anesthes

iTrader: (12)

Heads are trickflow 23. They flow 177cfm at .500 lift, and 183cfm at .550 lift (exhaust). I'm running a .528 exhaust lift cam, so say around 180cfm.

Looking at headers, options are Hedman 1 5/8 coated fro $500 or Hooker 1 3/4" coated for $650.

Hedmans are 14 gauge and have an inside diamater of 1.459"

Hookers are 18 gauge and have an inside diameter of 1.642"


Both headers are coated on the outside only, not on the inside.


The hedman headers, considering the following formula will flow 183.21 CFM.

1 5/8" - .166 = 1.459 ID. 1.459 / 2 = .729. .729 x pi = 2.29 sq in.
2.29 * 80 cfm = 183.21 CFM.


The hooker headers using the same formula flow 207 cfm.

1 3/4" - .098 = 1.652 ID. 1.652 / 2 = .826. .826 X pi = 2.59 sq in.
2.59 * 80 cfm = 207 CFM.


Assuming my math is good, do you think I should just go with the 1 5/8" tubes which appear to support my head flow?

-- Joe

Sonix

iTrader: (1)

Are you looking at these for your 'vette?
That flow number on the heads is based on NA, if you've got the blower your flow is SIGNIFICANTLY higher eh?

I've never seen math like that for headers. I'll be damned, I feel like such a hillbilly sometimes...

anesthes

iTrader: (12)

Yep the vette.

Well that is a good question. I'd think that during the overlap period (2 degrees on my cam at .050") the intake pressure would force the spent charge out the exhaust, so in theory maybe it changes the flow? Most of the data I've read is based on natural aspiration, and I can't find a whole lot if any data on primary sizing for boosted applications... I do know the scavaging effects are null when running boost because, well again, boost fill the cylinders not vac.

Now again, in N/A theory, oversizing the tube would result in a major loss of low end performance. Something that with a miniram or singleplane you would not want. I think in theory, a naturally aspirated engine with my heads should probably be 1 5/8" primary. However, what I'm not sure about is how the boost goes into play.

Hehe. If it was a $300 purchase I'd go with 'bigger, better' but since we're talking big money here for such a minor thing I want to buy the right part.


Now I do know a guy running low 10s on a LT4 engine in a '96 Corvette with short 1 5/8" tubes. Now, since he's a second faster than me does this mean 1 5/8" are fine for me, or does it mean he's really losing a few 10ths due to back pressure.

-- Joe

Sonix

iTrader: (1)

I don't have any direct experience to lend you, but my gut tells me that with blown/nitrous engines, the exhaust valve, and exhaust port are crucial. They are typically oversized because the blower will force more mix into the intake, but the exhaust can only rely on "normal" forces to expel the mixture. So any added size is of huge benefit.
On that token, I would assume the bigger headers would be better.
I don't think you'll lose much low end with 1 3/4" headers on a 350, you're not going too "wild" with that.

Fullsizewagon

anesthes, you forgot to square the radius, so the areas will be significantly smaller, especially concerning the 1-5/8".
I'm not sure though if cfm is easy to apply to primary tubes as pulse behaviour is the primary concern with them. Cfm is also related to a specified pressure drop and may be more or less length dependent. Where did you find that 80-number?
If I were to spend money on coated headers I'd get inside coated ones to ensure their durability. I don't have first hand experience with this but it makes sense to me to insulate the steel from the hot gases and I hear the outside coating can eventually flake unless the inside is coated too.

anesthes

iTrader: (12)

Ugg, you are right. So the 1 3/4" tube actually has an area of
2.1434332693206293 = 171 cfm. So I guess in theory 1 3/4" is too small.


The 80cfm per square inch number was taken from Dave Vizard. While I don't take him for gospel, nobody else has any numerical data.

-- Joe

anesthes

iTrader: (12)

Here is the corrected numbers:

he Hedman headers, considering the following formula of 80CFM per square inch, and 15 gauge will flow 133.74 CFM.

1 5/8" - .166 = 1.459 ID


Area = 1.6718621478590436 * 80 cfm = 133.74 CFM.


The hooker headers using the same formula flow 171.47 CFM.

1 3/4" - .098 = 1.652 ID.

Area = 2.1434332693206293 * 80 cfm = 171.47 CFM.


I made a math error previously, so this post is corrected.

It appears from these numbers that, a 1 5/8" header will support a stock application. However pretty much any aftermarket head flows 160+ CFM..

It seems as if, for good aftermarket heads you really want at least a 1 13/16" header. I'm not sure such a thing is available for C4's, but it makes sense why a lot of C5 guys are running 1 7/8" headers.

-- Joe