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By miker - 13 Years Ago
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There's been a lot of discussion about cam specs here, lot's of it pretty technical. Some of it I understand, at least the basics, of lift, overlap, measured height. More duration seems to come with more lift, more overlap causes rough idle with lower vacuum. Lobe centers, which I assume is the center-line difference of intake and exhaust, fully open, confuses me a bit. I've degreed in cams, but aside from following the instructions ( it seems they were normally retarded a couple degrees) I don't really understand what that means or accomplishes. Why not just grind it to go in straight up. Looking at Mummert's site, he has a 280-I-07, and a 280-I-11. The difference appears to be the lobe centers at 107" and 111" respectively. On a street V-8, of say 300 CID, what's the difference driving from, say 1500 or 2000 to 4000 or 5000 rpm? Assuming the heads, manifolds etc. are the same, and compatible to the cam, why does it cause the difference? And finally, how do (real general here) bore stroke/ over square/ under square affect this? I don't like to seem too dumb, but I'm pretty sure I'm not the only one, and there's so many combination out there, there must be some differences of opinion even among cam designers. How about a little remedial education for some of us.
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By lyonroad - 13 Years Ago
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Mike, I know a lot less than you (basically I saw a cam once) actually I have replaced a cam in a 351C, and two in my daughters cars (4 cylinder OHC). In the case of the one in the 351 I asked the parts guy for something better than stock and that was it. It seems to be ok. I anticipate that I will need to buy a cam for the Y-block. I have an ECK block with ECK-C heads (should be a '55 292 according to John Mummert's chart). It came with a 55 Fairlane Club Sedan and was supposed to be rebuilt. Its very clean but I have my doubts. From what I've read it is imperative that the cam have the oil groove so I will have to pull it out and check for that. I don't race. I just like to drive around, make a lot of noise and have fun. What kind of cam do I need. I'm at your mercy.
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By Ted - 13 Years Ago
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As a general rule, camshafts are installed 2-4° advanced and not straight up. This is for a couple of reasons with the first being to compensate for timing chain wear. As a chain stretches from use, the cam retards itself so advancing the cam initially helps to delay this. The second reason is chain elasticity where the chain stretches as the rpms go up and comes back in size as the rpms come back down. This can be seen with a timing light once all the ignition timing is in and where you can actually see the timing start retarding while watching the timing marks with a light and varying the rpms. That’s the camshaft actually retarding and in turn just dragging the ignition system along with it. Regarding lobe centerline angles on a given camshaft, as they get narrower the overlap increases which in turn reduces the vacuum signal. That’s simply due to both the intake and exhaust valves being open together for a longer period of time. Narrower lobe centerlines typically make the torque values more peakish with a narrower operating range while wider lobe centerlines have broader power bands with reduced peak values. Making the lobe centerline angle smaller also permits the intake valve to close earlier which allows the cranking compression values to also increase. This gives an immediate increase in low end torque. Because both valves are open simultaneously, this is where the choppiness or lope in an idle comes from and the more the centerline angle is reduced, then the more the lope or chop in the idle characteristics. Here are some past threads related to camshafts and their installation. Doing a search using different keywords will bring up a variety of discussions on the topic. http://www.y-blocksforever.com/forums/Topic38920-3-1.aspx http://www.y-blocksforever.com/forums/Topic42770-3-1.aspx http://www.y-blocksforever.com/forums/Topic61758-3-2.aspx http://www.y-blocksforever.com/forums/Topic61625-3-1.aspx http://www.y-blocksforever.com/forums/Topic55379-3-6.aspx http://www.y-blocksforever.com/forums/Topic46641-3-1.aspx http://www.y-blocksforever.com/forums/Topic57116-3-1.aspx
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By aussiebill - 13 Years Ago
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Ted, you are one of the very few people i,ve met that can explain clearly how things work in easy to understand terms and is great help to the forum. Best regards bill. PS this is 3rd attempt at posting this .
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By GREENBIRD56 - 13 Years Ago
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I've always liked some of this guy's thinking about engines - even if he does prefer to build Chebbies, he gets results and makes sense. http://www.popularhotrodding.com/tech/0607phr_camshaft_basics/viewall.html He makes some really good points about choosing the lobe centers and overlap.
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By miker - 13 Years Ago
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Well, the above is a lot to think about, but it helped me get my head around it. The relationship between overlap and lobe centers was wrong in my head, and this gets me started. I Never get anything till I re read it, and let it sink in. Thanks, all.
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By Oldmics - 13 Years Ago
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Download one of those freebie degree wheels.Then mark the intake and exhaust events with some tape or a marker. That will give you a visual idea of the overlap zone. Oldmics
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By Pete 55Tbird - 13 Years Ago
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One reason for a cam with the same basic lift, duration and different LSA is some people want a car with the sound they associate with power so they can impress all the guys in the Good Guys parking lot. Somewhat like " if it won`t go, chrome it". Pete
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By pegleg - 13 Years Ago
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Mike, Add to the conversation that with a blown motor you don't want a lot of Overlap. The centers need to be wider for a pressurized motor to allow pressure to build in the cyl without blowing it right back out through the exhaust valve before it closes.
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By miker - 13 Years Ago
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Actually, Frank, that's what got me to thinking about cams again. I took the blower off the bird and put a set of John's heads on, added the FPA headers, and ran a 2.25 exhaust. And a proformance throttle body, hooked to an Acell computer. It calc's at 10.2 to 1, and with the 110 lobe centers on the cam, we had to pull a lot of advance out down low to stop detonating. (It's a variant on the 274 cam, widened for the blower.). I don't want to do two cam changes, so I'm trying to remember all this stuff and do something right the first time. For a change.
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By charliemccraney - 13 Years Ago
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You should be able to get it to work fine. Mine's a copy of some blower grind with 112 lobe separation and I'm currently running naturally aspirated. Our compression probably works out to be nearly the same when the requirements for an iron headed vs aluminum headed engine are considered. I have mine set for 20 crank degrees by 3000rpm with 14 initial.
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By miker - 13 Years Ago
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You're probably right in most cases. We've been running it at full load on the dyno, so the heads are hot when we make the the pulls. But even in traffic, a sudden pull ( small hill) will let it rattle a bit, before it gets the revs up. The ignition curve (two of them, based on manifold pressure) is in the computer, so it may be the crossover point is wrong. The blower was an old Paxton, running at 4 lbs @ 5000 rpm, so it was a bit low, but the static was 9:1. It ran well. The 2.97 with the 3.31 rear is part of the problem with the new set up, as you put a lot more load against the cylinder fill than it did before ( the Paxton at that ratio made no boost till 3000 rpm). But I'll sort it out. But I think the lobe center/overlap/cylinder fill is still part of the answer. And it's not a racer, it's a driver for varied conditions and temperatures. Pulling the motor to change the cam is not something I need more practice at. I'd like to get it right.
Getting a bit off my original post, sorry for the diversion.
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By pegleg - 13 Years Ago
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Mike, Get Ted or John to see if they can grind you something with 112 degree centers. I have around 230 degrees on the intake and a 236 exhaust on the red car. But, factory manifolds, and 471's with flattops. So it's about 8.5 or so. I'd think you might get away with 9.5 on the 'Bird. As far as doing it right, gimme a break, when DIDN'T you do it right. Anybody wants to understand that comment, look a Mike's profile and check out his rides. I'd kill for that Roadster!
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