On going tuning saga.

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mooreken
Posts: 591
Joined: Sat May 09, 2015 12:25 pm

On going tuning saga.

Post by mooreken »

While tied down at home and still not quite ready to get back on the Triumph, I hope you are up to another of my long winded account of my saga. I have gone back to the Cobra tune.

Let me first say I'm not trying to get more power, I have all I need but I just can't leave the tune alone quite yet. Before the previous motor explosion, I had been dealing with timing and carb settings. This was before I borrowed Toms O2 meter. With the distributor I was going back and forth with using vacuum advance or not. Using the vacuum advance did help with the plug readings and some hesitation with the carb. But I was getting popping thru the exhaust at cruising speed with light throttle.
This went away when I didn't use the vacuum advance.
Now I have both heard from folks about not needing the vacuum advance for years and have often eliminated it. As I did this time to stop he popping. And I made the timing adjustments in both initial and total within the distributor. And as I said the power and performance was and is good.
However my plug readings once again just didn't feel right. Still showing somewhat dark. They had cleaned up with the VA.
I was never ever to do a WOT shut off to get a clean reading for the carb. (Too much infield at the track and high gear WOT on the road just didn't work out for me with a place to go that fast with a place to pull plugs)

With the new motor I kept the timing about the same (dropped 2degrees total with the increased compression), and using Tom's O2 meter I got both my 650cfm and the 750cfm carbs working well.

Now on to the meat of this posting. As the plugs still had more dark than I would like and the side pipes are more black than gray I still felt I was missing something. While messing with the idle screws today setting for max vacuum I still felt that the fumes in the garage even with the doors open were stronger than they should be. So I reconnected the VA and went out for test drives. I left the ignition settings the same. Sure enough the plugs have cleaned up quite a bit.
And the response seemed improved. That being said. With the light weight car and that much power it hides a lot of feel.
(I once came back from a day at the FIRM and happened to pull the valve covers and found a rocker stud had backed out and one of the rockers was no longer connected. I didn't notice any lack of power and with the "Thumper" cam the idle seemed ok)
After a couple of runs the popping at cruise was still there but not as bad.
My feeling with the first motor was that the VA as adding too much advance for my performance build. And I still felt that is what is going on now.

While searching the ol'interweb for how to adjust the VA I found a couple of articles that described my experience with the popping exactly as my motor is acting. So I hooked up my timing light and checked how much the VA was doing an it is almost 17 degrees. The bad thing is that MSD does not make or support an adjustable VA unit.

The fix is to back up the initial advance to compensate (now while I have lots of power let's not do anything crazy here) or find a way to restrict the VA.

With more internet searching is seams that the SBC MSD distributor is based on the sbc point style distributor and Accel makes an adjustable VA. So one is on the way.

With out trying to start a discussion on whether or not using the VA is necessary or not this is a street car most of the time and the advantages of more timing to increase cylinder pressure at low load mostly closed throttle, is something I think this motor needs.
Once again do to the low weight of the car, 90% of my street time the throttle is just barely cracked. So the motor lives in the area that more timing is a good thing.

I'll give an update on things after I get the new adjustable VA.

Now the only other non normal thing that I can't find on the internet yet, is why I have 245-250 lbs cylinder compression.
The compression ratio is 10.8 but that still is a lot. No pinging, no hard starting, no detonating that I can determine and I did have the heads off to fix the valve guides so I'm not really concerned just don't know why.

Well thanks for hanging in there. The story will continue.
Ken
Tom Wells
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Joined: Fri May 08, 2015 7:34 pm
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Re: On going tuning saga.

Post by Tom Wells »

Ken,

Well, I think I have one answer for you: your Cobra has a diesel engine! A cranking pressure of 245-250psi is a good number for a diesel having a compression ratio of 16:1 or 17:1. A compression ratio of 10.8 (times 14.7) gives a compression pressure of 158psi.

Problem solved. Or do you have a defective compression gauge?

Tom
mooreken
Posts: 591
Joined: Sat May 09, 2015 12:25 pm

Re: On going tuning saga.

