Cyl No. 6 Lean Out - Theory Vs Practical

[MattSR]

One thing people tend to overlook is that its the hottest cylinder as well - this causes leaness as well

[Shaun]

On my cars first dyno tune they just blew a Tomie head gasket on number 6. Engine is an RB26 with an aftermarket single turbo manifold. I think im going to weld bungs on the exhaust manifold (at least on #1 and #6 runner) so that we can use sensors to tune each cylinder perfectly with the Autronic SM4.

This would be using #1 (at least) as the control cylinder.

Can anyone see a problem with this?

As for the manifolding, screwed if i know what causes the problems. Maybe its a combination of many things; #6 runs hotter (further from water pump etc).

Shaun.

Edited June 18, 2006 by Shaun

[Duncan]

Interesting thread.

But in the first post you mentioned rb20/25/26/30, remember that the 26 has a totally different plenum design, one that would in theory result in more airflow to the rear cylinder. All the rest have the central intake but the 26 is front facing

[R31Nismoid]

Would have thought that the leaning out had more to do with the fuel rail then the intake manifold. Its not a big issue just tune a little richer.

same... and the added heat the rear as there is less cooling than the front.

Hence the best flowing injector to the end, and just trim up accordingly.

I ran over 300rwkw stock RB25 head/intake etc and i had not an issue.

The motor/car was worked very hard and it came apart for inspection after selling and was A-OK.

Just gotta do it right.

I think the problem is more prevalent in RB26 than RB25... and its just people not doing the right things (injector/trim/fuel) to prevent it really, being there is simple ways to do it than forking out extra for a new plenum.

[Beer Baron]

are you sure that all RB inlet manifolds flow more air at #6? I mean the difference between say an RB20 manifold and an RB26 one in design is quite huge. There are other problems too. the design of the rail (reg at one end) means that as the pump is running out of flow number 6 will lose pressure first. also it's posistion means that #6 gets hotter than #1 etc.

[Swiper the Fox]

Its the fuel rail design...its due to the fact fuel is fed in one end and regulated out the other...an easy trick...try putting the garden hose on the fuel rail, using a small cork with a hole drilled through the centre to act as a restriction (regulator) you will see why No.6 and even No.5 runs lean. The pressure differential over the rail is uneven, thats why a dual fed centre regulated rail will always perform better and even out the pressure over the rail.

[ISL33P]

busky2k - in terms of plenum flow yes this is a problem with the majority of factory inlet manfolds, regardless of design, each has their flaws.

shaun - no1 might run the cooles, but also has less airflow than that of 3 & 4, i would use 3, but that wouldn't be to hard to change. Turbine has the idea, put them on all exhaust runners.

duncan/beer baron - this is also the case with the rb26, but less prenowned, which is why i cant understand why a lot of rb26 owner's bag it out.

r31nismoid - well said, but not everyone has a good tuner or a computer capable of individual cylinder trim of injectors. this is about trying to do everything possible so no matter what a tuner does, within reason, it can be fixed cheaply and easily and to the maximum effeciency the engine can possibly run.

depends which way you want to look at it, "cure the cancer or prevent the cause".

plenum design, water temp and fuel rail design (dam where is that thread again) seem to be the general concensus and all contributing factors to the problem.

any other opinions/coments ?

Ariel

[Sydneykid]

I thought we had covered this subject quite thoroughly a couple of years ago. At the expense of repeating myself, there are 3 main issues with RB26's

1. #6 is the furthest for the water pump/radiator, hence runs slightly hotter. This is indistinguishable from the circulation differences which straight sixes always have.

2. #6 is the furthest from the fuel pressure regulator, hence has slightly lower fuel pressure. This is common hydraulic theory in practise.

3. #6 gets the most airflow, by a very small amount as it is at the end of the plenum and gets slightly higher air pressure as a result. Don't ask me to explain why, aerodynamics is not my specialty.

