Extra fuel for rear cylinders?

[Gav]

I have heard it recommended that if you have a programmable ECU fitted that it is worth richening up #5 and #6 cylinders as these run hotter on the RB series engines.

Any advice/experience?

[GiJOr33]

can you adjust the amount of fuel each injector puts in? or can you only adjust them all simultaneously?

[Roy]

To th best of my knowledge its only computers like Motec M800 and Autronic SM2 that have individual cylinder tables.

Unless you are lucky enough to have such a computer i think your best bet is to get your injectors cleaned and flow tested, and put the highest flowing injectors in cylinders 5 & 6

[Guest RedLineGTR]

mm hotec oh i mean motec

[Freebaggin]

Gav, where did you here this, as I think there's merit in the 5 & 6 running lean/hot when the RBs are pushed.

I have bought big turbo and have stalled its install to make sure I have good fuel supply to rear of motor. The delay is due to a couple of cars very recently here in SA have blown no. 6 pistons and also no 5 & 6 upper studs on my exhaust manifold have sheared off indicating to me greater heat cycling.

The issue I think is high pressure drop across the fuel rail at elevated fuel pressures and since it's a front feed system the end of the rail might see less fuel pressure.

Many aftermarket rails have centre feed, equalling out the pressure a bit.

[adam 32]

for some reason my car come over (modified) and with two fuel pressure regs. one at the back and one at the front

[Gav]

The power FC allows a percentage multiplier for each cylinder as well as increasing/decreasing the pulse time in 1ms intervals.

This applies across the entire fuel map.

[51jay]

Pretty sure my fuel rail is fed from the rear

[Freebaggin]

51jay, I had a look at my motor during lunch and I had it wrong re fuel supply point. The fuel comes in from the back as well (ie closest to no 6 cyl) not the front.

Spoke to a Chem. Engineer here at work and he said maybe there is higher pressure at no. 6 piston causing cavitation at the injector and consequently less flow. ????

[Steve]

'Tyndago' off the SDU forums drag races a bit and plays with GTRs in the US. He states there that he spoke to some japanese GTR guru (cant remeber name) and that he said go highest flowing injector to 6 then down to lowest at 1. So this is in line with the 6 and 5 running a bit leaner - might be worth flow testing your injectors before fitting them.

[Gav]

Ideally I guess you would measure exhaust temps on each cylinder when pushed hard on the dyno. Could then trim each injector to give similar temps.

[Sydneykid]

Hi guys, the main problem is the airflow in the GTR inlet manifold is biased towards no 6 and to a lesser extent no 5. This only starts to occur when they get up in the power output, over 280 rwkw seems to be the area.

It extra airflow isn't much, between 2% and 3%, which is enough to cause detonation on those 2 cylinders when the other 4 are fine. This of course only happens in an engine tuned very close to A/F ratio tolerance. So you can have very low detonation warnings, and think the engine is OK. But all the detonation is on 1 or 2 cylinders, not spread across the 6.

When you flow test 6 injectors you always find one or two that flow a little more than the others. This is an example of the set in our RB30/26 powered GTST

1 560 cc's

2 560 cc's

3 560 cc's

4 565 cc's

5 570 cc's

6 575 cc's

So, no 6 flows 3% more than no's 1,2 & 3. This is more than enough to allow for the extra airflow and ensure no leanness in no 6. Similarly no 5 is also safe.

As Gav said, if you have an ECU with individual cylinder trim (eg; Motec M48 Pro) then you can allow for this by richening up no 6 and no 5 (in comparison to the other 4 cylinders). If your ECU does not have this facility, then simply sticking the highest flowing injectors in 6 and 5 is a simple solution, albeit not as accurate.

The accurate way to test this is by having provision for a lambda sensor in each exhaust manifold primary pipe. That way you can measure the A/F ratio of each cylinder individually. Measuring temperature of the exhausts gas (or the primary pipes themselves) is a good substitute.

I have not seen this airflow biasing problem in standard RB20/25 inlet manifolds, as they appear to have very even distribution. It is quite possible that aftermarket manifolds with the throttle body at the front of the plenum may also suffer from this problem. Ditto standard manifolds that have had the throttle body moved. I have not tested this in practise, but it would seem most likely.

There is also an argument that says no 6 and no 5 are the furthest from the radiator and water pump and therefore may run hotter. This would exacerbate any leanness in those cylinders. Richening them up would also help this problem.

Hope that helps

[KamikazeR33]

sydneykid this is some very interesting stuff to learn about for ppl who dont know alot of the technical stuff like this of an engine

[Freebaggin]

Sydneykid, sounds feasible re RB26 manifolds when pushed.

So if the problem is compounded by no. 5 & 6 location relative to the radiator & water pump (that goes for any RB series motor), is there any merit in upgrading the cooling system when stretching RBs to ensure longevity?

If so, what would be a realistic stepwise uprgade path? Higher capacity water pump? etc etc

For background I have RB25 + very soon bolts-ons to obtain 250-260rwkW and want to make sure pistons last at least a little while.

[51jay]

With regard to cooling is there any benefit to swapping to an electronic water pump and fans

[adam 32]

ive also heard of people using large washers on the bonnet screws to lift the rear of the bonnet up a few cm's allowing the rear cylinders to cool

and it would only cost $1 at bunnings to do

[Freebaggin]

adam32, yep a few of us here have done that already (for about a year now), I also removed the rubber strip to increase gap width. Not sure how effective it is as I only recently noticed the exhaust manifold upper studs on 5 & 6 cylinder had fallen off!

[adam 32]

can anyone comment on how effective this modification is?

[Sydneykid]

Hi Guys, the problem is not one of "absolute" temperature, it is "differential" temperature. The sort of stuff suggested may well lower the overall temperature of the engine, but it won't make no 6 or no 5 run the same temperature as the other 4.

That's the basic problem, so you have to do something to cool only those 2 cylinders and richening up the mixtures is the best/easiest way. If the ECU supports it, you could retard the ignition timing a little, that will also assist.

Hope that helps

[Gav]

Sydneykid

what percentage would you recommend to richen up # 5 and #6 cyls?