Turbo D17

Did a search and couldn't find anything specific on topic...

If a turbo car is tuned on 93 octane at 8psi, and then for whatever reason decides to run 101 or 105 unleaded at the track, will the air fuel ratio change if the car runs the same amount of boost? Basically, if nothing else changes except the octane of gas, and is using an actual unleaded race gas, not pump gas with an octane booster. This is also assuming that there is no knock at either octane level.

Don't want to debate if it's worth it to use that high of an octane or not, just if octane has any relation to air fuel ratio.

pollito

I'm no guru on this, but I don't think it will change your AF ratio at all. It should merely let you go more "lean" safely. You'd have to tune with each fuel.

mos

It depends on the fuel. Could run richer, could run leaner, or the same.

boostn420

no no no

Think of it as insurance. If all you do is increase octane but make no changes to the timing, then you are just decreasing your chances of detonation.

mos

Theres more to fuel than octane.

kpt4321

No, the air to fuel ratio DOES NOT CHANGE. Your IPW is the same, and your airflow is the same.

However, that doesn't mean that the car will run the same if you toss race gas in and don't touch anything. Most of the time it will run worse, you need to tune it accordingly.

NoShoes

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kpt4321 &raquo;</TD></TR><TR><TD CLASS="quote">No, the air to fuel ratio DOES NOT CHANGE. Your IPW is the same, and your airflow is the same.

However, that doesn't mean that the car will run the same if you toss race gas in and don't touch anything. Most of the time it will run worse, you need to tune it accordingly.
</TD></TR></TABLE>

Most race gas has a higher specific gravity, hence the reason the car has to run a bit leaner for a proper burn.

tony1

What i've found in the past is that by dumping in race gas and not changing timing is that it'll run richer. The higher octane burns slower and without adding ignition advance it won't burn it all. Higher octane gas needs more timing to get a complete burn. Now, this is comparing 93 octane to 116 octane. I'm sure that the lower you go the less evident this becomes. It will also depend on how aggressive your timing is on the 93 as well.

HPinXS

The A/F will change!! The SG of the fuel will determine what the final A/F will be ,it is simple, the weight of the air is the same for all intents from day to day(allowing for atomsphereic conditions) but the fuel is not (race gas vs. street pump gas) hence the tune-up must change. Most if not all pump unleaded premiums have a SG of roughly .760-.775 depending on make-up and the amount of oxegenates in them ,this is also true of most unleaded race fuels (100-105 octane).Leaded race-fuels(108-118 octane) thou SG around .680 to .730 depending on make-up.So what we are really dealing with is chemically correct mixture based on weight,so an engine tuned on pump gas will require a different fuel map for race gas. As far as the octane slowing the burn down that is false,flame speed is dictated by chamber turbulence and dynamic compression,octane is resitance to detonation or pre-ignition,an engine only needs as much octane as it take to eliminate any or all detonation or pre-ignition ,after that adding more octane is wasted.The myth that race fuel in a low compression engine requires more ignition timing comes from the fact that these low compression engines were street engines. Race gas' make -up is such that it changes very quickly from good to bad when exposed to the enviroment. Race gas is photo-chemically reactive,by that I mean sunlight, if kept in a container that will allow light to pass through it ie: red or blue or white, the gas will be gone in 30- 40 minutes,FACT! The lead reacts with the ultra-violet rays in the sunlight,just ask any race-gas supplier.At the same time the light ends of its make -up vapor off very quickly,these are the componets that you hear as you open the container and hear hissing out,that hissing is you hear is your money going away. the light ends of the fuel are what gives it, it's cold start ability and throttle response.The care and storage of race gas is far above what is required of pump gas. So you can see where this is going now , The myth race gas that requires more timing in a street car is from the fact that the gas has "gone away" so to speak ,the additional timing is trying to cover up the loss of the light end componets,that allow it to combine the heavier fractions with the available oxegen.Race gas when stored should be kept in a tightly sealed steel container with the least amount of air space above it to keep the light ends from vaporing off,and in a cool dark space.Hope this helps.

QCKGSR

The A/f will change, i have run pump gas then drained the tank, and then adde some of the race fuel, the air fuel did change

kpt4321

Great post HPinXS, I totally forgot to think about specific gravity.

