Mixing fuels

[MoparMap]

Hoping there are some chemical engineers around here that can give me a hand before this weekend. Our club is doing our dyno day this weekend and I have some concerns regarding fuel in my car. I bought the car with an SCT tune and suspect it may have been set up for 93 octane. The problem is that I can only really readily buy 91 octane where I live. Normally this isn't a problem and I've been running it for over a year and a half with almost no issues because I don't flog the car on a regular basis in the worst case scenario conditions. However, at last years dyno day there was some audible knock in my engine as it was pulling through the mid range and I've noticed some myself with the top down on a full throttle hit from ~3000 rpm, so I wanted to try to get some better fuel in it before this year's dyno to see if that fixes the problem. There is a station nearby that sells 110 VP race fuel. I was debating tossing a couple gallons in to try to up my total octane rating, but wasn't sure if they really mixed that way. I didn't know if the 91 would basically stay 91 and there would just be pockets of 110 in there as well. Anyone have any experience with this?

 

[plumcrazy]

you can mix in race fuel anytime, it mixes fine. I was told to do that very thing in cold weather if i was going to be racing. if you hear knocking, thats a problem.

 

[MoparMap]

Turns out it may also be leaded fuel it seems, so that throws a bit of a wrench in the works. It really won't take much to get what I need (~3-5 gallons), so I'm guessing it won't have a large effect, but it does raise some additional concerns. They might have an unleaded fuel there too, haven't actually looked at the pumps to see what all they offer, but here there are 3 or so different race fuels at this particular station.

 

[ViperJohn]

If you still have the tuner, why not reach out to a tuner and have them load you a more fuel friendly tune?

 

[MoparMap]

Yeah, I've still got it, just haven't wanted to mess with it since it runs nice and makes good power. Put down 480/530 hp/tq at the wheels last year with just a filter, cat back, and tune. It isn't really a problem for normal driving and even the occassional spirited driving. I mainly wanted to try different gas this year to confirm that's the problem before changing things.

 

[Dom426h]

Your idea of solutions to this problem sounds ridiculous to me:

"Don't flog the car on a regular basis" --> Wow... Really??? That has got to be the lamest solution I can think of. FYI: Detonation does not have to be audible. It can silently(to you) be slowly destroying your pistons.

"Add race fuel" --> OK... So at every regular fuel fill-up are you going to attempt to calculate the ratio of race fuel needed to raise your octane to the appropriate level?

There is only two correct options to fix your problem:
1. Remove the SCT tune
2. Re-do the SCT tune

I assume the previous owner sold you the car and kept the handheld controller which is pretty lame considering it has your other tunes (including stock) on it. My SCT has on it the Stock Tune, a 91oct tune, and a 93oct tune. I can switch as often as I want between the tunes. You should be able to do the same.

I'll add that there is not much to gain in going from a 91 to 93 octane tune. Others with more dyno experience can chime in but I think were only talking about a ~5 ponies. In my experience the signifigant power increase from a tune comes from the added fuel in the lower rpms to compensate for the leaning out that headers and a free'er flowing exhaust system provide.
What are all of the modifications done to your viper?

 

[MoparMap]

I'm not necessarily asking on the race fuel because I plan to add it at every fill up. I have daily driven the car the way it is for over a year and a half and 30k miles with no noticeable issues aside from the very rare occassions I've talked about. My plan was to try to up the octane rating for this weekend alone for dyno day to see if that makes the problem go away. Even when I ran it last year it only had any sort of audible noise on two of three pulls and only very briefly for maybe 500-1000 rpm as it pulled through mid range, so I might have just had questionable fuel to begin with.

I do have the SCT that came with the car, but I think I have a very strange set of coincidences that keeps me from using a stock tune. The previous owner told me that he purchased the tuner that came with my car. However, I think the shop it was tuned at used their own tuner to apply the tune and just put copies of the tune on the handheld that the PO purchased. The reasoning behind this is that when I plugged the tuner in the first time myself to change from the cold tune to the hot tune that I got with the car when the weather changed, it asked to backup the "stock" tune. This "stock" tune never lit the skip shift light no matter how hard I tried, hence why I think the shop had already pushed a tune to the car with a different SCT. The tuner I got with the car still works fine and pushed a tune over without any problems, but I think what the handheld thinks is a "stock" tune is actually one that was programmed with a different handheld, so in reality I have no stock tune to fall back on. If I do the "return to stock" option on my handheld I still don't get a skip shift light, so something has to be different in the tune to do that (the amber arrow works on key on, so I know the bulb is good).

