Exhaust back pressure and mixture strength

Hi all,
A very basic question, does an engine tend to run leaner or richer as the exhaust back pressure changes?
regards
Alan

I think the general rule of thumb is the more open the exhaust the richer the mixture needs to be. Remember on an Amal you are dealing with 4 controls on the mixture depending on throttle opening. The idle circuit, the slide cutaway, the needle clip position and needle jet and the main jet. All may need to be made richer to get right mixture at all throttle openings.

htown16 said:

I think the general rule of thumb is the more open the exhaust the richer the mixture needs to be. Remember on an Amal you are dealing with 4 controls on the mixture depending on throttle opening. The idle circuit, the slide cutaway, the needle clip position and needle jet and the main jet. All may need to be made richer to get right mixture at all throttle openings.

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Started to click like - you sound right.

But I got to thinking about Triumphs with 1-1/2" pipes and Norton with 1-3/8" pipes, similar length, and similar mufflers. Triumphs use 190 (650 with concentric carbs) or 200 (750 T140 Twins) main jets. The original Triumph mufflers may have been more restrictive than original peashooters - don't know.

Generally (no nitpicking please): Triumphs are a little higher compression, have hemi-type pistons, and much earlier ignition advance. 750 Commands and T120R and T140 Triumphs have the same pilot circuit, the same needle and needle jet and the same slide so through the range, the carburation is the same (except maybe needle position)

So, the best I can come up with now - I'm confused - and would like to know too

When the very restrictive stock black cap silencers are replaced with the type of free flowing silencers used on the earlier bikes, the carb main jets must be replaced with larger main jets.
From that one can say that adding restriction will make the mixture richer, and opening the exhaust up will make it leaner, until the jetting is adjusted.

Glen

But does anyone know why? How does changed exhaust back pressure effect the effect the A/F ratio supplied by the carb?

I understand how things can effect overall draw of the intake charge, but how does it affect mixture ?

Fast Eddie said:

But does anyone know why? How does changed exhaust back pressure effect the effect the A/F ratio supplied by the carb?

I understand how things can effect overall draw of the intake charge, but how does it affect mixture ?

Click to expand...

Larger freer flowing exhaust pulls more air thru while stock jetting restricts the fuel volume intake in proportion. > CFM = Lean

Same with porting the head.

worntorn said:

When the very restrictive stock black cap silencers are replaced with the type of free flowing silencers used on the earlier bikes, the carb main jets must be replaced with larger main jets.
From that one can say that adding restriction will make the mixture richer, and opening the exhaust up will make it leaner, until the jetting is adjusted.

Glen

Click to expand...

Sort of the same for me again. To me, adding back pressure means less exhaust escapes which means that there is less "draw" for the intake charge so less A/F mixture burning, not a different A/F mixture. All guessing for 4-stroke engines. I know this to be true for certain very simple 2-stroke engines.

gortnipper said:

Larger freer flowing exhaust pulls more air thru while stock jetting restricts the fuel volume intake in proportion. > CFM = Lean

Same with porting the head.

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Is it that simple?

Surely a greater CFM through the same size port creates a greater velocity… and a greater velocity creates a greater Venturi effect ?

The more air flow, the greater the Venturi effect, the more fuel is pulled. What prevents this being the case? That’s what I struggle with.

gortnipper said:

Larger freer flowing exhaust pulls more air thru while stock jetting restricts the fuel volume intake in proportion. > CFM = Lean

Same with porting the head.

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OK, typing while you posted. Makes sense if I understand you right. More volumn of A/F means to have the correct A/F requires more F.

Fast Eddie said:

Is it that simple?

Surely a greater CFM through the same size port creates a greater velocity… and a greater velocity creates a greater Venturi effect ?

The more air flow, the greater the Venturi effect, the more fuel is pulled. What prevents this being the case? That’s what I struggle with.

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Over simplified but I think he is right. Yes, faster moving air tries to suck more fuel, but the jets are designed to meter fuel at a rate. Amal jets are cc/minute (if I remember right). The confusing part is the only real effect is at 3/4-WOT and maybe when on the needle.

BTW, 3am, have a cold, can't sleep, love this discussion.

Only exception to that rule I ever heard was from someone that'd fitted drag pipes to an xs1100
He and others have told me they had to go down a size or two on the mains

marshg246 said:

Over simplified but I think he is right. Yes, faster moving air tries to suck more fuel, but the jets are designed to meter fuel at a rate.

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Ok, that makes logical sense (to me at least).

