What's more important to a N54 engine? Ethanol or Octane rating?

[pbondar]

So an (at this stage) academic question..

Given you can now flex fuel a N54 with MHD and others..

What would tune better..

95 RON Octane fuel with 10% ethanol or..

98 RON Octane fuel with zero ethanol

Take it further, with a flex fuel sensor could you?

Get a really agressive tune with 98 RON fuel diluted with 100% ethanol in some ratio, varied from fill to fill?

 

[SlowE93]

More Ethanol = more safety IMO.

 

[wheela]

A couple decades ago back in college, I worked in a state lab that (among other things) tested the claimed octane values of different gasolines sold in the state. To test octane, they used variable compression ratio engines, gradually increased the compression ratio while running to find the threshold where knock began. That compression ratio was correlated with the octane value.

So the 98 ron fuel with no ethanol should have more knock resistance than the 95 ron with 10% ethanol. It also has the added benefit of stoich being at a higher AFR (due to no ethonal), placing less demand on your fuel system than a fuel containing ethanol. So between the first two, I'd run the 98 ron.

However if you add your own ethonal to the 98 ron, you'll get an even higher octane rating (more knock resistance), so you can run more aggressive tunes (as you mentioned), but the obvious trade offs of PIA factor mixing fuels, and you'll need a higher flowing fuel system due to stoich being at a lower afr with the added ethanol.

 

[pbondar]

A couple decades ago back in college, I worked in a state lab that (among other things) tested the claimed octane values of different gasolines sold in the state. To test octane, they used variable compression ratio engines, gradually increased the compression ratio while running to find the threshold where knock began. That compression ratio was correlated with the octane value.

So the 98 ron fuel with no ethanol should have more knock resistance than the 95 ron with 10% ethanol. It also has the added benefit of stoich being at a higher AFR (due to no ethonal), placing less demand on your fuel system than a fuel containing ethanol. So between the first two, I'd run the 98 ron.

However if you add your own ethonal to the 98 ron, you'll get an even higher octane rating (more knock resistance), so you can run more aggressive tunes (as you mentioned), but the obvious trade offs of PIA factor mixing fuels, and you'll need a higher flowing fuel system due to stoich being at a lower afr with the added ethanol.

Thanks for the detail…

I see my favourite fuel here in the UK is 99 Ron with 5% ethanol now..

As far as MHD tunes are concerned wondered whether I could toggle the 100 Ron option on as opposed to 98 Ron?

 

[wheela]

You could try the 100 ron option ans see what your cirrections are like, but I bet the 99 ron has the benefits of the ethonal baked into the 99 ron rating.

 

[pbondar]

You could try the 100 ron option ans see what your cirrections are like, but I bet the 99 ron has the benefits of the ethonal baked into the 99 ron rating.

Yes for sure..anyone know how / what happens with MHD (and other tunes) if the octane rating doesn't live up to the programmed expectations?

ie is it better to underspecify or ask for the moon and get into orbit?-

 

[RuskiRacer]

Lots of timing corrections if your using a lower grade fuel than what the tune is designed for

 

[carabuser]

The MHD octane ratings are really just a guide.
The higher octane maps run similar boost but more aggressive timing. If your car runs good on a 93 octane map with no corrections then you are safe to try the 95 map.

 

[pbondar]

Thnx chaps, given that my stage 2+ period will be limited pending the TTE500 turbo fitment in January I’ll stick at 98 Ron …I’ll get some dyno runs as is and with the airbox lid off and then also filter out just to see what effects they have.

 

[fmorelli]

Also consider not all fuels are the same quality. And in a gross sense, why so much fuel development goes on for F1 teams.

When we were tuning my car on 93 octane with 10% ethanol (before I went flex fuel) there were significant differences seen in timing retardation using various brand fuels at the same octane rating. In my case, and at least in my locale, the logs told us that Shell gas was superior.

As for ethanol - I don't know what the quality factors are. But in your case you are splashing a little bit in at most so.

But just for argument sake - the 99RON w/5% ethanol may be the best rated gas; whether it is the best gas will be demonstrated by your logs.

 

[pbondar]

Also consider not all fuels are the same quality. And in a gross sense, why so much fuel development goes on for F1 teams.

