V12 Engine Balancing

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Rudy van Daalen Wetters
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V12 Engine Balancing

Post by Rudy van Daalen Wetters » Fri Mar 28, 2008 10:24 pm

I am told by others that our V12s are "shaky" by nature. The straight six of my 1958 Aston Martin is incredibly smooth and vibration free. The V8 of my 1968 Maserato Mexico is turbine smooth! The GTE by contrast is a tad less refined for what I would expect for a V12. Working against it, is the practically solid mounting of the motor to the chassis and the lack of a
harmonic balancer. My question, to those of you who regularly rebuild these V12s is, what methodology do you use for balancing all the components of the V12? Are there other, more precision based procedures that may be particular to our V12s that would be employed to guarantee a vibration free engine? Any input on this subject is appreciated.

Rudy van Daalen Wetters
1963 GTE s/n 4001
1966 330 GT s/n 8705

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John Vardanian
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Post by John Vardanian » Sat Mar 29, 2008 8:25 am

Hi Rudy,

Short of weight balancing the reciprocating pieces and spin balancing the flywheel/clutch/pressure plate, there isn't much else. Remember, you are comaring apples and oranges here. Your Aston is essentially a Detroit V8 type phillosophy and your in line 6 is an old fashioned, long stroke low revving Brit iron.

The smaller bore Colombo engines operate on a different premise than your other cars. They have no dampers on the front of the crank shaft and a featherweight fly wheel.

Quickly press on the throttle of your 250 and let go, and do the same to your other cars and listen to the engine response. That should answer your question.

john
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John Vardanian
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Post by John Vardanian » Sat Mar 29, 2008 8:30 am

Rudy,

I forgot to add that my 250 also has a mild vibration in the 1800 to 2000 RPM range, but when it gets above 3800 it spins like a DC motor.

And, sorry, in my previous post , I mistook your Maser for another in line 6 Aston.

john
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Jimmyr
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Post by Jimmyr » Sat Mar 29, 2008 10:07 am

Many of the small vibrations can be traced to carb imbalance and ignition timing variations. If the advance curves from right to left banks do not match shutter takes place. Also when the main jets open unevenly there is the hesitation of one group of cyl causing a vibration. All of these are easilly fixed with a little extra effort on setting the parameters to factory specs. Jim

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v12 vibration

Post by mark » Sat Mar 29, 2008 1:13 pm

Jim is on the money. With careful attention to synchronizing the carbs, front to back and sise to side you can realize tremendous refinement.
This of course depends on how out of whack you are to begin with.

My limited experience with webers suggests that a lot can be attributed to air leaks. if the carb bodies are bent you will not be able to synchronize them.

Ignition is also a big issue. I had my distributors rebuilt which made a great deal of difference. It is difficult to find an old Sun machine, but the better shops still have them.

Jim, I was curious about your comment regarding the main jets. My impression is that the jets are fixed. Are you referring to when each carb
transitions to the main?

My car has a slight hesitation down low in the rpm range and I beleive it may have something to do with this. Your thoughts would be appreciated.

Mark
69 365 gt 2+2, 12659
98 M3, 02 Porsche 996
98 550 Maranello

Jimmyr
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Post by Jimmyr » Sat Mar 29, 2008 2:59 pm

Mark, yes the transition from the idle jet to the main jet is a big change in the overall smooth operation. The jet is fixed, but the change over is due to the vaccum in the chamber. If the throttle linkages do not all arrive at the same point there is a delay in some cyl mixtures and a vibration is felt. Jim

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TOMKIZER
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Post by TOMKIZER » Sat Mar 29, 2008 5:06 pm

Jimmyr wrote:Mark, yes the transition from the idle jet to the main jet is a big change in the overall smooth operation. The jet is fixed, but the change over is due to the vaccum in the chamber. If the throttle linkages do not all arrive at the same point there is a delay in some cyl mixtures and a vibration is felt. Jim
You people are terrible! I probably won't sleep tonight, trying to remember things from 1969, when I was doing carburetor development for Ford Engineering. Fuel injection erased almost all of my old memories.

Anyway, I remember when we used to set up the WOT ignition timing to MBT - Minimum spark for Best Torque (this was before unleaded fuel and detonation was not a problem - plenty of tetra-ethyl lead around). We just found best torque ignition timing at each speed and reduced the spark until we noticed a tiny decrease in torque. We then adjusted "manual carburetors" to find LBT (Leanest Fuel for Best Torque). That is to say, add fuel at a speed until the torque stabilizes then subtract fuel until the torque starts to drop - LBT. We tried to set up the carburetors to achieve slightly more fuel than LBT with the "Automatic" carburetors. For part throttle fuel calibration, we always searched for a calibration that we called LMV, Leanest fuel for highest Manifold Vacuum. For spark timing, we searched for "Borderline Spark Knock" minus about 2 degrees. It required a significant amount of mapping the engine at many speeds and loads. By the way, spark and fuel are interactive so it's an iterative process.

All the above BS as a background to ask the question,"Should we be setting up our carbs at idle and off-idle to find ITERATIVELY the Leanest fuel for best Manifold Vacuum (LMV) at a fixed synchronizer air flow using a vacuum guage to set the mixture and the synchronizer to set throttle angle relationships carb-to-carb and left-to-right?

