Modding The Standard Backbox

Hi folks, Here are the pictures of my modded exhaust. With some delay because my PC crashed and I first had to recover the data. For the record. I’m driving a 2.2 NA, not a Turbo, The goal of my modding was to raise the soundlevel, and to reduce the counterpressure so the engine could breathe out easier thus gaining in torque and power. The modding of the backbox wasn’t done by random, but was carefully calculated. The result fulfilled my expectations completely. The soundlevel is somewhat like a Milltec I presume. The sound became deeper, raising to sportlevel when accellerating. But when cruising at 120km it’s easy to endure (almost like normal) so long trips are no problem. For the lovers of screaming boxes, my mod will be somewhat disappointing. The powereffect is very noticable above 4000rpm. I still have to put my car on a testbench, but I had the chance to drive a standard NA and compare it immediately with my NA, and the difference was huge. Stepping from my car into the standard NA, that one it felt rather lazy above 4000rpm. While my car keeps on pulling up into the revlimiter (even in 5th gear), with the standard NA you could feel the torquecurve going down the higher the engine was revving. The fact my 2 cats are removed will also have something to do with it I’m sure. So I’m a very happy man now. Allthough I’m already considering other mods. Details about the mods: Catsection: removed the precat and replaced the maincat with a straigh pipe. This I did already 1.000km ago. I replaced the second probe by an electronic device so the EML-light doesn’t come on. Of course you have to keep a standard catsection in stock in case your car has to go to the annual checkup. Backbox: I made detailed plans and pictures of it before and after the mod, which shows the different sections indicated by an A or R-number. The A-sections are absorptionchambers, reducing the soundlevel by absorption of the soundwaves. The R-sections are reflexionchambers, where the gasses expand while passing through thus also reducing the noise. Original backbox: The gasses enter via a 48mm pipe in R1, expand and are pressed afterwards through a smaller 44mm pipe into R2. There they expand further and are pressed by two small 32mm pipes into R3 where they leave the backbox via the two 41mm tailpipes. The backbox alone raises the counterpressure with almost 16%! And then we don’t take into account the effect of the precat and the maincat. Modded backbox: I removed the pipe from R1 to R2. The gasses flow from R1 directly into R3 via the same 48mm diameter pipe. So no extra counterpressure there! Then I drilled 24 holes of 13mm into the intakepipe so part of the gasses flow directly into R2. The total diameter of the holes matches the total diameter of the tree pipes now connecting R2 with R3. So no extra restriction there eather! The only counterpressure generated by the modded backbox is by de detour the gasses have to follow before leaving the tailpipes. See schematic of the standard backbox

and a plan of my modification

Here a picture of the standard NA silencer

And a picture of the modified silencer

And last but not least a picture of the modded catsection

I love you Europeans. Your attitude to modifications is so much more scientific and considered. Well done Pipo! /goes away to look for SH backbox/

Brilliant job, 3 quick questions, 1, how or what did you cut it open with 2, what did you weld it back together again with 3, did you use the same piece as you removed to cover the hole up The reason I ask, I capable of doing it, but need to get some more equipment, so it could be a trade off against new equipment – v- new zaust Bob

I love you Europeans!

I thought technically we were Europeans

1, how or what did you cut it open with
2, what did you weld it back together again with
3, did you use the same piece as you removed to cover the hole up

I used a 1,2mm cutting disk for stainless steel, mounted on a small grinder.
To close the hole up again I used the original piece. But when you remove such a big piece of the backboxplating, the backbox pulls open a little bit. So when you lay the piece of plating back into the hole, there is a gap of about 3mm all around. So it's important to use a cutting disk as small as possible to keep that gap as small as possible too.
To close up the gap, you need a TIG welder. Because I only have a standard AC welder and an half-automatic wire welder, I brought my backbox to a professional company to do the welding for me. It did only cost me 38 €uro. Hiring the equipment to do it myself would have set me back 100€ !
The picture of the closed up backbox was lost during the crash of my PC, but except from the welding rig the backbox was as new.

Good info, thanks, I can see this being a weekend job coming up Bob

You can easily weld stainless with a MIG welder, use stainless wire and CO2/Argon mix gas.

You can easily weld stainless with a MIG welder, use stainless wire and CO2/Argon mix gas.

I have a MIG welder. But with a gap of 3 to 4mm to close up a TIG welder is a better choice.

Hi folks,

...I replaced the second probe by an electronic device so the EML-light doesn’t come on...

Hi Pipo,

Do you have more details about this ?

I have a detailed description at home. I'll mail it to you tomorrow.

I replaced the second probe by an electronic device so the EML-light doesn’t come on. Of course you have to keep a standard catsection in stock in case your car has to go to the annual checkup.

Pipo
Assuming I didn't remove the main cat, would I need to replace the second probe by an electronic device
Only reason is I can't be ar$ed to put it back on for the yearly checks
Thanks for your help so far

Bob

The second probe only checks the functioning of the maincat. If you leave the cat in place, then no need to do anything to the probe. I would remove the precat. It makes a huge difference to the engine, and it doen't make any difference for the MOT or the probes.

The second probe only checks the functioning of the maincat.  If you leave the cat in place, then no need to do anything to the probe.  I would remove the precat.  It makes a huge difference to the engine, and it doen't make any difference for the MOT or the probes.

I intend to remove the pre-cat anyway
Thanks again
Bob

Edited by trescoman, 02 February 2005 - 01:18 PM.

hang on a minute is this not going to vastly reduce the pressure in section R1?

Edited by walkes, 02 February 2005 - 02:20 PM.

hang on a minute is this not going to vastly reduce the pressure in section R1?

That's why they make the first reflexionchamber the largest. It's important for engineperformance that the gasses can expand as much of possible when entering the silencer. It's a principle used in all reflexion sportsilencers. Of course the engine must have some backpressure to function well, because if there is not enough backpressure the gasses are leaving the cylinders so fast they can suck the new injected mixure with them. Thus creating a mixure which is to lean to be good. This happens during the time the inlet and outlet valves are open at the same time.

I did a similar job on my TR6. The old backbox sounded fantastic. When it finally blew, I fitted a new one, which had none of the sound. Took it off. Did not cut as beautifully as Pipo, but point one Bob, you only need to cut three sides of the flap, which reduces the amount of welding required. All I did with the 6, was rip out the filling and left the internal pipework intact. Due to the slot left by the anglegrinder, you have to fill a 3mm void, which we all know is not easy. I filled some and plated over the rest. Perhaps it may be easier to either cut the original hole with snips (so no void) or use a fresh piece of steel to close the hole, slightly larger than the hole. Pipo, is our silencer full of stuffing and if so, did you put it back?