JohnEd

iTrader: (1)

JBRADY, Jamie and all,

I removed my center cat that turned out to be a resonator. It was a SS can filled with SS wool with a straight piece of perforated pipe running through it. It worked, as my exhaust note is deeper and only a tad louder. The result is what I "feel" is a significant increase in power down low and up on top. I always suspect people of wishfull thinking but this time it is me and really!!!! This mod only cost me $50 and I think I got a hp gain and another mile per gallon.

I am running Mich Pilots A/S and the tire has excellent ratings for traction. If I hold the brake and give it some gas and then release the brake and at the same time floor the gas I just light up the tire and sit there and spin and smoke. If I just mash the gas at launch I get a lot of spin half the time. To use my Gtech I have to feather the gas off the line. This is not normal for a stock 93 SC400, I am told. Something has changed for the better.

When I first tested the stock system I installed a O2 sensor bung, actually a 18 mm nut with a thread pitch of one thread per mm. aka 18x1, in the center section after the resonator and between the rear resonators and the "Y" and after the resonators. I used the O2 sensor hole in front of the first cat to get an idea of the total back pressure for the system. In those tests I read 2.5 pounds for the total system and 1 pound for everything after the center resonator.

After the resonator was replaced with a piece of 2.5 inch pipe I took readings and got a surprise. The back pressure for the rear half was down to "ZERO" psi. I made three runs to be sure. My explaination for this is that the center resonator was causing so much turbulance that the rear Y and everything after became restrictive. Without the resonator the gases flow smoothly. My best guess. I checked the pressure at the O2 port in front of the cat and found it still read the full 2.5 pounds psi of back pressure. You never look for an answer till u have a question. The answer is that that bung has a serious forward lean to it. I think the gasses are being ram charged into the fitting connected to my pressure gauge. I can't loose 1 pound in the rear half and still have the same pressure in the total system so the front reading is bogus. If the rear Y works so well I can assume the front Y is also non restrictive and I WILL NOT ditch my cats. Maybe "hi flo" in the future, but I will run them.

I will hit the dyno in a couple of weeks and get a reading on the stock system. In the meantime: can anyone tell me what a one psi drop in exhaust back pressure should represent in RWHP increase.

Almost forgot, I ran her up to 5.8Krpm, held the road speed constant and opened the throttle wide. That is the max flow condition I am sure.

Next up is the UNICHIP install.

John





Help! I want to measure the back pressur in my stock 93 SC 400 exhaust sytem. I want to do that at the input 02 sensor on the first cat. That should give me the back pressure for the entire system. I will need to measure that at WOT at 6 K RPM. My problem is in the design of the probe. I don't think a square cut tube that is even with the pipe wall will give true pressure for gases flying by and causing a pressure drop/rise at that tube tip. I can recess the tubing out of the gas flow at the 02 sensor...would that do it? It gets stickey as there is an effect if there is a weld ridge around the hole. Help!

I want to do this so I can properly evaluate the affect of the different ex sys mods I plan to do incrementally. I also want the base line. You guys have said things like "well, 5 lbs of boost will give you xxx.0739 Horse Power". OK! Fine! I can't argue with u and wouldn't think of it. Actually I believe what I read here cause if it is wrong a ton of stuff will hit quickly. Sooooo, if I reduce the back pressure by 5 psi, how many horse power did I pick up at the crank? Thats where I am trying to go. What range of pressures am I to expect from a stock system? 10psi...40 psi...? Makes a big difference when I buy the meter, the range and all.

Thanks for you help. And Jibbby, I believe you when you say you felt a significant improvement, truely I do. I am trying to tie a number to "significant".

