crvtkilla

Yea i guess i ought to call up rick anderson and see what he would recomend. seeing as he recommended the cam and all. thanks for the input

5spd GT

Many 302/306 custom camshaft guys will recommend a 1.625" header. A few will go with a 1.750" header.

Many 331/347 custom camshaft guys will recommend a 1.750" header. A few will go with a 1.875" header.

It all is dependent on the camshaft valve timing events and overall package goals.

Whether one sees gains or not is really going to be determined by your camshaft's valve events.

With a camshaft that has less and later exhaust activity, as opposed to "vice versa," you may see some gains. With later exhaust timing, and adding larger exhaust to what already exist, it can decrease cylinder pressure, and there-by relieving you of some torque you once had.

HP = Torque x RPM/5252. What happens to the end product of horsepower, if your "plugged-in" torque is decreased (loss of cylinder pressure)?

I would only upgrade the exhaust, if the camshaft intake/exhaust valve events have been constructed around the larger exhaust point. This is why choosing the camshaft should be done last and with the long-term goal of the car is going to be.

This is why you see gains with some small to large swaps, and losses from some small to large swaps. It is the entire package, mainly the camshaft events.

There is only an 1/8 of an inch difference between each, the 1.625" and the 1.750" ; 7.7% difference in o.d. Just because an engine runs with a certain set-up, does not mean it might not run better with another.

I am going to take the outside diameter as the inside area, hypothetically.

I get 2.072" (squared) for the 1.625" long tube.

I get 2.404" (squared) for the 1.750" long tube.

Now, long tube headers range in many different lengths: 30-38".

Using a 36" tube, no taper, and assuming uniformity you are getting near 1420cc for the 1.750" pipe.

Using a 36" tube, no taper, and assuming uniformity you are getting near 1225cc for the 1.635" pipe.

Could this have an effect? Possibly, but for your 347 it has the best chance of being a good possibility. Get with Rick on his suggestions.

The O.D. is not as drastic as one may seem, just like a cylinder bore. A 4.030" bore is .015" on extra each side. Now it suddenly does not seem so big, does it?

But run that down the entire length of the pipe and it begins to seem to matter.

2.404" - 2.072" = .332" of area difference. Now multiply this difference down the length of a tube, and you see the difference increase all the way down the tube.

Now factor in the different collector lengths, styles, lengths, design, and you see how not all headers are the same in power.

Either way, I would rather have a "small" diameter tubing, but a long pipe, than a large diameter tubing, and a short tube header.

Spinning it another way...

Hooker - 18 gauge
Kooks - 18-16 gauge
Mac - 16 gauge
BBK - 14 gauge
Bassani - 14 gauge
JBA - 14 guage

All are rated at 1.75" outside diameters right? Does a higher gauge (least thick) header have a larger cross section for exhaust flow?

Another question...

We all know that a 1.625" header works just fine on an average to higher performing 302. So...

302 - 1.625" header

A 347 displaces 14.9% more cubic inches than a 302. So should we increase the header to a 1.875" header (15.3% larger than a 1.625" header).

Or, what about a daily 408, which is 35% larger than a 302. So should we increase the header diameter to 2.2" (35% larger than a 1.625" header).

Then, we have to keep in mind that many of the exhaust ports are the same size or very close throughout a variance of smaller and larger cylinder heads.

AFR 165 - 68cc
AFR 205 - 70cc

Just more questions to ponder over and from an older post....

crvtkilla

wow now that was very informative and you make a very good point of volume of the primaries over just the outerdiameter. idefinatly appreciate the post. i will get in contact with rick to see what he advises. hey and when you said you would upgrade exaust if the valve events are constructed around a large exaust point are you referring to the exaust lobe having more lift than the intake. like this cam is a 544/568 lift.

5spd GT

Some of this was from an old post of mine, but it still applies. Lift is only one tiny little factor to consider when picking an exhaust.

The valve events are the intake opening and closing degrees and exhaust opening and closing degrees and how it is timed to the engine. This is where the custom cam guy (or Rick) will tell you what he feels is best for YOUR combination. Let the professionals handle this part, and give us an update.

crvtkilla

yea thanks man. and yea i was getting the point of cam events as being degrees of the lobes and lobe seperation andand all i was just asking if lift was what you were referring to in that particular sentence i quoted from your post. but yea you are definatly right i should just let cam munufacturer make the call. thanks for the input.

5spd GT

Nope, I was not talking about just lift. Let us know what they say.

crvtkilla

cool. yea im going to give them a hollar tomorrow. im pretty curious to see what they say now. thanks