Playing with my ZT SU carbs.

[AK260]

Thank you sir! That was a brilliant read and good food for thought. I definitely saw an increase in the dips after going to a more free flowing header / exhaust. It also seems there is hope to tune out, or at least minimise the dips. I do think I can get a few more ponies out across the mid range and the top end once the needlework is done. The torque curve quite consistently follows the AFR curve and drops off at the top as soon as the AFR starts to lean up (lean being relative - below 14 but above 12.8).

 

Ordering needles today, so the weekend should prove to be interesting.

 

In the interest of science, I plan to sort out the carb tuning to work well with the existing header, power run, then re-install old header / exhaust and do a power run to see the difference in the mid range and top end. It will cost me another 250GBP but as the man in Dirty Harry said (for those old enough to remember) - “I gots to know”, so it will be worth it!

 

And for those not old enough to know it ...

 

 

 

[Captain Obvious]

He also said "A man's got to know his limitations."  

[AK260]

He also said "A man's got to know his limitations."  

Utter genius!!! And so on point!!! [emoji1787][emoji1787][emoji106][emoji106]

[AK260]

So chaps, here’s a thought to chew over:

The video I shared has gotten me thinking and yes it hurts ;)

The intake velocity is high, the intake valve shuts rapidly, the energy of the intake air bounces against the back of the valve and sends a pressure wave back up the inlet tract.

With the balance tube on the SU manifold, the shockwave not only affects the carb that was supplying but travels through the tube to the other carb which potentially disrupts the airflow of the other bank.

On DCOEs, it spits harder through one inlet tract, say cyl 1, but cyl 5 starts to breath through an undisturbed tract.

So would this be one good reason why DCOEs behave better at the top end beyond the sheer size of each carb feeding each cyl?

Except for sacrificing low end smoothness what would happen if the balance tube was blocked? Surely that would be the same as The DCOE scenario? But without an idle circuit?

Also I think I better understand why foam filters on carb mouths have been known to disintegrate under the shockwaves and / or even catch fire when the carb spits [emoji33]

[240260280]

John Passini covered most of these items in the 50s and 60s on Webers.  He does not like balance tubes. He also and talks about different ways to deal with "pulsations" in the intake tract that reduce pull on the fuel from the well.  I would guess that the SU is similar.

Your analysis of the SU seems reasonable (3 pulsations (more) affecting a carb) however it also differs from Webers in that it draws fuel from the edge of the tract whereas fuel drawn through the Weber main is in the thick of the pulsations in the middle of the tract.  The pulsations may not affect the fuel flow at the side of the throat on a CV carb.

I would expect the fog you see is caused by the high velocity atomized droplets after reflecting off the closed valve. Here is a video of a DCOE and the "fog". http://www.240260280.com/Tech/Carbs/Weber/images/1 throat 3.mp4

 

Passini mentions that an extended (tall) aux venturi can be used to help the low end from pulsations but it makes little difference at higher rpms.

 

 

 

Edited June 18, 2019 by 240260280

[AK260]

Thank you sir! Loving the body of knowledge on this forum! [emoji106]

I need to read a lot more. Will likely buy his tune tips books to better understand the principles. (Even if I’m not running triples).

[Captain Obvious]

So do you think this is an issue on ALL of the carbed Z cars, or is this something specific to yours?

[AK260]

So do you think this is an issue on ALL of the carbed Z cars, or is this something specific to yours?

 What a great question! 

 

I would imagine all carbed cars will have some level of this due to the high velocity charge causing shockwaves bouncing back from the intake valve as it shuts. In fact all Engines will have the intake shockwaves. Even modern ones. Some use a Helmholtz resonator to capitalise on it - almost like a spring storing energy and returning it at the right point.

 

However cam overlap combines with this effect and magnifies it especially with performance cams. Exhaust velocity has a lot to do with it too. So if the exhaust is free flowing / has large primaries and the engine isn’t producing enough gas flow, the exhaust gases / pulses expand too quickly and lose their energy / speed and reversion becomes more prevalent. (God! That sounds like I almost know what I’m talking about!)

 

I’m sure more wisened heads than mine can provide a better explanation.

 

 

 

 

[AK260]

So I wanted to share one more thing for interest, feel free to shoot me down.

 

Given that a lean mix needs more time to fully burn, it stand to reason that an oxygen starved over rich mix also needs extra time to burn as fully as possible.

 

To put this to the test a few weeks ago, I used the 123 ignition to conservatively add a few more degrees at the dips building up to the troughs and then back again where the engine comes to life again.

 

Ignoring the anti-stall peak at 800rpm -which works brilliantly BTW - I call this my twin peaks timing advance (but sadly no Sheryl Lee - am I showing my age again!?) ...

 

 

The butt dyno told me that no longer did I feel a dip in power but I felt the dips behave more like flat spots. The flaw in this of course is that it is fixed regardless of throttle position or engine load. So at part throttle, I would have more timing than would normally be there - hence the conservative approach and not whacking it up by 5 or more degrees (above trend) at each point.

 

But I like the fact that it worked. Why? Because I’m a middle aged man in suburbia with 2.4 children, living the dream! Some men have golf, I have my carbs and my timing curves!