John IMHQ

Testing was completed using a turbo boost gauge to measure the back-pressure. 3/5ths of the back pressure was generated by the center cat in my 93 SC400 stock exhaust system. At the outlet of the first cat, I measured 2.5 psi. That isn't much but it is enuf to try to get rid of unless the cost is high. By replacing the center cat with a 2.5 inch tube you can reduce the "cat back" back pressure to 1 psi, approx. With that being distributed across two "Y"s, a muffler and a resonator and all the rear tubing, I am hard pressed to find another component that "needs" replacing. As jgscott put it "Lex did a pretty good job of designing this system." The cat is only part of the smog system in california equip cars. Others get a resonator for noise suppression and to deepen the ex note.

jgscott also noted that when he did the center resonator removal he experienced a significant bump in seat of the pants power. Others have posted that the cat removal did not have any affect on their smog test as they passed with room to spare after removing it. Be careful!

Hope this helps some of you.

John

Blizzy

iTrader: (-1)

I think you may be confusing back pressure with the psi boost from various forced induction methods.

Your engine is taking on air to mix with fuel in your engine to assist the combustion that is driving the pistons that are ultimately supplying power to the wheels. At sea level, atmospheric pressure is approximately 14.7 pounds per square inch (psi). When people are posting boost numbers from turbo or supercharging methods, they are saying that they are increasing the atmospheric pressure effect by X psi. Boosting to 10psi means that the total pressure ofthe air being forced into your engine is 24.7psi. More air means more combustion. More combustion means more power.

Back pressure is the amount of pressure exerted on a moving fluid against the direction of flow. In an exhaust system, large amounts of back pressure would mean that there is a large amount of pressure against the exhaust gases that are trying to escape the engine out the mufflers. An exhaust system with a large amount of twists, bends, and turns will probably have high back pressure. Back pressure, I believe, is commonly measured by comparing the flow of one exhaust system relative to another. These flow numbers are often measured in cubic feet per minute or CFM.

The stock SC400 exahust system has a number of twists and bends. These twists and bends originate at the headers and continue until the pipes straighten out through the stock mufflers. Constructing a more high flowing exhaust system (headers, high-flow cats, x-pipe, mandrel-bent pipe, to well designed mufflers) will minimize back pressure, increase engine efficiency, and thusly increase power. I would anticipate that an expertly designed exhaust system from the headers back would increase power by as much as 7-10% over your stock configuration.

When it comes to exhaust, I believe the convention is that you need 2.0-2.3cfm/hp. That is to say with 250hp, your exhaust system should be able to capacitate 500+cfm of flow. With two pipes splitting the flow relatively evenly, you would need about half that in flow from each of the pipes.

I am long winded, but I sensed some confusion and wanted to do my best to clear it up. Keep in mind that exhaust work is primarily science but a decent percentage an art as well. Nevertheless, a moderately well designed exhaust system should net you gains that are noticeable.

JBrady

I suggest a simple vacuum tube size nipple (I have used little screw in brass nipples in the past), a length of vacuum tubing and a standard boost gauge. Drill and tap the hole for the nipple, screw in, attach the vacuum line, run to the passenger compartment, make your test runs and record values. Helps to have someone "man" the meter and record numbers. Done quickly will usually not burn the vacuum hose.

Do not worry about gas flow past the nipple. I have never had a problem getting a clear measurement doing the above.

Stock systems will probably have between 5-15psi back pressure.

Power per PSI is dependent on system and cam profile.

JohnEd

iTrader: (1)

J,

Thank you for the advice and encouragement. I will be using the o2 sensor fitting to attach the hose. I expect a bolt into the fitting that is short enuf to not protrude into the ex gas flow with the nipple you sugested attached will be my access to the system. Thanks for the idea.

The 5 to 15 psi is good info. What is your estimate of the back pressure contributed by the first cat? The Y? Resonator? I will measure these and get back to you and all with my reading but I wonder at your educated guess. I have heard that the Y is responsible for most of the back pressure.

I expect that I will replace my ex sys from just before the Y to the muflers. I want to keep this car a Lex in terms of noise inside the cabin. Outside a little more V8 runble is fine if it goes with better breathing and economy. I have always stayed within the smog equip boundaries. Pushed them to the limit but never clearly broke them. Accordingly, I expect I will have to put another cat in each line. I think it would perform best if it is closest to the first cat. Do you agree? A good location for that second cat? Before the new X pipe or after.

