I put 22 gallons of gas in my truck last night....

[superpug1]

The higher the octane rating the higher the flash point of the gas. So for an engine tuned for 87 running on 91 the fuel wont burn as efficiently because the engine doesn't run hot enough

[penguindude132]

$2.15 east of mesa arizona

[AKHog]

Many turbocharged cars will run higher compression ratios and require premium. Also, many European models require it as in Europe - what qualifies for 'premium' in the US is equivalent to the lowest octane fuel generally available (91 R+M/2 - 95 RON).

My wife's Volkswagen has a 2.0 Turbo I4 used by many Volkswagen / Audi models. It requires premium.

I'm curious as to why mileage will suffer in a car tuned for regular, but running premium? I agree there's not necessarily going to be any benefit, but I can't see why there would be any detriment either. I would think it goes like this:

Two cars - 1 tuned for 87, the other tuned for 91 are filled up with regular. The engine management module in the 87 car detects no knocking so runs as normal and produces its maximum power. The 91 car detects some knocking so retards the ignition a bit to compensate and suffers a loss of maximum power.

Next, both cars are filled with premium. The engine management module in the 87 car detects no knocking so runs as normal. The engine management module in the 91 car also detects no knocking and so runs as normal and develops its maximum power.

So while there's no advantage to using premium in the 87 car, I don't see where the disadvantage comes from (other than wasted money). That said, the holes in my mechanical knowledge have been cruelly exposed on this board before.  Big Grin

The higher the octane rating the higher the flash point of the gas. So for an engine tuned for 87 running on 91 the fuel wont burn as efficiently because the engine doesn't run hot enough

Its actually more about the compression. The engine tuned for 87 simply does not have enough compression to burn all of the high octane fuel, therefor some of the fuel is being wasted. I think in many situations this would result in less fuel economy, less performance, or both.

Wooley, as far as I can tell the cars you mentioned have high compression motors (anything over about a 10 to 1 is high). In which case I would assume high octane is recommended by the manufacture.

[CAP1]

Many turbocharged cars will run higher compression ratios and require premium. Also, many European models require it as in Europe - what qualifies for 'premium' in the US is equivalent to the lowest octane fuel generally available (91 R+M/2 - 95 RON).not higher compression ratios.......but rather higher compression  pressures. there is a difference/.

My wife's Volkswagen has a 2.0 Turbo I4 used by many Volkswagen / Audi models. It requires premium.

I'm curious as to why mileage will suffer in a car tuned for regular, but running premium? I agree there's not necessarily going to be any benefit, but I can't see why there would be any detriment either. I would think it goes like this:it won't actually suffer per se, but it will not benifit from the higher octane rating.
Two cars - 1 tuned for 87, the other tuned for 91 are filled up with regular. The engine management module in the 87 car detects no knocking so runs as normal and produces its maximum power. The 91 car detects some knocking so retards the ignition a bit to compensate and suffers a loss of maximum power.

Next, both cars are filled with premium. The engine management module in the 87 car detects no knocking so runs as normal. The engine management module in the 91 car also detects no knocking and so runs as normal and develops its maximum power.

So while there's no advantage to using premium in the 87 car, I don't see where the disadvantage comes from (other than wasted money). That said, the holes in my mechanical knowledge have been cruelly exposed on this board before. 

[CAP1]

Its actually more about the compression. The engine tuned for 87 simply does not have enough compression to burn all of the high octane fuel, therefor some of the fuel is being wasted. I think in many situations this would result in less fuel economy, less performance, or both.

Wooley, as far as I can tell the cars you mentioned have high compression motors (anything over about a 10 to 1 is high). In which case I would assume high octane is recommended by the manufacture.

turbocharged cars run LOW compression ratios. this is due to the boost of the turbo.  taqke a car with 10.5 to 1 comprsession ratio, and boost it. most likely, you'll pop the head gasket. in extreme cases, even stretch the head bolts enough to literall lift the head off the block.
 run 8 to 1, and boost it. now you're running a higher compression pressure, but not blowing the gaskets out.
 compression pressure is determined by the cam timing, lift, and any power adders.
 compression ratio is determined by the size and shape of the combustion chamber, and the size and shape of the piston top.

[AKHog]

turbocharged cars run LOW compression ratios. this is due to the boost of the turbo.  taqke a car with 10.5 to 1 comprsession ratio, and boost it. most likely, you'll pop the head gasket. in extreme cases, even stretch the head bolts enough to literall lift the head off the block.
 run 8 to 1, and boost it. now you're running a higher compression pressure, but not blowing the gaskets out.
 compression pressure is determined by the cam timing, lift, and any power adders.
 compression ratio is determined by the size and shape of the combustion chamber, and the size and shape of the piston top.

One correction, compression ratio is also determined by cam timing and lift. In other words if the piston moves further up in the cylinder, you'll have a higher compression ratio.

