Discussion
Maybe stating the bleedin' obvious, I know, but I thought I'd share this with you...
I don't - correction, I *didn't* - habitually run my VXR on premium petrol. If I was just commuting and generally running around, mostly whatever came into view, went in. 95 RON, mainly.
The other day, I filled up at Tesco and stuck their 99 RON stuff in. Oh. My. God. What a massive performance hike! I've flitted between petrol types on other cars, and on sportsbikes (e.g. various GSX-R 1000s and other litre bikes) and never noticed much of a difference. But this was like someone let the brakes off!
One of the reasons I signed up for the RR day was that I was never convinced that the VXR felt like a 400bhp car. Now I know why. Will still turn up for the RR day anyway, see what happens, as thecar is 100% standard to my knowledge.
I don't - correction, I *didn't* - habitually run my VXR on premium petrol. If I was just commuting and generally running around, mostly whatever came into view, went in. 95 RON, mainly.
The other day, I filled up at Tesco and stuck their 99 RON stuff in. Oh. My. God. What a massive performance hike! I've flitted between petrol types on other cars, and on sportsbikes (e.g. various GSX-R 1000s and other litre bikes) and never noticed much of a difference. But this was like someone let the brakes off!
One of the reasons I signed up for the RR day was that I was never convinced that the VXR felt like a 400bhp car. Now I know why. Will still turn up for the RR day anyway, see what happens, as thecar is 100% standard to my knowledge.
So, I'm interested in hearing an experts opinion on this. My understanding is that the octane rating (RON) has no effect on the power generated by the car - it just allows you to tune the car higher (to get more power) without risking detonation (pre-ignition/pinking, etc) that could damage the engine.
Is this right? Higher Octane rating fuel should not give a noticable performance increase over lower octane fuel, but in a tuned engine it is essential to stop the engine from self-destructing.
Over to the experts....
Is this right? Higher Octane rating fuel should not give a noticable performance increase over lower octane fuel, but in a tuned engine it is essential to stop the engine from self-destructing.
Over to the experts....
Well, maybe something else changed. Maybe the air has been coincidentally colder and denser for the past 3 days. I don't know - all I can say is that without any expectation from me (I didn't even think much about it, just stuck the 99 in) the car is noticeably, tangibly faster and accelerates much harder.
I also run a Fiat Coupe 20 valve Turbo, and some of the guys on the Coupe forum rave about petrol grades making a difference, which I've never noticed. If I turn the boost up, sure, it's a lot faster, but simply changing petrol doesn't seem to make any difference.
Likewise the bikes - I'm on my 3rd GSX-R 1000, and own an old YZR1000R, and whilst I don't claim to be climbing all over the WSB guys, I ride and lap at a respectable pace on tracks and at the Ring, but I never notice any difference with which petrol I use. That said, few mere mortals, including me, will wring every last ounce of performance out of a litre sportsbike anyway, but still......
So, shoot me down if you like. I'm simply reporting what the seat of my pants felt!
I also run a Fiat Coupe 20 valve Turbo, and some of the guys on the Coupe forum rave about petrol grades making a difference, which I've never noticed. If I turn the boost up, sure, it's a lot faster, but simply changing petrol doesn't seem to make any difference.
Likewise the bikes - I'm on my 3rd GSX-R 1000, and own an old YZR1000R, and whilst I don't claim to be climbing all over the WSB guys, I ride and lap at a respectable pace on tracks and at the Ring, but I never notice any difference with which petrol I use. That said, few mere mortals, including me, will wring every last ounce of performance out of a litre sportsbike anyway, but still......
So, shoot me down if you like. I'm simply reporting what the seat of my pants felt!
Im sure this has been covered before but basically higher octane fuel is less volatile and burns with less energy than lower octane, but as you rightly say it can handle higher compression ratios and hence more timing than the lower octane fuels.
Now with a certain fuel, every engine has an ideal amount of timing, this is known as MBT Mean Best Torque or sometimes Min Best Timing etc. Basically its the amount of timing at which torque is optimised based on the rate of fuel burn and the point of peak cylinder pressure during the power stroke. You could drop timing and octane of fuel, or raise timing while raising octane, but inherently each engine has an optimial amount of timing. Some say the LSx series likes 28* total timing. Mine runs somewhat less at around 22* however I run 11.2:1 compression. We are talking at WOT here. Part throttle you may see as much as 40* due to the weak thin charge needing more time to burn.
So in summary then, read the manual (RTFM). I understand the HSV's are tuned to 98 octane for best results while the CV8 might do nicely on 95. I think Wortec stuff is tuned for 98 as well. Not sure about the other guys. I set mine up for 97 octane as Sainsburys is cheaper and closer to me.
Octane does make a difference to power definitely because if you are not running what the engine is optimised for you will not be running optimal timing as discussed above and therefore you will be further away from MBT.
Thats my 2p on the topic
Now with a certain fuel, every engine has an ideal amount of timing, this is known as MBT Mean Best Torque or sometimes Min Best Timing etc. Basically its the amount of timing at which torque is optimised based on the rate of fuel burn and the point of peak cylinder pressure during the power stroke. You could drop timing and octane of fuel, or raise timing while raising octane, but inherently each engine has an optimial amount of timing. Some say the LSx series likes 28* total timing. Mine runs somewhat less at around 22* however I run 11.2:1 compression. We are talking at WOT here. Part throttle you may see as much as 40* due to the weak thin charge needing more time to burn.
So in summary then, read the manual (RTFM). I understand the HSV's are tuned to 98 octane for best results while the CV8 might do nicely on 95. I think Wortec stuff is tuned for 98 as well. Not sure about the other guys. I set mine up for 97 octane as Sainsburys is cheaper and closer to me.
