MG Sports Cars

In Rob Ficalora's post about his car he complained about scraping his exhaust on a speed bump. [forum.britishv8.org] . I showed a couple pictures of some oval pipe I made to gain ground clearance for the exhaust pipes on my car and said I would show how to make them - so here goes.

First you need to make a section of oval pipe to go under whatever obstruction you have to clear. Get a piece of pipe half the diameter of the exhaust pipe and slit it lengthwise so you have two half circles. You then need to weld some 16 ga sheet on the top and bottom to form the oval pipe. The width of the center piece can be calculated to duplicate the the area of the main pipe plus about 10% to compensate for any flow loss the transition may create. This is the calculation for a 3" exhaust pipe that flattens to a 1 1/2" high oval:

Area of 3" pipe 1.5 x 1.5 x 3.14 = 7.06 sq"
Area of 1.5" pipe .75 x .75 x 3.14 = 1.76 sq"
Area of center rectangle 7.06 - 1.76 = 5.3 sq"
Width of center rectangle 5.3 / 1.5 = 3.53" plus 10% = 3.88" or about 3 7/8"

The following drawing shows why I don't make drawings for the stuff I make. This just to give you the concept and is not to scale nor is the perspective correct.



Now to the trasnition pieces. You now know that the width of the flat is 3 7/8". You want to have the bottom transition go halfway up the 1 1/2" tubing sections on each side of the oval or 1.5 x 3.14 / 4 ( 1 3/16" = one fourth of the circumference). You also want the bottom transition to go halfway up the 3" main pipe. Make a cardboard pattern (I like cereal boxes or the backing of a legal pad) that is 3 7/8 + 1 3/16" + 1 3/16" (6 1/4") on one end and 3" x 3.14 / 2 (4 3/4") on the other. It should be shaped something like this.



The lines you see are break lines for bending. This can be done in an actual sheetmetal brake or by bending by hand in a vise. You want the ends to be pointing up at 90 degrees and look like this.



With the piece sitting on the bench, the ends will not be perpendicuclar so square them up on a belt or disc sander (or you can file them). With your transition piece, main pipe and oval pipe sitting on the table - tack the parts together so the bottom of all pieces are in the same plane.



To make the top transition piece, take another piece of cardboard and wrap it around trimming so you get all the edges to line up and you have a piece similar to but not the same as the bottom piece. Cut out the shape in 16 ga sheet and lay out bend lines the same way as you did for the bottom. Bend it up and it should fit with some minor tweaking with a hammer and dolly. Weld it up and you just gained half the exhaust pipe diameter in ground clearance. You can see on my pipe that you can also change the in and out angles by changing the angle of the lower transition to the oval section.

Very nice directions Jim. I'm sure those'll be useful.

JB

Jim,
I like your work there with the custom exhaust. I believe you used 2" radius and 6" radius bent pipe to create your flattened curved piece. Where did you source those different radii pipes?
(There are more good photos of this exhaust work in Jim's project journal: [forum.britishv8.org])

Summit has them

Here's another approach to more exhaust clearance that's not quite as elegant as Jim's:

Here is another option.

[ovalexhaust.com]

Let's not forget the Hillbilly Technique; a 2X4 & a floor jack. ;-)

Ok, question for ya'lls collective wisdom -- what size to use. Reading up & looking at the how it was done articles, I think 2 1/4" exhaust for my mild street cam'd (E303 cam) should be sufficient (seems like most of the 302's on the site use 2 1/2" but talking with Ford Racing's tech & Summit Tech, 2 1/4" is what they all recommended & that's what Graham & a couple of others have so I think it'll be good). Question is when you go to oval pipe, does it flow the same or do you need to upsize to account for the (presumably reduced) flow from the pipe not being round?

Rob

I always add about 10% to the area of an oval over a round. I don't have any scientific basis for doing so but a round pipe has the minimum circumference for a given cross section area so the parasitic drag from the wall area is minimal - so I add 10%.area for an oval. My ovals are 1/2 the height of the round tube so if you want to make the ovals flatter you might want to add a little more cross section area. It's all Kentucky windage guesstimates on my part.

Jim

I was curious so I called "Dr. Gas Exhaust" over lunch. Figured they're the experts. Their tech rep said that the flow characteristics are only minimally different -- single digit for sure & in the range of a few % points. In my case, (302, mild street cam, etc.) I'm at the edge of 2 1/4" vs. 2 1/2" so he suggested upsizing to 2 1/2" would probably be preferable. The resulting flow would be about in the middle of round 2 1/4" vs. round 2 1/2" and the 2 1/2" oval pipe is only 1/8" taller than oval 2 1/4" so probably worth the compromise. Net - your Blackwood err, umm Kentucky windage assessment sounds like it was right on.

I'm no expert, but I'm very skeptical 2.5" pipes would perform noticeably better than 2.25" for a 302. How often will you drive at wide open throttle, where the difference shows up? Big pipes be a little heavier and a little harder to package. Bends will be larger, etc. They'll cost more.

If you're just trying to get pipes closer to the floorboards, maybe you should be thinking about modifying the crossmembers. If you notch and reinforce the crossmembers, your pipes can be moved right up close under the seats. This is the only photo I have of what I did. I wish it showed the tubular pass-throughs better.

Thanks Curtis. I'm going to have to redo my exhaust one way or the other. I have compounded errors.
1. I liked the sound of Steve Carrick's car so I sort of copied his. He has chambered exhaust -- 50 or 52" on each side if I recall. I wanted it to be a little quieter than his so I ordered mine @ 54" each side.