Yes I left out the part about turbos and supers for simplicity, although once everything is said and done it is still the final compression (however you may achieve that) that determines what octane gas will work best.

[CAP1]

One correction, compression ratio is also determined by cam timing and lift. In other words if the piston moves further up in the cylinder, you'll have a higher compression ratio.

Yes I left out the part about turbos and supers for simplicity, although once everything is said and done it is still the final compression (however you may achieve that) that determines what octane gas will work best.

nonono../....

cam timeing and lift will change your compression pressure only.

 you can take a 10 to 1 engine, and change the pressure according to the cam you put in. the only way to change the compression ratio is by changing the pistons, or the cylinder heads.

 i put an X303 cam in my 5.0L. with all of that duration, and added lift, my compression ratio remained the same(9.5 to 1 i think), although my compression pressure changed drastically over the stock cam.

and, yes.....that final statement is definitly true......

[AKHog]

In a normal piston engine the compression ratio is the ratio between the volume of the combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

If you install a cam with more lift, the piston will move further on each stroke, and the volume of the combustion chamber when the piston is at the top of the stroke will be less, while the volume of the combustion chamber when the piston is at the bottom stays the same, resulting in an increased compression ratio.

If you say your compression pressure increased, then in effect so did your compression ratio since it is only a measure of the difference between the uncompressed and compressed combustion chamber.

Correct me if I'm wrong but I don't think I am on this one.

[edog1977]

In a normal piston engine the compression ratio is the ratio between the volume of the combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

If you install a cam with more lift, the piston will move further on each stroke, and the volume of the combustion chamber when the piston is at the top of the stroke will be less, while the volume of the combustion chamber when the piston is at the bottom stays the same, resulting in an increased compression ratio.

If you say your compression pressure increased, then in effect so did your compression ratio since it is only a measure of the difference between the uncompressed and compressed combustion chamber.

Correct me if I'm wrong but I don't think I am on this one.

Cams adjust valve timing, not the distance the piston travels per stroke.

[AKHog]

Bahh sorry I was using cams in the place of crankshafts. DUH! Sorry I haven't even thought about this stuff in years.

So if you change the CRANKSHAFT to increase the piston travel it will increase compression ratio. Everything else I agree on.

[edog1977]

CAP1 already stated how to change the compression ratio, change the pistons or change the heads.

[AKHog]

CAP1 already stated how to change the compression ratio, change the pistons or change the heads.

OR by changing the crankshaft, which was what I was originally trying to say (but with the wrong terminology). "Stroker" motors have different crankshafts which move the piston further, essentially increasing displacement but also increasing compression ratio.

[CAP1]

In a normal piston engine the compression ratio is the ratio between the volume of the combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

If you install a cam with more lift, the piston will move further on each stroke, and the volume of the combustion chamber when the piston is at the top of the stroke will be less, while the volume of the combustion chamber when the piston is at the bottom stays the same, resulting in an increased compression ratio. negative. the piston only travels the same distance, regardless of camshaft. the stroke on a 302(5.0L ford) is 3.00 inches, with a 4 inch bore. the old 289 ford engines(predecessor to the 302)was 2.87 inch stroke with 4 inch bore.  the camshft cannot change this. the camshaft controls when, for how long, and how much the valves open. nothing more. this is how the camshaft changes compression pressure too. different cam profiles open at different places inrelationship to TDC.
 1985 and Later 302 Roller Cam 1.60 5° BTC 39° ABC 49° BBC 5° BTC 286° 286° .339 I .542 I 107° 117°
224° 224° .339 E .542 E


these are X303 cam specs. you can see a ton of duration, and overlap. this'll hurt combustion pressure a bit at idle, but once ya wind her up............
 

If you say your compression pressure increased, then in effect so did your compression ratio since it is only a measure of the difference between the uncompressed and compressed combustion chamber.

Correct me if I'm wrong but I don't think I am on this one.

In a piston engine it is the ratio between the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, and the volume of the combustion chamber when the piston is at the top of its stroke.

now, so ya don't misunderstand me, i'm NOT trying to berate you in anyway.........just having a fun discussion with ya sir!!

i love this crap.....could talk bout it all day!
<<S>>

[edog1977]

Hog, I knew what you were trying to say. My last comment came out wrong, please ignore it.

[CAP1]

OR by changing the crankshaft, which was what I was originally trying to say (but with the wrong terminology). "Stroker" motors have different crankshafts which move the piston further, essentially increasing displacement but also increasing compression ratio.

BUT.......

you can change the crank, and not affect the compression either. it also can be determined by the pin depth in the piston, the klength of the connecting rod, the stroke of the crank throws too.  

there was some weirdness with the late 60's ford 302's. some had shorter connecting rods than others did, but they had the same compression ratio. i'll try to find that one.....