Adapting Thin Flange SSI to Thick Flange SSI heat Exchangers

[Vincent Hill]

I have a 1976 911s with the Thin Flange SSI 1974 Heat Exchangers with a nice Stainless 2 in and 2 out Muffler.

The Problem is, I now have a 1982 911SC that uses the Thick FLange. My question is, can I shorten the studs and use the Short flange or is there an "Adapter" I can use between the thin Flange and the Exhaust Port to use the same gasket and mounting studs?

I also plan to cut in a 200 Hole small round Catalyst Converter on both sides and a Bung for the Oxygen sensor on the right side then see if I can pass emissions in Washington DC which uses a Sniffer at Idle.

[E Sully]

I cut spacers out of stainless steel pipe. It wasn't bad assembling them with the engine out on the stand, but it is difficult to bolt on the exhaust with the engine installed. I think it would be better to use the shorter exhaust studs than spacers, but it can be difficult to remove the old exhaust studs.

[don gilbert]

what i did, was order new barrel nuts, and drilled out the old ones to use as spacers. I was afraid to use just any old spacer, as there are thermal properties to think about. I did my 02 the same way.

[E Sully]

My 73 exhaust studs are Part # 900 061 008 01. They are shorter than the later ones.

[kiwiokie]

My mechanic cut the long studs with a Dremel tool. Just need to be careful not to damage the threads.

[James Brown]

you can thread on a nut prior to using a cut-off wheel to shorten the studs. when you remove the nut, it cleans the threads automatically. (old trick, like me)

[Jeff Higgins]

You can make the thin flange SSI's fit in a number of ways, but you really shouldn't. The thick flange provides a valuable heat sink, and the primary exhaust tubes sticking up above that thick flange act as an exhaust port liner. They work together to evacuate a great deal of heat out of the cylinder heads. The design is far superior to the early unlined exhaust ports and thin flanges. I would just sell the thin flange set and buy a thick flange set. The thick flange set has an O2 bung on it as well.

[tabasco]

Originally Posted by Jeff Higgins

You can make the thin flange SSI's fit in a number of ways, but you really shouldn't. The thick flange provides a valuable heat sink, and the primary exhaust tubes sticking up above that thick flange act as an exhaust port liner. They work together to evacuate a great deal of heat out of the cylinder heads. The design is far superior to the early unlined exhaust ports and thin flanges. I would just sell the thin flange set and buy a thick flange set. The thick flange set has an O2 bung on it as well.

I can help you get rid of your thin flange SSIs if you go the thick route !

-Guillaume

[175K911]

I called SSI and bought a set of 6 of their thin stainless flanges. Stacked on top of the flanges welded onto the SSI's, fit quite nicely. Need two sets of gaskets of course and the heat exchangers will sit a bit closer to the gound, but it was a simple way to mount them using all SSI components. Granted, that was 20 years ago but I'm sure they'd still sell you the flanges.

[Vincent Hill]

You guys are into everything! I wondered what the difference was and the quote that the Thick is a heat sink and has some of the tubing above the Flange to remove the Exhaust gas from the head sooner was good information! I also did not know if there was any "Mechanical reason that the Thin would not work and I see that it will.

My thinking is that the Thin flange because it is the 1974 style will not generate as much heat as the Thick Flange on the Newer style cross over Exhaust that has a Lot of Back Pressure considering I will also be using a 2 in and 2 out Muffler.

Second, I also wondered if the thick Flange used a Gasket or a one of those Hard metal rings and now it looks like Both use the flat Gasket. When I take it apart I will see how it looks in there and make my decision then.

[RarlyL8]

We make these for use with our thin flange headers.

[don gilbert]

before you go the dremmel route and get into a major headache (Ive done this conversion several times now) , make sure that you have threads. alot of times the exchangers have been run loose and have rubbed all the threads off. Getting all the studs out are no fun laying on you back. spacers worked well on my car for years. here is a pic off what i did on the last one, drilled them out to 10 mm and used hardened allen heads and washers. you can torque the crap out of them.but that easy one the stand.

[Jeff Higgins]

Originally Posted by Vincent A Hill

My thinking is that the Thin flange because it is the 1974 style will not generate as much heat as the Thick Flange on the Newer style cross over Exhaust that has a Lot of Back Pressure considering I will also be using a 2 in and 2 out Muffler.

Hmmm.... not sure what you mean by this. Nothing in the exhaust system generates any heat; heat is generated by the combustion process. How much heat that combustion process generates is, for the most part, completely independent of exhaust system design.

The thick flange absorbs and dissipates heat from that combustion process; heat that would normally be absorbed and dissipated by the exhaust port area of the cylinder head. The portion of the primary exhaust pipe that sticks above the flange and into the exhaust port is a critical part of this. It shields the aluminum head from the heat of the exiting exhaust charge, and helps transfer that heat to the thick flange, which then dissipates that heat.

Yes, the thick flange design will result in that area of the exhaust system running hotter than it would on a thin flange system. We would expect that in a system designed to evacuate more heat from the cylinder head. This is a good thing. It keeps the head at a more even temperature throughout, reducing deformation caused by uneven heating. The exhaust valve, its seat, and guide run cooler as the heat is drawn into that thick flange while being shielded by the pipe, reducing transfer into the head casting through the exhaust port.

[Vincent Hill]

Originally Posted by RarlyL8

We make these for use with our thin flange headers.

They "Look" Like Spacers that would go between the Head and Flange. but the Inside holes look to small! What am I looking at?

