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Race Fuel and Octane Choice


SK400
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Greetings everyone 

Long time no talk.   Lots going on in MI since I moved here..   a few developers are building a new race track 30 mins away north of Ann Arbor, MI, in Howell, MI, with garages etc.     Motorsport Gateway.  Check it out! https://www.motorsportsgateway.com/  .  Opening should be Spring of 2023.  Might be an FIA grade 3 or 2 track with tons of runoffs etc.  Really look forward to seeing this project come alive.  
 
On a very separate topic..  as some of you know, I have a new blue printed R500 spec engine that is a complete fury but I keep melting the tip of grade 7 and even grade 8 spark plugs indicating auto ignition it seems (not good).  The engine compression ratio should be right around 12.5:1 .. but perhaps slightly north of this given a few other head and block mods (squish gap etc, but not more than 12.75:1)..  hence very limit for 93 octane street / pump gas I assume.   Just wondering, if we had to buy race fuel, what would you recommend? 98 or 100 octane? if so, unleaded standard or unleaded oxygenated?  what's the difference?  Would you recommend separate octane additives to regular 93 octane pump gas? if so, what brand or product would you recommend? 
 
Thank you all
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Hi Seb - Can we validate the plug tip look first?

 

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From your description you are in clearly lean territory. 12.5:1 compression ratio would infer 93 octane is too low.  Thats not good since you are likely burning a hole in the top of your piston.  My rough rule of thumb has been anything over 11.75:1 should be at 98 octane or higher as that gives plenty of safety margin.    

 

Do you have a lambda reading or some way of assessing air fuel mix measure?  Particularly after upgrading octane to confirm you have no additional issue.

 

Go straight to the pre-mixed 100 octane from a reputable supplier.  You have an expensive engine and you should invest in the right fuel to protect it.  I'd hate to see it end up like Simon's engine where a piston succeeded in escaping at high velocity recently.

 

 

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Thanks Mike 

Simon also detonates and burns plugs (exact same engine) but his real issue is different.  He keeps a picture of me on his dashboard and he is still trying to beat my TBolt time.  His engine had to give up to give him an excuse.  
 

He is pulling his engine apart as we speak trying to figure out root cause.  Oil / lubrication on #1 Conrod bearing is an hypothesis.  Sorry Simon, I’m sure sure your ears are burning.  

 


 

 


 

 


 

 

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I agree with Croc, you on the edge of too lean/detonation disaster. Talk to you engine builder what fuel did they use? Pick a new commonly available race gas, and get a dyno tune so make sure it's compatible. 100 Octane is commonly available I think.  Oxygenated gas will give you more power, but it's expensive and not available at every track, you don't want to have to start carrying you own gas supply. We uses to run a VP oxygenated gas in the race bikes, it made your eyes water just fueling the bike up, we had to flush it out of the engine at the end of every race weekend because it caused corrosion. 

 

Graham 

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Thanks fastg. Good advice on oxygenated vs not.  The car has soo much power already, no need for more honestly. (Yes.. I said it, Croc).  Engine spec was designed with EU builder and we did discuss EU vs US gas pump risk, understanding that we might be at the limit. I guess we now know the answer to that question. 

 

Simon had the ECU tuned remotely with Steve from SDB (not dyno, but road tuning using car lambda, connected to Steve in the UK via WIFI .... pretty cool) which had a massive - massive - impact on power, torque and drivability from our stock Caterham R500 fuel map.  Steve added a lot more fuel than what the stock Caterham R500 map recommended.   We both have the same SBD fuel map now (not lean anymore), but still burning plugs after flat out track days.  I guess we need to upgrade to 100 octane and redo / refine the map again.  Nice winter project! 

 

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You need the help of a good tuner. It sounds as if you are experiencing autoignition. This is the precursor of detonation. Autoignition is caused by a hot spot in the combustion chamber. You might be able to cure it by adding more fuel, or reducing the ignition timing. If the engine was dyno tuned before you got it, the problem is highly likely the fuel and you need more octane if you want to maintain the same power it had off the dyno. If you have a lambda sensor, leaded fuel will cause the sensor to fail, but it takes time. Most octane boosters are toluene. Toluene is used as a paint thinner and is 125 octane. The sweet smell of burning race gas is toluene. Sparkplugs are hard to read with modern unleaded gas, but look at the porcelain. The side of the porcelain not where the electrode sticks out. You do not want to see any silvery flakes. those flakes are your pistons. 

