The manual says that me '05 5.7 ltr HEMI needs mid-grade unleaded. I accidentally filled up with standard (87 octane vs. 89), and my gas mileage increased dramatically! What kind of problems will I cause if I continue to use it?

Sealoch

 

You will have a nice ping in the engine... The lower grade will detonate under the higher pressure that your engine makes being more performance tuned than other engines. Where the problem is is that your pistons will not reach the top of their stroke before detonation and the spark plugs are not even doing their jobs. When it detonates it is now pushing against the combustion which has more energy pushing back against the piston. I am surprised that you saw an increase in mileage I would think that you would lose mileage but anyway, use what has been recomended if your tank is still full I would suggest going to an auto zone or somewhere similar to get some octane booster to pour in the tank to keep the ping away. No good will come of using lower grade fuel if it has been recommended not to.

 

The higher mileage surprised me, too, that's why I asked the question. Thanks a lot.

Sealoch

 

Not a problem welcome to the forum.

 

Something tells me maybe you got more fuel in the tank before you started running your test. If you don't fill the tank up until the neck is full and you can see fuel sitting there when you do your test you get false results. Also the pitch of your truck when you fill up does a lot so always fill up your ride at the same pump facing the same direction, and fill up all the way like I said before doing any test. As for running the wrong octane get some octane booster like Green said. But if memory serves me right your truck should be set for 87 octane anyway so check your owners manual for which fuel your ride is set to run. Just doesn’t sound right.

P.S. Your ride should know which octane your running and advance your timing or retard to run that fuel. Just don’t use 85 octane.

 

The owner manual recommends 89 octane, but it also states 87 can be run without issues. Just don't go below 87 or over 91 and you won't see any problems.

 

I see where you're coming from, Jason. In the SPECS portion it shows the 89 octane, but in the text it says 87 or 89. It also says to expect a little ping with 87 at low idle, so it should be all right. Of course, when you are driving at higher elevetions, such as in Colorado, they sell their gas at 2 octane numbers lower, so that creates another situation! Guess the best bet is just to rely on good old mechanical skills and listen to the engine, watch the guages, and use your head.

Thanks, eveyone, for your input.

Sealoch

 

The salesman filled my new Hemi with regular and then later told me to use midgrade. WTF? It ran fine on regular, although, being a new engine, I drove it gently. I have 89 in there now, but I might try 87 again after it's got a few miles on it.

 

I woulda smacked him around and told him to give me a new hemi LOL. The first 100 miles are the most important miles in breaking in an engine.

 

Hemis don't have a break in period. They are ready to run off the lot.

Yep up here in elevation, normal fuel grade is 85 octane which they sell at the price of 87 octane is the lower elevations. Unfortunately I have to pay a premium just to get 87 or 89.