Victor,

Stock is 2.75" and I am using some of the stock piping around the rear muffler so I was just going to stick with the stock piping diamter.

You forget that we have a TRUE dual exhaust and have 2.75" for each bank of cylinders. We'll be well over 1000bhp before we start choking out our dual 2.75" pipes

There are also a few other reasons I'm sticking to stock sizing - bigger is not always better my friend.

-m

 

Is it true that you want to maintain some backpressure for optimal power in all ranges?

 

Right now the design I have is the EVO headers come out at stock piping size which is 2.5" I.D. I believe. They flow into a "megaphone" cone which expands out into 3" tubing. This will keep the exhaust velocity high near the exit of the headers. The 3" tubing flows into a high-flow cat, which then flows out and into a 3" X-pipe with a very large and open cross-section. The X-Pipe then flows back out into 3" pipe and then into a reverse-megaphone back down to the 2.75" stock piping at the rear.

The reduction from 3" to 2.75" will cause a bit of reverb (backpressure) so I dont expect any low end torque loss.. We'll see if I'm right...

I've seen supercharged cars choke on 2.5" and 2.75" tubing on the dyno at 600rwhp. I want to support 700hp so I need 'dem BIG pipes.

 

I've always been taught that the exhaust system is as good as its smallest point. Why not jsut stay with 2.75" all around anyway?

 

Marcus have you tried this Co. yet?


http://www.burnsstainless.com/

Good Luck - - Bob

Sorry did not see that vrus had already posted this Co's Web address.

 

An exhaust system made up of 3" pipe with a 2.75" restriction someplace will flow a heck of a lot better than one made completely from 2.75".

 

vrus - On a F/I motor, exhaust backpressure is something you want to avoid. In relative terms, the exhaust has more energy in it than a N/A motor and wants to expand more rapidly. So, putting the 3" tube size in, is the best way to go, and reducing the size to 2.75" back at the rear mufflers should be ok because the exhaust heat will have gone down considerably at that point and will have also reduced in pressure and incraesed in flow.

As far as coating the SS exhaust, do just what is in the engine compartment for heat management purposes, after that you want to dissipate the heat out of the system, as much as possible, to reduce the pressure and increase flow. SS is a very poor conductor so you want to leave as much of it uncoated as possible. - Bob

 

I know that in an N/A motor you need some decent backpressure otherwise the car is a slug at the bottom end of the scale. On an FI motor I do agree that you want as little as possible, BUT, you do need SOME backpressure.

In otherwords, if I took 3" pipe and made it straight out to the tailpipes with no catalytic converters and no mufflers, that would give me 0% backpressure right? Well, you will probably lose a heck of a lot of torque doing that wouldn't you? This is the assumption I am making...

I want just a bit of backpressure in the system so that I don't lose the torque down low.

The reason for wanting to ceramic coat the exhaust system is for 2 reasons:

1) It helps to keep the undercarriage and radiated areas cooler.

2) Coating the pipes keeps the heat inside the tubes. The hot air maintains a higher velocity than cold air.. Or am I completely wrong about this?

 

Yes, some back pressure is beneficial at lower RPMs to prevent losing torque. This loss occurs because of valve overlap. If there is no back pressure at lower RPMs, the efficient scavenging of the cylinder will pull some of the intake charge straight out the exhaust valve opening during the overlap partion of the valve cycle. As RPMs build, this becomes less of an issue, since there is just not enough time for it to occur. Engines that have DOHC and variable valve timing can mitigate this problem.


Maintaining torque at lower RPMs and obtaining 700 HP at high RPMs in your car are two mutually exclusive goals. Putting on big pipes hurts you down low - maintaining back pressure hurts you up high. 700 HP requires air flow - lots of it. Unless you plan to up the boost or juice it, you probably won't get there.


No, you are absolutely correct. EM is wrong. As the hot air cools down, it becomes denser and moves slower. This is why you lose velocity going to larger pipes. As the hot gasses enter a larger pipe, they expand. This exspansion causes them to cool and thus, slow down.

