Highest CR on 87 octane
#11
03-06-2013, 08:44 PM
not to contradict the wisdom of an older member(or revive a dead thread), but you can run some pretty high compression on pump gas and be fine, no "crutching"... for example, i am building my 2.3L Ford at 11 to 1 compression and i should be able to run 13-16 degrees of initial timing advance with a stage one cam(on 91 octane gas)... i imagine that you would be able to run at least 10 to 1 on an otherwise stock Saturn without issue... most modern engines run between 9 and 10 to 1 compression stock and run on 87 octane gas all day, and some run up to 11 to 1...
#12
03-07-2013, 06:59 AM
The Basic reason a modern engine can run 87 octane with an 11:1 compression engine is that they are computer controlled using some form of knock or ping sensor to detect the on set of an operational condition that encourages the generation of pinging.
When sensed the computer de-tunes the engine to stop the pinging.
For those engines that are not computer controlled you have two choices.
You can build them with low enough compression to run 87 octane or de-tune them yourself to do so.
There is a third choice also. On more popular engines, read that to be that the aftermarket has recognized that there are people who will spend the bucks to buy high teck state of the art engine components that allow the use of low-test fuels, they are made.
But they also cost and they also have physical limits as to how much they can do in the drive to run 87 octane.
A fuel that will not run in a high compression engine with out de-tuning one. Simply speaking and with out any crutch factor to get things to work.
Compression determines the octane limits of the fuel used.
And if all that is that you are depending on plan limiting your compression to 8.7:1 if you insist on using 87 octane.
The question was what is the limit of compression on 87 octane.
My answer is 8.7:1 and having built engines of 8.5:1, I can assure you that I had to de- tune the thing to get it to run on 87 octane with out pinging.
Frankly I do not spend my money and time to build an engine just so I can figure out how to run the thing on garbage quality gasoline.
I build one to run as hard and fast as it can with a performance level tune. And in general that means it is going to run a premium grade gasoline which in my area is 91 octane so I limit the compression to 9:1 and buy premium. At my house 87 octane is used as weed killer and parts cleaner.
I spent good money a while back to purchase an engine management port fuel injection system that did not have a knock sensor. It came with instructions that rigidly stated USE NOTHING BUT 92 OCTANE with this package.
But it was designed when premium WAS 92 octane.
However I built the engine to the specification of 9.1:1 compression and as such it does indeed run with out pinging. A good thing too as the system purchased does not detect pinging.
I blew up two engines trying to second guess the physics of running garbage gasoline in a high compression engine.
Believe me, it is cheaper to run the gasoline based on the measured compression based on the available gasoline than it is to rebuild them because you ignored physics.
The answer to the question of what compression you can run on 87 octane based on physics is 8.7:1. Past that it depends on how good a tuner you are or how expensive the engine management system is.
AT my house and in my garage the answer is simple.
I don't use 87 octane so I don't worry about it.
When sensed the computer de-tunes the engine to stop the pinging.
For those engines that are not computer controlled you have two choices.
You can build them with low enough compression to run 87 octane or de-tune them yourself to do so.
There is a third choice also. On more popular engines, read that to be that the aftermarket has recognized that there are people who will spend the bucks to buy high teck state of the art engine components that allow the use of low-test fuels, they are made.
But they also cost and they also have physical limits as to how much they can do in the drive to run 87 octane.
A fuel that will not run in a high compression engine with out de-tuning one. Simply speaking and with out any crutch factor to get things to work.
Compression determines the octane limits of the fuel used.
And if all that is that you are depending on plan limiting your compression to 8.7:1 if you insist on using 87 octane.
The question was what is the limit of compression on 87 octane.
My answer is 8.7:1 and having built engines of 8.5:1, I can assure you that I had to de- tune the thing to get it to run on 87 octane with out pinging.
Frankly I do not spend my money and time to build an engine just so I can figure out how to run the thing on garbage quality gasoline.
I build one to run as hard and fast as it can with a performance level tune. And in general that means it is going to run a premium grade gasoline which in my area is 91 octane so I limit the compression to 9:1 and buy premium. At my house 87 octane is used as weed killer and parts cleaner.
I spent good money a while back to purchase an engine management port fuel injection system that did not have a knock sensor. It came with instructions that rigidly stated USE NOTHING BUT 92 OCTANE with this package.
But it was designed when premium WAS 92 octane.
