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Extra fuel for rear cylinders?

Gav

By Gav,
in Engines & Forced Induction

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Belly_up Contributor

Belly_up

Posted
Posted

I wouldn't think it would be a set percentage, you'd really need to have provision for EGT sensors in each primary, and monitor all the temps while you adjust the cylinder trim.

I've seen pics of RB26's (may have been one of the big jap drag cars) where the head and/or block is tapped at the back to accept a second coolant hose to better control temperatures. But that may have been due to the RB26 manifold enleanment mentioned above.

Still, if the fundamental problem is heat, and not actually mixtures, then more cooling at the back of the engine may fix the problem, rather than just treating the "symptoms", ie richening the mixtures.

GiJOr33 Experienced

GiJOr33

Posted
Posted

i would have thought that if there was less air getting to the back 2 cylinders (which i presume are no 5 and 6), then they would be running rich, as less air with the same fuel means a richer mixture.... or have i got it all wrong?

Belly_up Contributor

Belly_up

Posted
Posted

GiJOr33, you reasoning is right, however Sydneykid indicated that the airflow in the GTR / GTR Style manifolds was biased *towards* 6 & 5, rather than the other way round, so they run lean.

He also stated that this (air distribution) was not a problem on the 20/25's, so *if* there was going to be a cooling problem at the back of the engine, it would be cooling related, not mixture related, so changing your mixtures would be working against the goal of optimum tuning.

IF testing with EGT probes (not lambda in this case) or similar indicated that there was a problem with cooling, then you would be better off tapping in a coolant line to the back of the block to better distribute the coolant.

And upgrading the water pump and radiator on any high po engine, esp a turbo one, is never a bad idea. Provided your thermostat works properly, then you can't have too much cooling : )

GiJOr33 Experienced

GiJOr33

Posted
Posted

aaaahhhhh.... i was thinking that the first 4 cylinders were using up most of the air in the plenum before it reached the back 2. What must be happening is that the velocity of the air means if flows fast enough so that most of it hits the back of the plenum, meaning cylinder 5 and 6 get the most air and run lean.

Steve Grand Master

Steve

Posted
Posted

If you bow air into a tube from one end and block the other end, then most of the air will be where the tube is blocked, so feeds better into the rear cylinders that the front ones - this would explain the tapering off of the plenum at the rear too. If the plenum was sealed pressure would be equal on all surfaces, but once you open an inlet vavle, the flow dynamics of the entire plenum change - this is why it takes so much time to design any plenum properly, and the flow of each runner must be measured individually - to ensure they are matched as closely as possible.

haveing said that, it would follow that the stock RB20/25 plenum would be biased towards 3 and 4, perhaps they just got the design spot on? From Corky Bells books on forced induction, the plenum design for the RB20/25 is better than a front mounted throttle body.

Interesting subject.

Sydneykid Grand Master

Sydneykid

Posted
Posted

Hi Steve, you asked "As far as cooling for rear cylinders, could it be overheating on 5 and 6 because they are running a bit leaner?"

It's a bit of a circle, 5 & 6 run hotter so they lean out and they lean out so they run hotter. And around it goes.

As for the percentage leaner, I haven't seen any more than 5% and ours is between 2% and 3%. That's at 1.5 bar, at less than 1.3 bar it is hardly anything at all, less than 1%. More important than boost, we are running over 65 lbs of air per minute. That amount of airflow really backs up at the back of the plenum as Steve said in a previous post.

Hope that clarifies

adam 32 Grand Master

adam 32

Posted
Posted

by lifting the rear of the bonnet you would be cooling the rear cylinders more by allowing external airflow.

how effective is it to reduce engine temps from the exterior of an engine (venting the bonnet or raising it) compared to cooling the internals (coolant/fuel) ?

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