Exhausts and Performance

[Pete]

I know that the way that exhaust gases exit the engine are fairly important in the performance of it but just how important? Has anyone any anecdotes about changing exhausts and improvements in power output or to the contrary if they've found an exhaust actually made no difference or even made things worse. I know that short pipes were played with, side exits used on certain circuits, straight pipes where possible but how much difference does any of this make, silencers or bore size?

Having had my new Tony Laws manifold and exhaust fitted on the Unipower I'm slightly baffled as to why we've seen no improvement over the raggle taggle bent, dented effort that was on before. For some reason we're just not getting anywhere near the same power we would see from a similar 1293 Mini set up and I wonder if it'd due to the way the bottom part of the manifold has to bend around the rear frame on a Unipower. Will post pics later.

[Spider]

It's a pretty big subject for sure. There are a few schools of thought but I've not yet heard anyone bring them all together.

Shock wave is one, but what tends to blow that apart is that the shock waves vary with revs and load, so while it may for example be tuned at full noise, it can have detremental effects at other parts of the rev & load range.

A very good friend of mine, says that in the bigger scheme, exhausts play little part. His reasoning is that when comaring the Inlet Tract to that of the Exhaust is that we only have (in the case of N/A) at best 14.7 PSI to 'push' the Inlet gases in to the Engine, where as following combustion we can have a few hundred PSI to push the gases out (which obviously tappers away once the gases leave the cyinder). While this does have merit for sure, there is more to it than this.

A well designed Exhaust Manifold should scavange the gases out, but again, this will depend on Revs, Load, Engine Capacity and Camshaft timing as well as what happend after the gases have left the collector!

Many years ago I did seem some Tethered Model Cars racing around a concrete circle track. Seriously, they were going so fast you could hardly see them! Most of these had single cydinder, 2 stroke engines similar to those used in model aircraft, I think around 5 cc in capacity. Speaking to one of the old guys there (he'd been at it for over 40 years he said), he did say 'It's the Exhaust that makes them go' and he had spent much of those 40 years tuning the Exhausts of his cars, however I will also point out that these cars generally run at max revs, no less and once moving, the load is reasonably steady state.

From my own experience, I've found changing exhaust systems and set up on some of the performance minis that I have made none to little difference, these are bigger bore exhausts, however by comparision, I run a small pipe, of a longer length on the Moke it does makedly reduce top end performance, however it does bring up - dramatically so - the low end performance.

[ianh1968]

I kidded myself for years that my 1380cc,
"286 Scatter", 1.5 Rockers, 1.463"/1.218" valved
11.5:1 engine needed a Large Bore LCB...

This was specified for engines "over 100BHP",
so it should have been perfect, which it probably
was at the top end.

When I changed it for a "Medium", I was introduced
to a whole new world of engine flexibility and a far
superior bottom end that I never knew existed.

The larger bore would benefit the engine ONLY
on the occasions that it was being revved out,
at ALL OTHER TIMES it would be detrimental.

I am more than happy to sacrifice whatever I lost
at the top as the car is so much nicer to drive now.

Ian

[Vegard]

I know that the way that exhaust gases exit the engine are fairly important in the performance of it but just how important? Has anyone any anecdotes about changing exhausts and improvements in power output or to the contrary if they've found an exhaust actually made no difference or even made things worse. I know that short pipes were played with, side exits used on certain circuits, straight pipes where possible but how much difference does any of this make, silencers or bore size?

Having had my new Tony Laws manifold and exhaust fitted on the Unipower I'm slightly baffled as to why we've seen no improvement over the raggle taggle bent, dented effort that was on before. For some reason we're just not getting anywhere near the same power we would see from a similar 1293 Mini set up and I wonder if it'd due to the way the bottom part of the manifold has to bend around the rear frame on a Unipower. Will post pics later.

Have you changed the jetting or adjusted anything? There's not much point in adding another new fancy part, if the engine isn't adjusted accordingly to take benefit of the mod.

I mean, if you don't add more fuel or air, how could the engine be more powerful?

[mk1]

Only one way to get to the bottom of it Pete.

Hook the lump out get it on a Dyno (not a rolling road) & check out your manifold & system against an LCB & standard system back to back.

Personally I don't think that the manifold or system is the problem. but without back to back data it's impossible to get any further.

[ianh1968]

not getting anywhere near the same power
we would see from a similar 1293 Mini set up

Timing chain one tooth wrong?

Ian

[LarryLebel]

The book Tuning BLs A-series Engine has 2 chapters on exhaust. If that's not "bringing them together" I don't know what is.

[minimans]

I know that the way that exhaust gases exit the engine are fairly important in the performance of it but just how important? Has anyone any anecdotes about changing exhausts and improvements in power output or to the contrary if they've found an exhaust actually made no difference or even made things worse. I know that short pipes were played with, side exits used on certain circuits, straight pipes where possible but how much difference does any of this make, silencers or bore size?

