Racing gas in Roush motors
 

Forgive my ignorance with this question but for those out there with a Roush (or other big block). I have been running 93 octane up until now in the cobra with adding octane booster. Lately have picked up a hesitation under accelation I couldn't figure out. I got down under a 1/4 (which by the way I now understand why people say not to trust the gauge under 1/2 since it is nowhere accurate). Instead of filling up with 93 decided to try 104 octane available at the pump at a couple location around town. I hadn't done this since the car was new. I cannot believe the difference in power and accelaration as well as the hesitation went away. It feels like another 50-75 horses!

My question is this, Are there any long term risks in running this all the time other than being pricey? My engine is a Roush 511 with Fuel injection with a 9.5:1 compression ratio I believe. I dont want to damage the engine but if I can afford to, running that gas makes it a lot more fun to drive:MECOOL:
Ron

Sorry Moderators, posted in lounge by mistake. Can you please move to general board?
Thanks

You are not going to hurt your motor running the race fuel,if it runs better I would stick with it. I was running 111 octane in my motor and was having problems on the top end with hesitation, and after talking to roush I went to a mix of 91 and 111 and it runs great.I like the security the mix gives me against detonation over straight 91, but you can over do it like I did with the straight 111.

Be sure it is UN-leaded race fuel or you'll fry some FI sensors.I think the race fuel is masking the hesitation problem.
With a 9.5 to 1 motor,91 octane is sufficient to run the engine well.

There is also a formula to figure out the optimal octane for your combination,as too much octane is a not a good thing either.Then you can mix your gas to the correct level and it will be somewhat cheaper.

First thing, stop using the Octane booster, 98% of the time their a sham product to begin with. Many actually DECREASE performance. The can/bottle may say it will raise the octane number by one. From 92 to 93 octane? NOT! From 92 to 92.1, big freakin' deal.

Many engines respond well to more octane, many don't. The KEY factor is typically when the engine has computer controlled timing. Lower the octane, the computer retards the timing. Engine may run fine but isn't making the power it could/should. Raising the octane increases the timing, result is more horse power. Some engines may be doing a fine job with ignition timing set point all ready. In that case higher octane offers no benefit at all. The computer will not advance the timing any further anyway.

IF your timing is all ready advanced a bit to far for 92 octane all ready, THEN additional octane would make a noticeable difference in power and drivability. Adding more timing is almost always a good thing (up to the max reasonable curve). I suspect your distributor timing curve is bringing in max advance at a lower rpm than is optimum for 92 octane. Therefore, higher octane is more compatible with the timing curve.

If it were me I'd retune for lower octane and the street. I DID have a 12.5 to 1 compression side oiler that ran GREAT with race gas! But it's not reasonable for a primarily driven street car. I retarded the timing, lost a bunch of horse power, ran 92 octane and it was acceptable, barely. Going to the track? I'd bump up the timing, run race gas and say, "WOW, what a difference!" But no way I'm going to fill up all the time at $6.00 a gallon and have to drive to the only dam station in town that sold the stuff!

[quote=csx4910; Instead of filling up with 93 decided to try 104 octane available at the pump at a couple location around town. [/QUOTE]

Time to change your contacts.

That line is what prompted my octane "rant", but I agree, some new glasses are in order regardless. :)

I believe my compression ratio is 10.25:1. (Sandy, or anyone else with the same motor can confirm.) All I know is it runs like crap on 91 compared to even the 100 octane. It is unleaded 100 octane so it sounds like I am relatively safe running that or a combination of the 2 together. Thanks for the info everyone

I'm not sure if you've already done so...but a good dyno test and tune session might be in order. I thought my motor had issues because of gas, it was actually running too rich and the timing was off. I had noticed the same thing using 104 in the tank, but it was just covering up the bigger problems.

I concur, the car should run fine with lower octane if it's setup right. Even though it might make more power with higher octane.