ksconekiller

I know most V8 Miatas convert to dual exhausts front to rear, but i want to maintain a completely stock exterior, including a single exhaust outlet in the stock location. What will be the best routing for the pipes? At some point the two sides will have to merge into one; what is the best spot to do this? What will be the preferred pipe size? Probably 3" after they combine? Photos would be awesome if anyone has them.

Thanks,
Mike

gesso

in for answers, I'm wanting to do the same thing.

(LS1/t56) for me if it matters.

fmowry

I went from the 2.5 headers through the cutouts on the V8R trans brace into a y-pipe that splits to 3 inches then into a resonator then to the biggest Magnaflow 3 incher single in and out that fits in the stock location.

I have an old build thread at miata.net conversion section that shows the routing, though the y pipe is different now an some of the welds were done professionally.

XCLR8TN

^^That sounds like exactly what I want to do, nice!

corndogs

Shandell at v8 roadsters told me they tried a bunch of diffent configurations. 2 2 1/2 to a single muffler and then 3" out producted te best results.

charchri4

Old thread I know but will post anyway. If you sift through these pages you can see lots of info and photos of single exhaust.
Jim and Jenna build diary, Miata LS1 Conversion: Exhausted!

Jim and Jenna build diary, Miata LS1 Conversion: The 3rd half of the back half, 8.8 Ford axles in a Miata.

http://jimsmiata.blogspot.com/2013/0...le-bugger.html

Jim and Jenna build diary, Miata LS1 Conversion: Weighing in on the exhaust!

I am in the process of rebuilding my cut out now to make it cable operated and bigger so more to come.

So what did you guys end up doing? And where are your build threads for us to admire your work!

gesso

Just now getting to the point where I'll be working on the exhaust.
Build thread here:
http://www.norcalroadsters.com/forum/viewtopic.php?f=33&t=16557

charchri4

Nice! That is one fine build you have cooking there! You post up that link in our build threads section so we can follow along with you.

gesso

Placed my order from mandrel-bends.com last night, only enough to get me to the diff though as I haven't mounted it yet so I don't know what kind of clearance I'll end up with (might need to go with ovalized there).

2 1/4" from the cats to a y behind the trans in to a single 3" pipe.

cross sectional area of one 2 1/4" pipe = 3.98 square inches

two 2 1/4" pipes = 7.96 square inches

cross section area of one 3" pipe = 7.07 square inches

Just based on that I'll have some flow restriction from the 3" pipe but I'm building a daily driver so some compromise (noise, ground clearance) is acceptable to me.

Sunshine Guy

I know this is an old thread, but I figure other people like myself use the search function when they are seeking info for their own builds. So I'll add a thought.

There probably won't be any more back pressure in a 3" single tailpipe than in 2.25" dual exhaust pipes coming back past the transmission cross member. Such a system should be pretty well balanced.

The exhaust gases cool quickly as the cylinder pulses expand out inside the exhaust pipes. As gases cool they effectively shrink in volume, allowing the rear part of an exhaust system to be a little smaller than the front; no restriction is created because the cubic volume of the cooler gas is less towards the rear.

A generally accepted rule of thumb is to allow 2.2 cfm exhaust flow per horsepower. Keeping back pressure to a non-significant level, a 3" pipe ought to flow close to 750 cfm, and support around 340 crank HP. That should handle most of the 302/5.0L combos we typically see on street conversions, and would even be borderline acceptable for something like a mild, stock-mannered stroker motor. So, not a bad design concept.

charchri4

I think most folks run 2 1/2 rather than 2 1/4 but isn't the choke point the 1 1/2 header primaries?

Sunshine Guy

Actually, based solely on the math, the 1.5" primaries should support more horsepower than the rest of the system.

Here's a good table that compiles the generally-accepted calculations for exhaust size requirements: How To Calculate Muffler Size and Exhaust Pipe Diameter - Exhaust Videos | Exhaust Videos In it, pipe cross sectional area is based on actual, inner diameter for 16 gauge tubing in typically-used O.D. sizes.

As I suggested previously, it takes about 2.2 cfm of exhaust flow to support each horsepower without significant restriction. In typical exhaust pipe diameters, each square inch of pipe cross sectional area supports about 115 cfm of flow, which is the basis of these calculations. Now, if you insert very tight bends you will lose a few cfm, and if the guy at "Mel's Muffler Magic" gets carried away with the crimp bender you will lose more than a few. But this figure should be applicable to the thoughtfully designed, carefully executed exhaust systems found on most V8 conversion cars.

So, a 2.25" pipe has 3.55" of internal cross sectional area and will flow about 408 cfm, supporting about 185 HP. Multiply times 2 for HP capability with dual exhaust. You can see, as I suggested, that a 2.25" dual system will support most 5.0L displacement, naturally-aspirated street motors.

A 2.5" pipe has 4.43" of internal cross sectional area and will flow about 509 cfm, supporting 232 HP without significant restriction. Multiply times 2 for dual exhaust and that looks to nicely support most Ford strokers as well as similar displacement LS motors, both naturally aspirated.

