smudge67999 · May 4, 2014

hi again i know i seem to do a lot of posts but only because im in depth working on the engine side. so i have a disco1 1994 3.9v8 with cats. now i have made my own straight through exhaust system which i have taken away the middle box and back box left the cats in situ. now it was awfull loud so made some more modification. i made a d.b killer from a old exhaust box. took out the baffle and lagged the baffle with the proper exhaust lagging and must say its a nice sound and has dropped the noise level seriously low its got that nice v.8 sound. any way the good side motor is responsive very responsive through the gears but have noticed a down side. on start up in a morning it revs to about 1500 rpm then will drop after a bit also when i go to pull off there is a flat spot and you can stall it if you dont touch the revs a tag as it is a manual box. now oviousley its changed the running of the engine plus it dont get to temperture as fast and dont run as hot as the gauge use to go to half way it reaches the qaurter mark. now what do i have to adjust to or reset to account for what ive done or is there nothing i can do.

zen · May 4, 2014

it should make no difference to engine temp..low is good by the way..

flat spot...not surprised...but your fecked there as you cant tune..needs to be richer i would say at that point ..

also should make no difference to idle.

have you damaged the lamdas??

smudge67999 · May 4, 2014

hi dident relize you could damage them. but i do have a ecu mate and have run it to check and they saying ok.but i do have some spare original lambas as the ones ive fitted are after market. plus i brought a co2 tester and the reading read 5.5 but was told this is a high for my vehicle as it should be 0 something.

zen · May 5, 2014

of course the exhaust could just be a coincidence...who;s to know your maf, coolent sensor etc could be fecked.???

lightning series3 · May 6, 2014

Ive also messed around with exhausts and found that back pressure makes a difference a big difference on performance . Im now running cats . Terafirma straight center and the normal back box . Gives nice burble without annoying neighbours.

thesainte · Jun 19, 2014

We got one on ebay £98 on ebay very good ebay number 121303263087 in went on with all nww parts and has a very nice sound and now get 3 mpg more up to 23mpg on a run

ronsealdeath · Jun 23, 2014

How big the bore is makes a difference and as you say it affects the tune. Ideally you could adjust things if you were running a weber or something.
I did loads of research on this a while back and there's loafs of conflicting opinion. But essentially the velocity of gas through the system causes the difference in tune. Bigger bore means slower gas speed

discool · Jun 24, 2014

ronsealdeath said:
How big the bore is makes a difference and as you say it affects the tune. Ideally you could adjust things if you were running a weber or something.
I did loads of research on this a while back and there's loafs of conflicting opinion. But essentially the velocity of gas through the system causes the difference in tune. Bigger bore means slower gas speed

The idea of a bigger exhaust pipe is to remove more quickly the burnt gasses allowing more fuel/air mixture to take it's place, and with carbs, to do this get the benefit of a big exhaust system the induction has to be increased ie. larger jets, gas flowed manifold, bigger valves or a quick opening-slow closing 'fast road' cam is required, otherwise a big bore exhaust is a waist of money.
Just the basics of engine performance tuning.

Dr_evil · Jun 24, 2014

I'm in the process of fitting a bigger exhaust system........my old one was far too small for the Weber 500 set up I'm running

It'll be on dual 2.5" pipes instead of 1.25"

ronsealdeath · Jul 3, 2014

discool said:
The idea of a bigger exhaust pipe is to remove more quickly the burnt gasses allowing more fuel/air mixture to take it's place, and with carbs, to do this get the benefit of a big exhaust system the induction has to be increased ie. larger jets, gas flowed manifold, bigger valves or a quick opening-slow closing 'fast road' cam is required, otherwise a big bore exhaust is a waist of money.
Just the basics of engine performance tuning.

Yes I agree but if you put too big a pipe in for the tune of the engine, then you end up with a slower velocity of gas through the system which can bugger things up. There's an optimum for every engine and I'm sure if bigger was better ad nauseum then you would see performance cars (proper ones like Mclarens etc) with exhausts the size of the pipes from my kitchen extractor! But you don't cos they fit the diameter pipe that gives optimum exhaust efficiency. Think of a hose pipe the diameter that the fire service use running off your household tap. It would trickle out of the end (your bigger exhaust). But your thin garden hose gives a decent jet of water (optimum for average tap pressure). I think I'm just saying what your saying : )

jamesmartin · Jul 3, 2014

discool said:
The idea of a bigger exhaust pipe is to remove more quickly the burnt gasses allowing more fuel/air mixture to take it's place, and with carbs, to do this get the benefit of a big exhaust system the induction has to be increased ie. larger jets, gas flowed manifold, bigger valves or a quick opening-slow closing 'fast road' cam is required, otherwise a big bore exhaust is a waist of money.
Just the basics of engine performance tuning.

bigger bore is perfect for turboed engines, the bigger the better ,non turbos need exhaust tuning so pulses act to pull gases through, bigger is only better up to a point

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