You already have headers, the SSIs are a slightly improved version of what came w/ your car. Should your oems need replacement then SSI is a great choice. The MFI needs heat to function correctly, heat now comes from a tap in the left heater box, SSI are available w/ the same tap. Straight headers would need to have heat ducted from them via some sort of jury rigged duct or use an electric heater..

Just to be clear: headers and heat exchangers are two different things. Heat exchangers are the boxes wrapped around an exhaust to provide hot air for the car's heating system. 'Headers' is a term used to describe some exhaust pipe schemes, where the tubes tend to be longer, and of uniform (hopefully close to equal) length. For the first 10 years or so of the 911's production, all 911's came with headers (that happened to also have heat exchangers). EPA regulations mandated a change in 1975 (I'm guessing on that, but it was around that time), and the 911SC, Carrera and 964 had the exhaust from one side dump through a crossover pipe so that a single catalytic converter could be used for both sides of the motor. This is a less efficient arrangement. (It still had heat exchanger boxes).

The 993 went back to a header-type design, albeit one with shorter-length primary tubes. Aftermarket exhausts like SSI's mimic the early header-type design, and include heat exchangers. Some custom-made or race-only headers save the cost of adding the heat exchangers, which means your car doesn't get heat or defrost.

Heat exchangers do have the secondary benefit of removing some heat from the exhaust. It doesn't matter what position the cabin levers are in. Either you're venting that heat to the inside of the cabin, or it's being dumped outside. The system is moving air through the heat exchangers whenever the engine is running -- and even more when the heater's fans are turned on. The 964 (and 993?) both have a feature where the heater blower will continue to run for a period after the car is shut off if the engine is hot. This helps pull heat off of the exhaust and reduces the amount the exhaust is 'cooking' the engine.

Actually they are all headers, a header is just a way to take separate pipes and connect them into a single pipe, somtimes the reverse. The pipes generally are used as conduits for liquids or gases but not always.

We build headers to connect the 8 - 12 separate fuel tanks scattered around a boat, your home heating boiler will have a header to connect to the different heating zones in your house(or AC in warmer climes), the universe can be modeled as a 10 dimensional space-time manifold

the old 911 exhaust, ssi, SC, Carrera, 964, 993, 996 are all just different configurations of manifold, in mathematics there is a similar concept that conforms to a very precise set of equations, there are also intake manifolds.... but I digress

Everyone of the pics in this post is an exhaust "header" some of them have the sheetmetal wrapped around them to turn them into heating boxes, hence the term "heater box"


As previously stated there are many possible configurations and materials used for exhaust manifolds, the cheapest is an iron casting where no effort is made to smooth or equate the branches one w/ the other(think 70's chevy inline 6), the most efficient is an equal length header system.

When there is a 5^th cycle opportunity provided by valve overlap an equal length header can enhance cylinder filing sort of like a turbo, but only in a very narrow rev band. this happens when the rarifactuion part of the exhaust wave is present at the exhaust port when it and the intake are open(overlap) the enhanced vacuum sucks additional charge into the cylinder enhancing volumetric efficiency

http://forums.pelicanparts.com/uploa...1123883834.jpg

But modern smog cams have very little overlap, so the ability of an equal length system to enhance engine performance is severely limited and a less efficient design with adequate internal volume will work as well.

Equal length headers are also called "bundle of snakes" because to get the equal lengths the pipes are often quite convoluted

a 993 long tube header 29" pipes
http://forums.pelicanparts.com/uploa...1123884227.jpg

Bill, I'd agree with you all the way until the very end where you say modern cams have little overlap, where in fact the MOST modern camshafts such as those from Honda V-Tech motors and those like Porsche's Vario-cam plus use two different lobes, one which has more overlap and does allow the effective use of equal length tubes. Thus Honda can get 240hp out of a 2.0 liter inline for, and still be one of the cleanest burning engines around. Just too bad this technology is not available for our old tech 911 motors.

One should also separate the scavenging effect from adjacent cylinders from the effects of one cyl. on 'itself' at a later time....

Confused yet? Let me try some edits:

from JO: "headers and heat exchangers are two different" concepts. The stock early units and SSIs are headers with a heat exchanger wrwapped around them as in the BV pics.

A header really just means the proximal part of a manifold, but hot rodders for decades have used the term header to mean equal length exhaust headers used for peakier higher hp output engines.

911 people have often used the term headers to mean the headers w/o the heat exchangers - tho this is not really correct.

I am not gonna comment on the super-string theory troll...

kodabear, ya gotta remember that I am an old fogie, modern stops at 993, 996 and beyond are future fantasy.;)
Thats the whole point ogf variable cam timing and lift, to get the good idle and emissions from zero overlap at low speed and then to get the power surge from the 5^th cycle when the duration and lift are extended and the pipes can start resonating.

