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OK, we all know that today's engines have knock sensors that detect knock and automatically adjust (retard) timing to get rid of it.

We also know that retarding timing reduces power.

The question is, how do we know if/when the engine has detected knock and retarded our timing?

I ask this, because I used to own a 1969 Chev, a 1972 Valiant and a 1974 Olds Cutlass (6-cyl/slant6-cyl/Olds 350-later455). As the cars got on in age, they began to get knock - which usually told me that I needed to tune it up. Sometimes (especially in my Olds) I could just run the Pi$$ out of it and the knock would go away for about 10K miles. Or, if I could afford it, I'd put in either octane booster, a fuel system cleaner, or premium gas - which usually made the knock go away. Sometimes, I'd only need to do this in the summer.

A couple of times, I popped the heads and cleaned out the valves and cylinder (usually with lots of carbon deposits) and that would make it much better.

So, the related question is, why do engines start to knock when they get on in age? Is this only with carbeureted engines, or do today's engines have the potential for the same things that cause knock as yesteryear's engines?

And, if all engines are prone to knock when they get on in age, then might that explain why a mid-90's Cavalier might be down on power and mileage as it approached the 100,000 mile mark?

Lots of questions.
 

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I have absolutely no answers for you, but I have a question/request for 3rd party people out there. Why don't cars come with some sort of 'idiot light' for knock? I personaly think having something alert you that it's happening so you know that tank of gas you got at the cheap-o station was bad. Or to help trouble shooting too. It might exist, I've just never seen it on a car before.
 

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solsticeman said:
A couple of times, I popped the heads and cleaned out the valves and cylinder (usually with lots of carbon deposits) and that would make it much better.

So, the related question is, why do engines start to knock when they get on in age? Is this only with carbeureted engines, or do today's engines have the potential for the same things that cause knock as yesteryear's engines?
I think you pretty well answered your own question. As Walter Mathou side in A New Leaf, "Carbon on the Valves! It's always Carbon on the valves!"

But the difference is a lot has been done to reduce this thanks to the EPA. The same engineering that's needed to keep a car from turning into a gross polluter in just 50,000 miles keeps carbon deposits from accumulating on the valves like they did in days gone by. The cleaner, hotter burn inside today cylinders reduces total carbon production, a byproduct of inefficient combustion. Remember, all those carbon molecules that didn't stick to the valves would instead eventually plug all the tiny holes in a catalytic converter, a big no-no for EPA compliance. And just in the last few (no more than 5) years has GM had knock sensors on any of their engines. And these come mostly into play when running a lower octane gas. I know that none of the F-bodies that ended in 2002 had knock sensors. Instead they had a sticker that read, "use only premium, unleaded fuel." In a way it's tied into fuel injection, but only because that what keeps it running cleaner than a carburetor ever could.

Most often, now if your experiencing a loss of power in an older car, it's because the rings are no longer as tight a seal and you just don't have the compression you had at 150,000 that you had at 70,000..
 

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Knock is pre-detonation. It can be caused by a lot of things, but in older cars it's usually deterioration of the quality of the cylinders due to wear and carbon buildup, not to mention the ignition system itself. It can also be caused by running leaner, due to fueling problems. To detect it, you can get a scanner/computer installed, get one of the special lights that plug into the knock sensor system, or if it's bad enough you can hear it (pinging). In forced induction vehicles, knock can kill your engine before it's bad enough to hear it...

edit: and yes, todays cars are just as susceptible to it. Better fuels and computers mean we don't notice it as much or as soon. But knock is the primary enemy of all forced induction modders out there. I scan constantly to make sure my mods don't put me into knock, and if it does I either have to back off my mods or do something that gets rid of knock. For example, putting a smaller pulley on my supercharger increases boost, which increases power, which at some level will cause knock. Things you can do to get rid of knock include cooling the charge with an intercooler, backing off timing (as you mentioned), improving exhaust flow (headers, rockers, porting, etc.) , enriching the air/fuel mixture (via computer, higher fuel pressure, etc.), bleeding off psi (blowoff valve or in the case of my supercharger adjusting the bypass valve) or any of another dozen ways to control it.
 

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Picture the insides of your combustion chamber. Over time deposits will build up on the valves and on the pistons heads. The octane rating on your fuel determines the burn rate as well, the lower the octane the faster the burn. If you have pre or post ignition taking place the lower octane fuel makes this occur more in an uncontrolled rate. Running higher octane fuel allows for a longer burn rate and reducing the pre/post burn in the chamber.

Clean injectors for the best possible atomization of fuel, check plugs for proper temperature (not too lean or rich), make sure ignition is firing strong, and worse case have the valves removed and cleaned.
 

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Discussion Starter #6
Compression check on the 2.4 l Cavalier was fine - 115 to 125 PSI all four cylinders. I also know the "twin-cam" is basically a re-vamped Quad-4, a family of engines that fo' sho' had knock sensors and pre-det logic as far back as 1990.