Octane does make a difference to power definitely because if you are not running what the engine is optimised for you will not be running optimal timing as discussed above and therefore you will be further away from MBT.
Thats my 2p on the topic
Sort of right Nige.
There are 2 tables, a high octane and a low octane table.
Basically the car will run solely off the high octane table until it encounters knock as detected by the sensors. A value called Adaptive Spark is then decremented forcing the timing to be a blend between the 2 tables. The more knock the closer it runs to the low table values. This might be when you fill up with some muck from a dodgy station etc. As the fuel cleans up and knock no longer happens the ASPARK value will return to run off the high table again. The rate can be changed for those who mess about with such things.
But its a reactive event meaning you have already knocked the engine about before it pulls timing, so is best avoided where possible. (ie) Dont labour the engine and run it on shit fuel so it pings all the time or your performance will be suckfull.
There are 2 tables, a high octane and a low octane table.
Basically the car will run solely off the high octane table until it encounters knock as detected by the sensors. A value called Adaptive Spark is then decremented forcing the timing to be a blend between the 2 tables. The more knock the closer it runs to the low table values. This might be when you fill up with some muck from a dodgy station etc. As the fuel cleans up and knock no longer happens the ASPARK value will return to run off the high table again. The rate can be changed for those who mess about with such things.
But its a reactive event meaning you have already knocked the engine about before it pulls timing, so is best avoided where possible. (ie) Dont labour the engine and run it on shit fuel so it pings all the time or your performance will be suckfull.
To be honest Id imagine due to much increased pressures and higher risk of damage that FI setups run very conservative timing so that you dont smash too much up.
Therefore you are likely to be a fair distance from MBT so whatever fuel you use would seem similar. In which case lower octane fuel may be slightly more powerful.
Therefore you are likely to be a fair distance from MBT so whatever fuel you use would seem similar. In which case lower octane fuel may be slightly more powerful.
???? The knock sensor is sending you open loop? If too much knock is seen it will force you onto the lower octane map.
Since sorting out my cats I've seen a reduction in knock when running my new map. The sensors were picking up resonence from the system not the ignition. If you are looking for replacement sensors I'd be first speaking to vx about a warranty swap
Since sorting out my cats I've seen a reduction in knock when running my new map. The sensors were picking up resonence from the system not the ignition. If you are looking for replacement sensors I'd be first speaking to vx about a warranty swap
On NA cars the impact of fuel grade is marginal at best. You'll get more performance from the additives than anything else. For forced induction it allows you to increase the boost and shove more fuel in without wrecking the engine... but even then "marginal" is the word.
Also, RON value is pretty pointless - it's a indicator of resistance to pre-ignition/detonation whilst under low engine load and/or rpm.
All fuels also have a MON value which is the same index as the RON but indicates resistance to knocking at medium/high engine speed and load. This is always lower than the RON and when you compare the fuels available in the UK the difference is very very small. this is also why the US run fuels with values of 89/91/93 RON and still get good performance as the fuel still has a good MON value comparable to the UK fuels.
There was a (unscientific) study in a motoring mag last month that said there was zaero difference between the most budget fuel and the best performance one. The only benefits you'll get of the more expensive fuels is the extra additives to clean and prevent coking in the combustion chambers.
Also, RON value is pretty pointless - it's a indicator of resistance to pre-ignition/detonation whilst under low engine load and/or rpm.
All fuels also have a MON value which is the same index as the RON but indicates resistance to knocking at medium/high engine speed and load. This is always lower than the RON and when you compare the fuels available in the UK the difference is very very small. this is also why the US run fuels with values of 89/91/93 RON and still get good performance as the fuel still has a good MON value comparable to the UK fuels.
There was a (unscientific) study in a motoring mag last month that said there was zaero difference between the most budget fuel and the best performance one. The only benefits you'll get of the more expensive fuels is the extra additives to clean and prevent coking in the combustion chambers.
Thorney did tests on their dynos and found that Tesco's was actually the best over all. Of course it really hardly matters what you whack in as long as its good fuel.
If you been running shitty fuel and stuck on the low octane mape then go to good fuel it will feel like a new engine as the timings that more aggressive, it isnt the petrol giving you the power its just enabling the car to stay in the higher state of tune
If you been running shitty fuel and stuck on the low octane mape then go to good fuel it will feel like a new engine as the timings that more aggressive, it isnt the petrol giving you the power its just enabling the car to stay in the higher state of tune
Yes, changing fuel without optimising the tune is pointless. Just use what the book says or the tuner as mentioned above.
Most factory cars are designed to run on sewage so changing fuel wont do hardly anything as you mention and tests will show this also as mentioned.
BUT if the car is tuned right on the button then varying the octane will vary the power output or could even cause major ping when used with lower octane fuels. Hence why OEM's tune conservatively!
Most factory cars are designed to run on sewage so changing fuel wont do hardly anything as you mention and tests will show this also as mentioned.
BUT if the car is tuned right on the button then varying the octane will vary the power output or could even cause major ping when used with lower octane fuels. Hence why OEM's tune conservatively!
stigmundfreud said:
If you been running shitty fuel and stuck on the low octane mape then go to good fuel it will feel like a new engine as the timings that more aggressive, it isnt the petrol giving you the power its just enabling the car to stay in the higher state of tune
Well that seems to sum up my experience - I wouldn't say "shitty" fuel, just ordinary 95RON unleaded from whichever petrol station comes into view, but I'll be sticking with the uprated stuff now.Gassing Station | HSV & Monaro | Top of Page | What's New | My Stuff