Exhaust System

[lsrjunkie]

So, I had some questions about the exhaust I am building for my car.

First a few specifics. I'll be running a 302 inline and for those of you who don't know, the cylinder head is configured a little strange. The front cylinder has a dedicated exhaust port, cyls. 2,3 share a port, cyls. 4,5 share a port and finally number 6 has it's own.

Here's my plan. Use 1 1/2 pipe for the front and rear cylinders, and 2 inch for the center pairs. For no other reason than it will make it much easier to route the header. Then I plan to build a zoomie style header that will have all four pipes exiting the car behind the front wheel and parallel with the ground.

The questions I have are...

#1. How critical would it be to make equal length tubes if the header doesn't come into one common collector?

#2. Would I be gaining or losing anything by not using a collector

#3. Will the different tubing sizes effect performance in any way? Good or bad?

#4. What may I be overlooking?

Thanks in advance for any proven theories, opinions, or any other information!

[Milwaukee Midget]

Can open - worms EVERYWHERE!

Are you running supercharged?

Shared exhaust ports are always a PITA.  The Midget has the same lack of engineering forethought.  Fortunately, there has been 60 years of other people working on this engine with this issue, so here's what I can contribute.
 
We tested an equal length 3 into 1, a simple 3 into 1 header, and a LCB (Long Center Branch).  The best output we got on the dyno was with the LCB configuration that included a larger diameter pipe on the center shared port -



Essentially, it's a variation of the tri-Y setup one would have on a traditional 4 exhaust head, but with a twist.  The center pipe is larger and longer in order to compensate for the added exhaust it carries versus the outside pipes, which are joined together much as a traditional tri-Y is.  Then, they meet in a collector, making the virtual 3rd Y.

It works well in that the exhaust pulses are evenly matched, and there is good scavenging due to appropriate pipe lengths and volumes.

BUT -

While on a 4 cylinder with this set-up, you get exhaust events at 360 degree intervals on the shared port - equal time to blow down - the GMC only provides 240 degrees between the 3 and the 2, and the 4 and the 5.  The backside of that is that you have a long dwell of flow - 480 crank degrees.  So what you want to do is use the flow of the 1 and 6 exhaust to promote scavenging in the shared exhaust ports.

If what held true for me holds true on the Jimmy, I'm thinking the configuration you would need to create would be Y-ing the 1 into the 2 and 3 shared pipe, y-ing the 6 into the 4 and 5 shared pipe, and then Y-ing them together into a collector.

As to what diameters and lengths, that's a job for a Pipemax program. 

Seeing as you're starting from scratch, you do have the advantage of calculating where you want peak power and torque, and with the program, you can calculate lengths accordingly.

Now I have no idea what Zoomies will get you, but the cars that I've seen run 'em successfully tend to be supercharged and well over 1,000 horsepower.

I have a friend who posts here who has said it pretty concisely . . . "It's complicated".

[wobblywalrus]

Sometimes it is not worth it to put a lot of time and energy into a compromised setup.  Is there any way to adapt a different head or to separate the ports? 

[Milwaukee Midget]

I think it's a vintage category, which means he'd need to keep the stock port configuration.

[lsrjunkie]

Thanks for the info Chris! I figured I'd have worms crawling all over this topic. I am running N/A, if that helps in figuring this out.

Wobbly, I'd love to go with a different cyl. head design. Some people that are much smarter than me have devised a 12 port head for this engine, six intake ports on one side, and six extake ports on the other side. They flow amazingly better than the stock cyl. head. Two problems there. Number one, it changes my engine class from XO to XXO, Basically I would go from vintage engine to vintage engine with a specialty cylinder head. Number two, they are available but the last one I found was big $$$$.

[kiwi belly tank]

Zoomies are not desirable for a naturally asperated engine that needs to opperate through a broard power range & even more so if they have to be extremely long.
My fix for siamese ports over the years has been to build a separator plate to go down the port to the point of seperation & weld it to a header plate. Then build the two pipe header with a second plate on top of that.
Tuning a common point (cylinder sharing) fuel system without doing this is a compromise.
  Sid.

[SPARKY]

Sid how would you do a exhaust system that you wanted 1 90 just out side the body and the pipes to point straight back if you were not particularly concerned about a wide power ban---short stubbies like WW II aircraft or long equal length singles with out a collector or one like my "equal volume" with different pipe sizes that I know from the dyno is not as good as a good set of equal  length dyno headers by 20 hp..  But at 300+ I am guessing the headers would have more speed loos than the 20 hp would produce.

[lsrjunkie]

I think I understand what you are saying Sid. Could you elaborate a little?

I was thinking of zoomies purely from an ease of fabrication stand point. Mine wouldn't be terribly long, 30 inches at the most.

[kiwi belly tank]

Not sure what you're looking for here Sparky, WWII fighter's with stubbies were blown.
The center two on a SBC configuration is what you need to achieve here. A plate down the center to divide them into two seperate ports. You can do it on a seperate flange plate & put the header on top of it or weld it right to the header plate.
  Sid.

[lsrjunkie]

Gotchya!

Now, that being said, does the shared exhaust port gain anything by being shared. I have read some stuff about a shared intake port using all of the volume of that port, but does it even matter on the exhaust side?

[kiwi belly tank]

I have never seen a power loss by un-siamese-ing an exhaust port. I have also ported siamese type heads out far enough to need a new bolt hole but that might be an issue in a vintage class.
  Sid.

[Milwaukee Midget]

I have never seen a power loss by un-siamese-ing an exhaust port. I have also ported siamese type heads out far enough to need a new bolt hole but that might be an issue in a vintage class.
  Sid.

You will only be able to go so wide on these ports - the pushrod holes limit that dimension.

Pick up a copy of Vizard's book, "Tuning the A-series Engine".  The whole head section is applicable to what you're trying to do. 

Sid, I know it sounds like a good idea, but I think you're likely creating pumping losses rather than flow by fencing the exhaust ports.  I guess I'd put it on a flowbench and verify a benefit there before I'd commit to the arrangement.

Blame that on Fordboy . . .   

[Stan Back]

. . . and, back to the start, collectors are required in some classes.

[lsrjunkie]

I didn't know that. Which classes Stan?

[Milwaukee Midget]

Some of the production classes require an exhaust system that can be closed off.

I know it's that way for GT.