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  #1  
Old April 9th, 2019, 16:35
Black1990jeep Black1990jeep is offline
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Octane and power

I have Renix 4.0 inline six motor in my 1990 XJ. It has a knock sender. Is the timing advanced such that use of 91 octane verses 87 octane fuel makes a difference as far as maximum advance is concerned, or is the the system fully advanced already using the 87 octane?

On last road trip switched from 91 to 87 verses last road trip. Subjectively it seemed I might have had less power, but it is a very subjective measure, different routes even.

Anyways, does the ecu max out the advance using 87 octane under normal conditions, or would 91 octane allow the ecu to give more advance? If no, then I will stick with 87 octane, california gas is too expensive, paid over five bucks at one station, for 87 octane!

Thanks
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  #2  
Old April 9th, 2019, 17:23
lawsoncl lawsoncl is offline
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Re: Octane and power

As far as I know, the knock sensor only backs off the timing if it hears pinging. It doesn't proactively advance the timing. Stick with the 87 octane and save your money.
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  #3  
Old April 9th, 2019, 19:39
JeepNoob JeepNoob is offline
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Re: Octane and power

From what I've read, 4.0's kinda run the ragged edge of what's possible for compression ratios in an engine with a cast-iron head running on 87-octane pump gas. With that being said, I know of at least 1 TSB floating out there advising NOT to use premium/91-octane fuel in them (believe it was for the H.O. models though.)

With something as old as a Renix, I would be more concerned about carbon build-up on the pistons. This can raise your effective compression ratio (which can cause pre-combustion if the engine isn't properly designed for it) and also absorbs heat (which again, can cause pre-combustion.) If that's the sort of thing you're worried about you can try running a can of Seafoam with the little straw through the engine. An old salty mechanic if you don't want to shell out $13 for the Seafoam stuff is to spary or even slowly pour some water into the intake manifold to basically steam-clean the pistons.

I do NOT, NOT, NOT, NOT!!! recomend that last method if you don't know what you're doing. Too easy to hydrolock an engine. If you like, I can send you some videos via PM on how to do that sort of thing, 'cause I sure ain't posting them here for every Tom, Dick, and Harry...
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Old April 9th, 2019, 21:34
Heavyopp Heavyopp is offline
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Re: Octane and power

Quote:
Originally Posted by JeepNoob View Post
An old salty mechanic if you don't want to shell out $13 for the Seafoam stuff is to spary or even slowly pour some water into the intake manifold to basically steam-clean the pistons.

I do NOT, NOT, NOT, NOT!!! recomend that last method if you don't know what you're doing. Too easy to hydrolock an engine. If you like, I can send you some videos via PM on how to do that sort of thing, 'cause I sure ain't posting them here for every Tom, Dick, and Harry...

I am planning to do this very soon -- had my intake, exhaust manifolds off -- lots of carbon build up on the top of the valves and in the intake -- I physically removed what I could but figured a slow mist of water thru the air intake would help out with the rest

It's a new to me 96 XJ -- guy had a 160* thermostat in it -- I figure all the carbon is because it was running so cold all the time


I was planning to use a handheld sprayer with the nozzle adjusted to mist and just spray a mist of water over the top of the throttle body and let it suck in -- also I will remove the cat while doing this

Sound about right???
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  #5  
Old April 10th, 2019, 05:33
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cruiser54 cruiser54 is offline
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Re: Octane and power

Quote:
Originally Posted by JeepNoob View Post
From what I've read, 4.0's kinda run the ragged edge of what's possible for compression ratios in an engine with a cast-iron head running on 87-octane pump gas. With that being said, I know of at least 1 TSB floating out there advising NOT to use premium/91-octane fuel in them (believe it was for the H.O. models though.)

With something as old as a Renix, I would be more concerned about carbon build-up on the pistons. This can raise your effective compression ratio (which can cause pre-combustion if the engine isn't properly designed for it) and also absorbs heat (which again, can cause pre-combustion.) If that's the sort of thing you're worried about you can try running a can of Seafoam with the little straw through the engine. An old salty mechanic if you don't want to shell out $13 for the Seafoam stuff is to spary or even slowly pour some water into the intake manifold to basically steam-clean the pistons.

I do NOT, NOT, NOT, NOT!!! recomend that last method if you don't know what you're doing. Too easy to hydrolock an engine. If you like, I can send you some videos via PM on how to do that sort of thing, 'cause I sure ain't posting them here for every Tom, Dick, and Harry...

