allanallen 528 2 Cars Posted February 3, 2006 is there any benifits from removing the mid box, i know it wont go though an mot and will be loud but is it worth doing? Share this post Link to post Share on other sites
Dom9 2 Posted February 3, 2006 If you do a search it has been covered before and quite recently I believe. The Group N system is straight through the centre section, isn't it? It might give some gains, but it should be fine for the MOT and the exhaust note will change a little. If you have to replace it, you might as well go for the Group N exhaust anyway... Share this post Link to post Share on other sites
allanallen 528 2 Cars Posted February 3, 2006 i tryed a search and it showed a group n system im thinkin more home made Share this post Link to post Share on other sites
sonofsam 5 Posted February 3, 2006 i tryed a search and it showed a group n system im thinkin more home made Got all my bits from Jetex,have'nt made it up yet,but the quality is there Share this post Link to post Share on other sites
dodsworth_gti 11 Posted February 3, 2006 i have also thought abaout this aswell,but was told it wouldnt be able to build up the back pressure!!but i dont know Share this post Link to post Share on other sites
Dom9 2 Posted February 3, 2006 i have also thought abaout this aswell,but was told it wouldnt be able to build up the back pressure!!but i dont know It's myth, you don't want backpressure... Can't you use a section of pipe from a lower spec 205 without a centre box? Share this post Link to post Share on other sites
M3Evo 0 Posted February 3, 2006 It's myth, you don't want backpressure... Had this arguement with a chap at work (who has a degree in Motorsport Engineering don't ya know) I reckon this whole back pressure thing has come from the maintainence of high gas speed (which presumably increases back pressure a little) My arguement was that if I welded a penny washer into my exhaust system, I'd have lots of lovely back pressure, but I'm pretty sure it wouldn't increase the power somehow Share this post Link to post Share on other sites
Jer309GTi 3 Posted February 3, 2006 is there any benifits from removing the mid box, i know it wont go though an mot and will be loud but is it worth doing? I can't see any reason why it wouldn't pass an MOT without the centre box. Ive got the straight through centre pipe on mine and I'd hardly call it loud. Share this post Link to post Share on other sites
dodsworth_gti 11 Posted February 3, 2006 but would you benifit from it power wise?? Share this post Link to post Share on other sites
pug_ham 244 3 Cars Posted February 3, 2006 Fitting a straight through centre pipe alone won't make any noticable difference to the final power but will loose a few lbft of torque in the mid range. Mine has a slight dip that Wayne @ Chipwizards was uncertain for why until I mentioned it was a single box exhaust system & he said this could account for it. There is no reason why it wouldn't pass an MOT without one though, the group N system hasn'yt got one like Dom said & quite a few people on here have them without any problems. Graham. Share this post Link to post Share on other sites
pugrallye 0 Posted February 4, 2006 only thing you increase with back pressure is torque, yamaha patented the exup valve system, which partially shut the exhaust off at low rpm to boost torque levels but openned at high rpm to give the bhp. Back to basics here... if you run a car with no exhaust how does it run? (bar the noise)....like a bag of poo, so its mainly down to exhaust velocity, but equalled with an element of back pressure, if your designing your own system, its going to be a case of suck it and see Share this post Link to post Share on other sites
Dom9 2 Posted February 6, 2006 Had this arguement with a chap at work (who has a degree in Motorsport Engineering don't ya know) Ahhh... A jack of all trades and master of none, especially fluid/thermodynamics... No offence Motorsport degree people! only thing you increase with back pressure is torque, yamaha patented the exup valve system, which partially shut the exhaust off at low rpm to boost torque levels but openned at high rpm to give the bhp. You sure? I thought the Yamaha EXUP system effectively changed the exhaust from a 4-2-1 at low and mid range to a 4-1 at high rpm, giving the best of both worlds... i.e. slightly more torque at lower rpm, but more power at high rpm... In any case, ALL factory race Yamaha's bin the EXUP system as you do not need to compromise the exhaust as