Now, look at primaries. A 1.5" primary with 1.48 square inches of internal cross section will flow 171 cfm and support 78 HP. That's for one cylinder. Multiply these primary tube figures times the four cylinders feeding into one, 2.25" exhaust pipe and you see that you have, 5.92 square inches primary tube versus 3.55 square inches exhaust pipe.

By calculation, the 1.5" primary tubes x 4 will even outflow the 2.5 inch size exhaust pipe they feed, with 5.92 square inches of total primary area versus 4.43 square inches in the 2.5" exhaust. So, the primary tubes should handle more exhaust flow than either a 2.25" or 2.5" exhaust system.

As anecdotal support, the exhaust ports on my AFR 185 heads, known for good exhaust flow, are no larger than 1.5" primary tubes. There is meat left in the casting where they could have made them bigger. There's a reason they didn't; hold that thought.

With the wonderful sources of information and plentiful engineering science available these days, most of us have figured out when seeking optimum engine performance, bigger is not always better. We tend to act on this knowledge when selecting intake manifolds, heads, and cam. However, there is still a lot of "bigger is better" thought process out there regarding exhaust systems. If a raccoon can crawl inside the tailpipe to have her litter, it must be just right, eh? Um-m...no.

Just like with those other components, we should be seeking to maintain maximum possible velocity without creating a bottleneck to airflow. High velocity in the exhaust is as important as in the induction system, as it creates an inertial, negative pressure wave at the exhaust valve that helps quickly evacuate burned exhaust gases, as well as helps draw in additional air/fuel during the overlap portion of the cam timing.

Just like with heads and intake manifolds, the smallest possible, non-restrictive, exhaust cross section will increase gas velocity, raise volumetric efficiency, and thereby increase torque at every rpm. Matching primary size to your other flow-regulating components is most the most important selection criteria, but going any larger than necessary on the balance of the exhaust system will gain you no peak power, leave low and mid-range torque on the table, and unnecessarily make more noise.

So, to Jim's point, "how come" we often see one step larger primary tube sizes produce better dyno results than would be expected using these calculations? I think that is because these calculations optimize the 2.2 cfm flow requirement at the gas temperatures typically seen within the actual exhaust pipes. As anyone who has ever baked a cake on their intake manifold while the engine is idling knows, the gas temperature within the header primaries is hotter and is shedding that heat very rapidly, making the flow area requirement in the primaries a little greater than further down the system. I would also suggest that is why you generally see primary tube diameters designed to create a significantly greater cumulative flow area than the exhaust pipe they feed...as with the sizes discussed here.

That said, it is so important to remember it is the AVERAGE torque through the utilized rev range that accelerates a vehicle, not peak HP. If a smaller particular primary size gives you enough torque gain across a good rpm ------ in the mid range, it may actually offset a slight torque loss at peak rpm and thereby accelerate the vehicle faster overall, even for competition events. If the car lives on the street, smaller primaries will surely will make the car more fun to drive at the real-world rpm it sees most of the time.

As a final thought, I think some of the headers we see on these conversions use 1.625" O.D. tube primaries, which will support about 18% more flow than the 1.5" size.

Sunshine Guy

I wonder if the forum censoring software could be tweaked to give us back the word s-p-r-e-a-d (without the hyphens).

It sure is a handy word for discussing horsepower, torque, gear ratios, etc., and I seriously doubt it would be abused by any of the good folks in this community.

MX-Brad

------ ------ ------ ------?

sorry, just had to try it.

Gator Bait

And real world results can be even higher. I'm at 430 at the wheel, so in excess of the formula at the crank. But makes me wonder if the restriction is my headers or the down stream pipe. Guess I could get some open header pulls to find out.

Sunshine Guy

What size primaries are you running? Short or long tubes?

It WOULD be interesting to do back to back pulls, closed system versus open headers. I would expect the closed system to provide a bit more torque in the low and mid range, and the open system to excel slightly on peak power.

However, rather than looking at peak numbers, it would be most interesting to compare total area under the power curve, which is what accelerates the vehicle. I bet the net-net in performance would be fairly small.

But only the dyno knows for sure.

charchri4

Now guys we all know street racing is illegal and unsafe and we should not be suggesting it here. The fact is I felt the car miss and was just clearing it out and really have no idea what that yellow vette was doing back there. And for the record I was not racing the black Camaro on I-35 we just both happened to speed up to pass a truck at the same time. And I was certainly not racing those 2 morons in the Camaro, that was a full on ego whooping for being so disrespectful. Damn kids have no respect for old guys in little cars anymore.

Gator Bait

Short tubes. I believe the primaries to be 1.5 but don't know for certain. It's the FM headers.


Exactly what I love about this car. The same age guys in their vettes looking smug amuse me too. Only out to make one person happy with my choice and that's me. Bonus is my wife enjoys it too. Some times the gods smile on you.

ToySnakePMC

Jim makes me laugh too hard & Sunshine Guy makes me think too hard!!