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A friend and I have a chance to buy another guys project. Brand new unassembled parts. Dart block, Eagle rotating assembly, Super Victor, AFR 345 with CNC chambers, app. 10.2-1 compression, Comp lifters and 278-282 at .050 cam on a 110. Cam seems way too much for a max 7000rpm pump gas engine. If we can get him to a comfortable price buying a new cam will be an option. What would be a better choice of cam for this sucker?
 

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66 El Camino 57 Chevy pickup 2004 Tahoe
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something about 20-25 degrees shorter.
 

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10 degrees shorter max. I don't think that cam is that far off. Might peak at 7000 and carry power 300 more. I have a 268/271 at .050 in my 565 and I think it should be bigger. Peaked at 6700. My afr 357 heads are bigger too.
 

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10 degrees shorter max. I don't think that cam is that far off. Might peak at 7000 and carry power 300 more. I have a 268/271 at .050 in my 565 and I think it should be bigger. Peaked at 6700. My afr 357 heads are bigger too.
I agree here. I don't think the cam is THAT far off. I DO think the duration splits are probably not right for a Dart head. Mine have generally been 12 degrees.
 

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LOL...you guys crack me up.
Horton, the AFR 345's are an as-cast head and don't flow a ton of air compares to these other two guy's combinations (Jon and Ray). The less cylinder head, the more cam you're going to need.
If you want an idea of how that cam is going to work with YOUR combination, give Chris Straub a call. He'll give you an honest assessment and will tell you if it's close enough to not worry about, if you're leaving a few hp on the table, or if it's way off on left field. Other cams that may or may not work in other combinations honestly has nothing to do with your cam or your combination.
 

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I have a 260/270 on my 540 peaks like 6,200 and is a hydraulic roller, 10.7 compression ratio and ported 320 heads. Its not so much the peak power but you want it to idle nice. Its been my experience if it a pump gas street car with occasional use go with a slightly more conservative cam so it idles around town better and has better low / mid range response. You are not going to notice the difference between 740 hp and 720 hp but stalling at the stop light and the unburnt gas fumes all the time you will notice. That compression is way too low for that cam. At 10.2 you need to be in the 260 duration range or you low speed and static compression ratio will suck and if you have an automatic with a tight converter it will stall unless you get the rpm above 1,000 when you put in in gear. But if you race the car a lot and are looking for maximum effort I would still change cam. I see from you signature you have got a Bel Air with 4.30 gears, well with that 565 you wont need those and could probably drop to like 3.73s - you will probably make 150 ft pounds more tq over your 461.
 

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I have a 260/270 on my 540 peaks like 6,200 and is a hydraulic roller, 10.7 compression ratio and ported 320 heads. Its not so much the peak power but you want it to idle nice. Its been my experience if it a pump gas street car with occasional use go with a slightly more conservative cam so it idles around town better and has better low / mid range response. You are not going to notice the difference between 740 hp and 720 hp but stalling at the stop light and the unburnt gas fumes all the time you will notice. That compression is way too low for that cam. At 10.2 you need to be in the 260 duration range or you low speed and static compression ratio will suck and if you have an automatic with a tight converter it will stall unless you get the rpm above 1,000 when you put in in gear. But if you race the car a lot and are looking for maximum effort I would still change cam. I see from you signature you have got a Bel Air with 4.30 gears, well with that 565 you wont need those and could probably drop to like 3.73s - you will probably make 150 ft pounds more tq over your 461.
How does compression ratio effect the cam?
 

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I'm with Lew on this one...compression seems low for that size cam. I may not have this perfectly right and I am sure others will comment. I think the simple explanation is that the dynamic compression ratio of the engine (cylinder pressure) actually declines as the duration of the cam increases. As a result, for cams with large duration you need to start with a higher static compression ratio...if you don't, the resulting dynamic compression will be so low that the engine will simply not perform as well as it should. Sound about right?
 

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I'm with Lew on this one...compression seems low for that size cam. I may not have this perfectly right and I am sure others will comment. I think the simple explanation is that the dynamic compression ratio of the engine (cylinder pressure) actually declines as the duration of the cam increases. As a result, for cams with large duration you need to start with a higher static compression ratio...if you don't, the resulting dynamic compression will be so low that the engine will simply not perform as well as it should. Sound about right?
No such thing as dynamic compression ratio and compression occurs after both valves are closed. Cam has already done it's job evacuating and filling the cylinder. Compression is not an issue in cam design.
 