you only work at high rpm, where you do not want any back pressure! I've read a paper written by Lotus when they adapted an EXUP system for a road car and it actually worked very well, can't remember why it wasn't used now though! Anyway... At low rpm the smaller the diameter of the exhaust the faster the gas exits the exhaust... this momentum in the exhaust pulls it out of the cylinder quicker and probably helps pull the inlet charge in if the cam is running some overlap... As the rpm's increase the exhaust needs to expel more and more gas and then the small exhaust strangles the engine as it can't get the gas out quick enough, backpressure builds and the car makes very poor power at the top end... So, in order to make power, you need the smallest exhaust that flows enough to make the most power you are likely to see! No bigger, no smaller... You may lose a little torque at the bottom end, and this may not be so favourable for a road car, but I would sacrifice a small amount at the bottom end for a lot at the top end! Don't buy anything from anyone who has built backpressure into their system! Middle box may not help you but it may not harm you... Exhausts are complex things and you really won't know what is going to happen until you try it!! Share this post Link to post Share on other sites
hodgy 0 Posted February 6, 2006 (edited) i no off a lad who used to rally his 205 and he had a 1.1 mid section and group N rest didnt understand why tho.do now tho! only prob i have with my group N is that it only pops every now and then when id like it pop all the time! Edited February 6, 2006 by hodgy Share this post Link to post Share on other sites
j_turnell 137 3 Cars Posted February 6, 2006 The 1.1 centre section is designed for the Tu engine, a lot of the XS boys use them, but if you want one for a Gti just get a group n system its simple. Share this post Link to post Share on other sites
veloce200 3 Posted February 8, 2006 Anyway... At low rpm the smaller the diameter of the exhaust the faster the gas exits the exhaust... this momentum in the exhaust pulls it out of the cylinder quicker and probably helps pull the inlet charge in if the cam is running some overlap... As the rpm's increase the exhaust needs to expel more and more gas and then the small exhaust strangles the engine as it can't get the gas out quick enough, backpressure builds and the car makes very poor power at the top end... So, in order to make power, you need the smallest exhaust that flows enough to make the most power you are likely to see! No bigger, no smaller... You may lose a little torque at the bottom end, and this may not be so favourable for a road car, but I would sacrifice a small amount at the bottom end for a lot at the top end! Absolutely. Its largely the same on the inlet, anyone remember Toyota's TVIS? A variable length induction system. When are they going to invent metal that changes shape under an electrical current? We could start with a small bore 4-2-1 with long primaries then it could morph into a large bore 4-1 with short primaries as the revs rise Share this post Link to post Share on other sites
rossimura 0 Posted February 9, 2006 I made a de - centre boxed exhaust 2 weeks ago, using a bit of old Volvo 2inch exhaust pipe, replacing the original centre box. Havent tried it yet but I'll let you know how it goes. Share this post Link to post Share on other sites
Rippthrough 98 Posted February 9, 2006 Absolutely. Its largely the same on the inlet, anyone remember Toyota's TVIS? A variable length induction system. When are they going to invent metal that changes shape under an electrical current? We could start with a small bore 4-2-1 with long primaries then it could morph into a large bore 4-1 with short primaries as the revs rise We already have it with shape memory alloys. Same thing as muscle wire, pass a current through it/heat it up and it pulls tight, stop and it relaxes. Once saw a really long straight piece, that when you apssed a current through contracted up and coiled into a sculpture. Share this post Link to post Share on other sites
Nathan_marriott 0 Posted February 10, 2006 I made a de - centre boxed exhaust 2 weeks ago, using a bit of old Volvo 2inch exhaust pipe, replacing the original centre box. Havent tried it yet but I'll let you know how it goes. I used a Merc Sprinter bit on mine, sleeved it over and welded it up, i removed it coz i smacked it on an event and split it clean down the middle!!!!!! Share this post Link to post Share on other sites