If the problem goes away with better gas I'm pretty sure it's a tune issue. If it's still around it might be a valvetrain tick or something else. It certainly sounds more like valvetrain when I've nailed it with the top down as opposed to piston knock. My goal with different gas is to determine what the problem is before I go spending money trying to fix it. If I buy a new tune and it's still ticking away that's money I could have spend on lifters or something else that might have been the problem to begin with. If I had a stock tune I would have tried that a long time ago.

The mods I listed are all I have. I bought the car with a K&N intake (so basically just a filter of negligible difference), a Borla cat back, and the tuner. The factory manifolds and all 4 cats are still there. I've made no other changes since then aside from redoing my oil cooler lines when they started leaking and a set of Woodhouse mounts.

 

[steve e]

You can not run leaded fuel if you still have cat converters, you will destroy them. Just add octane booster 104 or any good brand at each fill up and your done, you can keep the tune the way you like it.

 

[Dom426h]

If I had a stock tune I would have tried that a long time ago.

OK, had me worried there for a sec that you might have overlooked the most obvious thing... lol

Guess I'll share my experience with detonation/knock to see if it can help you.
A few ways I can think of to describe the sound:
- Pouring coarse sand or BB's into a metal can
- Pouring dried corn into a popcorn maker
- A rattle snake rattling its tail
- Riding over Sand/gravel and the tires throwing it up into the inner fender.

Can be a faint sound to a loud one depending on both the degree of detonation and the environment you are listening in. I heard detonation on the road that was very faint but when tuning on an indoor dyno the sound was significantly louder since the higher frequency sound is amplified when bouncing off the walls and there is no road noise/wind to mask the sound.

-------------------------------------

I suggest doing headers or some other mod that would benefit from a re-tune so that you have an excuse to get-r-don

If you do the dyno day then make sure to have them hook up the wide-band o2 sensor so that you can see what type of A/F ratio that the motor is running and have them print it by itself at a large scale so that you can have others interpret it. Running too lean contributes to detonation. Also, Im pretty sure that det. can be visually seen on a dyno graph as the torque line going shaky/jagged. This is hidden with the high smoothing factor that most shops print at so you might want to print it with 0 smoothing to see what is what.

I don't recommend putting any race fuel in it before the dyno day because that doesn't really tell you anything. Of course its not going to ping with race fuel... Yea the sound could be something else but putting the stock tune on and just running full throttle down the highway late at night with minimal traffic & noise can tell you that. Why not just call ROE, DCPerformance, ViperSpecialy, or ACPerformance and see if one of them will email you the stock tune. You can upload it to the controller yourself and flash the car. I doubt they would charge you anything if you explain the circumstances. Then if you dyno you could email them the graphs(or even better the .DRF file) to have them interpret and explain to you exactly what they would tweak to optimize your tune.

good luck and let us know how you make out.

 

[MoparMap]

Had thought about asking for a stock tune, but didn't know if that was feasible since I figured it might have been somewhat unique to a car. Will have to look into that one more. The noise last year at dyno day was hard for me to hear and fairly faint. I just caught a little ticking, but nothing major and the shop guys noticed it as well and pointed it out. On the road when I've nailed it it's blatant. It's a very pronounced rattling sound as opposed to the tick/sizzle/"water in a frying pan" sound I've typically heard knock described as. It's almost more like something is loose and hitting something. I've only ever noticed it with the top down as well oddly enough, so it may be something completely different too.

I did get a printout of the runs and have the AFR on the same graph. I'll have to look it over real close, but stuff pretty much looked in check from what I remember. My guess was the former shop pushed the timing to the edge and without knock sensors (it's an 04) I don't have much safety blanket, assuming it's knock to begin with.

I have used octane booster in a pinch in the past (filled up with 89 as opposed to 91 without thinking), but from most sources I've read it's very limited effect at best. I've heard it's really not even good for a full point boost when used on a full tank.

 

[EllowViper]

Drop a can of TORCO in it and have a nice day. Use it when you need it. Keep a can stashed in the car for those unexpected times.

 

[EdwardETraylorIII]

Octane booster (of any brand) doesn't do anything for you. It's snake oil.

Yes, you can mix in race gas. However, leaded race fuel will ruin your catalytic converters and MORE IMPROTANTLY will ruin your O2 sensors.

