Unfortunately, Oregon's top gas is 91 octane. The Renntech (and others) tune is designed for 93 octane. I think it's equal to a loss of 35 HP (so a bump of about 50 instead of 85?).

I'm wondering if anyone else has done a 91 octane tune on a GT C and whether or not you'd recommend it.

This might make me move to another state . . . haha.

Andrew

 

Hi Andrew,

Do you have access to E85 in Oregon? Why don't you just blend in 93 using 91 and E85?

E85 (if it is 85. Sometimes it can be lower. Look at the pump sticker to get an estimate) is 108 octane. 9 gallons of 91 octane and 1 gallon of E85 would get 93 (92.7). Ethanol also gives a cooler charge than gasoline which is a benefit once ambient air temps go past about 85*F. Stoich is different between gasoline (at 14.7:1) compared to ethanol (at 9.8:1) but a 9 to 1 blend shouldn't cause a lean condition.

Not sure how you might use the 93 tune, but if you're just street, a tune is hardly used at all even if you're an aggressive street driver (maybe 5% or less). If you track the car, you can do a 10 gal mix for the track, then go back to all 91 for street driving.

For a 93 tune, 91 is hardly a negative impact at all, unless you're at the higher range of the RPM limits (such as tracking or street racing). At cruising speeds the timing won't be impacted at all, and if knock occurs the EMS will pull the appropriate timing (and why would you care if you're just cruising). But frankly, at cruise speeds, I've never logged knock with lower octane gas. I had to be doing a WOT pull, then I might see knock over 4500 rpm on a supercharged engine. Still, the EMS immediately pulls timing.

 

Thanks. Great thoughts. Yes, I can get 85 but it's a bit of a hassle. I do like to use a lot of the HP and yes, I like the track. But I have to admit, I'm like Enzo Ferrari, more horsepower is better . . .

 

Understand - however, BC of the difference in stoich I wouldn't suggest a fulltime diet of an e85 blend, just occasionally, then back to all gasoline so trims can adjust back. So, if you need the extra HP all of the time.....just let me know what streets you'll be driving so I can make sure I stay home...LOL...

The other option is if you can acquire a little race gas and blend that in the correct ratio to get 93. More octane than 93 will be wasted if the tune has been optimized for 93. It'll be very expensive.

And, suggest you not consider octane boost (the stuff in a can). The amount of product you need to get to 93 will get crazy and I personally don't like the product.

 

So I'm going to assume that no one has experience doing a Renntech tune when using 91 octane?

 

For this exact reason during road trips, I carry a couple of bottles of Octane Booster. Ran into this when having to get gas at a country store out in the middle of nowhere a few weeks ago.

Lucas Oil Octane Booster

 

Renntech has an "Intermediate" level of Tune on their Handheld: "Our intermediate ECU Upgrade is also developed for customer that drive their car mainly in regions where only 91 Octane fuel is available."

https://renntech-store.com/performan...mg-gt-bi-m178/

 

This looks like a good option for you Andrew.

 

Im in California also with the 91 octane. This looks to be the one. Anybody know the power differences between this intermediate hht and the advertised normal tune?

 

Hey,

I've been running the Renntech tune on 91 for about a year now. Both Renntech and Eurocharged mentioned the difference between 93 and 91 is marginal. It's really woken the car up, and feels like more than a 50hp bump. I'd guess the car is making ~625 at the crank.

For comparison's sake, my previous car was a 335i weighing about the same, making 515/530 at the wheels, and the GTS feels a bit faster than that. I also raced a ~650whp M235i from 50-130 and pulled about a fender (he had a ~230lb ballast in the passenger seat though).

Reliability wise, I've had the car go into limp mode twice total, many many months ago. Restarting the car cleared the issues.

 

I really believe this is more of a marketing idea than anything real HP wise.

 

Thank you, Hetzle, for a first hand and obviously qualified response. Appreciate it.

 

Are you suggesting that tuning is about marketing and that these tunes don't actually increase horsepower? I have a tendency to believe the plethora of dyno graphs and client reviews.

 

Yes, I know that but I was hoping to hear from people who have actually done the Renntech with the 91 tune. Thankfully, I've now hear from someone.

