Header Clearance Info

So, knowing how fond you people are of funky engine swaps, I'm sure you have had trouble with header clearance and maybe had to ding a tube or two to make it fit. Well, guess what, it doesn't matter when it comes to power, unless you go beserk.

https://www.youtube.com/embed/azPKIjxmmdU?feature=player_detailpageH

What do you mean "swaps"? I've had to beat headers to fit OE applications.

And then there was the Ventura that I saw, where the #1 header tube was hammered into a crescent moon shape to clear the steering. Not just D shaped but intensely concave for 6-7 inches.

Could it be that the headers were too big to start with ?

iceracer wrote: Could it be that the headers were too big to start with ?

Quite possible. Although I was expecting that even then, the uneven, turbulence-causing header walls would have had a worse effect.

iceracer wrote: Could it be that the headers were too big to start with ?

Could be, but these guys are pretty good at setting this stuff up and usually dyno shops have a ton of headers laying around for test purpose.

iceracer wrote: Could it be that the headers were too big to start with ?

Pontiac V8s are nightmares. The manifold flange faces DOWN. There's no room for tubes. There is barely enough room for a log manifold.

I'm sure it made sense in 1955, not so much when trying to cram a V8 into an X-body. They were a tight squeeze even in the relatively roomy A-bodies.

But hey, on the bright side, one generally can access all eight plugs while standing up, with a 3' extension and a swivel socket, and you NEVER have to worry about burning a plug wire.

Knurled wrote:

iceracer wrote: Could it be that the headers were too big to start with ?

Pontiac V8s are nightmares. The manifold flange faces DOWN. There's no room for tubes. There is barely enough room for a log manifold. I'm sure it made sense in 1955, not so much when trying to cram a V8 into an X-body. They were a tight squeeze even in the relatively roomy A-bodies. But hey, on the bright side, one generally can access all eight plugs while standing up, with a 3' extension and a swivel socket, and you NEVER have to worry about burning a plug wire.

Just need to do Pontiac headers like this!

[URL=http://s240.photobucket.com/user/NOTATA/media/The%2014%20Car%20Performance%20Therapy/003_zps61elounw.jpg.html][/URL]

Not enough internet to watch right now, but I suspect most engines have oversized headers on them, giving some breathing room (literally) for a dent or three.

edit: Watched, yeah, same thoughts, I'd like to see this repeated with a real monster 8000rpm engine, or the same 400 with smaller tubes.

In reply to Kenny_McCormic:

Versus a Cup car's stepped and tuned and "perfect" header. (The end of that log header is closed off, there is only one outlet)

Guess what the power difference was!

In reply to Knurled:

You've exceeded my knowledge on the subject, but I'd like to know more. I like the reflector cone (I'm guessing) on the dead end of the log collector.

IIRC, the difference was about 10hp and there wasn't really any spot where the power varied wildly, it was consistently within a couple percent, lower or higher.

Also IIRC the point the guy was trying to make was that, beyond the first eight inches or so, the header design isn't really all that important.

Knurled wrote: beyond the first eight inches or so, the header design isn't really all that important.

(provided the pipes aren't too small)

A lot of people go way overboard on exhaust sizing.

I remember a while back, I had a 200+hp 13B that still used the stock 12A RX-7 midpipe, which has an OD of 1 7/8".

I eventually did replace it with 2.5", but only because that was part of the process of adding mufflers to the car.

I've also seen 1000+hp cars with 2.5" exhaust systems. I get a real kick out of guys who OMG NEED 3" EXHAUST for cars making only 300-400hp.

As far as header diameter goes... how big is the port? Does the header NEED to have larger tubes than that? Stepped tubes are more for the "airflow diode" effect more than the need for more diameter.

Keep in mind, the maximum amount of power you can possibly make with a given exhaust size is far more than you'd optimally use that exhaust for.

On an N/A V8 making just under 250whp with a single 3" exhaust, I've seen someone put in a cutout and show a 12hp difference with it open vs closed. So while they could make that power with the 3" exhaust (and probably more if they upgraded other parts of their setup), going to a 3.5" system would likely have gained them a little power (and the difference would only get bigger as the pushed the rest of the combo further).