As far as what makes the heat? Yes, all of the heat is "Originally" generated by the engine, but when there is Back pressure in the exhaust system (as with cross over pipes and "Y" Connections and 1 outlet pipe) then more heat is retained at the exhaust valve opening and the entire Exhaust system.

On 2 Stroke engines that used Expansion Chambers you could practially put your hand on the exhaust and not get burned, NEVER on a 4 stroke and 2 strokes run Hotter because they have combustion every stroke.

I want to remove as much heat as is desirable to get the best exhaust flow no more and no less. I like the idea of the sleeve inside the exhaust hole limiting the amount of heat going directly to the head with the thicker flange. I totally agree with everything that was said about this. Now to get this done ans neatly as possible!

[Vincent Hill]

Originally Posted by don gilbert

before you go the dremmel route and get into a major headache (Ive done this conversion several times now) , make sure that you have threads. alot of times the exchangers have been run loose and have rubbed all the threads off. Getting all the studs out are no fun laying on you back. spacers worked well on my car for years. here is a pic off what i did on the last one, drilled them out to 10 mm and used hardened allen heads and washers. you can torque the crap out of them.but that easy one the stand.

That Picture makes my heart beat faster! Looks Real Good! I like the idea of 1o MM Studs and even better in Stainless 316 Grade that I can get for a good price from some friends in South Africa. One thing for sure, I am not making a move until I get the worked out! I sent a note to SSI to see what I can get from them even if it is the Flanges and some tubing and I make them up myself with some very careful welding!

[RarlyL8]

You're looking at spacers. They go between the flange and the nut that holds it on.
I cannot imagine that an extra 3/8" of flange metal would be a good thing. Thick flanges hold heat. Less metal means it is easier for the air to take the heat away.

[Walt Fricke]

If you use thin flanges and the longer studs you have, consider using flange nuts instead of the barrel nuts.

Get M8x1.25 12mm wrench flange nuts. The smaller wrench size will handle clearance issues. Using an external wrenching nut will allow you to compensate for the stud length. You may find that you have to install the headers in stages - let them hang down (or be propped up) so you can just get the nuts started without interference from tubing bends. Then move the parts closer together for more tightening.

I don't like barrel nuts.

For that matter, 10mm nuts make fine spacers, and are about all you need. That's what I did, as I recall, when putting the '74 headers onto my '82 engine.

Walt Fricke

[Won]

Originally Posted by Jeff Higgins

The thick flange absorbs and dissipates heat from that combustion process; heat that would normally be absorbed and dissipated by the exhaust port area of the cylinder head. The portion of the primary exhaust pipe that sticks above the flange and into the exhaust port is a critical part of this. It shields the aluminum head from the heat of the exiting exhaust charge, and helps transfer that heat to the thick flange, which then dissipates that heat.

Jeff, is this based on any empirical data? I can understand the extended primary tubing acting as a lining, but thicker flange taking heat "away" from the head?

Given the difference in thickness, therefore the mass, of the metal on the two different flanges, the thicker flange will certainly have a higher heat capacity from dead cold. But once the equilibrium is reached between the heat added from exhaust gas and heat extracted away from the flanges, the thickness of the flanges shouldn't affect the net heat flow, because given their limited surface area they themselves wouldn't really act as heat "sinks". Even if they did, the only difference in surface area between the two is the extra 1/4" or so around the side of the flanges, which I don't think is very significant.

So in my mind the extra heat "dissipated" from the thicker flange has nowhere to go; the extra amount of heat transfer due to surface area is miniscule and everything downstream stays the same. If anything it would be transferring the heat right back into the heads. I'm just trying to understand your explanation because I think it should make sense but it doesn't. Am I missing something completely?

[Jeff Higgins]

Won, there is no "steady state" of constant temperature at the exhaust port except under steady state throttle. Part throttle freeway cruising is the only "steady state" our motors will ever see. Exhaust gas temperatures are not high enough to be an issue under these conditions. The lined port / heat sink idea doesn't do much for us here.

The on the gas, off the gas, accellerating and decellerating, going up and down through the gears sort of driving we hope to do in these cars is where this principle shows its merit. Exhaust gas temperatures vary a great deal under these conditions. The spikes in temperature are quite high under high rpm / full throttle running.

Porsche has shown its later cast-in ceramic port liner to reduce cylinder head temperature by over 100 deg. F. I don't have the numbers on the earlier steel header tube liner (under discussion) from which these evolved, but one would assume them to be somewhat less efficeint. How much less, I'm not sure.

The thick flange acts as a heat sink, or a "reservior", into which the liner deposits the heat energy it absorbs. It is more or less a "damper" - an effort to maintain a more constant temperature in the exhaust pipe as the exhaust gas temperatures spike up and down. The advantages of this are less pronounced than those of the liner itself (with the SSI's, we have no choice but to get both). Porsche actually did this originally foe emissions, believe it or not, with the idea being to retain more heat in the exhaust system so that the heat made it further downstream, to the catalyst in the cat. converter. In the intervening years, we have discovered the performance advantages of retaining more heat in the exhaust system. These have been fairly well documented throughout the automotive and performance industries.

Empirical data? The SAE has any number of technical papers that discuss every aspect of exhaust heat imaginable.

[Won]

Cool Jeff. I was missing something - of course it is not a steady state condition. I also missed in your earlier post that the flanges help keep the heads at a more even temperature. I can see now exactly how thicker flange would be beneficial. Learn something new every day, thanks!