Funny!! My recommendation also.
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  #6  
Old April 10th, 2019, 07:18
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Tim_MN Tim_MN is offline
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Re: Octane and power

There is no correlation between octane and power.

Octane prevents the air-fuel mixture from igniting before the spark plug ignites it. Firing the air-fuel mixture at the proper time gives you the maximum power your engine was designed to get or capable of achieving.. Using higher-octane gasoline than your engine is designed to utilize is only wasting money.
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  #7  
Old April 10th, 2019, 15:25
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cal cal is offline
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Re: Octane and power

Quote:
Originally Posted by JeepNoob View Post
From what I've read, 4.0's kinda run the ragged edge of what's possible for compression ratios in an engine with a cast-iron head running on 87-octane pump gas.
I don't know where you heard this but its absolutely incorrect. In fact 4.0's are fairly low compression.

They do have really lousy quench, however, that leads to pinging.
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  #8  
Old April 10th, 2019, 16:12
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8Mud 8Mud is offline
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Re: Octane and power

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Originally Posted by cal View Post
I don't know where you heard this but its absolutely incorrect. In fact 4.0's are fairly low compression.

They do have really lousy quench, however, that leads to pinging.

I think he meant air to fuel ratio.
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  #9  
Old April 10th, 2019, 16:16
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cal cal is offline
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Re: Octane and power

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Originally Posted by 8Mud View Post
I think he meant air to fuel ratio.
No more true than compression.

They have shitty quench. Fix that and you have a lot of potential to work with.
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  #10  
Old April 10th, 2019, 17:46
JeepNoob JeepNoob is offline
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Re: Octane and power

Quote:
Originally Posted by cal View Post
I don't know where you heard this but its absolutely incorrect. In fact 4.0's are fairly low compression.

They do have really lousy quench, however, that leads to pinging.
http://www.huntsmachine.com/383ss01p3.html
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  #11  
Old April 10th, 2019, 17:54
JeepNoob JeepNoob is offline
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Re: Octane and power

Quote:
Originally Posted by Heavyopp View Post
I am planning to do this very soon -- had my intake, exhaust manifolds off -- lots of carbon build up on the top of the valves and in the intake -- I physically removed what I could but figured a slow mist of water thru the air intake would help out with the rest

It's a new to me 96 XJ -- guy had a 160* thermostat in it -- I figure all the carbon is because it was running so cold all the time


I was planning to use a handheld sprayer with the nozzle adjusted to mist and just spray a mist of water over the top of the throttle body and let it suck in -- also I will remove the cat while doing this

Sound about right???
Yeah, the 160 thermostat probably isn't helping anything.

Sounds like a good plan to me. Just make sure it's warmed up first. I'd also open throttle a bit so the engine's running at a good cruising speed, say 1,500 or 2,000 RPM. Even the Seafoam stuff likes to try and stall the engine out.
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Old April 10th, 2019, 17:55
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cal cal is offline
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Re: Octane and power

That has nothing to do with the 4.0 and its stock 8.8:1 nor is it actually accurate.

I ran 12:1 on iron with 96 octane in a jeep and currently run 11.4:1 on aluminum with 91.

Fix the quench and a lot of things improve.
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Last edited by cal; April 10th, 2019 at 18:06.
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  #13  
Old April 10th, 2019, 17:58
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Re: Octane and power

That may have well been accurate information in 1972 A but it holds no bearing on modern fuel injected motors, even our ancient fuel injection
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  #14  
Old April 10th, 2019, 19:29
Cummins90 Cummins90 is offline
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Re: Octane and power

FWIW I've put 15psi from a 20g into a stock 4.0 for years now. It has even held up to a few 20psi pulls all with good charge cooling. I actually dont even retard timing and force closed loop to hold for the first 4psi. I am on 93 at close to sea level.
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  #15  
Old April 11th, 2019, 06:37
RCP Phx RCP Phx is online now
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Re: Octane and power

Quote:
Originally Posted by JeepNoob View Post
From what I've read, 4.0's kinda run the ragged edge of what's possible for compression ratios in an engine with a cast-iron head running on 87-octane pump gas. With that being said, I know of at least 1 TSB floating out there advising NOT to use premium/91-octane fuel in them (believe it was for the H.O. models though.)
It depends on the build, I'm running close 9.4 CR on regular gas with no pinging issues at all.
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