Mixing Fuels - Pros and Cons 1

[JBirch]

G'day all,

I have just completed build a Nissan RB30DE (3 ltr twin cam)with approx. 11.5:1 compression ratio.

I am trying to determine what the best fuel to run this on would be. I was thinking a 50 / 50 mix of Avgas (100 RON) and PB Ultimate (98 RON).

However, I am unsure of the pros and cons of mixing fuels, especially seeing that Avgas is a leaded fuel and BP Ultimate is an unleaded fuel...

The car is designed for track use only so the use of leaded fuels is permissible.

Thanks in advance!

 

[patprimmer]

Run straight 100 octane avgas if the rules and law allow it.

It will have enough octane where the mix may not and it is a lot easier to control. ie just add fuel from the drum to the car.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[TStaples]

Running avgas isn't quite so simple. Avgas isn't optimized for automotive racing consumption. Based on some background club racing Porsches, I would run the 98 BP pump gas, instead of the avgas, if those are your only two choices, and you don't have detonation on either.

You did not mention whether or not you still have a catalytic converter in place (not likely), and whether carburetor, (carburettor, or carburetter, or carb depending on what part of the english speaking world you hail from) or fuel injected, closed loop or open loop.

If you have a carb, then realize you need to tune for different fuel densities (avgas vs street pump gas.) If you are tuning for a given track at a given altitude, then this isn't too much of a problem. This is less of an issue, or possibly a non-issue, if you are closed loop fuel injected. If you are still running closed loop fuel injected with a catalyst (i.e. some sort of controlled street car class) then low-lead or regular leaded avgas is not going to work.

Here's some good info:

http://www.osbornauto.com/racing/race2avgas.htm

-Tony Staples

http://www.tscombustion.com

 

[swall]

In the U.S. , at least, you can still find leaded race gas with 4g/gal lead content. Up to 112 octane, I believe.

 

[dickon17]

Do not mix significantly different types of gas. There is no way to predict the final octane number. It may actually be lower than both of the fuels.

 

[patprimmer]

I never saw the OP mention racing gas. He said avgas or BP Ultimate, which without further explanation I presume to be a pump gas in his part of the world. I just hate having to guess.

I presume his part of the world is Australia based on the G'day and the 3 litre Nissan, which is a common motor here.

Avgas will be made to a consistent tight spec whereas pump gas will be whatever is cheapest and meets the octane and SG requirements and maybe some evaporation rate requirement. I would much rather tune to avgas due to it's very consistent standard, even if it is not optimised for road use.

I guess the lack of optimisation refers to evaporation rate and SG.

To buy leaded avgas in Aus, you need to produce a race drivers licence and sign a statlatory declaration about use.

Last time I looked, it was A$1000.00 per cylinder fine if caught being used on public roads. It is only allowed by the EPA at special sanctioned events. The lack of availability and the potential fines is why I changed from avgas to methanol on our street class race car.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[cheapcharlie]

been running avi gas a long time.very little if any improvement running racing gas. avigas is less dense so diff fuel ratio may be required if no eng management sys is used.also worth mentioning is the cleanig avigas does to the fuel system if it,s dirty. on carburated systems this will amount to idle circuts stopping up, no big deal.fuel injected sys could be more of a problem if injectors plugged causing a lean out under load.I have no experience running avigas with injected systems. if your fuel sys is new should have no problems.

 

[thruthefence]

This is anecdotal,& tempered with my personal experience, but yeasrs ago I had a 1275"S" mini cooper, that I ran 3:1, premium unleaded to 100LL avgas. The Mini had 11.5:1 compression, I cant remember the timing, but the distributer had been aggressively (professionally)'curved'. Never had any pinging, or run on. I was told, that the avgas added just enough lead to the premium gas to work. The car seemed to pull well under load, & that's the only thing I had to go by.

 

[TheBlacksmith]

My uncle is a licensed pilot with a lot of experience including US Navy, Corporate Pilot and experimental aircraft. From his perspective, one difference is avgas has a much lower vapor pressure and is less likely to vapor lock and stall an engine at altitude. One of his planes is a high wing model with the tanks in the wings and a carburated engine. Because of the positive pressure from the tanks down to the engine and the carb, he has an FAA cert to use road gas, which is much cheaper. He said if the engine was fuel injected or had low tanks, he would be required to use avgas

 

[JBirch]

Thanks all for your replies.

Yes, I am in Australia.

A few more details:

The motor is fuel injected and is using an Autronic SM4 for the EMS.

There is no catalytic converter installed as it is not required for racing. The car is to be used for club days and the occasional drag race and does not have to meet any specific criteria.

What unleaded race fuels are available (in Australia)that are reasonably easy to source?

 

[patprimmer]

VP

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[rternie]

We use VP Race Fuels... they have several fuels for your applicaiton and list the specific weight of each. I use "C16" and "VPImport" Both are for boosted applications.

 

[JBirch]

I have had a look at the VP MS100 unleaded race fuel. There is a VP Fuels supplier nearish to me (about 100km away) so I will have to make some calls and see if they stock it or can get it.

I am now thinking it may be a better idea to use the unleaded fuel as then I can tune the car on the raod as well and not have to take it to the track everytime I want to use it.

 

[patprimmer]

Mobil premium unleaded and the shell grade with a splash of ethanol both have antiknock properties approaching 100 octane race fuel and can be had for a relatively cheap price (compared to VP) from your local servo (Aussie for Gas Station).

VP make some fine racing fuels, but they are very very expensive, with a great deal of hype about brand name. I know their methanol is over twice the price of "industrial grade 99.9% pure. They claim an advantage due to purity. I never found a difference in the 0.01%. I very strongly suspect that at least some grades are just traded by them from outside suppliers with a change of drum and a huge mark up.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[JBirch]

Being in Tassie the only 98 I have access to is BP Ultimate.

However, I have read that BP Ultimate with 10% Tolulene will push the octane towards ~102...

Several members of Skylines Australia have used it in High Boost applications with good results. This should work nicley in a high compression N/A motor. Some people have also siad that Avgas does not work well in high RPM motors due to the burn properties. I'm not sure what thay class as 'High RPM' but I am expecting around 7500 rpm out of mine, would this be too high for Avgas?

I hadn't chase any prices on VP fuels yet but I was expecting I'd need to be sitting down when they told me!

What would be the ratio of 98 and Methanol (not sure if I can get ethanol) required for this sort of application and would methanol be acceptable?

 

[patprimmer]

I have never seen an issue with avgas at high rpm.

It was used extensively as a fuel in touring car racing, boat racing and drag racing in Australia up until about year 2000 when it was effectively banned by the EPA.

The Shell fuel with the ethanol is a standard Shell retail pump product, well at least it is in Sydney.

I just looked it up. There are 2 products, one is a LRP at 10% ethanol and 94 octane and another is a blend of their 98 octane premium unleaded and 5% ethanol to give a 100 octane racing fuel. It is only available in Sydney, Melbourne, Brisbane and Canberra.

http://www.shell.com/home/Framework...or_motorists/fuels/vpower_r/introduction.html

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mattsooty]

Dont forget: -

When running oxygenates (like alcohols) with an open loop fuelling control system you will actually be running leaner than and may need to richen to compensate.

The oxygenates you add reduce the calorific value of the fuel but *may* allow you to usefully increase the spark advance without knocking - thus restoring bsfc.

MS