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post #1 of 8 (permalink) Old 08-30-2010, 05:18 AM Thread Starter
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BBC Headers or Mainfolds

I know this is an all to familar question but I need some advice on this subject. The purpose of my ride will be a cruise night/ weekend car; no racing; no stratosphere rpm's; just a simple 3500 rpm car.
My question is would headers really benefit me? I had header's before and my biggest gripe was heat even though I had coated and wrapped sets they still heated up under the hood. My stock BBC manifolds excel in this area but do they outperform header's in the lower rpm's? I know BBC's weak point are the exhaust runners but apples to apples would it be worth investing in a pair? This is a 68' 4 speed car so applications hardly warn you of this little tid bit of trans. info.
I know I can port and polish; invest in aluminum's all the basic upgrade's
but that will be in the long future. Thanks all.
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post #2 of 8 (permalink) Old 08-30-2010, 05:43 AM
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To be truthful yes headers make a real measurable difference. You will pick up about 25 horsepower on a stock 454 by running a set of headers over the cast iron factory log manifolds. But as you point out you are not going to be racing the car.

I would remind you of the times that you take your car out on a cool early fall morning and how much extra pep it has. That is about a 7 horse gain based upon nothing more than improved air density over your usual summer air that is also full of moisture (in the morning most of the moisture has condensated out as dew). So you can feel a 25 horse loss or gain, trouble is you get used to it and after a while you don't notice the header's contribution (nor would you miss it if it wasn't there).

A possible expensive compromise would be a set of Sanderson cast iron block hugger manifolds which offer the benefits of isolating the runners (so there is no cross cylinder contamination caused by exhaust gas reversion) with the quite and heat insulating capabilities of cast iron. If ceramic coated they look good and will not rust, plus it keeps even more heat in the manifold and your engine compartment cooler.

http://www.sandersonheaders.com/Sand...eader-Set.html

Big Dave
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post #3 of 8 (permalink) Old 08-30-2010, 06:29 AM Thread Starter
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So that +25 hp is at normal rpm range or peak which is where most companies love to sell the info. on? Not too concerned about torque due to it naturally being there on a BBC up to 2500-3000. I forgot to mention that this is a 70' 402 BBC 515 lift/228 dur. cam everything else is stock even the bore,so its sort of the 305's of the big blocks
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post #4 of 8 (permalink) Old 08-30-2010, 07:27 AM
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All horsepower numbers are peak as it is useless to talk about horsepower at any other point. There is only one peak horsepower (though it differs for every engine). But there are an infinite number of horsepower numbers for any given point on the RPM range. That is why it is more relevant to talk about torque which is what is actually measured on a dyno. Though with your cam it won't be as much as the 325 horse stock cam offered at low RPM. A longer duration cam takes the power you would have in the lower RPM range and moves it to where you rarely ever need it at a higher RPM. That is why with a very radical cam most dyno chart's do not show the engines power below 3500 RPM because there is so little power at that speed that the load applied by the dyno stalls the engine below 3500 RPM. Put that radical cam in your car it too will stall as you attempt to pull away from a light.

The marketing guys always try and make you believe that they are adding power to your car by installing their cam which is not true at all. It makes the exact same amount of total power that it always made with your old cam. The amount of power a motor makes is fixed by the amount of gas it can burn, and the amount of gas burned has to have an equal amount of air to combust. The amount of air is controlled mostly by your displacement which is why bigger engines make more power than little ones. There is no magic, or free lunch. This is why horsepower numbers are so useless, and why they are the coin of the realm in selling aftermarket parts.

Just like there is only one truth you can prove anything statistically if you ask the right question. Horsepower is measured peak torque multiplied by a fudge factor. The horsepower race started with the first salesman to use the term horsepower to sell his steam engine to mine owners. James Watt created the term horsepower to sell his engine that put out less than four horsepower by his own reckoning and was as big as a house. By his horsepower standard a real four legged horse was rated at 2.3 horsepower so it has been an inflated number from the get go.

Big Dave
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post #5 of 8 (permalink) Old 08-30-2010, 08:28 AM
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The story I heard was that one horse power is equal to the amount of work one horse can do in a full day. At a full gallop, a horse can put out about 7 horse power.

I just thought I'd throw that out there. I also don't agree about a cam having nothing to do with peak horsepower. I watched as we went from a .700 gross lift cam in a 540 N/A big block, and slowly did cam swaps (over weeks and a few trips to the track and dyno) and worked our way down to around a .640 gross lift cam. We were trying to reduce the load of the valve springs and lifters without losing too much horsepower. We watched the direct correlation between the drop in lift, to the drop in peak torque and horsepower. Nothing else in the engine changed. NOTHING. Same pistons, carb, intake, etc. We lost an over-all 50 horsepower and about .4 of a second. What we gained was two seasons without replacing lifters. (solid roller)

Don't listen to me, what do I know? I've only been doing this for 30 years.
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post #6 of 8 (permalink) Old 08-30-2010, 10:32 AM
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I didn't say it didn't affect peak horse power; I said the area under the horse power curve (total power generated) remains the same whether it is all pilled up at one end with nothing any where else, or a straight horizontal line running from idle to peak RPM (which isn't going to happen either). You can not have your cake and eat it to. So you either have your power evenly distributed, or favoring were you want to use it. On a street driven car that is from idle to about 4,500 RPM, with 90% of engine operation never getting above 3,000 RPM.