I have opted to stay with the 2.25 inch system throughout. I have a question on the cat selection. I expect to get the cats from a wrecking yard. The only one I am familiar with is the old Dat 280Z cats. They were what appeared to be a solid piece of bar stock 5 inches in diameter with a million tiny holes drilled in it. The things were 12 inches long and the Dat only needed one for 2.8 liters. Is this a good free flowing design? What do you recommend for a cheap alternative?

Thanks again for your excellent post.

John

jgscott

iTrader: (1)

JohnE I think I know what your desires are and its a tuff thing to get, and expensive to learn. From my own experience I would like to share some info with you for yourself and other. If anyone who reads this is interested in Mustang aftermarket Exhaust noise sound then this will NOT apply to them. I sometime ago decided, has you are posting about to " Engineer " my own system.
Thru research and flow data I detirmined that the best flowing was a 2 /12, high flow Cat system, with Mandel bends, a 10 " X-Chamber, thanks to Peter at Planet Soarer and JBradys input about the disadvantage of the off the self Magna flow X pipe ( AKA midrange killer ). And Dyno max Ultra High flow Mufflers. So a performance shop guy went to work building just what you see in the pics below and installed it for me. Looked great ! Flow rate was high, very high ( maybe too high ).

Problem was :

Great Top end at redline pulled hard.
Down low sucked, less TQ, ok midrange. Way too LOUD. No more Lexus quiet cabin sound. TOO LOUD and droned. Could have installed resonator but would have only toned down small bit.

I sold it for what I paided which was a deal, about $ 500. Any one else would have paided about $ 1000 or more.

My suggestion and what I learned :

All Test @ 15" we

2 1/2" straight pipe 521 CFM
2 1/4" straight pipe 365 CFM
2" straight pipe 281 CFM

Best mod for Money.
1. Behind the center section of the SC4 system right under the drive axles there is a mashed crimp in the system down to 2" or maybe smaller. There are 2 other one like this somewhere if I remember correctly. Have them cut out and weld in 2 1/4 pipe. 70 + CFM improvement with little or no noise increase.
2. Cut the center section right after it goes from 2 into 1 from the Cats. Right there is a double wall of pipe inside that joins the 2 pipes together going into 1 Cut out the center metal double walls that restrict right down the middle up and down. Do the same on the back 1 into 2 exiting the back of the center pipe to the rear resonators. Weld the 2 into 1 and 1 into 2 joint double pipes back together that there are no leaks at the double joining pipes. Weld the center pipe back in removing the Center Cat or resonator, depending on what SC4 you have. No 02sensor and still will pass emissions.

3. Take off the Cats and spray soak inside with gumout and or Seafoam and let set. Rinse with Jet spray hose and clean out build up in cats.

Results are at least 15-20 % improvement in flow of stock cats that are already high flow metal cats. Flow restriction improvement entire system and my estimate is 80 to 90 CFM based on pipe flow data above. That center section is there in a specific place to optimize the stock cams.
Remember you can only flow as much as the smallest restriction ( the 1 3/4 " crimped mashed sections ). This should cost about $ 100 to have done. Along with a correctly built pressurised BFI intake system you will feel the car wake up, and give you a little bit of rumble sound.

jgscott

iTrader: (1)

10 inch center pipe

jgscott

iTrader: (1)

High flow cats in front system layed on ground.

Blizzy

iTrader: (-1)

2.5" piping is too big. With 521cfm of flow in each pipe, you had over 1000cfm of flow in your system. The SC400 has ~250hp which requires ~600cfm of flow at the convention of 2.3cfm/hp.

Recall that JBrady said your exhaust should be as small as possible to not restrict flow. This maximizes exhaust flow speed and minimizes restriction. A 2.5" pipe is too big, causing the exhaust gases to actually slow down by scattering and losing momentum in the bigger pipe. When the engine was trying to pull at the low end, exhaust gases were actually being sucked back into the engine because their velocity was not sufficient enough for them to be able to fully escape. This effect would have changed as the engine was exhausting closer to the requisite gas for your 2.5" system on the top end. All of this adds up to why 2.25" pipe is the preferred size for the SC400.

