Cyl No. 6 Lean Out - Theory Vs Practical

[ISL33P]

why does cyl no. 6 lean out ?

after having read many threads lately in regards to this issue and knowing how much of a common problem it is for all (yes all rb's 20/25/26/30) i wanted to get peoples feedback on this.

when flow bench testing any standard rb inlet manifold there is a common issue which continually arsies and causes many people problems when it comes to tuning and maintaining a healthy engine. cylinder no 6 flows considerably less air than other cyliders which in theory would cause rich mixtures (remember fuel flow is partially dictacted by pressure and not flow i.e. same fuel flow - as other cylinders - contributing to a rich mixture) but there is constant comments indicating that issues with cylinder no 6 (mainly detonation causing collapsed ring lands, blown head gaskets etc) are caused by the plenum flowing too much air to this cylinder. this is most common within rb26 discussions.

is it really the lack of flow of oil/water to the rear of the crank/block ?

how would you fix it ?

does anyone have egt logs for each cylinder ?

note - pressure should no come into this equation as it equal regardless of flow.

ps - i am in no way trying to promote what i do. just trying to get some sort of consistency when it comes to the truth behind this, all to common, problem.

Ariel

Edited June 18, 2006 by ISL33P

[Zardos]

You bring up a very good point on rb engines, i will keep a good eye on this thread as i would like to know more info on this too...

[mad082]

if number 6 is running rich, shouldn't number 1 as well? and (design wise) an after market plenum would have to have more chance of number 6 running rich as it is furterest from the throttle body, even though they are bigger than a stock one. i know it flows more air, is it was the same size as a stock then it would richen up more on number 6.

[nizchic]

we looked into this when i chopped our plenum and i found that bouncing the air flow off the back of the chamber helped to distibute the air more evenly, this was acheived by curving the T/B neck toward the cam belt cover more.

The engine is in pices now and no 6 piston shows no signs of detonation.

i have built another plenum for another engine, a toyota, with bellmouths to eliminate these problems too, so far so good.

[ISL33P]

we looked into this when i chopped our plenum and i found that bouncing the air flow off the back of the chamber helped to distibute the air more evenly, this was acheived by curving the T/B neck toward the cam belt cover more.

air bouncing off the back wall would cause, what some people call, a "dead" spot in the plenum which is where the air is having no effect on the plenums performance and is not being used by any cylinders.

how did you come to this conclusion, was it just from looking at the piston/plugs/runners of the plenum or do you have egt's or flow numbers to confirm this ?

The engine is in pices now and no 6 piston shows no signs of detonation.

i have built another plenum for another engine, a toyota, with bellmouths to eliminate these problems too, so far so good.

there are theories that say bellmouths are no good and only increase turbulence and these same people support the chamfered edge runners as they come from factory. how do the bellmouths fix the problem ?

i am not at all doubting what you are saying, i am only asking these questions for the sake of everyone reading, so please do not take offence to it.

Ariel

[paulr33]

its already been covered i thought. put highest flowing in #6 and least flowing in #7 and it should balance out the fuel issue.

[Busky2k]

Alternatively just -1% on Cyl #6 on the Injector Correction on the PowerFC

[ISL33P]

what #7 ?

so you are saying #6 injector will be the cause in every car. just seems strange that every #6 injector would cause such a common problem ??

that brings to light the argument about fuel rail and the positioning of the supply and return lines.

Edited June 18, 2006 by ISL33P

[ISL33P]

Alternatively just -1% on Cyl #6 on the Injector Correction on the PowerFC

thats the easy way out and another bandaid solution

[paulr33]

mines use the stock fuel rail and plenum, i guess they were ok with it and their power target. they probably just dialed in the highest flowing injector into #6 and maybe did a slight trim like busty2k said

[ISL33P]

mines use the stock fuel rail and plenum, i guess they were ok with it and their power target.

just out of curiosity what was their power level

they probably just dialed in the highest flowing injector into #6 and maybe did a slight trim like busty2k said

probably doesn't provide answers

[RB30-POWER]

maybe we look at the wrong side of motor.

maybe the exhaust side is the issue, maybe #6 has better cylinder scavenging and actually runs leaner because of that.

more of the burnt air that exits, means more fresh air into the chamber, more air and the same quantity of fuel = leaner, compared with cylinders that don't scavenge as well.

do equal length turbo extractor engines still have this issue, all only stock manifold engines?

just trying to add to the conversation...

[ISL33P]

know thats the king of thinking i am after.

it appears to be a problem with std and equal length as i have seen it with both std, ebay and reputalbe aussie brand exh manifold but i like the theory and it is something that also needs to be taken into consideration.

Edited June 18, 2006 by ISL33P

[Turbine]

I like this subject because it has alot to do with aerodynamics.

The trick is to trim the engines breathing through all cylinders to be equal. Manifolding the engine correctly (physically), not by fitting the highest flowing injector, or increasing the injector end time in your ECU to be longer than the other cylinders. Do all that when you have a resonably ballanced air path first.

I've seen 2.0L Toyota six cylinder engines develope 700HP with very little work done to their intake manifold, running 65mm pipes to and from the intercooler.

I've read that the nissan RB20/25 DET intake is good for 300rwkw without many mods. Beyond that figure it must be unbalanced.

As for detonation/breakages in No.6 cylinder, well the air side of things is just the start of its problems.

[paulr33]

mines were somewhere near 600hp

stock fuel rail and stock plenum

[ISL33P]

I like this subject because it has alot to do with aerodynamics.

The trick is to trim the engines breathing through all cylinders to be equal. Manifolding the engine correctly (physically), not by fitting the highest flowing injector, or increasing the injector end time in your ECU to be longer than the other cylinders. Do all that when you have a resonably ballanced air path first.

i agree totally

I've read that the nissan RB20/25 DET intake is good for 300rwkw without many mods. Beyond that figure it must be unbalanced.

therefore its unbalanced from the start. that would mean the unbalancing is only having a major effect on air distribution when the airflow required for 300rwkw is demanded

this may explain why less powerful engines are seeing these problems when combined with 1 or more other influencing problems.

Edited June 18, 2006 by ISL33P

[nizchic]

air bouncing off the back wall would cause, what some people call, a "dead" spot in the plenum which is where the air is having no effect on the plenums performance and is not being used by any cylinders.

how did you come to this conclusion, was it just from looking at the piston/plugs/runners of the plenum or do you have egt's or flow numbers to confirm this ?

there are theories that say bellmouths are no good and only increase turbulence and these same people support the chamfered edge runners as they come from factory. how do the bellmouths fix the problem ?

i am not at all doubting what you are saying, i am only asking these questions for the sake of everyone reading, so please do not take offence to it.

Ariel

the skyline plenum we messed about on a flow bench changing the angle of the neck, and the bellmouths are of a design (in theory) that the air flows around the B/M that protude into the plenum so airflow isn't biased to any cylinder, ie pressure equalises before the runner draws on it, works for N/A, but ive only had it going a short time on a turbo toyota.

[Busky2k]

Is it just the RBs that are affected? What about SRs? 1JZs? etc...

[Turbine]

I have engine EGT thermocouples fitted to our race RB20's ehaust manifold, but the engine is yet to run (october this year). Does anybody have a similar setup on a running engine?

[rob82]

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.