Supercharger vs. octane

[Steve68Cougar]

My '18 RS2 had a Roush supercharger installed on it when it was sold new. The dealership ordered it at the time of purchase and was put on about 500 miles later. I bought the car last year with 4800 miles on it and currently am at a little over 18k miles. I've already had to replace a catalytic converter, and the other one has tripped the check engine light once already, but is currently not on. When they replaced the one catalytic converter (less than 1k miles ago), I had them upgrade form the 700hp tune to the 750hp tune. My understanding is that the 700hp tune is notorious for taking out catalytic converters, but the 750hp tune seems to be fine. One thing I've noticed with both tunes is that I sometimes hear a pinging/detonation sound under medium to heavy acceleration. I was "informed" by someone on another page that the knock sensors "won't let it do that", and I must be hearing something else. As far as gas, we only have 91 octane available locally, so that's what it typically gets. The 750hp tune is optimized for 93, but says 91 is acceptable. I do have a barrel of Sunoco 260GT that is 100 octane that I spike the gas with if I'm headed out for an autocross or otherwise know I'll be running the car hard. I don't know if straight 100 will clear it up, as I haven't had the car back with the new tune and been able to run straight 100. With straight 91 it's definitely making a few noises I don't like. Is there something "normal" that would sound like a bit of detonation? It mainly seems to be while I'm aggressively tipping into the throttle (I can't just mash it, as that would typically result in instant wheelspin in anything less than 4th gear). The car hasn't thrown any codes other than when it did the P0430 and P0420 for the cats. I've got Forscan, but haven't figured out how to find if there's a record of detecting knock somewhere in the memory. I'd like to get this figured out, as I know what detonation can do. It may just be that our local 91 gas is all junk, but I'd like to know if there are any other possibilities that it might be. It'll be next spring most likely until I can get the gas all drained and run straight 100 octane. I really hope that the $9/gallon solution isn't going to be mandatory for running the car hard. Oh, the joys of 12.5:1 C/R and a supercharger! Thanks for any insight you might have.

 

[KazarS197]

You think you answered your question in your question. 91 would make me nervous with either tune. The other thing you have to to look at is the fuel is probably a 10% ethanol mix which makes things worse.
You said you mix occasionally.
Here's a way to mix spot on to what octane you are shooting for:

Octane Mix Calculator - Wallace Racing

Calculate Octane Mix

wallaceracing.com

I ran some numbers trying to figure out something close to 93 and a simple numbers to remember.

10 gal of 91 with 2 gal of 100 will figures to 92.5 octane. 25 extra bucks is a chunk but if you look at it this way- Running strait 100 is something like 90-100 bucks extra / tank and a new engine is maybe 10 grand.

Another option is call a local Dyno shop and chat with them. Maybe they have options with a "better" tune. A better tune though will probably be richer which will not be cat friendly. I'm thinking the 750 tune really cranks the timing and leans the car- nothing good can happen from that when running below octane recommendations. Those are 2 things waiting for detonation. Just my thoughts.

Edit. I can't leave things alone and played around with the calculator for 5 gallons total - same as a fuel container - 4 gal of 91 and 1 gal of 100 figures to 92.8 octane.

 

[thelostotter]

You’re probably right, it probably is detonation. These cars can and will ping/detonate given the right conditions despite having knock sensors. I didn't heed advice to run race fuel on the track because I had my car conservatively tuned on 91 octane and thought I was fine. It was a mistake and I ended up cracking ring lands on 3 pistons. Now when I track the car I add half a tank of Sunoco unleaded 100 260GT or 100 Supersport as cheap (compared to rebuilding another short block) insurance.

 

[Stangra]

E85 is another consideration. It would require some fuel system upgrades, and there are pros and cons that accompany. My experience with a supercharger and E85 has been a hand-in-glove solution. Race fuel (or better) performance and protection at pump gas prices.

 

[Steve68Cougar]

You think you answered your question in your question. 91 would make me nervous with either tune. The other thing you have to to look at is the fuel is probably a 10% ethanol mix which makes things worse.
You said you mix occasionally.
Here's a way to mix spot on to what octane you are shooting for:

Octane Mix Calculator - Wallace Racing

Calculate Octane Mix

wallaceracing.com

I ran some numbers trying to figure out something close to 93 and a simple numbers to remember.

10 gal of 91 with 2 gal of 100 will figures to 92.5 octane. 25 extra bucks is a chunk but if you look at it this way- Running strait 100 is something like 90-100 bucks extra / tank and a new engine is maybe 10 grand.

Another option is call a local Dyno shop and chat with them. Maybe they have options with a "better" tune. A better tune though will probably be richer which will not be cat friendly. I'm thinking the 750 tune really cranks the timing and leans the car- nothing good can happen from that when running below octane recommendations. Those are 2 things waiting for detonation. Just my thoughts.

Edit. I can't leave things alone and played around with the calculator for 5 gallons total - same as a fuel container - 4 gal of 91 and 1 gal of 100 figures to 92.8 octane.