What I have always seen, is that if you go from good pump gas to good but not extreme race gas, the change is specific gravity is not enough to change the A/F ratio more than a few tenths. But, it could possibly have a greater effect that that, like you showed with your examples of what the SG can be.

Tony also mentioned what I was sort of hinting at in my post... if you run high octance gas with the same timing maps you use for pumpgas, you will lose power 95% of the time because the combustion will be taking place so late in the stroke.

tony1

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by HPinXS &raquo;</TD></TR><TR><TD CLASS="quote">As far as the octane slowing the burn down that is false</TD></TR></TABLE>

Are you saying that higher octane fuel does not burn slower? From everything that i've ever read and heard, that is false.

edit: a little bit i found about burn rate...
Burn Rates- That's the ticket!!
If you take that same fuel that worked well in the above V8, and run it in an engine, like a cbr900, with it's lower cranking compression and lower combustion chamber temps, it will, without a doubt, burn much too slowly at those lower temps and pressures and reach maximum cylinder pressure too far after TDC for best power.
Things generally burn slower when they are cooler and vice versa. Peak cylinder pressure will occur much past TDC - decreasing the power produced if you keep the same ignition timing.

You can advance ignition timing to try to recover power, but that will cause the air/fuel mixture to burn earlier in the crank stroke and spend, percentage wise, more of the energy produced by the expanding, burning mixture, pushing back down, trying to prevent the piston from rising up to the power stroke - robbing power.

If you MUST use a slow burning fuel, which USUALLY has a high octane rating, advancing the ignition timing will lessen the power loss, but the best results are usually obtained with the quickest burning fuel obtainable, that, of course, doesn't "knock".

All other factors being the same, except for burn rate - use the quickest burning fuel that doesn't "knock", light the spark in the middle of the combustion chamber, adjust ignition timing to reach peak cylinder pressure ~TDC and keep your mixture correct. When the ignition timing is correct, the engine will make best power for that fuel.

mos

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by boostn420 &raquo;</TD></TR><TR><TD CLASS="quote">no no no
Think of it as insurance. If all you do is increase octane but make no changes to the timing, then you are just decreasing your chances of detonation.
</TD></TR></TABLE>
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by kpt4321 &raquo;</TD></TR><TR><TD CLASS="quote">No, the air to fuel ratio DOES NOT CHANGE. Your IPW is the same, and your airflow is the same.
</TD></TR></TABLE>

Some of you guys should hold off on posting answers until you are sure you know what you are talking about.

DIRep972

iTrader: (1)

So a guy I know had his car tuned up to 16psi on pump gas and made 290/250. He went to the track and ran 12.6 @ 112mph on just 12psi of boost where he makes 250/220. He got very excited and went and bought some race gas from the track fill ups. He then turned up the boost to 16psi. He went back out on the track and ran a 12.8 @ 111.

moral of the story: DO NOT RUN RACE GAS IF YOUR CAR IS NOT TUNED FOR IT!

HPinXS

Combustion speed or "burn rate" whatever you want to call it ,goes like this .The intial thremal reaction which occurs between the addition of the ignition energy by the the ignition spark and the exothermal reaction of the air-fuel mixture is called the flame development phase. This phase is roughly constant over time, and depends solely upon the mixture composition. This means that referred to piston movement an ignition lag occurs which increases as engine speed increases and which varies as the air-fuel ratio changes. The course of heat release depends primarily upon the shape of the combustion chamber and the place at which ignition occurs,and combustion time is primarily a function of the speed with which the flame propagates (combustion speed).The combustion speed is greatest at approx. 10% excess air, ie:14.85:1 (not stoich 14.7:1) and is 20-40 m/s. Combustion speed is determined by difussion processes in the flame front as well as by the degree of turbulence and the temperature conditions in that portion of the mixture which has not yet combusted. The degree of turbulence in the combustion chamber and in the flame front can be influenced by a number of factors, including the design of the mixture inlet devices, the shape of the combustion chamber an the use of piston movement to generate so-called squish. However, the turbulence in the combustion chamber may occur spontaneously as a result of flame propagation and the attendant increase in pressure, and is in all cases a function of the engine parameters : compression,intake air temperature and engine speed . Pressure build up during combustion has a decisive effect on the end gas temperature, while the significance of thermal dissipation and radiation can be ignored. Low fuel consuption and high efficiency are promoted on the one hand by short combustion times, ie. high combustion speeds, and on the other hand by heat release at the proper location with respect to the piston movement. The greatest heat release should occur shortly(approx. 5- 10 degrees crankshaft) after top dead center. If most of the heat is released too early, wall heat heat losses and mechanical losses ( high peak pressure) are increased. If heat release occurs too late, the heat is used inefficiently and exhaust gas temp. increases. The correct position for heat release must be insured by properly selecting the ignition point while taking the following factors into consideration: the air-fuel mixture ratio,the fact that turbulence depends upon the engine parameters and flame development, which is constant over time. So as we can see here Octane has no bearing on flame speed or power output .