If you are racing you want as much of your horsepower as you can get it in the higher RPM range with your foot planted on the floor boards. This is because horsepower is torque applied over time. And the RPM is measured in minutes just like horsepower is. The more RPM's, the more power strokes you can get into a given minute, so you have more power you will have to work with.

Because air is a fluid just like water, it doesn't instantly start flowing the moment you open the valve. It has to build up speed by overcoming inertia and friction. Once flowing it doesn't want to stop either (that annoying physics thing again), and if you shut off a water spigot quickly you get a bang in your pipes, as the water piles up behind the valve increasing the pressure locally creating what is called a water hammer.

Your lumpy idling cam is ground to open the valve sooner than a stock cam, and to keep it open longer than a stock cam, and to close it latter in the compression cycle than a stock cam, just so you can get a column of air flowing into your engine at high engine RPM's. Trouble is at low engine RPM's your valve is open all the time which makes it real hard at low RPM to build compression (the reason a big cam with long duration kills static compression, so you have to have a lot more than a stock piston to run the big cam). In addition to the intake being open with no compression; the exhaust is opening sooner to let the burnt gasses out which is killing the power stroke at low RPM as well. Then there is the rumpity rump sound everyone craves because their hero's race car makes that noise so it must be the source of all of that power. It is caused by overlap. Which is the amount of time during passing through BDC that both valves are open and off there seats so that you have exhaust gasses from an adjacent cylinder which just dumped into the log style manifold pouring into your cylinder that you are trying to empty to get fresh air and gas into. With pressure traveling up the intake path it is even harder to get the air column to start to flow into the cylinder and there certainly isn't a vacuum in the cylinder to suck in the fresh charge. Which is why a big cam sucks at low RPM which is another way of saying driving on the street.

Getting back to the man who "invented" horsepower James Watt; he defines horse power as lifting a 550 pound weight one foot in one second or 33,000 pounds one foot in one minute. He was selling a steam powered water pump to mine owners who had to pump water out of their mines to get at the coal they wanted.

Wikipedia said it best.

"To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that ground corn or cut wood, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known.

Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today".

Big Dave
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post #7 of 8 (permalink) Old 08-30-2010, 12:18 PM
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Quote:
Originally Posted by Big Dave View Post
I didn't say it didn't affect peak horse power; I said the area under the horse power curve (total power generated) remains the same whether it is all pilled up at one end with nothing any where else, or a straight horizontal line running from idle to peak RPM (which isn't going to happen either). You can not have your cake and eat it to. So you either have your power evenly distributed, or favoring were you want to use it. On a street driven car that is from idle to about 4,500 RPM, with 90% of engine operation never getting above 3,000 RPM.
Ok, I mis-understood what you were saying.

Quote:
Getting back to the man who "invented" horsepower James Watt; he defines horse power as lifting a 550 pound weight one foot in one second or 33,000 pounds one foot in one minute. He was selling a steam powered water pump to mine owners who had to pump water out of their mines to get at the coal they wanted.




Wikipedia said it best.

"To help sell his steam engines, Watt needed a way of rating their capabilities. The engines were replacing horses, the usual source of industrial power of the day. The typical horse, attached to a mill that ground corn or cut wood, walked a 24 foot diameter (about 75.4 feet circumference) circle. Watt calculated that the horse pulled with a force of 180 pounds, although how he came up with the figure is not known.

Watt observed that a horse typically made 144 trips around the circle in an hour, or about 2.4 per minute. This meant that the horse traveled at a speed of 180.96 feet per minute. Watt rounded off the speed to 181 feet per minute and multiplied that by the 180 pounds of force the horse pulled (181 x 180) and came up with 32,580 ft.-lbs./minute. That was rounded off to 33,000 ft.-lbs./minute, the figure we use today".

Big Dave
I guess the story I heard was wrong then. I tip my hat to your superior knowledge.

Don't listen to me, what do I know? I've only been doing this for 30 years.
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post #8 of 8 (permalink) Old 08-30-2010, 07:53 PM Thread Starter
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I guess its like drinking a gallon of water. You have a fixed amount of volume --a gallon = c.i.d's, now its how you drink it--alot quickly then slower or slowly throughout or slowly until until the end = dur. and lift. it's all where you want to have it in you curve. To try to make it simple (er)
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