FYI: Flow through the exhaust system is not a continuous stream of hot gasses. It is a series of discrete pressure pulses that occur due to the opening and closing of the exhaust valves. These pulses are characterized by a very high-pressure leading edge and a very low-pressure trailing edge. This is what causes them to move down the pipe. The low-pressure trailing edge is what causes the scavenging effect. As one exhaust valve closes and the next one opens, the low pressure helps pull the gas out of the cylinder, which takes some of the load off the piston and frees up a little power. But, most V8 engines fire on alternating banks, which lessens the scavenging effect. That's where the X-pipe comes in. It allows the opposite exhaust bank to see the previous pulse and aids in scavenging. Ideally, you would like these pulses to line up one behind the other in a steady march down the pipe w/o hindrance. But, life as we know it prevents that on street machines.

 

Why yes, I can. But I can also say Failed Emissions. I don't know how stringent the smog laws are in Canada, but this setup might not fly in the US. When you replace a 400 cell-count cat with a 100 cell-count cat, you have reduced the catalytic reaction area by 50%. Losing two of the four cats takes this down to 25%. I gotta believe all that surface area was there for a reason. I donno. Perhaps the MB engineers installed all the cats because they knew how often these cars would be driven in open-loop mode, when the A/F ratio is lower and HC emissions are higher. Not sure. You may be alright for testing when the engine is in closed-loop mode. I think it would be worthy to find out before you finalize your exhaust system. Just a thought.

 

vrus - High temps. = high Pressure = relatively lower flow. The pressure drop, due to the cooling of the exhaust gases, causes the exhaust gas volume to be denser and occupy less room therefore it will move more easily thru the exhaust system. On your heat radiation concerns, the OEM heat sheilds between the exhaust system and the floor pan are all you need to keep the heat from radiating into the passenger compartment. The priority here, given the OEM heat shields, is getting the heat out of the exhaust exhaust system.

On the back pressure issue, yes, ideally you want ZERO back pressure on F/I motor. The Lysohlm Supercharger will develop all the bottom end torque (cylinder filling) you need. - Bob

 

Or Victor you can move to south Florida and not worry about emmissions. Florida did away with smog testing a few years back.

 

I picked up my car this afternoon and did some driving around.. Temperature was ridiculously high today (32C w/humidity index it was 40C !!!!) so it was hard to gauge real performance.

As promised I will provide a pic and some thoughts. Like I said right from the beginning I will share good or bad whatever I find...

Performance:

First the good news... NO CHECK ENGINE LIGHT!! YEAHHH!!! We got the placement of the O2 sensors perfect because so far I have not heard of anyone getting away with using 1 set of race cats and avoiding a check engine light. That part is taken care of.. I am very happy about that.

Now, the bad news... My first impression was disappointment to be honest. The car felt slower. Much slower! Was I spinning my tires uncontrollably?? Was the X-pipe too big and just taking away all the torque down low? Did I increase the flow so much that I leaned the car out even more? I have a nasty suspicion that the last one is the problem... I think the A/F is just too lean now.. I was at 13:1 in the lower RPMs.. I have a feeling I dipped even lower. The engine temp was at its normal 100C - 105C. It should have been at least 10C cooler with the improved efficiency. That's what leads me to believe its an A/F problem. It leaned out more and caused the engine temp to go up.

The car felt like it was getting a bit quicker after an hour or so behind the wheel.. Maybe the ECU still needs to relearn? After I had the headers installed, it wasnt until the next morning that the car was at its optimal performance level.. I'll see what happens on the drive to work tomorrow.

I will have to throw it on the dyno on Thursday or Friday and see what happens. I have a feeling I will have to get the ECU remapped for more fuel and ignition advance.

Sound:
Noise level was unbelievably QUIET! I was floored when I started the car.. I couldn't hear it.. I was like.. WTF ??

Anyways, the huge X-pipe that I have seems to have taken out all the noise in the car. It has a very raspy, mechanical sound to it at WOT (really hard to describe but its like nothing I've heard before). At idle, you can't hear the damn thing!! Can you believe it? 1 set of race cats, 3" straight pipe, no resonators and the SOB is QUIET????