However, you can purchase CSP (unleaded race fuel from VP) and mix in a couple of gallons with 91 to achieve 93-94 Octane..

As previously noted in this thread, the correct thing to do is have your car properly tuned if you can audibly hear knock. Raising your Octane by 1-3 points is absolutey NOT going to fix *audible* knock.

Do it right, buy HPT and get it dyno tuned (and they can data log knock to make sure you aren't getting _any_ during this process).

 

[MoparMap]

I don't have any knock sensors to be able to log to begin with or I'd hook up my OBD scanner and watch the knock channels for an indication. Car is an 04, so early gen 3 before they started putting knock sensors in them. How the Viper went so long without knock sensors is beyond me for a performance car, but that's a whole different discussion.

 

[MoparMap]

Well, for the most part dyno day went without a hitch, aside from one strange thing I still can't figure out. Car was nice and quiet, no audible knock/rattle/etc, though numbers seemed weird. Power was still pretty much the same as last year at 477 (was 479 or something like that last time), but torque was waaaay down. Made 530 last year and only made 480 this year, despite zero changes to the car. The last run was also strange because it made similar peak numbers, but was down probably 20-30 hp and torque all the way up to the peaks. I think I may have been slipping on the rollers, but at this point I'll never really know. Car still pulls and feels good, so I'm just going to tick this one up to something odd. Slipped on the rollers one run last year, so it's not out of the realm of possibility. I just can't see how I can lose that much torque and still make similar horsepower unless something was funny.

 

[VIPER GTSR 91]

For a great answer here whatever happened to Tom F&L? Have not seen his posts since the turn over??

 

[MoparMap]

I was hoping he would chime in. I know he has lots of chemical experience in this field and figured he could provide a good scientific answer.

 

[EdwardETraylorIII]

Mopar Map,

It's very simple - Petrol based fuels have certain properties that determine their BTUs, sussceptibility to burn and their gaseous release (volume) when burned. One of the primary factors (at least for the fuel we'd run in our engines) is Octane. This is similar to Hexane, PROPane, Cetane... There is no way to chemically alter the Octane of your fuel short of further refining it (a highly involved process that involves a lot of heat and pressure).

This is why octane "boosters" do no such thing.

 

[Tom F&L GoR]

MoparMap, I've been away from Viperhood for a while, but have come back... um... in the "other" site mostly. But I am getting back in the swing and traded up from my '94 to a '98 GTS. I'm getting my second wind.

As already said, mixing different octanes is essentially linear - when you buy 89 at the pump you are getting a mix of 87 and 91 octane. Also as noted, the aftermarket bottles talk in "points" which are not whole octane numbers, but hundreths of a number. And there is no magic additive like tetra ethyl lead to increase octane, and TEL does kill the catalyst and eventually O2 sensor. One commercial product contains Methylcyclopentadienyl manganese tricarbonyl, or MMT. This is effective, but is another illegal metallic additive as far as the EPA is concerned.

If your tuning gizmo is flexible enough, you might consider using E85. There is some dispute over how the octane of a non-hydrocarbon fuel should be measured, but the optimistic end is 113 and the pessimistic is 96 R+M/2. (How much octane benefit your engine gets depends on where it makes peak cylinder pressure; at lower or at higher speeds.) Modern engines (i.e. your Gen 3) are far more tolerant of high ethanol concentrations, even if not rated as "E85 capable" so it's low risk to use for a tank. A further complication is that regulations require a seasonal temperature adjustment to provide good starting ability when cold, so E85 is never really 85% ethanol; it is between 53% and 83% ethanol. Therefore the energy density and your MPG will change a lot - hope you have a wide band O2 sensor.

Edward, there is "octane" the chemical that is the specific molecule used to define the value of 100 for anti-knock performance, and then there is "octane" for the rating of a gasoline. You are correct that the specific molecule has a specific octane rating. However, "octane rating" is defined as "the resistance to preignition" and is a chemistry independent performance measure. Adding tetra ethyl lead to "octane" will raise the "octane." Further refining really means removing the lower octane components of gasoline (of which there are hundreds) and including more higher octane components (of which there are hundreds.) Gasoline is a mixture of many hydrocarbons and has a distillation curve - it begins boiling at ~120F and may end ~340F. So, I do not mean to disagree but only to point out that it is (no surprise) complicated.