Note, just as there are points inthe rev range where there is beneficial resonance there are also points in the rev range wher there is detrimental resonance, caused by a high pressure peak at the exhaust port,

What tuned pipes give at 1 rpm range they take away at another (w/o variable cam and intake geometry), thius can be alleviated somewhat w/ longer pipes that tend to smear out the peaks and troughs a little.

So, in practicality, there is no real benefit for me in getting SSIs on my 69T, beside having two less items to check for rust...

The SSIs are slightly closer to ideal equal length than stock, and they will not rust like early stock, towards the end the factory did start using beter grades of steel but never as good as that used in the SSIs

The factory philosophy in those days was to use the best steel in the pipe and a lesser grade for the cover. That way the end used always replaced the heater box/header before the pipe rotted and introduced CO₂ into the cockpit.

CO₂ into the cockpit was a major concern at that time.

Bill:
What's this fifth cycle term you used? I have never heard that before, and I'm an old school guy also.

That's CO -- i.e. CO1 not CO2 -- there is plenty of that in the cockpit all the time. CO, carbon monoxide, is highly reactive with O (Oxygen) and will actually 'pull' bound O out of the blood stream in the alveoli. That's why it is so deadly.


5th cycle is a handy abbrev. relating to the 4 cycle (Otto) engine we drive... as close as you can get to getting something for nuthin' and your tricks for free....

thanks, randy
Does that imply we should have a 0th cycle for the intake? I never did care for those staid old T-S and P-V diagrams, anyway.

Yes, I meant Carbon Monoxide(CO) not Carbon Dioxide(CO₂)

The so called 5^th is the overlap period where the low pressure at the open exhaust port is sucking the spent charge out faster and at the same time sucking the fresh charge in faster through the also open intake valve.

Smog cams do not allow for this to happen. It is the reason the 2.7RS was such a happy motor and the 2.4S was such a peaky motor

OK...IN the US, particularly California.....smog rules required restrictive exhaust systems....in the early 70s there were add on devices that while reducing emissions cooked the motor...this the 2.7 debacle.

Early 911 (pre 74) engines had "tuned" equal length Heat Exchangers (HEs). You got heat AND the engine had the best way to expell exhaust.

If you hot rodded the motor, LARGER diameter tubes would have helped....not so so with MFI...at least what I have seen.....

Once the 3.0s (78) came out....the exhaust (still HEs) started doing "U" turns and had a catylytic converter and was necked down restricting flow. So some hot rodders used EARLY HEs and got better flow and more HP.....

SSI, an aftermarket manufacturer started making early HEs in stainless...early HEs rust and there was a limitted supply....SSIs aren't cheap.....the 2.0, 2.2, 2.4 (carbed), the 2.7 and 3.0 (CIS) benefitted.

Headers...an American term from muscle car fame....were used to do the same thing....larger diameter headers were used for larger hotrodder motors....headers are cheaper than SSIs or early HEs (if you can find a non rusty set)....but don't provide heat.....

In CA and some other states...SSIs or early HEs are NOT smog legal for 76 and newer cars....even though tail pipe emissions may be within legal limits....


Clearer?

unless you are going to use your car primarilly for track keep the h.e.s on the car and keep the resale value. headers on a car with the displacement you mention that are above 1 1/2 in id (what you get with with ssi"s) kill your already poor low end torque (even with an msd system... ask me how i know) and only add a few near redline hp. I live in ct and only drive the car in the summer but for defrosting/fogging heat can be really nice..

I have had only SSI on my 3.0 and 3.2, both have obviously had heat, and good performance.

Good thread. Thanks to those who provide in depth technical details. Although it can be confusing as we relearn these and similar concepts , it becomes easier to understand.

I thinking about all this, and as it relates to a turbo or TT.

Regards,

Sorry if this is slightly OT, but I don't understand why a state should give a hoot what you have on your car as long as the emissions (and noise levels, where appropriate) are within the limits.

Neither do I, but it's California we're talking about here, and if there's a way the state legislature can control your life, they will find it. It doesn't need to make sense...

It does make sense and is easier to enforce. Emissions cannot be measured exactly without extensive tests - far beyond the little roller gizmo they use -- all parameters would need to be altered - weather, load on the engine, rpm's, mech. cond. & etc. The easy way out is to simply require the manf. to do those tests and do a simple check periodically (smog. test). An aftermarket co. _can_ get their product CARB certified but the tests are neccessarily costly for the reasons given above.

This is what happens when you pack 34 million primates with motors into a series of desert basins...

It's the same basic idea as why you are legally req'd to stop at a stop sign at 3 am when there is no traffic w/in a mile of you...

header exhaust
 

Hallo ,

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