L78 Plugs Color Of Combustion Residue

[Daniel K.]

Please see the attached picture of the plugs from my L78. Have been in the car since 4-10-2003 with about 2.5K miles. Essentially a steady diet of 100LL and oil used is a CJ-4 15-40W. Any concerns with color of the burn? Now on to adjusting the valves per John Hinkley and Duke Williams and the revised article of 9-23-08.

Thanks for your help.

Dan K.

[Joe C.]

Re: L78 Plugs Color Of Combustion Residue

Looks like a perfect situation for stagger jetting.
2-3-5-8 look to be leaner than 1-4-6-7, which look perfect.

[Dick W.]

Re: L78 Plugs Color Of Combustion Residue

For a street engine, the plug color is acceptable. I would not mess with the carburetor.

[John H.]

Re: L78 Plugs Color Of Combustion Residue

Now on to adjusting the valves per John Hinkley and Duke Williams and the revised article of 9-23-08.

Thanks for your help.

Dan K.

Dan -

I hope you're talking about a valve adjustment on a small-block with a "30-30" or LT-1 cam, not your L-78; we don't have rocker arm ratio variation data on any big-block applications yet on which to base cold lash recommendations.

[Daniel K.]

Re: L78 Plugs Color Of Combustion Residue

Went with .022 E and .020 I per Duke's recommendation for lash on the BB engines. Correct?

Thank you for your post.

Dan K.

[Duke W.]

Re: L78 Plugs Color Of Combustion Residue

Yeah, those settings are fine. The OE recommendation for L-78 is .020/.024". These were increased to .024/.028" in 1966 for L-72 - same cam - go figure.

I have some data on big block rocker ratio, but not enough to make a call, so in the meantime I'm recommending the '65 specs - or something very close - for all SHP big blocks.

It's interesting that the inlet and exhaust lobes are identical, yet Chevrolet calls out more lash on the exhaust side. The bigger the exhaust valve the hotter it can run, but I think more exhaust lash is only necessary if you race. In road driving you can't run full load for more than a few seconds at a time so the exhaust valve stem will rarely get hot enough to expand another .004".

The clearance ramp height is .012", so if the true rocker ratio is 1.7, 1.7 x .012" - .0204", but the lash point ratio is probably a little less. That's probably the origin of the '65 inlet lash and they added .004" for the exhaust side, which is basically arbitrary.

I have no idea why they increased the lash for '66. It's too loose.

Remember that valve setting clearances are just nominal values so the running clearances will be within a proper range over all engine operating conditions. The exhaust valve on average runs hotter, so the stem can grow a bit more and close up the clearance, especially if run hard for sustained periods, but that's why mechanical lifter cams have clearance ramps - to allow a wide range of actual running clearance.

If the clearance is set too tight, stem expansion can close up the clearance and hang the valve open, but if it's too loose the clearance isn't taken up by the end of the clearance ramp and the valve is jerked open and slammed shut at greater than clearance ramp velocity, which shock loads the valve train. If there is a lot of valve train clatter, this is what is happening. Proper valve clearance should create little if any noise.

Often, camshaft exhaust clearance ramps are higher to allow a wider range of running clearance (Duntov and LT-1 cams for example), but they didn't add any extra clearance ramp height to the big block SHP cam as they did on the Duntov.

The Duntov cam lobes are identical, except the exhaust side clearance ramp is exactly .004" higher, so at any point above the tops of the clearance ramps the exhaust lobe is exactly .004" higher, and that's how the peak lobe lift ends up at .2625/.2665".

Duke

[Daniel K.]

Re: L78 Plugs Color Of Combustion Residue

Thank you for your comments Duke. #'s 1 & 6 valves were very loose and the rest of the train was also a bit loose. Interesting information on the cam ramps and valve shock when set too loose. I won't be able to fire up the engine until Thursday and I am looking forward to listening for "sewing machine" noise (music?).....that is, if I can hear over the side pipes. This engine was rebuilt in the 80's by a well known collector on the east coast.

Thanks again for your help.

[Michael W.]

Re: L78 Plugs Color Of Combustion Residue

Daniel,

Just curious as to why you use Avgas.

[Daniel K.]