A larger pipe capacitates fewer reflections for sound waves from the engine to the muffler. Each reflection of the sound wave minimizes the intensity of the sound wave. Thus, a smaller pipe would have allowed more sound wave reflections which would have resulted in your exhaust noise being at a more tolerable level.

I bet your old exhaust would have pulled like a champ from 6-8k rpms...if only that were possible. A 2.25" system would still be top end biased as there is still more flow necessary than for the engine, but it would ensure that the power would be realized as it is available within the confines of the engine capacity. 2" pipe would risk losing top end performance when higher flow is required than is able to supplied by the piping, but 2" pipe is probably a better "mistake" than 2.5" pipe because the power is able to realized (2-4k vs 6-8k rpm).

JBrady

You are welcome.

How are you going to run the 02 sensor(s) if you use that location for testing?

I am unfamiliar with the 280z cat but highly doubt the old design can match current offerings. You can purchase metal substrate cats for aroun $150 each and ceramic core for $75 each. Salvage yards do not sell cats due to legal issues. Why do you want to change or add cats? If the stock ones are dirty or damaged replace with high flow aftermarket.

I will not guess to the pressure loss at each junction but do look forward to your data. My guess is the middle muffers and the 2nd Y (divergent) will be the highest loss. I would test stock. Remove rear muffs and test. Remove center muffs/Y and test. Finally I would cut the single pipe after it is joined by the first Y and before the center resonator and test. Noise will go up obviously at each test and will be VERY loud for the last test above. Even with a drop in pressure you may not see an improvement in power without a replacement system to continue the flow. The center single pipe is 60mm (2.36") but the resonator may be substantially smaller ID.

Further point on the pipe diameters. David Vizard states a 2.5" straight pipe flows apx 560cfm at only 10.5" h2o. JG is giving less flow at higher drop. He probably tested without a bell mouth on the pipe which causes restriction and therefore skews the data. Although 2.25" is a good choice 2.00" works very well. Stock is 1.97" (50mm). The trick is engineering a system that works better than stock. Harder than it looks!

JBrady

Definitely NOT trying to discourage in fact the opposit... I wish more people would test and try! But, people often hope to just throw parts together and have an optimized system. Truth is that sizes, locations, intersections, lengths, ect, can and will alter the traits, pressures, tuning, tonal quality and sound level. Testing and TRYING is a great plan. Copying the stock layout with improvements in muffler flow and capacity and merge construction is a good place to start.

Here is an except from the Vizard Engine Masters piece that all would be "designers" should read. Be warned, at various levels of experience people will pull certain parts from the article and put it on a car only to find out what I have already stated above.

http://www.popularhotrodding.com/eng...re/0505em_exh/

"When we dealt with collector length it was emphasized that it was, in most cases, more critical than the primary pipe lengths. Adding a muffler (even one with zero backpressure) to a system with already optimized lengths can alter the pressure wave response such that the tuning is now out of phase with what is required and as a result, power drops. The trick here is to install mufflers such that they don't alter the tuned lengths of the system."

jgscott

iTrader: (1)

Remember it was not to big in my case. I was running a big Nitrious shot. The biggest problem was noise. A Nitrious FI motor is a little different case. I was just trying to give some experienced advise to other with a stock N/A motor.

You mention about the 2" pipe being restrictive but did you read where I said the stock system is crimped down to 1 3/4 ". No matter what you do, you can only flow as much as the smallest restriction will flow. The stock system is mostly 2.25, theres no need to rebuild a New complete 2.25 system. Its a waste of time and money on a N/A stock 1uzfe. The Sound wave theory sounds good but........ its not really about all sound.
Here was some research I used to build a Zero Loss Nitrous exhaust system, I built fig 10 on page 2. This also may help you understand the the Sound wave theory correctly. http://www.superchevy.com/technical/...exh/index.html


Couple of other things here :
1. I've had the S&S Heades, a complete custom built free flowing exhaust free, F/I and Dyno testing.

