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MPG vs. CR vs. Premium Gas $, Lou?
https://www.slantsix.org/forum/viewtopic.php?t=23666		 Page 1 of 1
Author:  Slanted Opinion [ Tue Jun 19, 2007 6:19 pm ]
Post subject:  MPG vs. CR vs. Premium Gas $, Lou?
I recently scanned through a post where someone (I think it was Dart270) mentioned that he built all of his engines for premium fuel.

I've also archived an old post of Lou's from s6.com with a mention of some pretty good mileage numbers given the ET the Dart was turning.

My question: Is it worthwhile to build a car that demands premium (say 10-10.5 compression ratio) with the cost of premium gas today? In other words, does the efficiency of the high compression engine offset the extra cost of premium fuel? (Assuming that the camshaft, etc is chosen for "performance street" driving).

Thoughts on this???

-Mac
Author:  knuckledown [ Tue Jun 19, 2007 6:24 pm ]
Post subject: 
In my opinion, it all depends on what YOU want to achieve with your ride. Do YOU want a high compression car? Is it your daily driver? Are you willing to shell out the cash for premium fuel?

If so, great. If not...

You get the point.
Author:  SlantFred [ Tue Jun 19, 2007 6:48 pm ]
Post subject: 
I did the exercise with my 91 camry. Since it have a knock sensor, it advance the timing with the use of premium fuel (advance the timing untils its pinging than retars it a bit with the computer). I got around 6 to 8% better mileage with the use of premium. In my area , premium is an extra 8 cents a liter, if the price 90 cents and up a liter, and even more if the price goes up, since premium is a flat rate and mileage is a percentage.
Author:  dakight [ Tue Jun 19, 2007 7:27 pm ]
Post subject: 
Gas is is around 2.90 a gallon with a 10 cent step between grades. That means from 87 octane regular to 91 octane premium you pay about 7% more. If your gas mileage can increase by 7% you break even assuming you don't have to much fun with the new-found power increase. My experience has been that few people get better mileage from a performance engine, if for no other reason, because they just can't keep their right foot out of it.
Author:  Aggressive Ted [ Tue Jun 19, 2007 10:35 pm ]
Post subject: 
I can run either 89 or 92 octane Cenex gas with not alot of change with my engine. However, on 96 Cenex it is a blast and that is the problem. It is too tempting to get the posi locked and light both tires at least once a day. So 89 in a 9.5 compression ratio engine is fine, no ping.
Author:  oldgoat83 [ Wed Jun 20, 2007 2:28 pm ]
Post subject: 
I've been habitually tracking my mileage in my 94 Honda Accord (vtec) since I got the speedometer fixed. I've got not quite two years worth of data. I have only ever ran regular in it. With the change in weather, fuel emissions additives the state puts in during the winter and my ever changing driving habits, my economy varies between 26 mpg and 33 mpg. Due to the price of gas I have slowed down and reduced acceleration. My last tank of gas got me 32.9 mpg over 493.9 miles. I mentioned to the guy I bought the car from how far I had gotten, to which he suggested running premium to get over that 500 mile mark. I can and have gone back to calculate the cost per mile. I plan on my next tank filling with premium to see what I get. As was mentioned, if the economy improves to the point that the cost per mile is decreased, I may stick with it. I haven't crunched any numbers as to what I need to achieve yet, but I've got the data at least. And it won't be a psychological improvement that Dan would be sure to mention. :P
Author:  Dart270 [ Thu Jun 21, 2007 4:50 am ]
Post subject: 
I'm not sure it is worth designing for mileage based on fuel choice. As Dave pointed out above, the difference between reg and prem is only around 20 cents a gallon, or 7%. To me, the fun factor of having a quick motor is worth this any day. Let's say you drive 20000 miles in this car at 20 MPG (not hard), that's about $3000 of regular vs. $3200 of premium gas - not much money compared to other investments in the car.

You should see a gain in mileage with a higher comp motor, BUT it depends on so many factors it's difficult to design for ahead of time. I can tell you that 22-26 MPG with a big cam and high comp is a reality!

Lou
Author:  Slanted Opinion [ Thu Jun 21, 2007 2:30 pm ]
Post subject: 
Hey Lou,

Do you define "high compression" as 10:1? 10.5 to 1?

-Mac
Author:  oldgoat83 [ Thu Jun 21, 2007 2:37 pm ]
Post subject: 
Data log for my Honda in case anyone is interested.
I have only about four data points for my truck, the last being from September of last year when I last filled up.
Author:  emsvitil [ Thu Jun 21, 2007 3:03 pm ]
Post subject: 
Quote:
Data log for my Honda in case anyone is interested.
I have only about four data points for my truck, the last being from September of last year when I last filled up.

Looks like my dad's spreadsheet..........


:wink:
Author:  Dart270 [ Fri Jun 22, 2007 8:20 pm ]
Post subject: 
For pump gas, I define anything over 10:1 as high comp. 10:1 is a pretty solid jump up from stock 7.5 or 8:1 in terms of performance and efficiency. 11:1 is getting up there for pump premuim, but is doable.

Lou
Author:  Slanted Opinion [ Sat Jun 23, 2007 3:50 am ]
Post subject: 
Well, that makes sense. Digging into the engine spec books of the 1960s reveals that most performance engines came with relatively high compression right from the factory. 10.5 to 1 was a common number for factory street performance.

It seems that only the slant six got left behind in the HP race. :(


-Mac
Author:  dakight [ Sat Jun 23, 2007 5:13 am ]
Post subject: 
The slant didn't get left behind; it just was never viewed or built to be a performance engine. It was granny's grocery getter and was treated as such. The original Hyperpack and then later the 2-barrel Super-Six option were about the only factory nods to higher performance. The emphasis was always on reliability and economy. It took a few hot-rodders to realize the little mill's capability
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