Guest Thurem Posted May 4, 2003 Share Posted May 4, 2003 In my quest to plan my motorbuild, I have had the goal all along to be able to run it on the shittiest gas available, ie. 87 octane. But tonight I tried to figure the actual savings by using the cheaper gas, which is typically 20 cents cheaper a gallon. over the life of the motor, 100000 miles at 20 miles per gallon, the savings come to 1000 bucks figured per mile 1 cent. So now I'm like is it really worth it. My plan involved tight quench, polished pistontops and chambers with 9.3:1 compression plus some overlap to bleed off pressure at low RPM's. So my question is how far should I go with 89 or 91 octane gas. Set in stone are my Vortec cast iron heads, that are sitting in the closet. What gains should I expect with higher compression in a 400 hp motor and how far should I go, and maybe even get a comfortable margin for detonation. I also plan to use 6" rods cause the larger rod to stroke ratio seems to be advantagous to counter detonation. Please discuss, and let me know if I overlooked anything. Thure Quote Link to comment Share on other sites More sharing options...
Mudge Posted May 4, 2003 Share Posted May 4, 2003 I'd save $12 a month between 87 and 91 (California), if I were getting 20 MPG @ 300 miles a week. So for myself, its not worth it, I'd rather have what horsepower, compression and timing I can, on an older perhaps milder SBC, then no biggy if it runs fine with a little timing pulled. Quote Link to comment Share on other sites More sharing options...
Guest 1320the260 Posted May 4, 2003 Share Posted May 4, 2003 just run premium. its a race car not a honda civic. the cool thing about lower compression tho is that it will be very boost friendly later on. Quote Link to comment Share on other sites More sharing options...
Guest Aaron Posted May 5, 2003 Share Posted May 5, 2003 I actually did a paper once on what is exactly the "cheapest" gas. I found that is varies greatly based on the car. Lower octane gets less mileage than higher octane, but higher octane obviously costs more. I ran three tanks of each octane grade and calculated the average mileage for each grade. I then took the gas prices at the time I completed the test and divided the miles per gallon by the price. The result is miles per dollar. On my '93 Mazda B2200 and my wifes '90 Pontiac Sunbird, I found the highest MPD with 89 Octane. On my '98 Frontier and my wife's '96 Pathfinder, we get the highes MPD on good old '87 Octane. If the cheapest gas is your quest, make sure you do this test. Quote Link to comment Share on other sites More sharing options...
Guest Your Car is Slow Posted May 5, 2003 Share Posted May 5, 2003 Lower octane gets less mileage than higher octane This is just a silly comment. Perhaps its true for YOUR car....as it probably wasnt designed to run on 87 octane and the computer takes timing out upon sensing detonation thus reducing power...and costing you MPG. However cars that are designed to run on 87 octane...will experience no milage increase when switching to a higher octane fuel. Quote Link to comment Share on other sites More sharing options...
Pop N Wood Posted May 5, 2003 Share Posted May 5, 2003 There is a difference between octane and energy content. Methanol has a very high octane rating but (I think) only something like 3/5 the energy content. Thus you need to burn 5 gallons of Methanol to release the same amount of energy as 3 gallons of gasoline. Or conversely your tank need to be 2/3 larger to get the same number of miles between fill ups. That is why I don't like ethanol blend fuels. They don't have the energy content per gallon of gas, so gas mileage always drops (noticeably too). What I don't know if there is any difference in energy content between regular and premium. In the old leaded gas days I would have said no. But since premium gas is more highly refined regular gas, then I don't know. If there is any difference, I would think it would be very small. The statement that a car designed to run on regular will not get better mileage using premium is true. But the reverse isn't true. If a modern, computer controlled car is built and tuned for premium, then switching to regular could cause a loss in efficiency when the computer retards the timing to compensate for the bad gas. But getting back to the original question. For a car that is not a daily driver, I say build for HP and count on premium gas to do it. BTW, If you get 100,000 miles out of a performance engine, then you are probably not driving it the way it was meant to be driven. Quote Link to comment Share on other sites More sharing options...
Mudge Posted May 5, 2003 Share Posted May 5, 2003 Yep, MTBE for example raises octane but kills approximately 1-3% power versus "normal" gas. Alcohol also has high octane, but low energy content and runs cooler, and is "corrosive" if you dont run the engine often (some people run oil with the fuel). I have a stoichiometry table of various fuels somewhere if someone is curious. Quote Link to comment Share on other sites More sharing options...
Guest benjlv Posted May 6, 2003 Share Posted May 6, 2003 hot rod magazine ran a article some time back that was title "the motor chevy should ahve made" or something like that. It made a little over 400hp on 87octane. They did it by increasing the storke and some head work. But i would think 400hp would be plenty for a daily driver. Quote Link to comment Share on other sites More sharing options...
Mudge Posted May 6, 2003 Share Posted May 6, 2003 I have that article I believe, still not my cup of tea though. I just dont see a big deal about $0.20 difference, its the total that ticks me off Quote Link to comment Share on other sites More sharing options...
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