John Holowczak on Exhaust Design
John Holowczak on Exhaust Design, January 18, 1998
The following are some comments and observations on exhaust system design that I've absorbed over the years, as applied to the SHO. Hopefully they will provide some meaningful guidance to any of you that are pondering designing your own custom system. I welcome any input or debate anyone wants to make here, other than the "Doug sez they rattle" or "Ted sez they're better in his book so it must be so" dual vs single debate we've heard too much about. All of the below is IMHO, and FWIW, I do not design exhaust systems for a living.
First off, the ability of one pipe vs another to flow gas can be roughly approximated by their cross sectional areas (a paper towel roll being easier to exhale through than a straw, for example). For a stock SHO, as well as the Contour SVT 2.5 liter, the intermediate pipe is 2.25 inch in diameter, for an area of 4.0 square inches. For a 5.0 H.O. in a Mustang, dual 2.25 inch pipes are used for an area of about 8 square inches. The interesting thing about these two comparisons is that the SVT has a high winding 2.5 liter DOHC engine, similar in some respects to the SHO engine, but with lower peak horsepower. The engineers at SVT felt a single 2.25 inch pipe was ideal for high peak power, without reversion hurting low end torque. If true, the larger 3.0 liter SHO engine should need a 2.5 inch intermediate pipe (area of 4.9 square inches, 22% increase) for its 20% higher displacement, and 13% higher peak HP. The Mustang engine is added here because its factory exhaust is often noted for having very low backpressure for an unmodified 5.0l V8 engine, and because its peak horsepower is equal to the SHOs, or very close. In order to effectively burn enough fuel/air to make 215 to 220 hp, the Mustang has double the rough exhaust flow capacity of the SHO engine, even though, at peak hp, the amount of fuel/air consumed is roughly the same.
The reason for this difference in size has to do with reversion, which is back flow of the burned exhaust gases back into the engine. The bigger that the pipes get, the better the chance that flow can reverse and contaminate the incoming fresh air/fuel charge. On a Mustang, the large displacement keeps the total volume of gas going though the exhaust high (at low rpm), and a moderate amount of reversion is even desirable to help reduce peak cylinder temperatures and keep NOx down (the 5.0 having a basic layout and chamber design that is not ideal from an emissions standpoint). With the smaller displacement, bone stock SHO engine, dual 2.25 inch pipes would likely allow too much reversion at low RPMs, hurting low rpm torque. This latter effect can only be mitigated somewhat through additional fuel/spark via an LPM, but it cannot be eliminated completely, regardless of what you may hear elsewhere.
It is well known and widely accepted that an otherwise bone stock SHO can pick up 8 to 12 hp through a less restrictive catalytic converter Y pipe assembly. On a stock MTX, the pipes from the exhaust manifolds are 2", and on the auto 3.2L they are 2.25 inch. The rear bank 2" on the MTX has a restrictive 90 degree turn into the face of the converter "brick". SHOshop's replacement features a smooth bend, and 2.25 inch pipes into and out of the cats.
The problem I have with the SHOshop Y pipe is that it's configured to fit into the OEM space (i.e. the exit flange terminates at the same location as stock). The rear bank's 2.25 inch pipe joins the front bank's pipe immediately after the rear bank cat, and the flow is forced down to a 2.5 inch diameter area immediately. I think Vadim keeps this flange location so that the Y pipe and cat back can be done separately; he wisely knows that a lot of us cannot afford to upgrade the whole exhaust at one time. I think this design leaves a few HP on the table, one which the dual exhaust "pioneers" are able to pick up on. Vadim's testing of various cat-backs on otherwise complete bolt on stage SHOs, while admirably admitting that most are worth only 3 to 5 hp, is missing the small overall volume of the rear bank converter/collector and its potential effect on HP.
Vadim is further quoted in the FAQ as saying that a customer with a dual exhaust SHO was able to use the same LPM program as that used by a hi flow Y-pipe/single 2.5" cat back user, and surmises from this that the added exhaust flow capacity was not adding HP, a conclusion that I'm not totally comfortable with (and it's not clear if that customer with the duals had 2.25" pipes between his cats and exhaust manifolds. True, if "true duals" really did make an additional 15 or so HP over a good single system (as claimed in Ted's book) you would expect to need some additional fuel at WOT, however, such a (dual) system with the same program could still be making say 3 to 7 hp over the hi-flow single system simply due to reduced pumping losses, without requiring additional fuel.
If all this sounds like an argument in favor of dual exhaust, I'm now going to let you all down. I do want to note to those planning dual systems that Borla, and others, have dual inlet/dual outlet resonators, and mufflers - I used such a dual in/out design on my LTD LX 5.0 H.O. due to space constraints imposed by my secondary fuel pump. In your designs, pay most careful attention to the area downstream of the manifolds; remember that the exhaust temps immediately after the cats are still 500 to 800 F and need volume to prevent backpressure.
Once past decent cats & collectors, the pipes can likely be combined (further towards the rear of the car than stock), with common sizes being 3 inch, 2.75 inch, and 2.5 inch. While a single 3" system offers 7 square inches of flow area, any resonator or muffler will not quiet much, and the 3" diameter tailpipe will act like a megaphone. Also, once the exhaust is down near or past the shifter, it will have cooled such that a 3" pipe is unnecessary. Since 2.5 inch components are commonly available, a short 2.5" resonator and single 2.5" in/out low restriction (i.e. Borla) muffler is likely all that's needed.
A dual 2" system offers 6.3 square inches of flow area, and is probably warranted for those going to aftermarket cams (note, 2.25" pipes should be used up near the cats). A dual 2.25 inch system makes perfect sense for those with - or planning - superchargers; 350 flywheel hp engines are going to need the flow capacity, period. I don't know whether larger pipes can be fitted to a SHO given the space constraints.
For my 95 5-speed, the cats are still under warranty, and I want the car to stay legal in Gstock. Since SCCA rules permit any mod after the cats, I'm thinking of adding 12 to 18" 2.25 pipes, with nice transitions from the cats themselves, mating downstream into a section of 2.75 inch pipe. Then neck down just ahead of a 2.5 inch resonator (unless I can find a 2.75 inch in/out resonator) and on to a 2.5" in/out Borla or Walker Superturbo. I know that, going this far, I should get rid of the 90 degree turn ahead of the rear cat, but that technically violates the rules. I would appreciate any comments on this proposed exhaust system design. [Frankly, I wouldn't bother at this point. -ed.]
