications you can get to where you want the power to be...whether it’s from 1500-3000, or from 4500-6000, or whatever. But there are always compromises. Which is why modern engines have variable lift/ duration/ and cam timing....to have the best of everything, all the time.
 

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Well I have always found it interesting that I can drive my car in 5th gear at about 1500 rpm on level road and it pulls great with a LSA of 108. 10:1 compression, premium gas.
Travis’ GT autocross car uses an F-306 cam with 106 LSA, and that‘s in a 1.9. As I recall from his chassis dyno run, he had 90% of his peak torque available from 2200-6000 rpms. It had a ridiculously flat torque curve. I drove the car and can attest it really didn’t matter if you were at 2500 or 7500 rpms, it pulled everywhere quite well.
 
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There is no substitute for displacement(except a turbo or a supercharger). A 2,6 liter engine has enough torque even if the cam isn't exactly spot on. And a higher static compression ratio brings back some of the lost low speed torque from a narrow LSA high duration cam.
 

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Opeler
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Discussion Starter #80
Travis’ GT autocross car uses an F-306 cam with 106 LSA, and that‘s in a 1.9. As I recall from his chassis dyno run, he had 90% of his peak torque available from 2200-6000 rpms. It had a ridiculously flat torque curve. I drove the car and can attest it really didn’t matter if you were at 2500 or 7500 rpms, it pulled everywhere quite well.
So based on your previous thoughts did it have a rough idle? You recommended a 112 LSA for a big streetable engine. If I understand correctly the wider LSA offers a smoother idle and should move the torque curve even lower?
 
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