Compression Pressures - SU/Hitachi and running on.

Compression alone won’t be the reason for running on, there must be other factors at play. Interesting that you both went to hotter plugs. The higher octane fuel must help lower chamber temps also?
 

Rob Gaskin

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Interesting you say BP6ES to BP6ES , I have swoped to a hotter BP7ES plug in mine because the plugs were getting to hot.

Never had a problem with mine running on, it was about 210 when we tested it on the dyno a while back and I run it on shell V power or the tesco 99 octane

Interesting - what carbs?
 

atomman

Club Member
Compression alone won’t be the reason for running on, there must be other factors at play. Interesting that you both went to hotter plugs. The higher octane fuel must help lower chamber temps also?

Dyno guy looked at mine and said to go to 7 , my understanding from it was the hotter the combustion chamber temps i'e from high compression you use a colder plug which gets the heat away from the tip quicker to stop it burning out, that's if i understood it right,

Interesting - what carbs?

weber 40 DCOE 151
 

Rob Gaskin

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Dyno guy looked at mine and said to go to 7 , my understanding from it was the hotter the combustion chamber temps i'e from high compression you use a colder plug which gets the heat away from the tip quicker to stop it burning out, that's if i understood it right,



weber 40 DCOE 151

Yes I agree on plug theory. I used 7 in my old track car from advice from the engine builder.

Weber - yes assumed so.

This Thread is confirming that very high compression with SUs results in 'running on' which is what my engine builders have said.
 

jonbills

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When my engine was melting the tips of bpr6s, I tried bcr8s but they did foul up a little if I let it idle too long. I'm now on bkr7e.
I think it's possible Rob that the 8s might be running below the self cleaning temp and collecting carbon which is then red hot and causing running on.
Might be worth trying a hotter plug if you haven't already.
 

AliK

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OK I'm out of the box again! Boing!!!

5fddd0c277245d0b7b331df5d8d3668f.jpg


This Thread is confirming that very high compression with SUs results in 'running on' which is what my engine builders have said.

I see it as quite the opposite and let me explain why:

We all agree that the higher compression will be heating the mix more than the norm. That is a constant regardless of what is outside of the chambers I.e. the type of carbs.

We've determined that DCOEs will work just like SUs and continue to supply a mix as long as the engine is sucking.

As has been said already, if the heat of compression alone was sufficient to ignite the mix, then the engine should keep running like a Diesel engine until you cut off the air/fuel supply; and not be stopping after a second or two.

The carburation making a difference can therefore only be because the DCOEs are naturally running richer than SUs and keeping the chambers cooler - especially as they don't cut anything off on power down.

So does that mean you should go richer on the SUs to avoid running on?

While the engine builders have said it's "normal" to run-on with SUs, did they help you understand the logic / give a technical explanation as to why SU carbs make a difference to the compression itself being responsible for raising temperatures to the point of ignition?

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Rob Gaskin

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Guys, no matter how much theory you post it's a fact that modified (high compression) L6s with SU type carbs have a tendency to 'run on'.

What I'm trying to establish is what compression pressures folks are running first of all because mine seem excessive (or very good). Also if someone comes on here and says they have 210psi plus on SUs and not getting the running on I'd like to know their configuration and/or solution.

It appears that not many people measure their compression so perhaps that's the end of this Thread. I've altered the title so that it might attract more response from owners.
 
Rob, I’ve asked a ‘bikey’ friend who looks after a HSCC car, it makes very strong power(all in regulation), on SU’s, apparently stops dead.

He wouldn’t really say what would be causing it ‘as it could be a combination of many things’ but felt a bit of setup optimisation could help maybe. He lives in Dorset if you wanted him to have a look. Would have to fit around his new job with an world endurance team.

Who is your engine builder?
 
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