SOUNDEFFECTS

So we have established that on some cars not (knocking) or needing 116 race gas, a lower octane street gas(93) might be favorable, correct?

Also the higher octane gas does require more timing to run the same power as a lower octane gas would as long as there is no knock, correct?

Tony1, on street cars that primarily run on 93 pump and switch to 116 for higher boost; What kind of changes do you usually recommend for the timing maps? Do you alter the lower boost settings as well or just change the higher boost settings when the race gas is used?

HPinXS

As to the examples cited above ,the CBR 900 with its low cranking compression and lower chamber temps, what you are seeing is just the opposite, the low cranking compression rises as rpm increases due to the dynamic cylinder filling at peak torque rpm. These low cranking compression engines are VEing in the low 120-125 % range so now compression has actually jumped ,due to the fact that the cylinder was filled beyond a 100%, as far as the cold chamber temp is it a geometric realationship , with all current high specific output engines in that they are highly over square ie: large bores and wide combustion chambers and short strokes,all of these conditions expose more area for the available heat to "sink " into at low rpms, removing heat from the expanding mass with in the cylinder which translates into less work, the combustion process didn't take any longer (20-40m/s),but the speeds at which the piston approaches TDC and moves away from it does. As for sparking an engine early , yes you will see a reduction in net power as the intake charge is ignited too early and the engine has to now expend power to continue the piston up the bore ,at the same time this engine is also trying to lift the cylinder head off of it so in short this engine will fail as peak cylinder pressure is occurring too close to TDC ,the crank has not rotated enough to put the rod in a position that will allow it to transfer the maximum amount work and allow it to move away from TDC easily.Again octane has nothing to due with flame speed , air-fuel mixture ratio does. Take one highly tuned race engine and richen the mixture , what happens?? Low power and black smoke out the exhaust, right?Why? There was more fuel than there was oxygen available to support complete combustion, the initial oxygen was consumed and the remaining fuel was left what did the fuel do?It absorbed the heat created by the fuel that did burn reducing the net heat that was expanding the air in the cylinder.A lean engine does just the opposite, it pops when you wack the throttle ,right? and generally experiences high rpm misfires as there is now more oxygen available than there is fuel to support normal combustion as the peak pressure is occurring too early.Because the oxygen is the accelarant it is what controls the flame speed ,right?This is the same reason power will vary slightly from day to day, less oxygen means less heat,because we now had excess fuel in the cylinder, and the same reason we cant tune to get it back .

kpt4321

Just wanted to say good post, keep up the good work.

lazerus

upgrading to race gas on a aftermarket ecu will do nothing, unless the setup is designed to advance timing untill it sees knock, then retard.

some factory turbo ecu's do this, some do not.

Toyota supras are a car you can tell a difference on, if you have some BPU mods done, if you run 18 psi on pump gas, and then 18 psi on race gas (with stock ecu and twins) you will notice a difference in power.

If the car is properly tuned on 93/91/whatever octane, upgrading to a higher octane with the same boost pressures will not affect power.

It seems to me that race gas tends to make the afr slightly more rich, probably due to the burn rate.

When going from a tuned 91 octane map, and only switching the fuel, if the tune is properly done and not on the close limits of the fuel, you will not notice a power difference, but might notice a slight afr change if you have an accurate AFR meter (Wideband) you could probably datalogg the difference.

just my thoughts..

Tofurky

hate to beat the dead horse on this one....

but yeah... running 100 on a car that's tuned for 91/93 will only cause the car to run richer. However, I think running a gallon or two of 100 when running 91 octane might help deter knock a bit.

I know on my Evo when running I fill up with some 100 it seems to run a lot better than with 91... I found that at least 1/3 100 octane gets the best results.... which would put you around 94ish