Changing my mind on gearbox and final drive ratio

[pmansson]

On this particular S1 FHC of mine, I got the car with an original EJ box, a very, very strong engine and a fresh 2.88. I was adamant on changing to a 5-speed and 3.54 which I have on my S1 3.8 OTS. I like that combo a lot. Very good acceleration and very quiet on the motorway with almost 25 MPG.

But the more I drive the FHC with its 2.88, the more I like it. It?s a different animal altogether. If I can change the exhaust system to something more silent, it will be a very comfortable cruising car, with fewer gear changes. The car is opal dk green and in a beautiful condition. Not at all the raw beast that the 3.8 OTS is.

My s/s exhaust gives horrible resonance between 2500-3500 which could perhaps be reduced by mounting it slightly differently. I read that somehwere, but best of all, I would like to get a system in mild steel.
Anyone know if someone makes these?
Peder

[Heuer]

Peder

Suggest you use a s/s ClassicFabs exhaust system as they can provide you with their 'Swiss' venturi which sits in the silencers and dramatically reduces noise levels below 4,000 rpm whilst maintaining power. http://www.classicfabs.co.uk/



I did have a mild steel system and agonised over the timbre of the exhaust note but in fact it is little changed with the s/s system as it is affected only by the tail pipes (after the resonators). So you could fit an all s/s system and simply weld on mild steel tails but it may look rather odd.

[PeterCrespin]

"I did have a mild steel system and agonised over the timbre of the exhaust note but in fact it is little changed with the s/s system as it is affected only by the tail pipes (after the resonators)."

Excusez moi? Where does that come from David?

1) I agree that modern stainless systems can sound different.

2) I'm not sure if any/all of them use OEM silencer/resonator baffling - which would certainly account for significant differences in some systems.

3) Even if they do, I agree that stainless is stiffer and *may* affect timbre.

4) I believe much of the change noticed with a new system is because the old one was carbon-lined, not merely mild steel. It is never the case that a 'woolly' mild steel system is immediately replaced with a equivalently coked-up stainless system, so this comaprison is never made and therefore the point never clarified either way.

5) I cannot conceive, with my rudimentary physics and metallurgy knowledge, that the timbre of an entire exhaust system if affected audibly by the composition of the last foot - ceteris paribus .) I would guess that timbre changes are the result of less pressure-induced acoustic absorbtion/deflection with stiff higher-tensile work-hardened drawn stainless components, compared to more malleable mild steel.

7) If that were so to any extent, such differences would accrue mostly where pressure fluctuations were highest and the acoustic signal most marked. That surely applies the further forward you go. It applies least of all in the final foot of outlet, by which time the gas is more of a smooth stream than sharp pulses, having passed through expansion boxes and baffles.

Pete

[Heuer]

Try striking a stainless steel tube with a hammer and then do the same on mild steel - they will sound different because different steels exhibit different harmonics. The reason the tail pipe construction alters the timbre of the sound is because it is only supported at one end so it is subject to resonance. In fact try touching a tail pipe whilst someone holds the revs and that tingling sensation on your finger tip is the resonance. The pipe acts like a transmission line loudspeaker (which uses tuned lengths of pipe to amplify certain frequencies) but unlike a hi-fi speaker the end is free to vibrate. Another example, close to my heart, is the different sounds shotguns make out in the field. A gun made with Whitworth Fluid Pressed Steel barrels (e.g. a 1930's Purdey) sounds totally different to a modern Beretta, which uses a totally different steel composition, when firing the same type of cartridge. Guns with 30" barrels sound different to those with 27" barrels - still have the same end result, obviously.

This effect is well known in audio circles and it is not just steel - why does a Stradivarius sound different from a more humble stringed instrument? I am a die hard vinyl record lover and I know LP's sound better than CD's despite the science and can prove it to anyone within five minutes. I have also heard the difference changing a single cable in a hi-fi system can make (even a digital link) which really screws with my mind as science or I cannot explain it.

Going back to the sound of the exhaust my own experience says mild steel produces a softer note whilst s/s is more harsh and (according to my wife who has the ears of a bat) not particularly pleasing.

[pmansson]

of silencers. I have over 20 collector cars and have both systems on some of my BMW coupes from the early 70s, so I can really tell the difference from experience. Same on my first Jaguar, my father?s old S-.type which I restored in 1984-85. I replaced the terrible sounding Falcon system (with its 25 year guarantee) after 2 years, when I found a complete NOS system in mild steel which has served me well for 22 years. I drilled small drain holes at the lowest point of the 4 silencers. That probably added to their life span...

[Tony]

The stainless steel exhaust on my S1 OTS is as quite as a church mouse. I am in work so I am not sure what system it is. A lot of noise comes from induction noise but as long as you have the stand E type filter box this should not be a problem. if you are interested which exhaust system I have let me know and I will dig out the notes.

[PeterCrespin]

All interesting stuff, and amendable to a little experimentation methinks....since the aforementioned resonator tube is so accessible.

I have a mild steel system, apart from the downpipes I put on the other week for Mr Picky the MOT man. It definitely sounds what I would call soft and woolly. This is not a mixture issue (rich mixture sounds soft) as I know from looking inside the headers the other day that all carbs are fine. I think it's because it's mild steel and has a mildyl acoustic shock absorbing carbon lining.