2. Going back to JonhEd's 1 st post. I don't think testing at 6000 rpm is going to give you anything other than a good flow rate at 6000. If I remember correctly the NON VVTI makes peak power at around 5600 rpms. Flow rates thru out the rev range are what would be needed.

3. JBrady in regards to pipe flow rate. As long as my test flow data #'s are all consistent at 15" of h20 It would give ME consistence results for comparison rate of apples to apples, to compare at 15". I was always told that the more lift you use with lower #'s the more accurate the flow comparisons would be.

JohnEd

iTrader: (1)

Scott and J,

I seem to keep shooting myself in the foot. The decisions I make apear straight forward but I always seem to get the curve ball.


I bought the Magnaflow "X" pipe. I did that because of a test(timonial) that I read condeming my favorite H pipe. Dyno test verified the superiority of the X. I didn't get that the length of the x legs was critical as they become the len of the pipes connested to them. If the Mag X pipe is not good, what do you recommend? Excluding the 4 inch collector single pipe in the middle of the system.

I have 2 Flow master delta 50's in 2.25. This is a Lex and I don't want noise in the cabin. My only objective here is to get the power being robbed and to pick up some MPG if I can. Are these going to be too loud? Again, I thought I had chosen the quietest low restriction muffler around.

I was not going to replace the o2 sensor with anything when I test. Just use its hole. Will that somehow upset my readings? I can add a fitting and leave the sensors alone if need be. I was going to install ports in the rear of the ex sys anyway so doing that up front wouldn't be a stretch. Directions?

I intend to cut both of the Y's out and replace them with two 2.25 straight pipes connected to the remaing dual system. Can this possibly have a down side. Noise maybe? The X was going to be inst at the front.

I was interested in the cat because my 93 SC400 has a third cat and I will need another "TWO" to balance my system and keep that cat. function. One would be left unconnected to an O2 sensor. Is ithe third cat nec to the ECU?

I am hard pressed to convey the sincerity of my gratitude to both of you for saving me time and money that are both in short supply. The discouragement factor of my failing to achieve my modest goals goes with out saying.

Quote me: I came into this life with NOTHING and so far I have hung on to most of it.

And: The older I get the better I was.

John

Supra Dr

iTrader: (14)

Ahhh, nice to see informative posts. Nice write ups everyone.
John, O2 bungs & plugs are available if you want to drill your pipe just
for the pressure sensor. I have a extra in my downpipe for tuning, I
put the dyno's wideband in it, that way I don't disturb/disconnect any
of my sensors. I just plug it up when not in use.

Snapon, Mac & Matco all make exhaust pressure sensor kits just for the
purpose of testing cats. Comes w/ O2 sensor fitting, high temp hose
& gauge. You could make one also as mentioned above. I beleave the
O2 sensor threads are 18mm x 1.50.

JohnEd

iTrader: (1)

Supra,

Thank you for your advice. I called around and found that the muf shops stock the O2 bungs but not plugs. The bungs sans plugs were 6 to 7 bucks each. Holy bat farxs, Batman! I want to install 6 ports in the system and $42 is a gager. The info that the O2 sensor is a 18mmX1.5 pitch thread was very usefull. I verified that at the parts store. I looked at Khnects Auto store and came up with a oil drain plug of that size and bought seven. The nuts I had to get at the hardware. Grand total of $18. More better than $42. I will install them this week. Maube get the test done this week also.

Thank you for your help.

John

Supra Dr

iTrader: (14)

Yep, older subaru oil drain plugs work. Get the cooper/ metal gaskets too.
Also you can get a barbed fitting w/ a threaded end, adapt the threaded
end up to the 18mm x 1.50. The barbed end will hold your hose on well &
not fall off. You can use regular fuel hose to run to the gauge. The hose
will melt over time, but it's cheap. ($1.00 / Ft) Real high temp hose is $$.
I'm interested in your results & good luck.