Having had my new Tony Laws manifold and exhaust fitted on the Unipower I'm slightly baffled as to why we've seen no improvement over the raggle taggle bent, dented effort that was on before. For some reason we're just not getting anywhere near the same power we would see from a similar 1293 Mini set up and I wonder if it'd due to the way the bottom part of the manifold has to bend around the rear frame on a Unipower. Will post pics later.

Have you changed the jetting or adjusted anything? There's not much point in adding another new fancy part, if the engine isn't adjusted accordingly to take benefit of the mod.

I mean, if you don't add more fuel or air, how could the engine be more powerful?

+1 You really do need to get a base line of what you have and go from there, as has been mentioned exhaust size and length do make a difference, sometimes a huge difference in HP. But HP is not always the deciding factor in a road car drive ability IS what's good for the track will not be good for the road, so you have to decide on what you want first.

[mk1]

On this issue, I remember when I took my 1071 Hillclimber to Harry Ratcliffe years ago one of the things he did was remove the back box & slide lengths of exhaust pipe over the end of my existing pipe & slide it back & forth to see what a difference it made. the exhaust ended up cut off just in front of the back of the rear subframe. Doing this did make a little power & Harry obviously considered it worthwhile.

As I say though, I don't see how you can make any progress with this issue without getting some sort of comparison data on a decent Dyno with a competent operator at the helm.

The time, effort & money this would take is nothing compared to the cost of just stumbling round in the dark trying stuff at random.

[Astro]

Maybe Harry Ratcliffe liked this way. The BVRT Racer is looking the same. Just for fun. Our 8-port hill climber cracked his competition maniflow rear box last month. OK, I had a standard rear box from maniflow in the shelf and now fixed it to the big megaphone. No more horses, no less.

[mk1]

Maybe Harry Ratcliffe liked this way. The BVRT Racer is looking the same.

Good point, but the engine specs were "similar" too.

[David Reid]

I have done quite a bit of work with a non a series 1400 engine in my Autograss car.
I found massive gains from intake length tuning (changed in 10mm increments in the runners) small gains from changing the exhaust manifold runner length (sliding the collector up and down) and no change at all from different exhaust lengths.

To truly see an advantage from any of the above on an a series I believe you would need mappable ignition.

[In the shed]

Riiiight. I'll chime in with my 2p.

Just because you've got a highly strung race engine doesn't mean you have got a highly strung race engine. The 60s were totally different times with totally different approaches to developing and maximising power. We don't see ridiculous compression ratios, nor do we see overly overlapped profiles.

I don't think exhaust requirements are so sensitive due to this. You're not looking at ultimate power spikes, but more of a user friendly band. I don't think the finer points of exhaust design apply so much and make much difference, purely because the engines are less sensitive. When you are looking at an ultimate 320/320 cam, there is so much going on, a small pressure anomaly can really mess things up, whether it's a shockwave pulling mixture out of the exhaust, or whether it's lack of velocity causing flow to reverse. Things are less "cammy" and nasty. Sure, they are, but not like they were.

I'd say that sizing is optimal in order to get the right sort of velocity aided low pressure extraction at the right rev range and to flow right and not choke up if required (3>1 over LCB) at very high flow rates caused by high capacity/rpm.

I remember having the maddest 2" system on my +60 with 286 cam. It sounded "off the scale". I ended up putting in a back box from an Audi 80 D and it silenced it totally without altering the performance at all.

Very interesting point about the "big bore vs not".

My 320/320 1430 came with a medium bore 3>1 and I thought it looked a little restrictive. It does have a 7" manifold and 60mm trumpets though!

However....in this, are we really hellbent on taming really terrible low RPM manners down?

The answer is often "no".

[Pete]

Only one way to get to the bottom of it Pete.

Hook the lump out get it on a Dyno .

Agreed. Will be doing imminently and report back.

[ianh1968]

However....in this, are we really hellbent on taming really terrible low RPM manners down?
The answer is often "no".

A "road car" with a 320 cam and massive exhaust system and a 2.6
first gear is going to make you look pretty stupid at the "lights" when
you lose 3 seconds to your "competitor" in a bog-standard Fiesta as
you wait for your engine to come up on-cam...!

Sure, you'll hopefully catch him up afterwards, but as far as he is
concerned, he beat you...

You might have 2bhp more between 7500 and 8000, but the rest of the
time you have a bag of nails.

Each to his own...

Ian

[In the shed]

It should be like a 1275 with a 649.

I have quite a spirited driving style in the mini. Not too worried about cam-lag!

I think I will stick to a medium LCB, purely because of personal hunch reasons relating to the size of the exhaust ports. I think unless you have the head carved out to huge exhaust ports, it's unnecessary.

My head has HUGE inlet ports and 37/30 valves. The exhaust ports are moderate. They have been worked, but not overly so. I'm not aiming to overly wring the shit out of it. It will get wrung, but not too often!

The mini isn't a road car. My vauxhall 70mpg and 100mph van is.....and it's tedious.

The mini has always been a road going racing car and half the fun is doing a full bore start at every junction/set of lights, because it's the only way!