Re: L78 Plugs Color Of Combustion Residue

Michael,

I've used pump gas and 100LL and have found that the cars (I try to use in my 62, 340hp and 63, 340hp) simply run better from start to stop. All the cars start quickly when they have been sitting for a while with 100LL and when they have pump gas in them, it seems to take a few more sessions with the start process. When on the road, acceleration seems to be much improved and there have never been any issues with run-on when turning off the cars. Especially true on the 62 as it tends to run hot and is punched out to I think .060. Now for the less empirical, as if the above is, I like the way it smells both when filling the tank and on combustion.... I know, I'm an odd duck....... Then, since this fuel is blue, it becomes clearly evident if there are any fuel leaks. I also like the way the exhaust tips look when the cars have been on the road at highway speed. Nice grey/tan color, like in the days while I was terrorizing the town in my 55 Chevy and the 62 Vette (very early 70's). So the long and short of it is, it makes me feel good and I believe it makes my cars perform better. I'm sure I could attempt to tune the cars timing wise to get ok performance from pump gas, but the airport is just a short distance from my house and its fun to drive the cars there to get gas. As long as there are no detrimental effects on my engines (just the wallet at this point), I'll continue to use 100LL. Thanks for your note. Dan K.

[Joe L.]

Re: L78 Plugs Color Of Combustion Residue

Michael,

I've used pump gas and 100LL and have found that the cars (I try to use in my 62, 340hp and 63, 340hp) simply run better from start to stop. All the cars start quickly when they have been sitting for a while with 100LL and when they have pump gas in them, it seems to take a few more sessions with the start process. When on the road, acceleration seems to be much improved and there have never been any issues with run-on when turning off the cars. Especially true on the 62 as it tends to run hot and is punched out to I think .060. Now for the less empirical, as if the above is, I like the way it smells both when filling the tank and on combustion.... I know, I'm an odd duck....... Then, since this fuel is blue, it becomes clearly evident if there are any fuel leaks. I also like the way the exhaust tips look when the cars have been on the road at highway speed. Nice grey/tan color, like in the days while I was terrorizing the town in my 55 Chevy and the 62 Vette (very early 70's). So the long and short of it is, it makes me feel good and I believe it makes my cars perform better. I'm sure I could attempt to tune the cars timing wise to get ok performance from pump gas, but the airport is just a short distance from my house and its fun to drive the cars there to get gas. As long as there are no detrimental effects on my engines (just the wallet at this point), I'll continue to use 100LL. Thanks for your note. Dan K.

Dan-----


For the amount of mileage on them, I think these plugs show a lot of combustion residue. I'll bet there's a lot more in the combustion chambers and piston tops. I'd say this residue was directly related to the use of the avgas.

[Michael W.]

Re: L78 Plugs Color Of Combustion Residue

Dan-----


For the amount of mileage on them, I think these plugs show a lot of combustion residue. I'll bet there's a lot more in the combustion chambers and piston tops. I'd say this residue was directly related to the use of the avgas.

Yup, beat me to it and that's the reason I asked the question above.

[Duke W.]

Re: L78 Plugs Color Of Combustion Residue

What heat range plug are you using?

Duke

[Daniel K.]

Re: L78 Plugs Color Of Combustion Residue

Thanks Michael and Joe for your input on the 100LL and the amount of residue on the plugs.

Duke, I'll get back to you on Thursday on the plug heat range when I'm back at my garage.

Next question; what can I do to ensure when using pump gas, I get better performance and ease of starting? I would need this information for the 62 340hp and the 63 340hp cars too.

Thanks again for your help and the education you all bring to me.

Dan K.

[Duke W.]

Re: L78 Plugs Color Of Combustion Residue

Avgas has two "advantages" - if you want to call it that - over pump premium. First, it has higher octane - about 104 PON compared to 91-94 PON for pump premiums depending on where you are in the country.

The second potential advantage is higher vapor pressure, which will generally prevent vapor lock problems that some vintage engines have, but the engine might be harder to cold start.

So it comes down to whether you actually need either of the above. Most OE engines will run on pump premium, but big blocks have a bit more octane appetite.

It also comes down to the actual CR of the engine. OE built engines are about a half a point lower than advertised and most rebuilt engines end up even lower, so very few engines actually need more octane than pump preimum, and if they do, about a 25 percent blend of 100LL avgas with 75 percent pump premium will usually quell any detontation.

Duke

[Clem Z.]

Re: L78 Plugs Color Of Combustion Residue

Yup, beat me to it and that's the reason I asked the question above.

100LL has 1 to 2 CC per gallon of TEL where sunoco 260 back in the bad old days had 4 CC per gallon. in a BBC engine most of the spark plug deposits were from oil because of the poor ring design and lack of a positive seal on the intake valve