 

[MoparMap]

Hmm, never really thought about converting to E85. I live in the midwest, so availability isn't all that bad, though it does seem to have dwindled some since it first came out. I think there are some interesting misconceptions about it as well. Everyone seems to think it's corrosive, but I believe the reality is that it just mixes with water where gas does not, so the water can get where it normally wouldn't in the system and cause corrosion.

 

[Tom F&L GoR]

To be more clear; my comment about newer engines being more tolerant was mostly in regards to materials (elastomers, gaskets, fuel lines) with ethanol and not so much being able to consume a greater than 10% ethanol content in the fuel (as gasoline is these days.) You are correct, most of the issues are from the water that the ethanol helps to attract. As an example of why it's less of a corrosion problem, however, look at your fuel tank - it's a polymer, not the steel/solder tank that could rust through.

There is a push to market an E15 gasoline (15% ethanol) but many pre-2004 (I think that's the year) models don't have a wide enough fuelling range programmed into their ECUs and would run lean. That makes it tricky at that gas station because it's not clear who is liable should an older car purchase E15 and then have problems. However, E15 would be fine in a Gen 3.

Besides the added octane, there is the higher latent heat of vaporization; to evaporate ethanol takes more energy and so it draws heat out of the incoming air. (This is why alcohol feels cold on your skin - it draws heat from your body to evaporate.) The cooling effect helps get a higher density charge into the cylinder and has a benefit in resisting knock.

Ethanol has about 2/3 the energy density of gasoline, so your fuel injectors would have be sized to provide 3/2 times the volume. Then it's up to you to advance timing, tune mixture exactly, etc to take advantage of the octane and cooling effect. This must be fairly routine because even the car companies do it!

 

[MoparMap]

Yeah, I'm a little familiar with the differences between the fuels from some work I did in college. My masters project was setting up a control system on a small two cylinder injected generator to use as a test bed for alternative fuel development. I had a pump gas tune working first and to prove the concept ran it on E85. I did a rough 30% fuel map bump as a starting point and it actually worked pretty well and was fairly close. I know we used it in my bachelors senior project of an FSAE race car. You could actually tune more power out of E85 due to the higher knock resistance over pump gas, but that's just it, you had to tune for it. Wasn't good for economy by any means, but it was a race car after all, lol. I wonder what sort of limits the SCT has regarding fuel map changes. I suppose just changing the injectors out and keeping the tables could potentially get around that issues, but the cruise mode would have to be deactivated since it would try to hunt for 14.7:1 fuel ratio which is no good in E85. Would be running mighty lean at that point.

 

[Tom F&L GoR]

Yes, ethanol is more tolerant of being a little lean or a little rich and still running fine. I don't know your system, but would the (wideband) O2 sensor not also hunt for the correct O2 level at cruise and add fuel volume? (I don't want to say "richen" the mixture, since it would only bring it back to stoichiometric.) Want a job at a big chemical company?

 

[MoparMap]

The system is still the stock one with just the tables modified as far as I know (SCT tuner on stock ECM). I'm not sure what sort of flexibility it has, but I don't think it can handle a wideband input. May have some sort of cruise parameters that can be tweaked so that it doesn't try to hunt 14.7:1, but without a wideband I don't think it would adjust to environmental conditions as well. Technically narrowband sensors still read a range, though the output curve is more like an on-off switch than the nice linear wideband. I made a little LED display once that I used with a narrowband on my 5.7 Hemi swap in my 67 Dart. Was just a set of 6 LEDs or so, 2 red, 2 yellow, 2 green. I programmed them to light up at certain voltage outputs of the O2 sensor to give me a rough idea of how my carb was running to see what I needed to do for jetting. Entertainly little project, though admittedly not the most accurate gauge in the world.

I bet the job would be interesting, though I'm guessing a mechanical engineer might be a little out of his league in a chemical company . Would I get shipped off to the most remote oil rig in the middle of the ocean for newbie training? lol

 

[Tom F&L GoR]

I'm an ME and my group does the engine dyno, vehicle chassis, and fleet testing to develop fuel and lubricant additives. Our challenge is to translate new lab chemistry into a credible performance improvements. We run standardized tests but also design new tests to support individualized consumer product claims for regional and global oil companies. You'd love it.

 

[MoparMap]

Wow, that sounds pretty interesting. Where are you based if you don't mind my asking?

 

[Tom F&L GoR]

BASF has a location in Tarrytown, NY. Looks like an earlier post with link was deleted, so search for BASF Fuel and Lubricant Solutions.

 

[MoparMap]

I didn't think they had fluids that far north, I thought it was all ice