 

You're very welcome, hope it helps you make a decision. I also forgot to add, I have driven the car back to back between the stock/tuned maps and the change in character of power delivery and midrange torque is very obvious.

Such a suggestion is completely at odds with my own experience, dunno what that guy is talking about

 

As a tuner, be assured tuning has nothing to do with marketing, other than to sell tunes.

Manufacturers (ALL manufacturers) leave hp on the table on performance cars. They do that BC they have to warranty a powertrain for thousands of miles and multiple years. They would not be able to do that if they shipped a product that performed at 100%. It is why it is so easy to find HP/Torque if you have access to the EMSs data tables. Any car I've ever touched with a factory tune is running pig rich in PE and timing is nowhere near what it could be.

 

Let me clarify.

I think the marketing is between the 91 and the 93 tune which is a negligible HP difference. But the tune on a stock car is remarkable for sure.

 

There is a technical (and measurable) difference between a 91 and a 93 tune which will yield different HP. It appears on the high-octane timing tables (the 4.0L platform uses 4 complete high-octane tables circled in red in the first pic). The timing values in terms of performance between 91 and 93 octane fuel impacts the timing values approximately highlighted in the second pic below. The entire table, technically, will have lower values for 91 octane fuel, but the area circled in red is where/when it might be felt or seen on a dyno. Fueling won't matter much. If it is dialed in correctly, in won't matter much between 91 and 93 octane.

If a 93 octane tune is done and 91 octane fuel is used, then the ECU will be making on the fly timing adjustments (MAYBE - based on ambient air temps, altitude, and a couple other metrics.) using the table to the lower right titled KNOCK OFFSET.

Ideally, the correct octane is used for the matching tune. You'll get away with 91 on a 93 tune w/o much notice, if it is a 91 tune with 93 fuel, that's just a waste of money, with no extra power as the timing tables are at high limits and cannot grow, but they can shrink by design.

 

I have a Renntech tune for my 2020 GTR and live in NC... we do have 93 here and I was asked what state I lived in... so I'm assuming my tune was mapped for 93 max octane... that being said, If i'm on a 200-400 mile trip I will almost always tank up with 89 octane... Then when back local, 93 goes in for perceived fun/performance on my WOT moments... my seat of the pants experience tells me there's not some "oh my god" diff between the two octanes...In fact, the performance seems very similar... I will say that there was diffidently a difference between tune vs. stock... I get your question about the 91 vs 93 tune but I suspect only a timing sheet from a drag strip will reveal a difference. Get the 93 tune, put 91 in and enjoy the torque that the tune is going to deliver...

 

Hmm...not something I would do with the 89 octane on a 93 octane tune, but I try not to judge others' decisions. Manufacturers too, understand that because there are options out there, such as 89 (or 85) octane fuels, owners will make those choices, so the EMS (on any performance engine platform, not just MB) has built in safeguards.

Just a quick primer:
Octane is a way of controlling fuel burn rate, the higher the octane the slower the fuel will burn. It is important to have this control feature in fuels BC it allows an engine designer to vary compression ratios in different engines to attain desired hp/tq outputs. Essentially, the higher the compression ratio (and especially with forced induction platforms) the slower we want the fuel to burn.

When cylinder pressures and temperatures rise, fuel has a tendency to spontaneously combust, but with higher octane fuel under the same circumstances, spontaneous combustion is curtailed. Spontaneous combustion is, in essence, an explosion. This explosion causes what is commonly known as Knock. Knock is Kryptonite to an engine. I won't go into all the gory details about how unchecked knock eventually will cause an engine meltdown, but gone unchecked, it's lights out. So, mfgrs design EMSs to aggressively attack knock. When the ECU receives a knock signal, it reduces timing by a similar amount to the knock as a survival mechanism.

Although under any circumstances other than WOT, using a lower octane fuel with a high octane fuel tune, knock is probably not felt. Some claim they can feel/hear the knock at higher RPMs, but the ECU correction is so fast, that you don't realize it is happening. Nonetheless, knock is still occurring. With a fuel rated at 4 numbers below a 93 octane tune, the ECU is very busy (like whackamole) beating down knock and constantly reducing timing.