When we were tuning my car on 93 octane with 10% ethanol (before I went flex fuel) there were significant differences seen in timing retardation using various brand fuels at the same octane rating. In my case, and at least in my locale, the logs told us that Shell gas was superior.

As for ethanol - I don't know what the quality factors are. But in your case you are splashing a little bit in at most so.

But just for argument sake - the 99RON w/5% ethanol may be the best rated gas; whether it is the best gas will be demonstrated by your logs.

Shell is my local station so I use it most of the time (99 Ron with 5% ethanol)…

When I dyno the car I ensure it’s on Shell and had at least a couple of tanks of the good stuff!

 

[Martha_Farquhar]

So I've just joined as a new owner of an E92 with N54. Previous owner fully rebuilt the engine with all new gaskets and valve stem seals, 19t hybrids, bigger intercooler and uprated charge pipe, catless DPs and uprated exhaust. And obviously a custom MHD tune to tie it all together. I've owned the car about six weeks now and am very much enjoying it. I was fuelling it on E5 99 octane, but at the moment there's nearly 10p difference on the litre between that and the e10 95 octane so I thought I'd downgrade for a while just to see how much worse I'd find performance.
So after about two half-tanks of the E10 95 I was more than a little surprised to see a moderate but not insignificant *increase* in fuel economy in urban driving, like 2mpg or so. The car certainly doesn't feel slower or sound different, and honestly although I've not had much opportunity to really open it up fully to see what the top end is like yet, I canno see it being significantly worse. However all my older BMWs, many of which also had custom maps all responded well to E5 99, with around 3mpg increase and smoother running.

I'm wondering if it's because the sensors are expecting the extra ethanol? Either way, until I get further insight I'll be sticking with the the E10 for now!

 

[langsbr]

So I've just joined as a new owner of an E92 with N54. Previous owner fully rebuilt the engine with all new gaskets and valve stem seals, 19t hybrids, bigger intercooler and uprated charge pipe, catless DPs and uprated exhaust. And obviously a custom MHD tune to tie it all together. I've owned the car about six weeks now and am very much enjoying it. I was fuelling it on E5 99 octane, but at the moment there's nearly 10p difference on the litre between that and the e10 95 octane so I thought I'd downgrade for a while just to see how much worse I'd find performance.
So after about two half-tanks of the E10 95 I was more than a little surprised to see a moderate but not insignificant *increase* in fuel economy in urban driving, like 2mpg or so. The car certainly doesn't feel slower or sound different, and honestly although I've not had much opportunity to really open it up fully to see what the top end is like yet, I canno see it being significantly worse. However all my older BMWs, many of which also had custom maps all responded well to E5 99, with around 3mpg increase and smoother running.

I'm wondering if it's because the sensors are expecting the extra ethanol? Either way, until I get further insight I'll be sticking with the the E10 for now!

You always want to use the lowest possible octane - anything higher than what you need is wasted money and mileage. In your case I would say it was odd as the o2 sensors will typically compensate for the slight increase in ethanol and I'd have expected your mileage to drop to reach stoic at cruise.

 

[Martha_Farquhar]

Exactly what I would expect also but perhaps something in the mapping is forcing them to stay on a certain range? This is the stuff I'm trying learn about, am OK with n/a theory, have a little ownership experience with previous gen diesel stuff that is a bit simpler but this is my first time owning a petrol power turbo car.

 

[KClemente]

Exactly what I would expect also but perhaps something in the mapping is forcing them to stay on a certain range? This is the stuff I'm trying learn about, am OK with n/a theory, have a little ownership experience with previous gen diesel stuff that is a bit simpler but this is my first time owning a petrol power turbo car.

Unless the car is fitted with an ethanol content sensor, it will not know what percentage of ethanol you fill up with.

These cars run on full time closed loop fueling and so it will actively correct short term and long term fuel trimming with some knock factor correction too.

Your custom mapping has a base fueling and ignition target that the car will actively try to push for and depending on how aggressive your tune is, 95 vs 99 ron may or may not make a difference.

If you really want to know how it is running without any speculation, you can always take multiple datalogs between the two fuels and compare