4. We have done lots of testing on RB26's with individual cylinder EGT and A/F ratios readings. We have 3 EGT sensors and 3 lambda sensors that use the same fittings. Regularly we use infra red temperature sensing via a hand held pyrometer to check the individual cylinder tune. It is much quicker than swapping EGT and Lambda sensors between primary pipes and gives very reliable results.

Fixing one of the problems (as above) does not guarantee a perfect result. So don't just think that by feeding the fuel rail centrally you have fixed the problem. In fact our testing showed this (#2) to be the least of the 3 problems. (Hence I suspect why the Mines and Gibson Skylines retained the standard fuel rail).

So what can you do to fix the problem? Well this is what we have done to equalise the EGT's and A/ F ratios on our race engines.

A. We always put the highest flowing injector into #6 (then the next highest into #5 etc). Differences of 1.5% in injector flow rate is not unusual, regardles of brand, aftermarket or standard.

B. We then tune the individual cylinder injector trim to give equal primary pipe (exhaust manifold) temperature using a $100 infra read pyrometer. On a tighly tuned engine (over 12.5 to 1 A/F ratio) we check the results using the 3 EGT sensors and 3 Lambda sensors by swapping them around amongst the 6 cylinders.

This is not a cheap tuning process, due to heat soak contaminating the results this can take several days on the dyno. I would expect that a road engine tuned to 12 to 1 and with highest flowing injector in #6 would not require this amount of money to be spent on it. With a drag or circuit race engine that is tighly tuned, I would suggest it is worthwhile.

Hope that has been of some use

cheers

Edited June 18, 2006 by Sydneykid

[R31Nismoid]

r31nismoid - well said, but not everyone has a good tuner or a computer capable of individual cylinder trim of injectors. this is about trying to do everything possible so no matter what a tuner does, within reason, it can be fixed cheaply and easily and to the maximum effeciency the engine can possibly run.

I think if your at the point of injectors and ECU upgrade... you'd only want the best people to be tuning it.

If you choose the ECU that cant do that type of detail... then your gonna be in trouble, and its no real issue but your own.

Again though, a professional would know about the problems in RB's in respect to the rear and should be advising you accordingly.

If they are not, then they are not professionals with any decent knowledge behind them

[RedDrifter]

I found that on the RB20 I fried (due to a bad fuel pump and 16psi of boost) that pot one was the one that melted and pots 2 and 3 were scorched(very badly). Why would this be different to what I have been reading here?

[Sydneykid]

I found that on the RB20 I fried (due to a bad fuel pump and 16psi of boost) that pot one was the one that melted and pots 2 and 3 were scorched(very badly). Why would this be different to what I have been reading here?

Bad fuel pump is not the same as uneven fuel pressure.

Bad fule pump means #6 gets ot use the fuel first as that's where the fuel inlet is

Then #5 grabs some of what's left.

Then #4 etc.

So when we get to #3 and #2 there wasn't much fuel left, hence lean running.

Maybe #1 survived because there was zero fuel left.

Which is not leaning running, it is not running at all.

cheers

[RedDrifter]

Bad fuel pump is not the same as uneven fuel pressure.

Bad fule pump means #6 gets ot use the fuel first as that's where the fuel inlet is

Then #5 grabs some of what's left.

Then #4 etc.

So when we get to #3 and #2 there wasn't much fuel left, hence lean running.

Maybe #1 survived because there was zero fuel left.

Which is not leaning running, it is not running at all.

cheers

Ahh cool that explains it.

Thanx.

[Turbine]

On my cars first dyno tune they just blew a Tomie head gasket on number 6.

Can anyone see a problem with this?

Shaun, in your pictures this looks to me like an assembly problem to me, not a lean out of fuel/air mixture in the cylinder No.6

Correct me if I'm wrong here,but it looks to me like the corrosion has come from the water jaket to the cylinder and blown through their.

If the cylinder head is not clamped down tight enough, water gets underneath it and heads towords the cylinder. Check to see if your aftermarket exhaust manifold does not crash into the block on the water jaket channel directly under the header flange. If it's the same as for RB20 (real close less than 0.010" in some spots) then grind some material off.