But, the main jet size is only going to be a limiting factor (as described) when the absolute peak flow (WOT, at peak intake lift, at peak revs) exceeds the jets ability.

So, what I struggle with next is how the exhaust back pressure can have such an effect? By the time the engine is at this point, the exhaust valve has been closed a good while.

I‘m thinking it might have more to do with pressure waves and pulses than outright flow.

Think I’ll stop hypothesising now and dig out a book or two off the shelf…

Last point, and further to Baz’s post, my Maney style pipe requires weaker needles in the FCR carbs than the small diameter stock headers and peashooters. Totally counter to what I’d expect.

baz said:

Only exception to that rule I ever heard was from someone that'd fitted drag pipes to an xs1100
He and others have told me they had to go down a size or two on the mains

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That also makes some sense to me. As I understand it, back pressure is not only how freely the gas flows out about also about the push/pull of the gasses in the pipe. Short straight pipes don't add to the suction, they just let the gasses flow.

baz said:

Only exception to that rule I ever heard was from someone that'd fitted drag pipes to an xs1100
He and others have told me they had to go down a size or two on the mains

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I had an early Triumph T509 Speed Triple and wnen I put an Art can on , the fuel mixture had to e lowered (dunno the terminology for injection) so was exactly the opposite of what I thought it needed.

Fast Eddie said:

I‘m thinking it might have more to do with pressure waves and pulses than outright flow.

Click to expand...

Can't tell if you guys type faster than me or start earlier

Not sure about "more to do with" but that's basically what I think too. Very long straight pipes will have high and low pressure areas (stopping flow and sucking) and mufflers change it all. Very short straight pipes just get the gasses out. Think of dragsters - to hell with backpressure when you have a supercharger.

A 2-stroke expansion chamber to designed to increase suction while allowing the gasses out - quite the balancing act for performance. My stepson's 72 H2 was missing the exhaust when he bought it. He found some old racer/tuner that that a set of stainless expansion chambers for racing. Took that peaky bike to a whole new level of performance but you better hang on for dear life in the first three gears when it "comes up on the pipes". 4th and 5th will bring the front wheel off the ground if you open the throttle to far before it hits the pipes. He had to increase the main jets buy 50% if I remember right - the old guy told him what jets to change and the size. Gas milage - quite low!

marshg246 said:

4th and 5th will bring the front wheel off the ground if you open the throttle to far before it hits the pipes.

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Thanks. Now I understand why my RDLC racer is so eager to lift the front wheel out of the bends.

A 2-stroke expansion chamber is designed to resonate and stuff mixture back through the exhaust port. And so is a 4-stroke exhaust system. If you replace the silencers on your Commando with megaphones without jetting to suit, you are likely to burn a piston.
If you advance the exhaust valve opening point, you work the pipe harder, and get more torque, but you also get more noise. Exup on GP bikes is probably more about noise reduction when the valve timing is advanced. The overall effect is more torque with less increase in noise.
I had a 2 into 1 exhaust on my Triton. I started with a tail pipe the same diameter as a header pipe. I immediately lost 2000 RPM of the top of the rev range. THen I started cuttinh the collector bac and fitting larger tail pipes. When the tail pipe was big enough to accomodate the total gas flow, I was 1000 RPM down, but had my top end power back. Instead of 10,500, I had 9000 - but much better midrange. The bike became rideable, instead of stupid and dangerous.
When you change the exhaust, you need to adjust the valve timing and mixture. A 4-stroke is similar to a two stroke, but different. You would need a sidecar if you fitted an expansion chamber to a 4-stroke.

Mike40M said:

Thanks. Now I understand why my RDLC racer is so eager to lift the front wheel out of the bends.

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Most people get a two stroke pointed straight before they give it the berries. It is a pretty brainless past-time. You can do a lot more in corners with a 4-stroke. Later 2-strokes develop more torque. THey have reed valves and variable exhaust ports. Have a look at an RZ350.
My 85 year old mate took his RZ350 to a meeting and blew-off a decent Seeley 750, with ease. The RZ350 has to be as fast as a TR3.

Fast Eddie said:

But does anyone know why? How does changed exhaust back pressure effect the effect the A/F ratio supplied by the carb?

I understand how things can effect overall draw of the intake charge, but how does it affect mixture ?

Click to expand...

Yes.
Cylinder emptying is less complete with a restrictive exhaust.
SO, there is residual exhaust gas left in the cylinder, which in turn reduces the intake efficiency as the next charge is drawn in.