Exhausts and back pressure

[cleverusername]

The current issue of Car Mechanic magazine has a long article on exhaust systems. Which shows that modern exhausts are horribly complicated but it also talks about type approved exhausts and non type approved exhausts. Saying that the latter can be problematic due to cheap half filled cats and other issues.

One the things they talk about is back pressure and how fitting after market sports exhausts or home made exhausts with the wrong diameter can actually reduce performance and even damage engines.

Now I can see that being an issue with a modern car, with complex emission equipment and engine management systems but is an issue with a Dolly? My understanding was a freer flowing exhaust increased power and economy because the engine didn't have to work against restrictions.

Do the engines in the Dolomite range need back pressure to function? What does back pressure actually do? What would happen if you took it to extremes and ran an Dolomite engine, of any model, without the exhaust manifold (apart from noise and fumes). Would it damage the engine?

[cliftyhanger]

A dolomite needs no back pressure. In fact, a well designed manifold will scavange the exhaust gases, IE suck them out the engine...(within rev bands etc etc)
Modern cars? Who knows, but I hesitate to believe all these articles. Do agree about poor aftermarket stuff. (Eg when I scrapped a fiesta CAT, I got £40 for it a genuine. Reprp ones about £2 as hardly any platinum in them.

[Carledo]

The current issue of Car Mechanic magazine has a long article on exhaust systems. Which shows that modern exhausts are horribly complicated but it also talks about type approved exhausts and non type approved exhausts. Saying that the latter can be problematic due to cheap half filled cats and other issues.

One the things they talk about is back pressure and how fitting after market sports exhausts or home made exhausts with the wrong diameter can actually reduce performance and even damage engines.

Now I can see that being an issue with a modern car, with complex emission equipment and engine management systems but is an issue with a Dolly? My understanding was a freer flowing exhaust increased power and economy because the engine didn't have to work against restrictions.

Do the engines in the Dolomite range need back pressure to function? What does back pressure actually do? What would happen if you took it to extremes and ran an Dolomite engine, of any model, without the exhaust manifold (apart from noise and fumes). Would it damage the engine?

For the amount of time you would would be able to stand the noise, probably no damage to the motor. Wouldn't like to say the same about the body, with flames shooting out of the head ports onto the chassis leg though. Yes, the engine does need SOME back pressure to run properly! I recently set up a 1500 (more or less) that had a down pipe joint blow. Then went back the next week, swapped out the manofold and down pipe and had to set it up all over again!

Like so many other things, exhaust design is a compromise between flow, space, noise and a dozen other factors. A modern turbo charged car is actually easier to tune the exhaust on as all you need do is bolt on the biggest bit of drainpipe you can find, the turbo itself provides the back pressure and a good bit of silencing too!

TA and non TA cats are well known in the trade! I ALWAYS fit a TA cat, if one is needed, they cost more, but you won't be replacing it every MOT like you will with a non TA model! But that's nothing to do with engine efficiency, just emission regs!

Like everything else, car makers put a lot of research and testing time into a factory exhaust and it is more than possible to make the performance WORSE by messing with it! When I first built the Carledo, I used a "parts bin special" exhaust, made up from bits I had available for purely cost reasons. Later I fitted a slightly altered (to fit the short tail shell) Rimmers Sports system and gained what felt like 30bhp (probably more like 10 in reality) but a significant and noticeable improvement in performance whatever! I also lost a slightly annoying mid range "flat spot". So basically, I got lucky! Unless you want to spend a LOT of time trying different setups on a rolling road, that's what you do do, try it and hope! The only consolation is that the original Sprint exhaust is hopelessly convoluted and almost anything that straightens out some of the worst bends will be an improvement!

I was told, many, many years ago, of one trick for tuning exhaust length, which is virtually free and can add a couple of horses to the stable. Remove all paint etc from the rearmost foot or two of tailpipe, then paint with anything handy that is NOT heat resistant. Now take out for an Italian tune up! When you get back, you should notice that the paint has burnt off in a series of rings around the pipe at 3-6" intervals. Simply cut off the pipe at the ring nearest the back of the car and it's "tuned"! This works because the gas goes down the pipe in pulses or waves and where it burns the paint is where the "wave" hits the pipe. Cutting here lets the gas expand cleanly, adding a little more power. Or so I was told!

As for potential engine damage, I reckon that's just a scare tactic!

Steve

[cleverusername]

I was told, many, many years ago, of one trick for tuning exhaust length, which is virtually free and can add a couple of horses to the stable. Remove all paint etc from the rearmost foot or two of tailpipe, then paint with anything handy that is NOT heat resistant. Now take out for an Italian tune up! When you get back, you should notice that the paint has burnt off in a series of rings around the pipe at 3-6" intervals. Simply cut off the pipe at the ring nearest the back of the car and it's "tuned"! This works because the gas goes down the pipe in pulses or waves and where it burns the paint is where the "wave" hits the pipe. Cutting here lets the gas expand cleanly, adding a little more power. Or so I was told!

As for potential engine damage, I reckon that's just a scare tactic!

Steve

That is very clever, what you're saying is the exhaust gases behave a bit like a soundwave and if you make the out match the end of the high pressure section, it leaves more cleaning.

As for engine damage, I was a bit surprised by that and couldn't workout how it would happen

[xvivalve]

What sort of exhausts did piston engined fighter planes, tuned to perfection for ultimate performance and speed, have?

[cleverusername]

What sort of exhausts did piston engined fighter planes, tuned to perfection for ultimate performance and speed, have?

If I remember correctly, just short pipes sticking out of the engine.

However Aero-engines are very different to car engines. They are designed to run for long periods at very high loads, whereas car engines are designed to run at cruising speeds at low revs, with occasional demands for large amounts of power.

Plus aircraft engines are rebuilt at regular intervals. There are several articles about using car engines in planes, because they are cheaper. The consensus is, is it isn't generally a good idea. They are meant for different applications.

[Carledo]

A 27 litre V12 Merlin engine tops out around 2200rpm! With emergency boost override on the final 2 stage supercharged version you could get 2500rpm for a short period (and only below about 15000ft altitude) A horrible abuse that shortened engine life dramatically, if it didn't just blow up! Aero engines ARE better built, to much finer tolerances than any car engine and, predictably, go out of tolerance much quicker. They are more like the nitro-methane burning top fuel dragsters which produce 2000bhp from 8 litres, but life expectancy is in MINUTES! Oddly enough, these dragsters run open pipes around 18" long too! But they are only designed to run best at wide throttle openings, a more "normal" race car will have a carefully crafted extractor manifold, even if there is no silencer attached, because race cars run at part or trailing throttle and my understanding is that these situations are where a little back pressure is useful, like providing engine braking.

Steve

[Jod Clark]

I was told, many, many years ago, of one trick for tuning exhaust length, which is virtually free and can add a couple of horses to the stable. Remove all paint etc from the rearmost foot or two of tailpipe, then paint with anything handy that is NOT heat resistant. Now take out for an Italian tune up! When you get back, you should notice that the paint has burnt off in a series of rings around the pipe at 3-6" intervals. Simply cut off the pipe at the ring nearest the back of the car and it's "tuned"! This works because the gas goes down the pipe in pulses or waves and where it burns the paint is where the "wave" hits the pipe. Cutting here lets the gas expand cleanly, adding a little more power. Or so I was told!

As for potential engine damage, I reckon that's just a scare tactic!

Steve

That is very clever, what you're saying is the exhaust gases behave a bit like a soundwave and if you make the out match the end of the high pressure section, it leaves more cleaning.

Its the same maths as finding the correct (best compromise) port length on a bass reflex speaker cabinet.