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RB26DETT Vacuum

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Hey guys, my first boost ITB engine, the vac/boost gauge is connected to the vac log (same place as the stock FPR was) and I noticed at cruise the vacuum is alot lower then my RB25DET.

ie cruising at 100kph its only pulling about -2 to -3psi

at idle it is -9.9 tho (well thats as low as the gauge goes)

Im guessing its just a characteristic of a ITB-T motor but im assuming the vac log takes its signal from behind the throttle plates so im a little confused on why it would be any different from a normal single throttle setup in that regard?

Yes, this is typical with the RB26. It doesn't take too much effort to end up around 100 kPa in cruise if you're going up a slight hill. With a multi throttle body setup, manifold vacuum post throttle plate is no longer a good representation of engine load. That's why you can see 100 kPa at relatively low throttle openings, and why we need to use TPS as the load input.

I'll generally tune these areas around 100 kPa and even slightly into positive boost for a leaner target AFR in order to help keep something remotely resembling fuel economy.

Hey Andre, ive attached a screenshot from the haltech stock R32/3 GTR basemap. For a stock engine, stock turbos (14psi max) and wanting a max amount of economy (as its my daily driver) what would you set your targets at? I know its never going to be a 10L/100km econobus but id like to get as good as possible, But of course do not want to damage the engine. Ive also attached a target MAP of what I thought would be good, let us know what you think... am I abit to far towards the lean side then you would be comfortable with?

Also on the topic, if your running leaner the combustion and EGT's will be hotter, If your not detonating is there any other negatives? possibly cause the engine to run hotter overall (coolant temp) will it accelerate wear on the valves/turbo? Is it quite risky with the stock ceramic wheel turbos?

Attached Files

Your target AFR table looks pretty sensible to me. In fact I'd be quite comfortable running at stoic out to the 2.9 psi load zone and then tapering richer from there. For a sanity check it's quite common to see a factory turbocharged engine run in closed loop with a target AFR of 14.7:1 out to 3-5 psi. Ultimately we're trying to control combustion temp with the richer mixture, but you're still not actually applying much load at this point and hence the combustion temps are still low in comparison to what you might expect to see at 14 psi and 11.5:1 or similar.

At these sort of low boost levels you're not going to see any negatives from targeting 14.7:1, and the RB26 is normally pretty insensitive to knock until you get up in the boost, even on pump gas.

Thanks Andre,

In regards to not being at much load... The AFR target is RPM x MAP but would it not be possible to be at the same rpm/boost level but at quite different Throttle positions... In relation to AFR is Boost the only load source we are worried about? and actual engine load doesn't change that much if its the same boost but different TPS

is 15.5 about as lean as you would go in the light load/cruise areas? I think I remember you saying if you go much leaner then power drops off and you end up needing more throttle hence fuel anyway.

Is a EGT Probe Pre Turbine beneficial here? are EGT temps a direct relation to combustion temps?

If so what ranges do you consider correct/safe for each region?

bump :)


Sorry Viper, missed your questions.

Personally I would define ultimate load by mass airflow. Unfortunately neither TPS or MAP directly correlate to mass airflow under every situation. This is why the combination of MAP and TPS is necessary. For example if you had the MAF sensors still on the engine and logged mass airflow vs MAP you would find that you could see significantly different airflow at say 3 psi boost and 35% throttle and 3 psi at 100% throttle.

So to answer your question, yes since the AFR target MAP is only using MAP on the load axis we do need to apply a little common sense here. I'm aiming to target an AFR that will give good economy in the areas that the engine is operating in under cruise conditions which as we've discussed will extend out slightly into the positive boost areas. The consideration though is that while we may end up in these same areas briefly at full throttle, under these conditions the engine is making more power and will tend to both accelerate in rpm, as well as spooling the turbo(s) to increase boost and move up the load range and hence target more typical AFR values pretty quickly.

The general guide I have is that I lean the AFR in the cruise areas until I see the torque drop by about 10% - Anything leaner than this will be counter productive. In all honesty though the improvement in economy you're likely to see will be very minor.

Yes EGT is a good guide to what's going on in the combustion chamber and can be used to help guide your AFR targets.