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I didn’t give the second source of auto ignition. If the fuel doesn’t have enough octane the fuel will start to burn normally but as the temperature and pressure rises a second flame front will start. Generally this is a hot spot, but it doesn’t have to be. A Englishman did a lot of work on this. If you look on a BMC engine valve cover you will see a list of patents given to Mr. Westfield

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Just a bit of caution on boosting octane from a chemist. Toluene has been used for a very long time to boost octane. Pure toluene may be a bit toxic on its own, so best to avoid skin contact and breathing fumes of additives or race fuel, or higher octane pump fuel for that matter. But as refined from crude oil ( a process called distillation) the manufacturers try to separate out the benzene, but some toluene sources have contained significant amounts of benzene. Benzene is a known carcinogen that definitely causes big trouble like leukemia if you are exposed too much, so try to avoid sloshing fuel around and never use it as a parts cleaner as was common practice a long time ago. Living around an oil refinery is not a great idea either. i also know about burning pistons and

am probably way too cautious with 108 octane racing fuel in that regard.

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Thanks all for the thoughts and comments.  As you said, I think it is a simple octane increase solve.  I might be right at the limit of auto ignition for NA pump fuels so a little octane help wi go a long way.  
 

im looking at the Sunoco 260 GT fuel. Anyone has experience with this fuel? Any other equivalent fuels or brands to recommend?  
 

thank you 

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17 minutes ago, SK400 said:

Thanks all for the thoughts and comments.  As you said, I think it is a simple octane increase solve.  I might be right at the limit of auto ignition for NA pump fuels so a little octane help wi go a long way.  
 

im looking at the Sunoco 260 GT fuel. Anyone has experience with this fuel? Any other equivalent fuels or brands to recommend?  
 

thank you 

VP also is a good line and maybe a dealer nearby 

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Hi Seb.  I’ve used the sunoco gt 100 in my yellow car. 12.4:1 CR and VP race fuel MS 109 in my orange car.  13:1 CR. Then again I don’t bear the crap out of my cars like you and Simon do. :driving::918766748_biggrinjester(1):  Tom. 

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Thanks Tom. I think I ll give the Sunoco GT 100 a try.  
Here are some of the pictures.  Grade 8 spark plug cathode melted after track day (Simon’s). I had the same happened to me with grade 7 and street driving. And grade 8 spark plug after 15 minutes of use.  The ceramic is sparkling white. 
 

 

5BE88B87-7167-43EA-9CCD-BDD62508F49D.jpeg

D02F6DBB-1934-429B-9386-8B640FA438E9.jpeg

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On 11/19/2021 at 7:43 AM, SK400 said:

 

Simon also detonates and burns plugs (exact same engine) but his real issue is different.  He keeps a picture of me on his dashboard and he is still trying to beat my TBolt time.  His engine had to give up to give him an excuse.  
 

 

@simon450 Have you thought about pasting Seb's picture to your Tillet seat and sitting on it?  Seb is not a religious icon you know.  Well not in a good sense. 

 

 

 

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1 hour ago, Croc said:

 

@simon450 Have you thought about pasting Seb's picture to your Tillet seat and sitting on it?  Seb is not a religious icon you know.  Well not in a good sense. 

 

 

 

Ive forgotten Seb’s exact thunderbolt lap time but I was at least -1sec so well worth sacrificing an engine. I think the piston will buff out and my son helpfully suggested I can seal the hole in the block with Flextape, the guy on the TV says it seals anything. 

2A0ECCFB-16FA-4FA4-9817-CD327FF2FBC9.jpeg

D539E428-0566-4400-A7B0-4079C75F5B0F.jpeg

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a word to the wise-- "wide-band"  (air fuel meter) and if you wish to keep your engine, in the dash preferable.  When leaned out at high power an engine will resort to burning aluminum. and quickly.  At 14,000 feet, in the sprint to the finish, I lost 3 pistons in a Corvair engine with 30lbs on the manifold. It happened in less than a mile. Drove it down the hill, and when torn down the rings were still intact but you could see into the crankcase where half the pistons used to be.  Besides a wide-band will make tuning easy even with a carburetor engine.   john

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Seb That's bad news, melted plug electrode pumped thru your motor, cannot be good!! Is it possible that the squish band directs a stream right at the plug electrode.

Do you index your plugs? Is it always the same cylinders or position of the electrode in the cylinder?

I would look into going with a retracted gap plug, at lease on race day.  I have never run them on the street, they probably would not last very long, but better then a new motor. If you have a killer "CD" ignition system you could try surface gap plugs 

Dave W

Edited by Dave W
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Amazon sells a six pack of 15 oz bottles of Lucas octane booster for 

$60. If you add 3 oz to 5 gallons of gas, it adds 3 octane points(91 becomes 94). You can do the math and see how that compares to the roughly $10-12 gallon for 100 octane at the track. It is also a lot more convenient than carrying cans and pouring your own gas at the track.

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From the photos, you do not see any detonation on the two plugs. Which might mean the condition is for a very short period. Tip coloring indicates slightly less timing needed, back off 2 degrees. The single plug show very rich at idle and lean at WOT.  The best indicator for WOT is at the base of the ceramic.  You need a clean throttle chop, while de-clutching for a good plug read.

Dave W

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