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Old 08-18-2006, 10:23 AM   #1
linsavy
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Default Hotter plugs for improved mileage?

Gas around here is consistently over $1/litre so improvements on mileage are always welcome.

So a long time ago I read a thread on Edmunds about a guy with an Outback 2.5l Phase I. He claimed to get way better mileage like 15% or more, from changing the spark plugs to one step hotter and running premium gas (91?) to avoid pinging. You would need at least a 7% mileage increase to cover the higher gas price. This seems somewhat incredible to me.

Do any of you gurus believe in this theory?
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Old 08-18-2006, 10:30 AM   #2
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wouldn't he be paying the same amount for fuel by using 91 for saving fuel as much as paying for 87 and getting the gas mileage??
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Old 08-18-2006, 10:42 AM   #3
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If that was really possible, wouldn't the OEM do it in the first place?
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Old 08-18-2006, 11:40 AM   #4
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Quote:
Originally Posted by Master2192
If that was really possible, wouldn't the OEM do it in the first place?

Yes that is what I thought too, but there is a heavy marketing stigma associated with requiring premium gas. Many Joe Average types shy away from that.
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Old 08-18-2006, 01:55 PM   #5
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So who makes hotter plugs for the ej2.5, I would be willing to give them a try if there arent any down sides to using hotter plugs, they are a cheap upgrade if it works
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Old 08-18-2006, 02:01 PM   #6
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Only use hotter plugs if you plan to step up to higher grade fuel, as they can quite dramatically increase your engine's susceptibility to pinging/knock. Safest way to find out for sure would be getting a knock gauge or some scanning equipment into the car to monitor knock. It's possible you could safely run 87 octane with a step hotter plugs, but the risks are greater.

EDIT: And anybody that makes stock-replacement plugs for the EJ25 will make different heat ranges. In the case of NGK plugs, the higher the number, the colder the plug (lower number for hotter). Stock plugs are NGK FR5AP-11 (BKR5E-11 also works just peachy), the heat range on NGK's scale is 5, so 4 would be hotter (BKR4E-11), 6 colder (BKR6E-11, what I'm using) than stock. One thing that's important to point out, BKR6E-11 plug, which is one step colder than stock for 03+ n/a EJ25's, was spec'ed as the factory plug on the SOHC EJ25 up until the 02 model year, so really Subaru has already upped the temp on the plugs for better gas mileage under 87 octane. Another step hotter might not work. Likewise if you race/autox your car, BKR6E-11 will up your car's knock resistance in those high-demand situations with very minimal gas mileage hit....I get better gas mileage on new BKR6E-11's than I did on the stock, well-worn FR5AP-11's.

NOTE: Heat ranges ARE NOT universal between manufacturers. Everybody has their own way of marking their plugs.

Last edited by FalconRS; 08-18-2006 at 02:09 PM.
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Old 08-18-2006, 02:05 PM   #7
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If I can get 15% better gas mileage, its worth it to me to constantly use 91 grade, I use it often enough anyways.
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Old 08-18-2006, 02:11 PM   #8
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Quote:
Originally Posted by BURTONRIDR
If I can get 15% better gas mileage, its worth it to me to constantly use 91 grade, I use it often enough anyways.
Actually, using 91 octane on stock plugs will degrade gas mileage and HURT performance, as you'll start to get incomplete burn if combustion chamber temperatures don't reach the optimal level. You need to know how plugs work and what they actually do before you go messing around with heat ranges. If you were to go and now advance your timing to take advantage of the 91 octane, yep, you'd see more power and better mileage.

EDIT: That's what this whole thread is about, upping your plug temperature to make use of the additional knock resistance 91 octane will afford you, but still getting complete burn. If you don't do other things to compensate for the change in octane, you're essentially throwing money away.
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Old 08-18-2006, 02:20 PM   #9
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Quote:
Originally Posted by FalconRS
Actually, using 91 octane on stock plugs will degrade gas mileage and HURT performance, as you'll start to get incomplete burn if combustion chamber temperatures don't reach the optimal level. You need to know how plugs work and what they actually do before you go messing around with heat ranges. If you were to go and now advance your timing to take advantage of the 91 octane, yep, you'd see more power and better mileage.

EDIT: That's what this whole thread is about, upping your plug temperature to make use of the additional knock resistance 91 octane will afford you, but still getting complete burn. If you don't do other things to compensate for the change in octane, you're essentially throwing money away.
Good point

Incase anyone wanted to know what the cost savings are if you were to gain a 15% increase in MPG, assuming you get 25 mpg before and 28.75 after, and assuming gas prices are $3.50 per gallon for 91 oct, and $3.40 per gallon for 87 oct. You would be saving about $60 every 3000 mi. or every oil change.
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Old 08-18-2006, 04:16 PM   #10
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There's something else at play if someone claims they got a 15% improvement in mileage simply by changing plugs. Did the guy on Edmunds test using new regular heat range plugs before trying the new hotter heat range plugs? If not, then some (all?) of his mileage improvement may have been simply due to replacing old plugs with new plugs.

A 15% improvement in gas mileage is pretty drastic, I just find it hard to believe that changing plugs will net you anywhere near that.
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Old 08-18-2006, 04:36 PM   #11
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I'd tend to agree. Hotter plugs + higher octane could produce improvement, but to see improvement that drastic, I'd tend to think the plugs he replaced were pretty worn. I replaced my stockers with colder plugs, which generate worse mileage in general in normal cruising situations, but my gas mileage went up pretty significantly, to the tune of probably 60-70 highway miles to a tank, but my old plugs were 10,000 miles overdue for a change, so that's to be expected.

I should also note, I'm using the colder plugs for summer autox season, and going back to range-5 NGK plugs for the winter.
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Old 08-18-2006, 04:59 PM   #12
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I would like to point out that the DOHC Ej25's used a stock plug that was one heat range hotter than the SOHC Ej25's initially used. I see ZERO reason not to go one step hotter and not change the octane rating of the fuel you use.

As has been discussed time and time again, the ECU can't appropriately compensate for the needed timing advance to run 91 octane fuel and completely burn it at stock compression ratios. Don't mess with the fuel, just the plugs.

I won't even get into what plugs I personally run because not only is my engine heavily modified, but I am at 5000 ft, which is like having extra octane points thrown in, so it wouldn't be a very easy comparison...
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Old 08-18-2006, 05:30 PM   #13
G.Subramaniam
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I use 93 Octane mostly

What are stock and 1 step hotter spark plugs for
1. 1995 Legacy 2.2L SOHC
2. 1997 Impreza 2.2L SOHC
3. 2000 Outback 2.5L SOHC

Can the +1 hotter plugs be used for 11:1 CR and heads ported and polished with 93 Octane
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Old 08-18-2006, 08:01 PM   #14
Kavoom
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I heard the NGC plugs were plenty hot already.
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