Conclusion:
I will have to wait until tomorrow to make my final decision on it. I may get the shop to mock up an H-Pipe to put in place of the big X-pipe and see if it makes a difference.

You exhaust GURUs.. Look at the pic below and tell me what you think about the X-pipe and layout. Do you think the design and layout is good or is there some suggestions on how to make it better?

P.S-> This is a mockup so it isn't what the final product will look like in terms of weld or quality... And, the final product will be coated once I am done testing and find the best setup for power.

P.S.S-> This setup was done with 304 stainless and not the 321 stainless.

 

Thank god for 24" monitors.

Where are the racecats installed? Before the x-pipe right?

 

Nope. What you're referring to is the Bernoulli Equation, which requires a restriction. Go here for more info:

http://hyperphysics.phy-astr.gsu.edu/hbase/pber.html

What we're referring to is more like water flowing through a hose. The higher the pressure inside the hose, the more water that flows.

 

You are correct Grumpy, however, without changing the size of the hose you have to ADD more water in-order to increase the water pessure, thus more water flow. In vrus's case we are talking about a given volume of exhaust gases and the effect of temp. drop, pressure drop, and a larger I.D exhaust system.

BTW, Thank you for the technical link. - Bob

 

Did you check to see if any DTCs were set, short term fuel trim, etc?

 

Nope.. Didnt have my datalogger with me. I will hook up the datalogger tomorrow morning and record my trip to the office.

Do you think the X-pipe having a very large cross section at my current horsepower level is inappropriate? I know it will work well at higher power levels, but maybe it is just too big for right now? What do you think?

 

The race cats are the very first thing you see on the right most side of the picture. They are the shiney cans you see at the edge of the pic.

 

To make it simple for us layman, the front of the car is on the right-hand side of the photo. You can see the last O2 sensor at the end of the race cat just after the first weld joint at the top of the photo.

Victor, that's great the check engine light didn't come one. I'm guessing since they're race cats the O2 sensors and ECM are still working to adjust A/F to compensate based on what the last O2 sensor tells the computer. Do you think this could be contributing to your leaner-than-expected A/F?

BTW, visually the setup looks great. A work of art and you're not done yet.

Cheers,

Darren

 

vic, the very nature of the x-pipe you installed was designed to increase torque and lower the sound level over a conventional h-pipe. Magnaflow first experimented with that design when they developed their c5 catback system and were very impressed with the result. It is a high quality unit from stamped SS. They have spent countless hours on r&d on that unit on various v8's, maybe you should contact someone in their tech department for some advice, they may be able to help you with placement and size ......

 

Victor great job on the exhaust. Are you still planning to dyno today or tomorrow ?. BTW I agree with your choice of piping diameter the dyno will show an increase in peak torque and peak horsepower if your AFR are inline. The off idle torque you are referring to is only for showing off and esp activation. Keep us posted.

 

I did a datalog on the way to the office this morning. Car feels a little stronger, but still not like its usual beastly self.

As I suspected, I think I am just way too lean now for some reason. My short term Fuel Trims on both banks were 99.2% which means it is VERY LEAN. The ECU is trying to compensate by dumping more fuel but I dont think with the stock programming it is within its range of values. Just guessing....

The second thing is.. I had another thought last night.. I should of changed the location of the X-pipe. I am going to put the X-pipe where the resonator delete pipes are and put straight 3" pipe where the X-pipe currently is. The exhaust gases get cooler as they travel further away and slow down. Having them enter that big X-Pipe chamber closer to the front of the car is probably not the best thing.

P.S-> This car was making around 550hp at the crank and the A/F was already out of whack. I wouldn't be surprised if I approach 600hp at the crank with proper A/F tuning at the stock boost levels with this new exhaust setup.

I wish I could buy the ECU tuning equipment... Anyone have one for sale?

 

Do you know how lean it's running? On a boosted car, I'd be very worried.

 

Yeah.. I am a little worried.. I have O2 voltage readings in the datalogger software but I am not sure how to convert those values to Air:fuel ratios.

I might have to go on the dyno to get a "reading" and see where I am at compared to last time.