this is the wierdest thing

[fatality]

Can anyone explain this situation to me? I've got a 87T, all the standard stuff done to it. K&N intake, no cats, high flow muffler,only 2.5 in pipes new 3 piece metal head gasket. the car is well tuned, and I recently shimmed the waste gate so it boosts like crazy. the thing that I dont understand is, I always put shell supreme gas in my car. I get roughly 15 mpg in the city and about 20 mpg on the highway. recently my brother borrowed my car and actually filled up the gas for me (yeah I couldn't believe my ears either). The thing is he put the 87 octane shell instead of the 93 that I normally put. the car gave me darn near 20 mpg in city driving. so when I went on a trip to Baton Rouge from Houston I thought maybe I should test this and put in regular. the car performed well over 25 mpg for the whole trip. I was under the impression that the higher the octane, the better fuel economy. also the engine is running at a lot lower temperature. can anyone explain what is going on. I just bought the car less than a year ago. maybe the previous owner always put regular gas and the car just got used to it (lol). should I switch to putting regular gas into the car or stick to filling up with supreme.

 

[mazzer]

when octane numbers increase you actually take the burnability out of the fuel

 

[bigaaron]

Can anyone explain this situation to me? I've got a 87T, all the standard stuff done to it. K&N intake, no cats, high flow muffler,only 2.5 in pipes new 3 piece metal head gasket. the car is well tuned, and I recently shimmed the waste gate so it boosts like crazy. the thing that I dont understand is, I always put shell supreme gas in my car. I get roughly 15 mpg in the city and about 20 mpg on the highway. recently my brother borrowed my car and actually filled up the gas for me (yeah I couldn't believe my ears either). The thing is he put the 87 octane shell instead of the 93 that I normally put. the car gave me darn near 20 mpg in city driving. so when I went on a trip to Baton Rouge from Houston I thought maybe I should test this and put in regular. the car performed well over 25 mpg for the whole trip. I was under the impression that the higher the octane, the better fuel economy. also the engine is running at a lot lower temperature. can anyone explain what is going on. I just bought the car less than a year ago. maybe the previous owner always put regular gas and the car just got used to it (lol). should I switch to putting regular gas into the car or stick to filling up with supreme.

It is not a good idea to run regular on a turbo because it could cause pinging or detonation under boost. So yeah everything seems fine until you punch it hard on a hot day and really bad things start happening quick! The only way I could see it affect the gas milage is maybe because the ecu is retarding the timing under boost? I am not so sure those mpg measurements you are making are all that accurate. Something else would have to be going on.

 

[PorterzSupra]

No, the ECU is not retarding on regular gas with 6-10 PSI. It should run fine on reg with stock to slighty over stock boost leveles with that compression. I am running a T66 on a high compression NA block and the ECU is not retarding yet on super!! I even have a knock meter!! Maybe if it was a super hot day u might get SLIGHT KR, on reg if the IC got heat soaked, the ECU would cut back so your not at risk either way. For every degree of KR you only loose 2 HP, so basically you can run reg but mid would be the best for your setup esp during the warmer months.

 

[bigaaron]

I recently shimmed the waste gate so it boosts like crazy

It should run fine on reg with stock to slighty over stock boost leveles with that compression.

I don't think "boosts like crazy" qualifies as "stock to slightly over stock boost levels".

 

[Shawndude]

PorterzSupra, how are you monitoring timing advance on a stock ECU? Knock displays don't tell you the advance, only if it's knocking or not (and even then not all that good).

Unless you drive the same route day in and day out, it's hard to gauge mileage. Not to mention many gas stations are changing from winter to summer formulations now, and that will throw everything out the window as well.

But as said, higher octane does only one thing, and one thing only....prevent knock in engines that are heavily loaded, or designed on the limit of knock. Not sure where people got all the magic power idea of high octane. It has less energy in it than regular stuff, and should give you very slightly less mileage but nothing like what is being claimed here.

Maybe he added air to your tires, that would do it. :icon_razz

If you're going to beat on the car and it's warm outside, put in high octane for an extra margin of safety. If you're commuting and want to save some cash, put in regular.

 

[IJ.]

Another in a long line of classic Porter Gump posts !

 

[johnathan1]

so running 91 rated gas all the time will give you a good margin of safety all the time? No matter the outside temp or driving habits?

 

[bigaaron]

so running 91 rated gas all the time will give you a good margin of safety all the time? No matter the outside temp or driving habits?

GIVE THIS MAN A PRIZE for the best possible comment in this thread!

 

[johnathan1]

haha^^^ Wasn't expecting that, but yea...i try...

ps: Yes, I could definitely use some prizes!!:biglaugh:

 

[PorterzSupra]

"PorterzSupra, how are you monitoring timing advance on a stock ECU? Knock displays don't tell you the advance, only if it's knocking or not (and even then not all that good)."

My knock meter does not only display knock, it's a meter so it will display any pre knock sounds or if there is no sounds at all, the computer does not change timing if its not getting any voltage back, I am not getting any noise, there is Ready (ON) , Normal (NORMAL ENGINE NOISE) , Caution (SLIGHTY PREKNOCK), Danger(SOME KNOCK VOLTAGE, possible KR), KNOCK (POSTIVE KNOCK, computer KR)

 

[fatality]

Well my idea of boosts like crazy might not be what most people consider alot of boost. around 15psi is good enough for me, I'm not really looking to wrap my car around a traffic light pole or anything. so the consensus of everyone so far is to use plus or regular, dont go with the supreme?

 

[Supracentral]

I wouldn't put too much weight in your mileage numbers. Here's an example. My daily driver MKIV (the one on the right in my sig - stock excpet for the suspension and tires) gets around 20 mpg here in Atlanta. Atlanta is lots of highways, I rarely spend time on a surface street except for the mile or so from my house to the freeway. I always assumed I had a good feel for the cars cruising range on a tank of fuel, which I thought was around 300 miles.

This year I drove that car to TX2K6 since the other cars are down. Wow.. I thought I knew it's mileage. As I was driving, looking at the gas gauge I started worrying when the trip odometer passed 325 and I still had over 1/4 of a tank... Got really freaked when we passed 375 miles and it wasn't empty yet.

In the end the car averaged 28 MPG from here to TX on the highway and does over 400 miles on a tank of fuel.

In town in Houston it wend back down to it's predictable 20 - 21.

Now my point is I *thought* I knew that cars mileage. So if, for example, I had decided to try Jim Bob's Bullshit Fuel Additive (with Slickron) on the long trip, by time I got back I would have been calling this stuff amazing. I'd be telling people Jim Bob's Bullshit gave me 100 miles more per tank of fuel! Woo Hoo! And I'd be 100% full of shit and not even know it...

My point is, before making any assumptions like this, you need to run a test. A controlled test. With all variables considered.

So put together a controlled test and test the assumptions again.

As for running cheap gas, *I* wouldn't.

 

[bigaaron]

Well my idea of boosts like crazy might not be what most people consider alot of boost. around 15psi is good enough for me...

15psi on regular 87 octane gas?!? I will say a short prayer for your engine tonight.