Post by mooreken »

Well about the gauge it has always seemed to work before but it is old. (Like Most of my stuff...lol)
Maybe I'll drive down and use yours sometime.
BlackSnake
Posts: 345
Joined: Mon Jul 13, 2015 4:09 am

Re: On going tuning saga.

Post by BlackSnake »

Ken, FWIW, I don't run VA on my motor and have never felt a need to add it.

I agree with Tom, that cranking pressure is extremely high for 10:1 CR. My motor is 9.8:1 CR and my cranking pressures average 190 or so.
mooreken
Posts: 591
Joined: Sat May 09, 2015 12:25 pm

Re: On going tuning saga.

Post by mooreken »

Mike, I also have been running without the VA and as I mentioned it has Plenty of power and no real issues, but the motor does seem to the extra advance. Also its just in my nature to keep tweeking my stuff...whether its my cars or computer, or just about anything. Sometimes it doesn't go well and other times it's a home run.
Mostly I'm trying find out why it seems to be not burning the fuel at idle. I've leaned it up a lot even to the point that it just wont go...and I'm still showing dark plugs...but when the extra advance is in play...the plugs get white almost right away. I'm sure it has something to do with the "Thumper" cam and the "Twisted Wedge" trickflow heads. Both are a bit out of the normal for a street car build.
I don't know why I'm reading so much compression either. When the motor was first built, maybe even before I started it I did a check and got the same numbers. I expected it to be a little lower now with several thousand miles on it. So I think I need to check with a different gauge and compare.

Ken
mooreken
Posts: 591
Joined: Sat May 09, 2015 12:25 pm

Re: On going tuning saga.

Post by mooreken »

The Accel Vacuum advance came in. Now to be politically correct I am not trying to change anyone's mind here on using or not using a vacuum advance. This is just my experience on my motor.

It was a direct replacement for the MSD. (a point style Chevy advance). I was even able to get it in without pulling the distributor. This way I could see any changes from the base line. As it turned out I think the rod that pulled the advance plate was a little bit shorter as my static timing changed about 10 degrees. However this did not effect the final install.
By starting the motor and using my hand vacuum pump I was able to adjust the pot with the motor running and see the changes via my adjustable timing light.
The MSD pot was pulling about 17-18 degrees I really didn't try to see it any closer. I set the Accel to 13 degrees total vacuum advance. this gave me around 25 degrees advance at full vacuum instead of the 30ish that I had before.
The result is what I hoped for. The exhaust popping has stopped and cruising speed is a little smoother. A side benefit is I can now set the idle about 100 rpm lower. Before I had to be around 950 to keep the motor from doing what I called "fighting itself" it just didn't like idling any slower.
I'm sure all of this is from the Comp "Thumper" cam that has a lot of valve overlap to get that nice "thumping Idle" while cruising the car shows. My research on this has lead me to believe that the overlap has lowered the cylinder pressure at idle and the extra advance does raise the pressure.
With a short run on the interstate and the neighborhood speed back to the shop I pulled a plug and things are looking good. I need a new set to do a finale check.
I am using manifold vacuum for the VA as I wanted the advance when idling. For a less radical cam, ported might be a good option.
The research also stated that fuel mileage should increase...only time will tell but I'll take it if it shows up.
I hope this gives a little information that you can file away in case you ever find something similar going on.

Ken
Tom Wells
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Re: On going tuning saga.

Post by Tom Wells »

Ken,

What a shame :mrgreen:

Now you'll have to find something else needing correction....

I have a similar list; the thermostat moved itself the top. Our latest cruise showed the engine (radiator and oil) temps to be too low. Like about 140F or so. When I looked, the water level was also a little low.

So I bought a thermostat and gasket and removed the old one. One of the bolts showed a bit of a lack of threads evidently due to corrosion. Naturally when I reinstalled it, it stripped. The other one wasn't much better. :GR::

I'm now patiently awaiting the 21/64" drill bit and HeliCoil kit to fix both holes in the aluminum Edelbrock manifold. Grrr...

Tom
mooreken
Posts: 591
Joined: Sat May 09, 2015 12:25 pm

Re: On going tuning saga.

Post by mooreken »

If you have any trouble getting the heli coil kit I probably have one. I could drive down and use your compression gauge and offer advice from the proper 6' distance. ~~(::
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