[Fot] Crank Harmonics and Carburetion

Tue Aug 25 11:50:26 MDT 2020

Sounds like the open carb is the problem, check for sticktion and spring pressure. The way the needles are designed the piston should rise slowly to its max point and stay there. the incorrect operation looks like the pistons opening to their max point instantaneously. Most race SU's function like the latter and there is nothing wrong with that as long as the AFR is appropriate at all points, where the problem lies is when you can't get from your trailer to the pit late without having to rev the Sxxx out of it, that's the classic example.
On a side note a motor can only use so much air and if your carbs are larger than necessary the pistons will find the place they like to live and that's what you get. So just because the pistons are not all the way up doesn't mean they are not working properly, it just means if you want the pistons up all the way you will need more displacement, haha.
Rick

"Doing the right thing is always the right thing, regardless of consequences"

    On Tuesday, August 25, 2020, 12:14:18 PM EDT, Phil Gott via Fot <fot at autox.team.net> wrote:  

 Thanks everyone.I’m glad to know that we’re not alone. The carbs were rebushed by Joe Curto last winter. Running red, yellow, blue and no springs. It happened in every case, with slight differences in final height depending on spring rate.   Checked for vacuum leaks: we run the graphite hi performance manifold gaskets. We did find a slight gouge in one of the rear Manifold gasket faces. Dave did some harmonic calculations, treating the manifold and carb like a recorder and found that there may be a high pressure area under the two air holes of the piston at just about 5300 rpm. That slight gouge could have been contributing to some high pressure harmonics that would make the piston drop, just like a recorder will squeak if each air hole is not closed completely. Have not been back on the dyno to check the effect since, but that supports the idea that it might be due to a vacuum leak.Cranking compression is all within 10 psi of each other.Using my last stash of 10W oil in the dampers.Could not swap carbs due to linkage issues, but we did swap pistons. No change: rear still dropped.We did see an increase of about 0.5 in air/fuel ratio when the piston dropped. That is because a fatter part of the needle closed down the fuel toroid, as it should.  That change in air/fuel ratio went away when we manually held the piston open.
I have to agree that it does not seem to impact power much because the peak HP on the dyno was about the same whether we held the piston open or not.  Intuitively that does not make sense, but there it is.....But to return to the question, it does not seem related to the crank harmonics, I surmise after seeing all the responses.For what it’s worth, we are running a Tilton Aluminum flywheel ( I forget the Weight) with a TR6 pressure plate and three puck disc. As a left-over from “back in the day,” I run a Corvair damper. It’s relatively massive with high inertia due to it’s diameter. It seems to work well. Have had no fretting of the nose if the crank that we’ve seen with the small dampers available in kits today, or with none at all.
Sent from my iPhone

On Aug 25, 2020, at 10:42 AM, Kas Kastner <kaskastner at gmail.com> wrote:

I encountered this magic move the carburetor piston many many times on the engine dyno.  Could find no reason for the action and never did.  It did not seem to affect the power (you'd think it would) and was not always there. My last thoughts about this wasa the cross over balance tube may have some effect. I'd stop that particular run, idle the engine a minute and start over and the event would NOT occur again. It was just plain weird to stand there watching the pistons of the S.U's   and all of a sudden one of them would slowly drop down about halfway. My only suspect was harmonics.  

Never be beaten by equipment.

On Tue, Aug 25, 2020 at 4:50 AM Phil Gott via Fot <fot at autox.team.net> wrote:

Can anyone help with this weird phenomenon? The discussion of the second harmonic being at 5200 rpm on the TR4 crank got me thinking:
On a chassis dyno we could watch the SU (HS 6)  pistons during the dyno Pulls. We started each pull at 3000 rpm. The pistons went all the way to the top almost immediately after going to full throttle. The front carb piston stayed all the way up for the entire run. The piston in the rear carb DROPPED DOWN to about 3/4 open at around 5300 rpm and stayed there for the rest of the pull. This repeated consistently.
As these depend on air flow via Bernouli’s law, I examined the valve train thoroughly, fearing a stuck valve, problems with the valve train, a bad lifter etc. but everything looked fine. No noises from errant valve operation either. 
The crank harmonic discussion got me thinking. Could it be something to do with vibration that would upset the air flow or cause the piston to drop? Note that the piston was steady when it dropped. No apparent fluttering. I’m stumped.
The car runs very strong nonetheless. You see the results in Dave’s latest video at Thompson. He and I are thinking about putting a go pro camera and a light under the hood to watch that carb in real life track situations to see what happens.
Any thoughts as to why the Piston in the rear carb only should drop at about 5300 and above?
Phil Gott

Sent from my iPhone
_______________________________________________
fot at autox.team.net

http://www.fot-racing.com

Donate: http://www.team.net/donate.html
Archive: http://autox.team.net/archive http://www.team.net/pipermail/fot
Unsubscribe/Manage: http://autox.team.net/mailman/options/fot/kaskastner@gmail.com

_______________________________________________
fot at autox.team.net

http://www.fot-racing.com

Donate: http://www.team.net/donate.html
Archive: http://autox.team.net/archive http://www.team.net/pipermail/fot
Unsubscribe/Manage: http://autox.team.net/mailman/options/fot/rick.parent@att.net

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://autox.team.net/pipermail/fot/attachments/20200825/3579bba1/attachment-0003.htm>