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Old 10-31-2012, 09:27 AM   #7
Devs93
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Are there any aftermarket y-pipes available that are mandrel bent?
Yes, Edelbrock makes them and they are 2 1/2” down pipes into a 3” intermediate pipe. It is sold as an off-road unit, but it would be emissions-compatible with the addition of an O2 sensor bung and a 3” catalytic converter. It is pricey at $160. Another option is using Magnaflow’s y-pipe for 94-95 trucks, 2 1/4” mandrel bent tubing and includes their high flow catalytic converter for $150 – a great deal. Although it is listed for direct-fit applications for 2WD 94-95 trucks, it can be used in 88-95 trucks by cutting the 3” end off and welding on an intermediate pipe to connect it to your muffler. JBA also makes a larger mandrel-bent y-pipe for 4WD models.

Another option for a true dual exhaust setup that is mandrel bent is to use Magnaflow's catalytic converter/head pipe replacement unit from the 96-99 5.7L. It has 2 1/2" pipe and two high-flow cats. Cut the ends off and run 2 1/2" pipes to dual mufflers and you've got yourself a true dual exhaust setup. Cost on the Magnaflow unit is $316.


Do exhaust upgrades make a difference?
You bet they do! Brian at TBIchips did some dyno testing before and after exhaust modifications with his "Black Magic" and "Blue Whale" projects. The conclusion is the factory setup is very restrictive: http://www.tbichips.com/blackmagic.htm. He pulled another 40 rwhp and 50 rwft-lbs by changing to a 2-1/2" y-pipe. From the text, it sounds like he was using shorty headers and a 3" Magnaflow cat in both dyno runs. The results are mainly from the y-pipe change, although I'm sure some revised PROM tuning contributed as well. Installing headers and a catback exhaust is not enough. You need to change everything!!


Do I need a catalytic converter? Will removing it make more power?
This is somewhat of a loaded question. We all know that catalytic converters are designed to reduce noxious gases that harm our ozone and contribute to global warming. Their fault lies in the fact that they can restrict exhaust airflow and therefore can limit hp. Some people remove their catalytic converter in the quest for more power. Is this the right thing to do? Well let me provide some objective evidence and you can make the decision yourself. A recent article in Sport Truck magazine tested the airflow characteristics of their high flow catalytic converters against stock GM, Ford, and Dodge units. Here’s where auto math becomes important. As mentioned earlier, most people over estimate the CFM requirements of their engine. CFM requirements are based on 3 factors: max engine RPM, intake manifold design, and volumetric efficiency (VE) of their engine. VE? What the heck is VE? Probably the most important factor influencing CFM engine requirements. To estimate your CFM requirements, go to this link: http://www.wallaceracing.com/intakecfm.php. Don’t all assume you have a performance engine. If you haven’t changed heads or cam on your TBI engine, VE at 4500-4700 RPM is only 67%. On a 350 TBI engine, CFM requirement is 383-473 CFM. Shocking I know!! If you have a hydraulic roller camshafts, performance heads and other performance goodies, you can choose a VE between 90-100%, but I bet there is <5% of the TBI crowd out there with that engine combination.

Back to the Sport Truck article, their 3” cat is rated at 360 cfm at 1.5inHg (quoted 422 cfm at 28” H2O) and their 2.5” cat is rated at 307 cfm at 1.5inHg (quoted 360 cfm at 28” H2O). This was surprising and I now know why catalytic converter manufacturers don’t cite their airflow characteristics. No one would buy them!! This is just a misinterpretation of the kind of airflow needed for an exhaust system. Let’s take our TBI engine above with performance exhaust – needing 383-473 CFM of air. Will a single 3” cat suffice? – Sure. Will it affect hp – a little bit, but limits it only by 2-4% or about 4-8 hp. Most people wouldn’t even be able to tell the difference. Let’s say now you’ve got an all out performance 350 with the works, hydraulic roller cam, long tubes, the best performance heads money can buy (95% VE – it is rarely 100% at max RPM!!), shifting at 5700 RPM (computer-limited). You’ll need a lot of air - 658 to 823 cfm and dual 3” catalytic converters (2 x 360 CFM = 720 CFM). Most engines are not that potent though. My engine, for example, with Trick Flow heads, Comp hydraulic camshaft, shifting at 5500 RPM has a VE of 83%. My engine needs 555 to 693 cfm. Dual 2.5” cats are more than enough not to limit hp. However, TBI engines only come with a single 3” cat and it is certainly not high flow. Removing the cat on a 2-into-1 exhaust system will improve power on a heavily modified engine – there’s no question. So depending on how you feel about saving our environment versus the additional expense of 2 catalytic converters and a custom exhaust, only you can make that decision. All I can say is TBI engines with stock heads and cam do not need to remove the stock catalytic converter, unless it is plugged or faulty (or the ancient pellet-type). There is no significant change in power with or without a high-flow catalytic converter. Vortec 350 engines already come with 2 catalytic converters from the factory that is more than enough to meet your performance upgrade potential.


What is a smog pump (on pre-93 trucks) and can I remove it?
Your smog pump (AIR pump) is simply what it sounds like. It is a pump that sends air to your catalytic converter via a little round tube. This helps complete the combustion of residual exhaust gases in your catalytic converter along with quickly heating up your cat during cold starts. Replace the catalytic converter with a modern unit and the AIR system becomes redundant. It is located on the lower side of your accessory bracket right underneath the AC compressor on the passenger side. It has an upper and lower bolt that attaches it to the bracket. These bolts can be removed and your smog pump will come right out. Here is an article on smog pump removal: http://www.thirdgen.org/serpentine


I have a lot of performance components already on my 305 engine, but I want more power. What should I do?
Well, any performance upgrade that you can do on a 350 engine can also be done on a 305 engine. If you want more than that, then add a supercharger or buy a 350 long block or 383 short block with aftermarket heads and swap over the parts you want. 350 engines are so plentiful that IMO, there is no excuse for not having one for the serious performance-minded individual.
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