2. I went with the IRS which, by itself isn't an "error" but the lower control arms are "wider" than an axle tube -- net, the chambered exhausts extend a little past the LCA's forcing the exhaust tubes to be lower than otherwise would be needed.

3. I may have gone a little bigger on the chambered exhausts than Steve. I got 2 1/2" units -- that's the inner tube. Add about an inch to that for the outer tubes so my exhaust tubes are about 3 1/2" diameter. (Not to mention that I didn't know at the time that the bigger diameter also makes them louder -- defeating the objective of the longer tubes.)

4. Because the chambered exhausts are so long, the H-tube is under the front half of the tranny & has to dip down in the middle to clear...

Net all adds up to two or three inches of precious wasted space.

They're also way heavier than I realized before I got them.

I think it'll be easier to switch out the exhaust than it would be to try & adjust what I have but, I haven't looked closely yet. I've fixed the shifter piece that was blocking me from getting into 1st & 3rd & I flipped the fan around last night. Plan to test the cooling again tonight & do a few other things to make it more drivable while I research what to order for exhaust. But, sounds like 2 1/4" round or 2 1/4" or 2 1/2" oval will be the ticket.

Rob

Don't feel bad about having to re-do the exhaust, I threw away/repurposed everything from the header connection back to get mine up where I wanted it. Financially it hurts at the time you do it but at least you won't swear at yourself everytime you go out of your driveway for as long as you have your car.

Original exhaust system



Latest exhaust system less the rear section with the muffler



Here's the muffler section





Edited 1 time(s). Last edit at 03/09/2011 03:24PM by Jim Stabe.

In the muscle car era Detroit was using 2-1/4" duals on all their performance small blocks and lots of them had very outstanding performance. Plus they were considerably longer. I don't think what you might gain with the 2-1/2" tubes will justify the aggravation. As for how to route the system for best clearance especially with the IRS, the exhaust on the MG-Roadmaster is a great example of what can be achieved. There, Dave VanWyck routed the tailpipes up along the frame rails and over the half-shafts and it was all tucked in real tight. I'm sure there are some photos in that thread, probably from the Virginia meet where we had it up on the lift. Even on that engine it seems to me we ran 2-1/4" tubes, and the tailpipes might have even been 2", which would be consistent with what Detroit was doing. I realize modern theory is that going big all the way back is helpful, but the real question is, does the benefit justify the inconvenience? Personally I don't think so.

As for smashing your tube, there is a simple formula. Calculate the area of the new circle size for the smashed pipe (new radius of the sides) using cobbler (pies are square) and the area of the rectangle between the sides as L x H and add them up. Then run cobbler on the 2-1/4 and 2-1/2" size round tube and compare. My Ky windage suggests it's more than 10% but that's just a guess.

JB

Looking through the April issue of Super Chevy, John Parsons had a formula for calculating the horsepower that a tube can support.

"Engine requires about 2.2 CFM per horsepower, and exhaust gas flows about 115CFM per square inch."

"Assuming 1,400 hp/2( since there are two exhaust pipes) means we have 700 HP per exhaust pipe. Multiply by 2.2 cfm/ per horsepower and we see we need 1,440 cfm. Divide that by 115 cfm/ square inch, and we need 12.5 square inches of pipe area. The area of a 4 inch round pipe is equal to ( pi * r ( squared )= 3.14 x 2 ( squared) = 12.6 square inches. So a 4 inch exhaust is just barely big enough to support 1,400 HP "

The article was about building up a set of SS headers for a mega horsepower engine. Very good article and some interesting food for thought.

Bill

Did he say what a muffler does to the calculation?

Rob,

Don't know if this helps at all, but take a look at the Spintech Cruisers I'm using on the Healey. Only 2 1/4" tall: [forum.britishv8.org]

Here's a link to another article from February, 2009 Super Chevy magazine by David Vizard. He's published quite a bit of stuff about exhaust systems.

[www.superchevy.com]

Good info Phil.

Rick, any idea what the spintech's sound like yet?

What you have to consider is that with these big exhaust pipes you are in the area of diminishing returns. What "...barely big enough to support 1400hp..." really means is that going any larger brings no significant increase in power while going smaller gives small but gradually increasing losses. But how much horsepower do you need? Remember, it is torque which does the real work. Horsepower only comes into play at the top end. So in the real world it's only in a drag race taking it to redline that you will be able to tell the difference. Again going back to the muscle car era it was standard for a 300 hp 350 cu.in. engine to exhaust through a 2-1/4" exhaust with a 2" tailpipe. Switching to duals would allow you to pick up maybe 20 hp which is a less than 7% gain. Certainly significant and worth doing as many of us discovered, but we were doubling the flow area from 4 to 8 square inches. Going to a 2-1/4" tailpipe is therefore going to net you probably less than a whopping 1% improvement. Plus the shorter overall length compared to those cars might net another 1% if you are real lucky. So now you really are into diminishing returns. At this point how much gain is to be had in going to 2-1/2"? If you do the math it would seem like you'd have almost a 25% improvement based on flow area but this is deceptive and does not automatically convert into horsepower increases because we're dealing with a logarithmic curve. Plus the actual gain is less than 2 square inches. Even if we weren't into the diminishing returns area that would only be good for 3-1/2% and as it is it may only be good for 1% or less. So on a 300 hp engine is an extra 3 hp worth the trouble? Not to me. In a 2400 lb car 300 hp is silly power. That much of a difference is only significant if you are racing the car on a consistent basis.

JB