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No such thing as dynamic compression ratio and compression occurs after both valves are closed. Cam has already done it's job evacuating and filling the cylinder. Compression is not an issue in cam design.
I must need to go back to school. Yes, true compression occurs after both valves are closed, but to me what you are describing is exactly what you are dismissing...you are describing the resulting dynamic compression ratio (or cylinder pressure that can be measured) which is the result of two factors: 1) the physical static compression ratio of the engine 2) the cam's profile/duration. Regardless of what static compression ratio you start with, all things being equal, if you select a camshaft with 280 at 0.050" you are going to have less cylinder pressure than if you have a cam with 236 at 0.050". It is that drop in cylinder pressure associated with large duration cams that requires more initial static compression.

Also, I didn't mention compression as an issue in cam design...only in cam selection.
 

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I must need to go back to school. Yes, true compression occurs after both valves are closed, but to me what you are describing is exactly what you are dismissing...you are describing the resulting dynamic compression ratio (or cylinder pressure that can be measured) which is the result of two factors: 1) the physical static compression ratio of the engine 2) the cam's profile/duration. Regardless of what static compression ratio you start with, all things being equal, if you select a camshaft with 280 at 0.050" you are going to have less cylinder pressure than if you have a cam with 236 at 0.050". It is that drop in cylinder pressure associated with large duration cams that requires more initial static compression.

Also, I didn't mention compression as an issue in cam design...only in cam selection.
That's not necessarily true.
Valve events (not just duration) are figured around an engine's displacement, rpm, induction, power and a few other factors, none of which are compression. If compression is not taken into consideration with a cam's valve events then how can it be part of the selection?
At least you made the correction from "dynamic compression ratio" to a reference to cylinder pressure.
I've seen some pretty big cams on some low compression engine make some pretty big power. How about a 10.0:1 565 pump gas deal with 270/278 that made >920hp @ 6800? but then we have a 14:1 565 that made 965 hp @ 7000 with 272/278? Or a pump gas 540 with 10.5:1 that made 909 @ 6200 with 270/277?
With what you're saying, you're only taking into account one valve event and that's intake valve closing. There's just way too much going on to isolate one event like that an make those generalizations.
 

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Basically yes both valves are closed but they are closed further up the cylinder so part of the air has been blown out the exhaust valve. In the compression stroke the valves are still slightly open still then close quickly. You will notice in the cam selections in the manfactures catalog they say needs at least 10 or 11 to 1 compression as the cams get bigger. It is really more of a low/idle speed item when the air has the actual time to be blown out of the exhaust. I find that the high compression seems to burn the gas better at idle - less smell.
 

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My comment is not so much max power but the lugging around town and sitting at idle, power brakes, and the issues you run into a big cam when you do that.
 

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Basically yes both valves are closed but they are closed further up the cylinder so part of the air has been blown out the exhaust valve. In the compression stroke the valves are still slightly open still then close quickly. You will notice in the cam selections in the manfactures catalog they say needs at least 10 or 11 to 1 compression as the cams get bigger. It is really more of a low/idle speed item when the air has the actual time to be blown out of the exhaust. I find that the high compression seems to burn the gas better at idle - less smell.
The ex valve has been LONG closed when the piston starts on the compression stroke. The valves are not still slightly open. That happens on the exhaust stroke during overlap. The intake valve closing is what determines cranking compression but that has very little to do with a running engine and dynamic cylinder pressure.
Catalogues are for selling cams and that's why it's always best to talk to an actual cam designer when looking at a cam selection.
 

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You are correct not the exhaust I meant the intake valve is still open a little at compression stroke. I was thinking of the scavaging of the exhaust when both intake and exhaust are open which is different part of the cycle.
 

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My head hurts...I didn't think that the concept of big duration cams preferring more compression was so controversial...sorry for the generalization =). I will stop now before I get quoted again and reminded that I don't know what I am talking about.
 

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I agree here. I don't think the cam is THAT far off. I DO think the duration splits are probably not right for a Dart head. Mine have generally been 12 degrees.
I made a mistake in this. I see now you have afr heads and a DART block. Disregard my comments about the split. I run dart heads. I've heard afr doesn't need as big a spread, but don't know exactly how much.
 
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