Need some help! Engine knock!
Thread Starter
Advanced
Joined: Jan 2007
Posts: 82
Likes: 1
From: Portsmouth Virginia
Need some help! Engine knock!
Ok......350 I do not know the miles on the motor car has 120,000 so i'm going with that for miles. Top end was re-built alum heads etc. Runs great pulls hard. Oil pressure stays between 20 and 50 on the stock gauge.
When its cold the motor sounds fine.
When it gets hot it knocks. I can feel the knock in the pedals!!
Sounds like its only at idle but that could just be me.
Could it be octane? I run 93 but it does sit.
Could it be a bearing and still sound fine when cold and at 4000rpm even when warm?
When its cold the motor sounds fine.
When it gets hot it knocks. I can feel the knock in the pedals!!
Sounds like its only at idle but that could just be me.
Could it be octane? I run 93 but it does sit.
Could it be a bearing and still sound fine when cold and at 4000rpm even when warm?
Le Mans Master
Joined: Jul 2000
Posts: 6,663
Likes: 162
From: columbia city in
i would get a pool stick or something like it .put it were you think the noise is coming from .that will help you get a better idea what you have .if you can feel the knock on the pedals you will need to call 1-800-crate -motors 
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
Ok......350 I do not know the miles on the motor car has 120,000 so i'm going with that for miles. Top end was re-built alum heads etc. Runs great pulls hard. Oil pressure stays between 20 and 50 on the stock gauge.
When its cold the motor sounds fine.
When it gets hot it knocks. I can feel the knock in the pedals!!
Sounds like its only at idle but that could just be me.
Could it be octane? I run 93 but it does sit.
Could it be a bearing and still sound fine when cold and at 4000rpm even when warm?
When its cold the motor sounds fine.
When it gets hot it knocks. I can feel the knock in the pedals!!
Sounds like its only at idle but that could just be me.
Could it be octane? I run 93 but it does sit.
Could it be a bearing and still sound fine when cold and at 4000rpm even when warm?
Thread Starter
Advanced
Joined: Jan 2007
Posts: 82
Likes: 1
From: Portsmouth Virginia
Thanks...thats one I had not thought of.
I think if I was to pull it down to the flywheel to get at the bolts, I would just rebuild the motor and put in a new clutch etc.
I feel thats the way im heading. I just wish that it didn't run so good.
If it's not one thing it's another!
Burning Brakes
Joined: Jan 2009
Posts: 1,224
Likes: 1
From: Clarksville Indiana
no you dont take anything apart.. just take a wrench under the car, well you may have a tin shield over the torque converter that you have to remove and then just slide the wrench up in there and check the bolts. If the one you reach is loose, tighten it and then pull harder to turn the engine enough to be able to get to the next bolt, go all the way around
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Number one what compression ratio is the motor? If it is anywhere close to the stock ratio then you are running way too high of octane. The detonation will be delayed and it will create carbon. A large enough carbon deposit can chip off and get a "carbon knock". If it is a higher compression with the new heads then possibly it could neccessitate the 93 octane, another question is which oil you are using.... either way I would add a full can of Seafoam cleaner to the tank. It is an excellent way to rid your engine of carbon and clean up the top end. If you DO know it is still a stock or low compression motor then STOP using 93, go to 87 or whatever is the lowest in your area. Octane is only there to delay detonation in a higher compression motor. A good rule of thumb on any motor is to use the lowest octane you can get by with and have no predetonation (pinging). There can be many other reasons for knock, some can be real bad.....Good Luck
Wait a minute! Are you saying that running too high of an octane gasoline will carbon up the combustion chamber, CAUSING detonation?
I just wanna make sure I am understanding this properly, 'cuz I have a stock L48 (8:1 compression) 1980, and have been running 92 octane gas in it for the last few months, thinking I was doing it a favor.
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Drifting
Joined: Aug 2004
Posts: 1,883
Likes: 124
From: Lubbock TX
Octane rating determines resistance to detonation. The further gas is refined, the higher the octane rating, but the slower the gas burns. You may cost yourself power by running high octane if your engine doesn't need it.
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
Your car needs to be run on 87. Octane is basically the amount of pressure the fuel will take before it turns into a vapor and spontaneously ignites. The objective is to create the perfect state for the fuel to burn completely exactly when the engine sparks. If too low of octane is used the fuel will ignite prematurely causing pinging or predetonation knock(one premature flame front from too low octane slamming into the secondary flame front caused by spark= knock or ping) If too high octane is used it will not cause predetonation or knock, but it will not be vaporized properly before ignition and the remaining unburnt liquid turns into harmful carbon deposits. Your L-48 needs the low octane gas. Higher octane is not a "better" fuel, just a different application for higher compression engines. Rule of thumb: Use the lowest octane fuel that can be ran without any pinging.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Wait a minute! Are you saying that running too high of an octane gasoline will carbon up the combustion chamber, CAUSING detonation?