I agree David that steels sound different when struck (or dropped) and this can be noticed even when handling lathe bar stock. Interesting to hear about the guns. My cheapo Baikal over and under magpie-eliminator just sounded boring, though you could tell differences depending on the cartridge type you were shooting with.

But the point about striking is more or less where I was coming from. The pulses are harshest where they are least muffled - i.e. closest to source. I am struggling to see how there is very much 'striking' going on in the resonator tail pipe after the shock waves have traversed six feet of muffler and gas column absorbtion and attentuation. It's very much more a woofle by that stage, compared to what you hear with only the downpipes, or a full straight through system come to that. So with less energy input due to losses along the system, I'm struggling to see how most of the signal change happens to the least energetic part of the column?

Having said that, it ought to be a doddle to clamp some significant masses to the tailpipe at varying points along the last foot. I guess a heavier mass at the furthest extremity would have the most damping effect. By your logic therefore, the note ought to sound very different if I came long and clamped a pre-formed heavy steel circle around your tailpipe (correction, the CAR's tailpipe )

Maybe we should try it sometime and record the noise. My bet is that played blind to others they would not know which was which, or even if we played the same sound twice. Opinion is cheap though, so I may make something up one day and try for some actual data. Maybe even a sheet of lead wrapped around and held with a Jubilee clip.

I suppose to make it conclusive though, we ought to make vinyl recordings of the sounds and play them on a Goldmund Reference turntable :-)

Pete

[Heuer]

Ideal way would be to clamp the tips of the pipes to the car bodywork rather than try physically damping the pipes with lead or lead substitute (in case 'Elf & Safety are reading). The silencers reduce sound volume and do not, I believe, attenuate frequency. The frequency of the noise changes with rpm - presumably x6 ignition pulses - and there will be point's where it matches the harmonics of the car body and the tail pipes which is when things start to resonate. Actually a local mate of mine has a S1 OTS with a full mild steel system (including manifolds) but with no resonators, the tail pipes go right back to the silencers held only in the middle by a body mount. It sounds great but produces a much higher pitched noise than my ClassicFabs system, Very rasp, the fabled 'tearing calico' sound. I assume this is because the very long tail pipes behave like a fishing rod with lots of 'whip' in them.

[christopher storey]

I can add something which may be of interest to this discussion . When I had rebuilt my engine but hadn't got the car back, I ran it using the cast manifolds, the downpipes , and the resonators i.e. tailpipes stuck on the end of the downpipes. It was amazingly quiet. I have two films of this, with sound, and if anyone can tell me how to post them here I will do so . It rather suggests that the tailpipes do attenuate the noise considerably, certainly on a short length system

[Heuer]

Christopher

You will need to upload them to YouTube first and copy the link here preceding and ending it it with a click on 'YouTube' button in the post editor.

e.g.: [youtube]link to your video[/youtube]

[PeterCrespin]

Ideal way would be to clamp the tips of the pipes to the car bodywork rather than try physically damping the pipes with lead or lead substitute (in case 'Elf & Safety are reading).

Bit dodgy that if it introduces sounds/vibes into the body. All OEM (and industry practice) mounts are flexible to isolate the system. I was just thinking of heavy masses very short term to judge the results. In the event it gave a benefit something more elegant would be called for.

Actually a local mate of mine has a S1 OTS with a full mild steel system (including manifolds) but with no resonators, the tail pipes go right back to the silencers held only in the middle by a body mount.

You've lost me there. The resonators come after the silencers so how can tailpipes 'go right back' if the silencers are half way along?

It sounds great but produces a much higher pitched noise than my ClassicFabs system, Very rasp, the fabled 'tearing calico' sound. I assume this is because the very long tail pipes behave like a fishing rod with lots of 'whip' in them.

The exhaust note in an unsilenced or partly-silenced system depends a lot on compression ratio and cams. Is it a tuned engine?

[Heuer]

Pipes come straight out of silencers, no resonators. A bit like Jenks had on his car. Yes, I assumed a metastic connection to the body. Car has a blueprinted engine but otherwise standard.

[Tony]

1962 S1 OTS Original cast manifold

Exhaust System by The Hutson Motor Co Ltd

My parts list are:-
--------------------------------------------------------QUANTITY
JCB2066 EXHAUST PIPE FT RH S1/2 SS------------- 1
JCB2066A EXHAUST PIPE FT LH S1/2 SS ---------- 1
JCB2066C SILENCER TWIN S1/2 SS------------------1
JCB2066G INTERMEDIATE PIPE LH S1/S2 SS -------1
JCB2066F INTERMEDIATE PIPE RH S1/S2 SS -------1
JCB2066H SILENCER REAR RH S1 3.8 SS ----------1
JCB2066I SILENCER REAR LH S1 3.8 SS------------1
EFK23 EXHAUST FITTING KIT -------------------------1
C8397 EXHAUST MOUNTING STRAIGHT FT---------- 4
JCB2021 EXHAUST MOUNTING ANGLED RR----------1
C19018 EXHAUST HOOKED SAFETY BRKT------------1
C17462 EXHAUST BRACKET REAR---------------------1
C18975 TAILPIPE TIE PLATE---------------------------1

These are taken straight off the invoice in my paperwork 2008.

Best double check before buying

[Heuer]

Tony

Any idea what bore the system is? My ClassicFabs one is 2" and I think the OEM system was 1.5" (confirmation required?).

[Tony]

David,

My O/D is 1inch 7/8.

Not sure if that equates to 1.5 or 2"