We have several stations around that advertise no ethanol in their premium, and that's what I have been using. I did the math a while ago and figured that if I add at least 3 gallons of 100 octane during a fill up then I'll be at a minimum of 93 (assuming it's actually at 91 to start with). I'll have to check out the link and make sure my calculations were right. I'm still under warranty so I'm not really wanting to touch the tune yet. Overall I really like the drivability and everything else about the tune otherwise. Heck, the car got over 22 mpg for our trip to Boise, ID earlier this year that was almost all between 75 and 85 mph. Any of the tunes I've heard of have typically been more aggressive than the Roush tune. It's designed to make it through a 3/36 (or 5/60 if it's a new RS3) warranty and be 50 state legal, so they've done a significant amount of testing on it. If I hear detonation I really try to keep my foot out of it until I get better gas put in it. It sounds like I need to continue that trend. It just stinks that our local gas requires the extra work and $$$.

You’re probably right, it probably is detonation. These cars can and will ping/detonate given the right conditions despite having knock sensors. I didn't heed advice to run race fuel on the track because I had my car conservatively tuned on 91 octane and thought I was fine. It was a mistake and I ended up cracking ring lands on 3 pistons. Now when I track the car I add half a tank of Sunoco unleaded 100 260GT or 100 Supersport as cheap (compared to rebuilding another short block) insurance.

I hope I don't end up with piston issues like you did. If something should happen to the engine, I'll definitely lower the compression ratio. I hope it doesn't come to that, though. I'm fortunate that my neighbor is an engine builder and has given me a "friendly neighbor" price on the fuel. I've been getting Sunoco 260GT. I could possibly get the VP version, too, but it was more expensive. Most of the places I run the car at also have at least 95 octane gas, which is nice. I realized when I bought the car that this scenario was a possibility, so I'm not completely caught off guard. It's just annoying that I end up having to be so careful with it.

E85 is another consideration. It would require some fuel system upgrades, and there are pros and cons that accompany. My experience with a supercharger and E85 has been a hand-in-glove solution. Race fuel (or better) performance and protection at pump gas prices.

I know a number of people that run E85 and are happy with it. I may look at that once my warranty is up.

Thanks for the replies.

 

[Steve68Cougar]

Update for 2022. Check engine light back on and Forscan shows codes P0420 and P0430, so both cats this time. Looks like it's time to head back to the dealer and get them replaced again. Roush has a memo out to put the latest tune in if this happens, but I don't know if there's been another tune since I had mine done last year. I'm contemplating just going to the Kooks setup at some point, but don't know where I'd get a tune done for it. The car is a lot of fun, but this is getting pretty frustrating.

 

[Scootsmcgreggor]

There’s a thread on m6g that goes in depth on “cat protection”. With a supercharger it only takes like 1-3 secs of boost to elevate cat temps where they will overheat and break down and deteriorate. All the standard and aftermarket tunes dump a shiznit ton of fuel to try and cool the cats but it only buys time. You will keep killing cats with boost.

 

[Mad Hatter]

Would log the engine ASAP and see how bad the knock is. A decent tune will not take the car to detonation. I would rather give up some hp and be able to use it full throttle then be waiting to have a rod daylight.

With a SCT logging is very simple and you can do it without a dyno as well.

 

[Steve68Cougar]

So, with my ongoing catalytic converter issues, I'll ask this: What would you do? I've determined that continually replacing cats is not an acceptable long term solution. We don't have emissions testing of any sort, so visuals and sniffing tests aren't currently an issue. That being said, I would prefer keeping cats on it. One "off road" option I've considered is just hollowing out the current exhaust and running defoulers. It may not keep the CEL off, but wouldn't melt any more. Long tube headers with high flow cats is probably the most logical (albeit a ~$3000 option once it's tuned). I've also wondered if GT500 manifolds/cats would work or if the engine configuration is enough different that it wouldn't fit. I'd be open to other options, too. Both on-road and off-road setups would be considered.

 

[302BOB]

This won't help you...just a reflection...........N/A works best....superchargering will give you more now, but will cost you more in the long run......one way or another........................................

 

[Hye Power]

Put longtube headers and have downstream 02 turned off by the tuner.

 

[Beermoney]

As far as the octane, I've heard of success with Boostane to up the octane rating which is probably a more economical solution that spiking it with racing fuel.

The Cats I'm not sure, besides getting rid of them since you don't have emissions.

 

[Steve68Cougar]

This won't help you...just a reflection...........N/A works best....superchargering will give you more now, but will cost you more in the long run......one way or another........................................

The flip side of that is that jumping into a 750hp N/A car and cruising across the country at 25mpg is not really possible. It doesn't really matter which route you take - you gotta pay to play.

 

[302BOB]

you gotta pay to play.

That's a truth that can't be denied!!!!!...............My Boss ADM still stings me after ALL these years...............