I just wanna make sure I am understanding this properly, 'cuz I have a stock L48 (8:1 compression) 1980, and have been running 92 octane gas in it for the last few months, thinking I was doing it a favor.
I just wanna make sure I am understanding this properly, 'cuz I have a stock L48 (8:1 compression) 1980, and have been running 92 octane gas in it for the last few months, thinking I was doing it a favor.
Drifting
Joined: Aug 2009
Posts: 1,635
Likes: 8
From: Enid Oklahoma
No myth
Read my post about how ocatne works. YES it can harm it!!! Simple, a perfectly setup engine using proper octane fuel will ignite that fuel right before that fuel will want to spontaneously ignite and is perfectly atomized and pressurrized to basically a vapor. When too high of an octane is used for a given low compression motor, that fuel is not completly atomized with the air and still contains a fair amount of liquid that when burnt will turn to carbon and leave deposits that can be harmful over time, and can even chip off causing harm to a motor. Also the fact that the fuel does not completely burn it looses power and basically acts like a lean mixture and the engine can run hotter than normal. If it were OK for every application there would only be one octane for ever vehicle. It is not a myth I've rebuilt too many engines that someone ran high octane just because they thought it was better, it won't blow the engine up but yes it can harm it and will defineately cause wear, and decrease longevity. Not a myth , science.
Le Mans Master
Joined: Nov 1999
Posts: 5,768
Likes: 3
From: San Diego CA
no you dont take anything apart.. just take a wrench under the car, well you may have a tin shield over the torque converter that you have to remove and then just slide the wrench up in there and check the bolts. If the one you reach is loose, tighten it and then pull harder to turn the engine enough to be able to get to the next bolt, go all the way around
Ok......350 I do not know the miles on the motor car has 120,000 so i'm going with that for miles. Top end was re-built alum heads etc. Runs great pulls hard. Oil pressure stays between 20 and 50 on the stock gauge.
If rod or main bearings were bad enough to cause knock I would expect it to show up on your oil pressure readings, those are well within norms
When its cold the motor sounds fine.
When the engine is cold I would expect bearing knock to be worse, after warming up thermal expansion would take up some of the clearances.
When it gets hot it knocks. I can feel the knock in the pedals!!
Disconcerting to be sure - you dont say which pedals or is it all of them
Sounds like its only at idle but that could just be me.
If its at idle only then it sounds like something is loose and then the slack is taken up once rpms increase, could be the flywheel idea. if it were rod knock it would increase with engine rpm
Could it be octane? I run 93 but it does sit.
You dont need 93 octane
Could it be a bearing and still sound fine when cold and at 4000rpm even when warm?
I doubt it very much
If rod or main bearings were bad enough to cause knock I would expect it to show up on your oil pressure readings, those are well within norms
When its cold the motor sounds fine.
When the engine is cold I would expect bearing knock to be worse, after warming up thermal expansion would take up some of the clearances.
When it gets hot it knocks. I can feel the knock in the pedals!!
Disconcerting to be sure - you dont say which pedals or is it all of them
Sounds like its only at idle but that could just be me.
If its at idle only then it sounds like something is loose and then the slack is taken up once rpms increase, could be the flywheel idea. if it were rod knock it would increase with engine rpm
Could it be octane? I run 93 but it does sit.
You dont need 93 octane
Could it be a bearing and still sound fine when cold and at 4000rpm even when warm?
I doubt it very much
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
I read your posts and responded accordingly. Your basic understanding of this issue is incorrect. Atomization of the fuel has nothing to do with octane rating, and you've added another myth that high octane gas makes engines run hotter.
I'm not going to get into any sort of argument with you. Sorry. My post was to respond the person who questioned your statement.
I'm not going to get into any sort of argument with you. Sorry. My post was to respond the person who questioned your statement.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
Octane ratings are irrelevant for engines at idle or very light loads- you can run almost any engine on concentrated gnat's pee at start, idle or small throttle openings without preignition/knocking. The conditions required to induce the problem don't exist.
Burning Brakes
Joined: Jul 2009
Posts: 1,132
Likes: 14
From: A town SC
87 for low compression,thats all it needs. if a healthy engine 93 will do no harm. race fuel is used to slow the rate of fuel burn to prevent pre ignition in high comp engines. If you put 116 in an 8:1 motor it will lose some power. Its been proven over and over.
Le Mans Master
Joined: May 2002
Posts: 5,084
Likes: 786
From: Lebanon Township New Jersey
Absolutely incorrect. An increase in pressure, as in the combustion chamber, does not cause gas to vaporize. Exactly the opposite is true. Think of it as LIQUID propane in a pressurized tank, becoming a vapor as the pressure drops when it leaves the cylinder. The only down side to too much octane is wasted money.
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Why couldn't you run the engine on diluted gnat's pee? Not enough octane?
I'M SORRY, I COUDN'T RESIST! I TRIED, BUT I JUST COULDN'T HELP MYSELF!
Scott