However I built the engine to the specification of 9.1:1 compression and as such it does indeed run with out pinging. A good thing too as the system purchased does not detect pinging.
I blew up two engines trying to second guess the physics of running garbage gasoline in a high compression engine.
Believe me, it is cheaper to run the gasoline based on the measured compression based on the available gasoline than it is to rebuild them because you ignored physics.
The answer to the question of what compression you can run on 87 octane based on physics is 8.7:1. Past that it depends on how good a tuner you are or how expensive the engine management system is.
AT my house and in my garage the answer is simple.
I don't use 87 octane so I don't worry about it.
#13
03-07-2013, 08:10 AM
i only use 87 in my new car and my wife's car, and i will no longer be using it as soon as i can get the desired head work done on my ranger. i was running 93, but that was deemed overkill when it was running the factory carburetor, coil, plugs, and CR(9 to 1). it still runs fine on 87 octane fuel, just costing me less with each fill up than when i was running premium. that being said, some engines are more fragile and can be more easily damaged from pinging than others... those in the dirt track community and those building four cylinder rangers (there are a few of us) tend to agree that the 2.3L ford tends to like higher CR when it's naturally aspirated, and can withstand some small amounts of pinging with no consequence. what is small i don't know, but there is one guy running 11.2CR in his 2.3 mustang and he claims it runs fine on 91 octane gas with 16 deg of initial timing advance... then again, he has the EFI, and i plan on running a 2bbl carb on mine, so i'm not entirely sure if i would surely be able to go that high, but i do plan on aiming for 11.0CR adn if it turns out to be high, i can always run premium and add octane booster...
#14
03-08-2013, 07:43 AM
I am old school both in age and in thought. I own and drive a street racer I built when you could still buy 101 octane leaded gasoline. I also blew it up twice during the transition from 101 octane to the new method of identifying octane rating. All on trying to crutch the thing when using gasoline that no longer performed as it used to. There is as far as I can tell no direct method of comparing the two ratings.
I finally had to drop the compression ratio from the factory rating of 10.5:1 to about 9.5:1. I still had to detune the thing in order to get it to run with out pinging on 91 octane.
I have driven that car back east where 93 and 94 octane is now available and I can assure you that by comparing how that car runs and what I can do to tune it todays 93 or 94 octane performs very close to yesterdays 101 octane.
I am not talking about adding octane boost. I used to buy Moroso 104 by the gallon. I can tune a 9:1 compression car to run like a stripy axx ape. on 91 octane. Or I can tune something with 10:1 + compression to run and not blow pistons into the oil pan but it will run like a dead pig.
I firmly believe that on todays gasoline with out something computer controlled to automatically de-tune itself when ping or detonation is detected that two engine identical except for compression a 9:1 engine tune to run will perform better than a 10.5:1 engine de-tuned to not break on what passes today for premium pump fuel.
Mores Law says if a little is good, more is better.
When it comes to compression, basic science says there are limits to how much compression you can run with out breaking things on a given octane of gasoline and wishful thinking or bragging rights do not change that. Mores Law then does not apply.
I regularly attend cruise nights and listen to people who some how think using 87 octane is a panacea to some form of engine capability. That spending tons of money to build an engine and then de-tuning the to run on garbage quality gasoline some how is impressive. I see and hear these things run and they run like crap.
BUT they use 87 octane.
No Thanks.
Some of the Aluminum Cylinder heads available today for a variety of engines will let you run another point in compression. And compression does develop hp when you can tune for it and use a premium grade gasoline.
Sub standard gasoline did not work when it was leaded and 101 octane and nothing has changed since then. It still does not work.
Technology has developed means to crutch around that for sure, but it is still a crutch.
If you have a given engine and in my opinion I don't care what it is, a 2.3 Ford or a 500 cu in Cadillac, and it has NO MEANS to detect detonation and retard the timing, it can not and will not run on 91 octane with 11.1 compression with out some form of de-tuning.
And I would bet cold hard cash that the same engine with something on the order of 9.5 or less can be tuned to perform better and will out run it. On the same quality of gasoline
And at 20 cents a gallon difference at a current price of $4.00 a gallon the money saved filling the tank with 87 octane will not pay for building a new engine.
The highest compression engine I ever built for street use was 13:1 and if you did not have 101 leaded in the tank you had better have deep pockets to pay for pulling piston parts out of the oil pan.