When a custom tune is done by hand (which is all I do), the tuner's job is to first get fueling correct, then maximize timing (by modifying the cells in the timing table) to maximize performance, then back off slightly to ensure there will be some engine longevity. Also, a custom tune like this starts with the fuel in the tank. Whatever fuel is in the tank is what the tune will reflect. So, start by filling the tank with the fuel that is available to you.

I pulled up a datalog I did some time ago as I thought it would be a good illustration of the tuning challenge and how knock appears and is resolved. This is a V8 engine with a supercharger (non MB, but same principle). It is a 91 octane tune as all that is available in the area. The illustrations below are still shots of a motion picture that I see in the DL, but BC of proprietary SW I am unable to post the log as a motion pic.

These 3 shots are a sequence of the same event. The first pic is a stillshot of all the parameters I watched leading up to a serious engine knock event (where timing in the individual cells were too high). The second and third pics progress through the knock event. I set up the arrow markers of importance to observe as you move through these three pics.

In the first ROW watch RPM and the vertical white cursor (which is constantly moving in the motion pic version).
Note in the white field to the left, commanded throttle is at 100%. Commanded throttle is me. I was doing a WOT pull.
Watch in the third ROW both timing (white line) and Knock retard (red line).

I am coming up to a pretty serious knock event under hard acceleration meaning in a region on the timing table that I showed in an earlier post, the timing is too advanced (or similar to what I would see if I could drain the tank and put in 89 or 85 octane instead of the 91 in the tank). Notice the timing at this moment is 14.5* advanced. KR is 0. Watch how the ECU reacts when the knock appears in the second pic.

One point of observation, but not germane to this discussion, I was using a wideband on this car and had already set fueling just the way I wanted it. It is reflected by the yellow and orange lines in the first row. Orange is the actual AFR, yellow is the Lambda.



Now the white vertical cursor is moved over to the peak of the knock. Here you see knock reported as 6.9 and the ECU has pulled timing down to 7.7*, a reduction of 6.8*. Also note the RPM in the first row continues to climb and I continue to command 100% throttle (WOT).


This last pic now takes us to the point immediately after the knock event, notice that RPM is still rising, but it is the moment I decelerated and commanded throttle now shows 35.7%, so I was in the very moment of letting off with RPM almost the highest point in this pull. Note that knock retard in the third row is back to 0 and timing has recovered to 16.0*.

So this illustrates what the ECU will do with fuel that starts to spontaneously combust. BUT, if I tune it to 93 and then put a tank of 89 in, the red Kryptonite spikes seen in the third row will be continuous throughout the RPM range, not as severe under lighter loads and cylinder pressures/temps, getting more and more serious under heavier loads and pressures/temps.

BTW, to finish up this tune the way a good tune is done, I made a regional adjustment (downward) in timing advance, then back to another WOT DL to be sure this red spiked area was gone.

 

That's great. You drive a $200k car and save $6 a fill up with not putting the right gas in it for the tune. Why even pay the money for the tune to begin with?

 

That was a seriously thorough response and I really appreciate it! I have found 92 octane near my house so will ask that the tune I get (going with OE) is set for 92 octane and I will always seek it out. I really appreciate you taking the time to educate me further. Kudos.

 

@AndrewFerg I’m learning more about the different stage 1’s out there and being in a 91 octane state in Cali, what edged OE over the others like RT and EC?

 

I think I've read EVERY comment on this forum as well as several others and OE Tuning generally gets very positive comments from AMG GT C / R owners and I like the current "sale" price. It checks all of the boxes, and the sales folks have been very responsive answering my questions (via email.) I will let everyone on here know how it goes. I love just about everything about my current setup (throttle map, shifting responsiveness (paddles), engine sound and responsiveness, exhaust burbles (turned loud with Race engine selecte), etc. I just wish it had a bit more oomph to offset the "extra" weight (my last two cars were a 458 and a Panoz GTLM.) So I think once I get a bump in HP, I'll feel more at home with the power to weight ratio. Fingers crossed.

 

Andrew, What do you mean by current "Sale Price"?