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  #31 (permalink)  
Old 12-20-2012, 08:10 PM
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The thing about how new cars w/o a lumpy cam can run higher CR than we can w/sbc carbed engines is mainly because we don't have electronic feed back from sensors to a computer that can retard timing when knock occurs, trim the fuel/air ratio, and change the cam timing just to name a few.

Now that's not to say we cannot use higher than 'normal' compression on a finely tuned engine. But w/only about a 4% (according to prevailing opinion) gain- all else being equal- to be had for each point of CR, I see no reason to run at the ragged edge of detonation. And I don't believe that's what TI is saying to do, either, but I do not presume to speak for him- he can do that just fine w/o my help.

Too much of ANYTHING is not good. Like the medical profession says, "The only difference between a poison and a cure is the dosage." I keep the CR for pump gas engine w/iron heads in the 9.5:1 range and aluminum a point higher, max. BUT- the Vortec (and other) modern fast burn chambers allows more CR w/o detonation than what could be safely used in the older heads. Obviously cam timing matters, as does ignition timing, air/fuel ratio, plug heat range, air inlet temps, engine coolant temps, etc. So there is no ONE right way. ALL the parameters have to be considered.


DCR vs. the real world:

The thing about DCR is (IMO) it DOES try to account for what an engine "sees" in in a running condition. That's the whole reason in calculating DCR in the first place. Now, if DCR used cranking pressure as a datum point, I could see how an argument could be made that it doesn't reflect what a running engine sees, but it doesn't.

DCR is not the end all, do all of engine parameters. But it IS a 'tool in the chest', so to speak. I think the more info the better, at least if that info is able to tell something useful. And DCR has- in the right hands- been shown to do just that.


Vortec heads vs. cam timing:

The following are my opinions.

The L31 Vortec head is probably the most over-cammed head GM has ever produced!

Guys think that because it's said to flow better than the old Bow Tie heads, they can run 240 degrees @ 0.050" duration, no problem. If it were only that easy...

The facts are the heads do not continue to flow well cammed much more than about 0.500" lift and 224 degrees on the intake side. And even there the exhaust is lagging well behind the intake, ratio-wise, and needs help. A split pattern cam will help but the exhaust port actually should be ported to gain the intake to exhaust ratio that's needed. And to use more cam effectively, the head needs help on both sides.

I say to use the Vortec head as-is (with the usual clean-up, valve job/guides, etc, as needed). If more head flow is needed, go to the aftermarket.

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Last edited by cobalt327; 12-20-2012 at 08:25 PM.
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  #32 (permalink)  
Old 12-20-2012, 08:19 PM
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Originally Posted by ap72 View Post
Just because it revs to an RPM does NOT mean it makes power there. A lot of people think that just because they can "feel it pull" at a certain RPM then it must be making big power. That's part of the reason everyone that owns a sbc with an aftermarket muffler is making 500+hp, they just hope and wish and think about the power until it's magically there.

well obviously i shift past peak. But who doesnt to some extent? In some applications a lil over rev is very helpful.. like in mud.. or climbing up a sand dune for example.. I believe people sell these heads short. most cookie cutter street builds have low compression ratios and streetable torque converters, so of course they work best with the smaller cams... small cams dont pull as far as larger ones...true story. If a statement was made like "vortec heads are done at 5500 on street engines" i would agree with that, but circle track guys have proven over and over there is more hp potential to be found with these.. it comes at the expense of streetability... and i have already stated that is no concern of mine. its purely a toy.
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Old 12-21-2012, 08:32 AM
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Originally Posted by cr480r View Post
well obviously i shift past peak. But who doesnt to some extent? In some applications a lil over rev is very helpful.. like in mud.. or climbing up a sand dune for example.. I believe people sell these heads short. most cookie cutter street builds have low compression ratios and streetable torque converters, so of course they work best with the smaller cams... small cams dont pull as far as larger ones...true story. If a statement was made like "vortec heads are done at 5500 on street engines" i would agree with that, but circle track guys have proven over and over there is more hp potential to be found with these.. it comes at the expense of streetability... and i have already stated that is no concern of mine. its purely a toy.
almost everyone revs past peak power when racing, that has nothing to do with the point. Your cam is too big for your rpm range, by a very good margin, you can think and wish and argue about it all you want but it is what it is. It will run, but you're giving up a LOT by not using a smaller cam. Power, torque, fuel, durability, etc.
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Old 12-21-2012, 08:47 AM
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you new hydraulic cam is not all that big really. What are the rest of the specs on the cam?lobe separation
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Old 12-21-2012, 09:20 AM
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Originally Posted by ap72 View Post
almost everyone revs past peak power when racing, that has nothing to do with the point. Your cam is too big for your rpm range, by a very good margin, you can think and wish and argue about it all you want but it is what it is. It will run, but you're giving up a LOT by not using a smaller cam. Power, torque, fuel, durability, etc.
Check out the 280xfi cam from Comp, but have it ground on a 108-110 for your combo instead of the 113 they usually offer.

It's a good match for ported vortec heads.
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Old 12-22-2012, 01:06 AM
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you new hydraulic cam is not all that big really. What are the rest of the specs on the cam?lobe separation
240/244. 108 sep. .335 lobe lift.. will give .536 with my rockers
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