I imagine it wouldn't take more than about a week to get a good smooth idle and a good off-idle transition without hesitations, sags, or backfires on a three carb, two distributor V12 setup.

Or, on the other hand, maybe we shouldn't worry about it.

Any opinions?

Tom Kizer
So many sidewalk cafés - so little time left.
1969 365 GT 2+2 S/N 12293 (Gone but not forgotten)
1967 230 SL 4-spd (Currently on CPR)

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Post by whturner » Sat Mar 29, 2008 8:56 pm

Tom:
I agree we should - usually - not get too stressed out about the idle. I cannot get mine to idle perfectly at any setting, and it is fresh (now about 3500 miles) from a complete tune pursuant to the engine rebuild. And the best idle does not give the best/smoothest road performance, so guess which has priority. With Ferrari exausts one can fine-tune the carbs pretty well at speed by ear. Drive along a steel guardrail with the window open. Just like 50 yrs ago in a Flathead Ford with Dual pipes and Carbs.

I have found it interesting, however, how tempersture sensitive my setting was. After driving all summer with temperatures at 60 to 70 or higher, when we started to Tom Yang's shindig this year in a cold rain, I had to tweak all the carbs about 1/8th turn richer to get rid of the spit-back each time I moved the throttle. Maybe it was running a bit lean to start with.

Cheers
Warren
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Rudy van Daalen Wetters
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Post by Rudy van Daalen Wetters » Sat Mar 29, 2008 10:36 pm

My thought is that by the laws of physics, the V12 design should lend itself to be the most naturally balanced of motors around. I can see John's side about apples and oranges on the comparisons, but vibrations have a source that causes them. I would think that if all were balanced correctly you really should not be noticing much of any. My Lancia Flaminia has a very interesting design that allows the fitment of small individual weights to be attached to the driveline to assure a vibration free transmission of power to the rear 'propeller unit' (trans and diff in same unit) where the clutch sits.

With respect to the carbs, I have to believe mine are spot on. The idle is very good, the take off is without hesitation and the power is there. I balanced the carbs at 2500 RPM first, and then at idle. Numerous test runs indicate the same equal readings with the synchrometer.

The passsenger side distributor may have weak advance springs as the timing mark on that side flutters at about 2800 RPM and onward. The flutter can range up to a 10 mm deviation. The driver side timing mark stays quite steady throughout the RPM range. Locating replacement advance springs may be a difficult or impossible task. Another thing I notice is that there is 'play' in the drive gear coming off the cam and the drive gear going in to the distributor on both sides. I am not sure what affect this play may have in contributing to an actual vibration in the motor itself. Sometimes I feel that maybe two six cylinder engines are fighting each other. "Can we all just get along here?"

Rudy van Daalen Wetters
1963 GTE s/n 4001
1966 330 GT s/n 8705

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John Vardanian
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Post by John Vardanian » Mon Mar 31, 2008 11:26 am

Hi Rudy,

I think, if all things equal and in good order you will notice that the front engined 2+2 Lambos would also be far smoother than the 250. I have experienced a 400GT and I can tell you that the way the car surged forward under power reminded me of a large engined Buick Skylark.

The heavier flywheel affords these engines a certain smoothness, however, makes the cars less responsive to throttle.

john
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Rudy van Daalen Wetters
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Post by Rudy van Daalen Wetters » Tue Apr 01, 2008 12:51 am

Hi John,

You really need to drive my Aston Martin!

Rudy van Daalen Wetters
1963 GTE s/n 4001
1966 330 GT s/n 8705

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John Vardanian
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Post by John Vardanian » Tue Apr 01, 2008 7:16 am

Thanks Rudy. I take that as an invitation. Think I mentioned to you I had a DB2 once.
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abrent
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Post by abrent » Tue Apr 01, 2008 9:35 am

[quote="Rudy van Daalen Wetters"]The passsenger side distributor may have weak advance springs as the timing mark on that side flutters at about 2800 RPM and onward. The flutter can range up to a 10 mm deviation. The driver side timing mark stays quite steady throughout the RPM range. "

Hi rudy, just a thought about this. Have you checked the timing chain tension. I remember on my car that what you describe was fixed somewhat when I tightened the chain.

Regards,

Andrew.

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vibration

Post by mark » Tue Apr 01, 2008 4:15 pm

Thanks all for the input.

Jim: Your point is well taken. After reflecting on the intent of what you said it becomes a matter of figuring out a way to evaluate and synchronize the three carbs at the transition point. I believe there are two components to this point of transition. 1. When the progression holes in the carb bodies are exposed. 2. When the vacuum becomes high enough to draw fuel from the main jet.

I have heard of folks just manually sighting into the carb. bodies as the linkage slowly opens the butterfly valves. This seems crude but could be very effective.

Once again, your thoughts would be appreciated.

Mark
69 365 gt 2+2, 12659
98 M3, 02 Porsche 996
98 550 Maranello

Jimmyr
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Post by Jimmyr » Tue Apr 01, 2008 6:06 pm

Mark, you can start by making sure that the linkage rods are all the same length, and their action on each carb is exactly the same. Using a sync gauge on eash carb at various RPMs can give you an idea of when the transitions will take place. The controling parts are the emultion tubes on the top ledge. Be sure they are all very clean and have matching jet and body numbers on the parts. It is important that all ignition work be completed prior to attempting any carb adjustments. Jim

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