That would make sense, e.g. look at 4 stroke single cylinder kart race motors where the exhaust is often just a short pipe of somewhat tuned length, they'd be using something better/longer if it existed/helped.

I found this discussion on it, builder claims it lost 8-14hp, on a 790hp 8800 rpm SB2 cup engine compared to the usually 4-2-1 header used, and followed that curve. 8" 1.75" tubes (the usual size for one of those) stuck 0.5" into the 3.5" log.

http://webcache.googleusercontent.com/search?q=cache:eRpoeC7i7j8J:speedtalk.com/forum/viewtopic.php%3Ft%3D8305+&cd=1&hl=en&ct=clnk&gl=us

Also, these guys set some records with a similar principle.

In reply to rslifkin:

I have a feeling if you changed that single 3in 250whp v8 to a dual 3in you wouldn't gain that 12hp. Cutout probably makes the difference just reducing the number of bends (pressure drops after the bend) in the system.

Perversely, a cutout can compensate for a too-LARGE exhaust because if the exhaust is large enough that velocity is lost (or the muffler design sucks, like for example Flowmaster, which follows the naming convention of products like Duralast (not durable, doesn't last) or the People's Democratic Republic of Whoever) because... I'll start over.

Barring other issues like a really crappy muffler ( cough Flowmaster cough ), a cutout can benefit a too-large exhaust PIPE because the exhaust moves too slowly, so it can't pull itself out with its own inertia, it needs more effort to be pushed out. Too large of a pipe will create its own backpressure.

None of this really applies to turbos, though, since the useful exhaust inertia is spent on spinning the turbo. At that point, you can't really be harmed by going larger.

chiodos wrote: In reply to rslifkin: I have a feeling if you changed that single 3in 250whp v8 to a dual 3in you wouldn't gain that 12hp. Cutout probably makes the difference just reducing the number of bends (pressure drops after the bend) in the system.

Yeah, I figure the cutout had to exaggerate whatever gains would be seen with a bigger exhaust, considering there was a high flow cat and straight through glasspack-style muffler on the thing that it was bypassing and there was less pipe to push through as well. But going to a single 3.5" after the merge (it was 2.5" off each header) with correspondingly bigger cat and muffler would probably have gained a decent chunk of that 12hp.

FWIW, GM felt that a 3.5" single was necessary on the 400-ish hp 6.2l SUVs, so I doubt it's oversized enough to hurt for the high 300s to low 400s power range (at the crank).

GM also does great things with the OE manifold design. They have a kind of taper collector thing going that terminates the exhaust acoustically before the converter. Basically, after that, the exhaust doesn't matter because of the velocity at the merge.

You don't gain much if anything with shorty headers, and it is easy to lose power with longtubes. GM did their homework.

When we did a big PITA swap with one of those engines a little while back, I argued in favor of stock manifolds, arguing that there was minimal power loss but large thermal benefits in the cramped-confines situation we had. I was vetoed, because the owner wanted headers, and that was that. Ended up making some complicated heat shielding, and I felt vindicated that the headers didn't really grant the engine any more power.

Knurled wrote: As far as header diameter goes... how big is the port? Does the header NEED to have larger tubes than that? Stepped tubes are more for the "airflow diode" effect more than the need for more diameter.

From what little I know I'd think you wouldn't want any bigger than the valve seat ID or port area.

Another thing to consider is the exhaust is cooled as it goes through the head and down the tube, making it denser and presumably reducing the tube size requirement, e.g. look at the pea shooter tailpipe on a lot of 80s and 90s crapcans, 80-100hp through a tube I'd almost get my thumb stuck in, because it's 8ft down the line.

iceracer wrote: Could it be that the headers were too big to start with ?

The engine only made 559 hp with 1500 cfm of carburation....the headers were most likely too small to begin with. I would have looked for 1-7/8" based on the cubic inch and intake system, but I don't have any idea what the head flow was so you can't say the 1-3/4" were wrong or not. Ultimately they may have started with 1-3/4" in order to be too small to begin with and potentially see a greater impact with the damage. But the combo was definitely odd.

We took a set of box Hookers, modified 4 tubes and made 26 HP more. We don't race dynos, but every top running team has quality headers that are not beat up. And I only know of one car with records that has beat up headers, but it is blown.