Frankly my personal belief at this point in time the fact that Detroit, Tokyo and Stuttgart will supply you with a turn key car that can run on 87 octane (at a premium price mind you, keeping in mind that in 1970 the average price of a car was $3542 and the price of gasoline was 36 cents a gallon and nothing was computer controlled and gasoline was leaded and today the average price of a car is about $27,000 and the difference between grades of gasoline then and now is still 10 cents a gallon. Back in the day you needed premium to get the maximum performance from a given engine. That has not changed and is still true.
Except using garbage level gasoline has some how become a badge of honor.
No thanks, personally I think it is stupid.
But I am old school. Take it as you will.
It has been stated initial timig advance is 16 degrees. So what? That means little or nothing unless you have some idea what the advance curve is. The average engine will have a total advance of some where around 36 to 40 degrees advance. With most of the advance dialed in at a fairly low rpm setting with some exceptions, so unless the advance curve for a given ignition is a known, the initial setting is of no value.
It is a bit like asking how high is up?
I finally had to drop the compression ratio from the factory rating of 10.5:1 to about 9.5:1. I still had to detune the thing in order to get it to run with out pinging on 91 octane.
I have driven that car back east where 93 and 94 octane is now available and I can assure you that by comparing how that car runs and what I can do to tune it todays 93 or 94 octane performs very close to yesterdays 101 octane.
I am not talking about adding octane boost. I used to buy Moroso 104 by the gallon. I can tune a 9:1 compression car to run like a stripy axx ape. on 91 octane. Or I can tune something with 10:1 + compression to run and not blow pistons into the oil pan but it will run like a dead pig.
I firmly believe that on todays gasoline with out something computer controlled to automatically de-tune itself when ping or detonation is detected that two engine identical except for compression a 9:1 engine tune to run will perform better than a 10.5:1 engine de-tuned to not break on what passes today for premium pump fuel.
Mores Law says if a little is good, more is better.
When it comes to compression, basic science says there are limits to how much compression you can run with out breaking things on a given octane of gasoline and wishful thinking or bragging rights do not change that. Mores Law then does not apply.
I regularly attend cruise nights and listen to people who some how think using 87 octane is a panacea to some form of engine capability. That spending tons of money to build an engine and then de-tuning the to run on garbage quality gasoline some how is impressive. I see and hear these things run and they run like crap.
BUT they use 87 octane.
No Thanks.
Some of the Aluminum Cylinder heads available today for a variety of engines will let you run another point in compression. And compression does develop hp when you can tune for it and use a premium grade gasoline.
Sub standard gasoline did not work when it was leaded and 101 octane and nothing has changed since then. It still does not work.
Technology has developed means to crutch around that for sure, but it is still a crutch.
If you have a given engine and in my opinion I don't care what it is, a 2.3 Ford or a 500 cu in Cadillac, and it has NO MEANS to detect detonation and retard the timing, it can not and will not run on 91 octane with 11.1 compression with out some form of de-tuning.
And I would bet cold hard cash that the same engine with something on the order of 9.5 or less can be tuned to perform better and will out run it. On the same quality of gasoline
And at 20 cents a gallon difference at a current price of $4.00 a gallon the money saved filling the tank with 87 octane will not pay for building a new engine.
The highest compression engine I ever built for street use was 13:1 and if you did not have 101 leaded in the tank you had better have deep pockets to pay for pulling piston parts out of the oil pan.
Frankly my personal belief at this point in time the fact that Detroit, Tokyo and Stuttgart will supply you with a turn key car that can run on 87 octane (at a premium price mind you, keeping in mind that in 1970 the average price of a car was $3542 and the price of gasoline was 36 cents a gallon and nothing was computer controlled and gasoline was leaded and today the average price of a car is about $27,000 and the difference between grades of gasoline then and now is still 10 cents a gallon. Back in the day you needed premium to get the maximum performance from a given engine. That has not changed and is still true.
Except using garbage level gasoline has some how become a badge of honor.
No thanks, personally I think it is stupid.
But I am old school. Take it as you will.
It has been stated initial timig advance is 16 degrees. So what? That means little or nothing unless you have some idea what the advance curve is. The average engine will have a total advance of some where around 36 to 40 degrees advance. With most of the advance dialed in at a fairly low rpm setting with some exceptions, so unless the advance curve for a given ignition is a known, the initial setting is of no value.
It is a bit like asking how high is up?
Last edited by uncljohn; 03-08-2013 at 07:51 AM.