What do you think?

[GTRgeoff]

Aero is one of my specialities and my thoughts are that the extra air into 6 are related to the velocity pressure. It was mentioned before that pressure is the same everywhere. Not true. Air has mass, and that mass causes pressure due to the velocity it travels at, otherwise aerofoils woild not work and a car would experience no resistance due to air.

I would predict that it is not a steady state across the rev range, the extra air in 6, but would change. It only needs to occur at a time when fuel flow is inconsistent and temperatures at the rear of the engine are high. Then you have a deadly (to the engine) mixture of events.

Shaun, I recommend O rings and a good treatment for your coolant to reduce the deposits and rusting. Looks like the head was not clean or not entirely flat at the blowout at 6.

[Squishy]

My RB20 has this same problem...

It was blowing lots of smoke and sh1t so i compression tested it and what do ya know... the first 5 cylinders were at 125 and #6 was at 70!!!

So now i have a choice of replacing a piston or two or doing an engine conversion...

[discopotato03]

GTRGeoff I was thinking along the same lines , static pressure vs velocity pressure . Hugh Macinnes mentions in an early book on turbochargers that a static pressure probe is not affected by the velocity of the gas . By static probe he means tapping into a duct with air flowing through it at 90 deg to the direction of travel . A total pressure probe looks like an "L" shaped tube with its entry facing the approaching gas .

Actually it makes me wonder how boost gauges and wastegate actuators would go if they worked off total pressure instead of static pressure . May be an interesting way of regulating the turbo/s at the point of the boost threshold to make the transition from atmospheric to positive inlet manifold pressure

a bit more tightly controlled , better handle on torque surge and detonation at that point .

Gone OT , sorry , out .

[GTRgeoff]

Not really off topic at all I think. Adds to the knowledge.

What you are talking of is a pitot tube. Aircraft use both a static pressure sensor and a pitot tube for velocity pressure. Usually mounted close together as a straight probe and the L probe.

I think the boost guage still needs to show static or instantaneous boost and a guage for flow rate would be interesting to log as well. Each inlet is so small though to get individual figures you run the risk of disturbing the airflow.

Actually regulating the boost transition with a wastegate might better be done off the AFM with a stepper so you could use a big turbo and reduce the rate of power increase. Certainly something to tease the brain with for a few hours.

[Shaun]

Turbine and GTRGeoff,

Number 6 piston showed signs of detonation which, im told, resulted in the head gasket failing. The engine was seeing 1.2 Bar @ 3000rpm (turbine housing too small) and apparently this introduced excessive heat into the cylinder. Im sure that this was just the straw that broke the camels back, and i was the victim of several unlucky circumstances which i wont continue to mention. Im assurred that the head was torqued up to ARP specs, but what else would they say?!

The engine was quite dirty inside the water jackets; my mistake due to lack of coolant. This is a mistake that i wont make again because the rusty water inside the bore after #6 blowing, pitted the cylinder and neccessitated a rebore which meant my pistons were now useless. I certainly didnt think that would happen within 2 days! Ill include a pic of the bore.

Turbine, i dont understand what you mean by "Check to see if your aftermarket exhaust manifold does not crash into the block on the water jaket channel directly under the header flange".

O ringing would be a good idea, but it shouldnt be neccessay for my application, as im only after about 320rwkw.

Thanks for your thoughts,

Shaun.

Edited June 19, 2006 by Shaun

[mad082]

just to add some more info. apparently the rb25 throttle body is designed so that the butterfly never reaches horizontal. it stays a few degrees off horizontal to direct the air downwards.

[Turbine]

Shaun check out this picture.

Look at the bottom half (or the exhaust side) of the bore openings.

You can see that the gasket has not sealed as well as the intake side. kind of the same problem you have with the No.6 water jaket leaking into the bore.

Does this help?

Turbine