Replacing the OEM headers with aftermarket headers does not seem to significantly increase power based on what I have seen on here from others' results, notably: Les (velociabstract).

This has bothered me. Headers are so pretty. On V8 engines good headers can dramatically increase horsepower.

I have been trying to understand why this might be. The OEM headers have 1.75" primaries, which is great. The log style is not great, nor does the 1.75" outlet seem to be.

Here is what I have come up with. On V8 [and inline 4] engines, 4 cylinders feed into each header. For an even-firing engine, the exhaust pulses are separated by 180 degrees of crankshaft rotation. On a 90 degree V6 there are only 3 cylinders per exhaust bank, with 240 degrees of separation between pulses.

I think the scavenging effect in the collector may be largely lost on a V6 because of the increased separation between the exhaust pulses. I have tried to graph this below. With 4 cylinders per bank, the closer exhaust pulse spacing would overlap the low pressure part of the pulse with the opening of the next exhaust valve in the firing order.

v6exhaustpulses_zps3af40da5.jpg

 

Where did you get the 1.75" primary/port info. The 32's, 320's, 430's and 500's are 1.5" ports. I do not know for the 55's, sc or not. 1.75" is too big for primaries on the V6's based on every bit of research I've done unless they are only ~20" long each which would help power up top on the rev range but that isn't where I would focus to build power if it were me.

The M113's (V8's) aren't even spaced firing, yet 55K guys still pull massive power from headers.

It's just a matter of the right engineering for construction, material, and the one thing I was bashed on that I now agree with is space constraint. With how many bends that would have to go into each primary to pull off fit, it would be extremely hard to make it smooth flowing enough to really pull good power. I have an idea to get around this but we'll lose ground clearance.

 

I also suspect that you might get some gain on the NA. With the boost forcing the exhaust out on the SRT6, the scavaging effect is negligible.

 

I measured it. The OEM primary tubes are 1.75" OD or the nearest metric size. I would expect the ports to be slightly smaller and incorporate an inversion dam.

 

Gotchya.

Yeah the ports are tiny. I've often wondered why they used the same ID size for all the different displacement's but they seem to have thought that it was fine. Probably less cost so they said screw it lol

 

I'll throw in my 2¢. The supercharger cam and the relatively low redline are the culprits. But I'm just guessing.

Les

 

What a complete bunch of crap. In any kind of automotive engine in any cylinder configuration and number of cylinders the following will be true: If the headers are designed and constructed to be less restrictive than the OE manifolds or psuedo headers, and are in fact less restrictive than what they are replacing, they will be more efficient in allowing exhaust gases to exit the engine more freely. The engine will be more efficient and make more power.

 

If you say so Mr. Complete Bunch of Crap. Just so you know, I did a pre header dyno, installed long tube headers on my SRT-6, did an open header dyno and finally a dyno with the stock resonator to muffler. 4 hp gain. Not a lot for the money. You can add up the cost. I did around 10 dyno pulls, the cost of the headers, expense of the install, retune. Perhaps a bit more could be found with a true dyno tune but even then you won't find much IMO. Almost all the gains I found in the exhaust were from removing the cats.
So be like me, don't believe it and do it yourself based on results from cars you've had in the past.

Les

 

Initially It sounded like it was just a "feeling" you had based on conjecture, rather than a documented process and verifiable results.The results tell you one of two conclusions: The OE setup is very good and not restrictive or the headers are not very good and in fact as restrictive as the OE setup. Probably the former.

 

Well that is not entirely true. On a boosted engine headers are much less important than on NA engines since you are forcing (forced induction) the charge in and with care in the valve overlap that can also be used to force the residual gases out. As long as unclogged and the flow is essentially lamiar, you could just stick a tin can on the exhaust (why supercharged aircraft enginess often just had stub stacks).

So a header will not do much on a SRT.

With a naturally aspirated engine the rules are different. The more exhaust gas you can suck out, the more charge you can suck in. Intake is a whole different subject that we will skip for now.

OK now six cylinder engines are special as long as even fire (why the XF has offset journals like a 1987 Buick) in that the exhaust pulses overlap. You can tune a street 6 very well with a single tailpipe. OTOH a race engine with a lot of cam may need duals to avoid collisions (and a proper set of duals like a Jag with bananas or a Corvair with trombones has the sweetest sound in the world).

Probably should mention that I really like a good six. Know what to do with a proper V8 particularly with FI but just like sixes better.

That said a street six that doesn't wind past 6,000 just needs a single exhaust and a tuned equal length exhaust manifold can really be sweet, exhaust pulses every 120 degrees can be used to scavange (the exhaust pulse has charge inertia that can create a vaccuum behind it to help suck the next pulse along. Doing this requires maintaining lamiar flow and a near constant charge speed which is more art form than science.

So a proper exhaust will help suck the exhause charge out of the cylinder and make room for the next intake. Some people have trouble getting their minds areound the fact that both the intake and exhaust charge have both mass and inertia.

Bottom like: with a boosted engine particularly with a belt driven supercharger, the exhaust flow dow not really matter, it will get pushed out of the cyl. A nine exhaust will make a small difference but nothing turning up the boost wouldn't do.


OTOH a NA engine needs all of the help it can get, both intake and exhaust. That said if you understand that the XF 3.2 really is a product of the last century with all of the limitations that implies, it can still do a lot and make the best use of the technology then available.

So the title should really be "Why headers don't increase power on supercharged v6 engines"

ps to really understand an IC engine I'd suggest beginning with a copy of Sir Harry's "The High Speed Internal Combusion Engine". Mine is a 1958 edition.

 

My headers are of the highest quality. I made 26 hp over the stock exhaust with the Needswings downpipes open and another 4 hp with the headers. 26 + 4 = 30 so they do make 30 over stock but the power comes from removing the cats. The advertise 30 hp over stock more or less.

Les

 

All good points and understood. Though . . . a less restrictive header or manifold will be be better that any setup that is more restrictive.

 

Nick, your not crazy. Every car I've put headers on, before this one, made nice gains, even on low reving engines. This is my first factory supercharged car and headers truly aren't worth the expense, for whatever the reason. I should clarify that my prior experience was on the choked smog era cars ie 70's and 80's.

Les