Mysterious soot on the exhaust valves 3

[mrwizz]

Engine configuration:
361 cube oval racing small block Chevy geared and tuned to run up to 7600 rpms - Dart Aluminum heads with straight plugs - 55cc combustion chambers - cam with 256 deg intake and 264 degree exhaust at .05 lift. - the flat top pistons and chambers are configured to maximize quench/turbulance - .034 clearence piston to head cold - and total of 34 degrees spark advance.
I've been building this style engine for many years, so I generally know what to expect when setting up the carb and valves on the dyno. Usually the carbs are pretty close right out of the box with maybe going one or two sizes up on the jets. This is where things get strange. It took three sizes up on the jets to make smooth broad power. Race day approaching dictated that there was no more time for dyno tinkering.
At the track the engine snapped and popped like it was lean. One more jet size up helped power and the plugs looked better. After many runs it still sounded on the lean side. Added more jet size. Now with a total of five jet sizes up, the engine was clearly top dog both out of the corners and at top end. It was better than several cars that were know to be running outside the rules with roller cams etc. When my motor was protested - torn down for inspection - found to be completely legal - They could not believe it.

What has me puzzled is the exhaust valve tulips (combustion chamber side)are sooty following several high speed shutdowns to read the plugs and check piston to head interference. The plugs, piston tops, and intake valves are clean. The only other soot is just a small smug ring on the spark plug thread/head interface.

What could keep the ex. valves wet and cool enough to stay dark? The next step will be a little more cam and back off the timing a little.

 

[Fabrico]

Something changed in that carb to require/allow going up 5 jet sizes. That's beyond what most other changes would do.

Even a high speed shut down is not instantaneous. Things can still happen and leave their mark.

It sounds like a good combination to leave alone. Sometimes, you mess with something like that and never see it again.

 

[SMOKEY44211]

Just a thought is your fuel supply been changed to an ethanol blend? That would explain the need for the larger jetting.-------Phil

 

[kenre]

My guess is that for whatever reason it needs to run much richer than per normal, you are having more fuel to cool the chamber. The exhaust valve being the hottest part letting carbon build up from the richer mixture.

Ken

 

[mrwizz]

I switched carbs early on to eliminate it as the cause - no diff. The fuel used is always custom racing fuel that is supposed to be ok to 14 to 1 compression. I like things to make sense and follow the rules becauase it shows I understand the fundamentals. The ex. valve soot was a surprise. Until someone comes up with something better, I'm inclined to think that possibly the turbulance is spinning some liquid onto the valve and it doesn't have time/air to fully combust.
Dick Maskins (bought out Dart) makes the custom racing heads I'm using. He says: If it makes more power richer --- "What the hell is the problem?"
I justlike to learn from mistakes and improvements.

 

[automotivebreath]

"What could keep the ex. valves wet and cool enough to stay dark?"

Just a thought; an over effective exhaust system could be pulling excessive intake mixture past the exhaust valve during camshaft overlap. This may be cooling the exhaust valve enough to cool it form the carbon deposit.

 

[automotivebreath]

Mrwizz you ask one question about the carbon on the exhaust valves, in reality the extra fuel that you have added could be the source of the carbon. If that’s the case I would consider the need for the extra fuel as an indicator that combustion is poor. In my opinion if you can resolve the need for excessive fuel the engines performance will improve.

As for the exceptional performance I relate it to improved volumetric efficiency. I would be trying my best to understand this. It could be that Dart is using you as a test bed. I understand that they have a top cylinder head man in R&D.

I love protest, the jealousy adds to the fun of competition.

 

[patprimmer]

Very high octane petrol normally contains a lot of aromatic hydrocarbons. These are inclined to leave soot.

As others have said, a lot of overlap at TDC and an exhaust that is savaging well can draw extra fuel into the exhaust and cool the valve. Also if the manifold is cold and the mixture is not well emulsified, and maybe has dead spots in the air flow path where puddles can form, when these puddles are forming, the engine will be lean and will need enrichment, later these puddles might be sucked into the engine causing rich mixture soot. If the puddles are sucked in mostly during overlap, a lot of fuel will go out the exhaust.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[evelrod]

For many, many years there has been a sign that hangs on my wall..."If it ain't broke, don't fix it"

I have had so called "identical" engines that just made more of whatever--- giving me on at least three occasions lap records at some pretty impressive races. I, too, have been thrilled to see the look on my competitors faces as they ponied up the tear down fee...great stuff!

Rod

 

[automotivebreath]

"If it ain't broke, don't fix it"

evelrod, If an engine runs better than expected it does so for a reason. If you can understand the reason you have a chance to duplicate the performance next time around.

 

[GregLocock]

I'm sure a newbie like Rod will appreciate the advice.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[evelrod]

Thanks, Greg...I still feel like a 'newbie' sometimes.

When I started building engines for race cars, oh, some 50 odd years ago, I found that I could not build two identical engines...they all performed within a narrow range, but not identical. In my SBC faze in the early '60s I could get them within five percent or so and, finally, with the advent of much better machines to build engines with and quality products available over the counter...I have built the last three Lotus twincams within 1/2hp and equal torque. That's great until one of them suddenly pulled some ten more hp and about 12 more ft/lbs with a clear 400 rpm advantage. Yes, when it came time to freshen it up a bit I paid close attention to "Whats different". When it went back together it was still 'better' until the block cracked above the left motor mount and all the water blew out going into turn nine at Riverside...not the greatest place to have that happen. Never got another engine to make that much power...oh well, I race Minis now a days...cheaper and more fun in vintage. I still have the first Lotus Cortina in the shop, for my son (if he ever gets it finished). One thing I have learned, in spades, I do NOT know everything there is to building the ultimate engine. I learn something every time I do an engine, even if it's to just not do THAT again!!!

Just an FYI: The newest engine I built to the specs of a professional race engine shop, even let them do my valves...my engine dynoed (on the same dyno) TEN more hp and TEN more ft/lbs torque over a 1000 rpm wider range!!!
That was last year and I am STILL smiling to myself.

Rod

 

[automotivebreath]

If I did, I wish never to offend thee again. But I like you are always in the search of the why? If I know why something works, perhaps I can duplicate it, or even improve it. That’s what puts the smile on our face.

My bet is the 361 CI in question has a superior cylinder head, perhaps Dick Maskins at Dart knows something we don't. If so I’ll bet he’s smiling.

 

[whitevette]

Hi, PatPrimmer!
Lots of input & opinions on fuel, "spots", improper combustion, etc., etc.
Has anyone considered the fuel quality? Over the years (decades?) the quality of gasolines has slid. From pump gas to avgas, from racing gas to "whitegas"...she ain't what she used to be! Comment?

 

[evelrod]

I have a comment. We had our local fuel supplier change from Sunoco to VP this year. I re dynoed my engine a couple weeks ago on the new VP fuel for the first time. The engine has a season on it but the cyl head is completely fresh, all other specs the same 14:1 CR, etc...the engine oil for last year was Mobil 1 and this year it is Red Line and, obviously the fuel is different. I compared both MSDS sheets from both Sunoco and VP and, from what I read, the formulations are approximately the same octane but different chemical formula. What I mean to say is, with those differences there can be no direct scientific comparison...

Bottom line...this year I'm down a tiny hp and torque and rpm range is a bit less. Mixture strength is equal , both years with equal jetting. These were corrected figures, but I swear the Sunoco just "FELT' better. Just my opinion. It really matters not as I no longer have ready access to Sunoco. VP it is, then! Oh well, all I race is vintage these days and I have lost, over the years, more than enough skill to make up for any fuel differences.

Rod