When I had about 15,000 miles on the car, I averaged about 28.5 MPG yearly, and the car was about mid 8 seconds 0-60. I ran premium fuel at the beginning, and the first attempt at trying to figure out if there was a difference between premium and regular fuel came out scratch - no difference. Then I ran regular for most of the time except every once in a while.

HOWEVER, I noticed the mileage, after about 80,000 miles, began to drop. At about 90,000 miles, I was only averaging about 25 MPG and had not changed my driving habits. Tried a couple of 0-60 checks, and even on cool days the car wouldn't go better than low 9 seconds. Besides that, it just "seemed" a bit sluggish.

Which lead me to new plugs, compression check, etc. Checks were fine, old plugs were the platinum tipped type from the factory and they really didn't need replacing.

So, that's when I decided to use premium, and guess what? Mileage began to average up to about 28 again, and a later check on 0-60 at about 100,000 miles was high 8 seconds.

Yet, you see all these mythbusters/John Stossel/20-20 reports that get all the experts saying if your car isn't recommending premium fuel, you are wasting your money.

This is what lead me to think that if engines start to knock due to age, and the sensors dial back the timing, you get a loss in power. Put higher octane fuel in, and ta-da, you get "better" performance (when really you are just getting the performance back to where it was). Early in the car's life, fuel doesn't matter, later on in the engine's life, IF it starts to knock, then it DOES matter.

So, now the next question, if I have a higher mileage engine with good compression that starts to knock, is there some special additive that can get rid of the deposits or whatever things start to cause knock?
 

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I've had a few people recommend this 3-step fuel system cleaner from valvoline. I've never used it though. If I remember correctly, it's like 50 or 60 bucks.

Valvoline 3-Part Fuel System Cleaner
 

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The best procedure is to have the carbon professionally removed. My ignorance on this is showing, but my mechanic has a procedure/equipment he uses to clean the carbon build-up through the entire intake system and combustion cambers. I defer to the mechanics on what this process really is. Bottom line, my 1987 Ford 150 is no longer pinging and running great on 87 Octane fuel. Like you, I really didn't want to spend the extra 20+ cents a gallon on premium any longer.
 

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Discussion Starter #9
Ah, but as the price of gas goes up, it becomes more and more economical to use premium.

Regular and Premium are usually $0.15 to $0.20 different in price. This seems to be regardless of the price of regular gasoline.

20% difference in yearly fuel cost if the average cost for regular is $1.00 per gallon. 600 gallons per year is $120 per year more for premium ($720 vs $600). If you get a 10% increase in mileage or performance, then you're losing a bit.

However, at $2.50/gallon, premium is only 8% more than regular. It's still only $120 more per year for premium for the 600 gallons you'll buy that year, but that's only $120 more than the $1,500 you are already going to spend.

If you get an 10% increase in mileage or performance, you are slightly ahead of the game.

The more expensive the regular fuel, the more economical it becomes to use premium.
 

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solsticeman said:
....

So, now the next question, if I have a higher mileage engine with good compression that starts to knock, is there some special additive that can get rid of the deposits or whatever things start to cause knock?
Try BG 44K:

http://www.bgprod.com/products/fuelair.html

Recommended by one of the top mechanics in our area. If the gas tank application doesn't work, a shop that carries this product line can perform a carbon depletion service with the BG chemicals.
 

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solsticeman said:
So, now the next question, if I have a higher mileage engine with good compression that starts to knock, is there some special additive that can get rid of the deposits or whatever things start to cause knock?
The fiero forum guys always recommend some product, that I can't remember at the moment. Plenty of them have tried it - even examining the combustion chambers/valves with those flexible camera hose thingies - and got "like new" combustion chambers/valves from using it. I can't remember what the product is - it might even be the aforementioned valvoline product. I can't get to the fiero forum from work though, so I can't give you a positive id on it. But at least you know that yes, something out there somewhere is supposed to work...

Wish I could be more help.


One more thing - I'd check your catalytic convertor. Exhaust obstructions can increase backpressure, causing knock.
 

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Discussion Starter #12
SkyCaptain said:
The fiero forum guys always recommend some product, that I can't remember at the moment. Plenty of them have tried it - even examining the combustion chambers/valves with those flexible camera hose thingies - and got "like new" combustion chambers/valves from using it. I can't remember what the product is - it might even be the aforementioned valvoline product. I can't get to the fiero forum from work though, so I can't give you a positive id on it. But at least you know that yes, something out there somewhere is supposed to work...

Wish I could be more help.
No problem. Maybe I could just wait for the head gasket to fail and dump coolant into the cylinder :lol:

That happened on two friend's of mine's Quad 4's. Cylinders 3 & 4, including the exhaust valve and port, were almost sparkling, since they were pretty much steam cleaned... ;) The steam cloud left behind could get rid of mosquitos for weeks.
 

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solsticeman said:
This is what lead me to think that if engines start to knock due to age, and the sensors dial back the timing, you get a loss in power. Put higher octane fuel in, and ta-da, you get "better" performance (when really you are just getting the performance back to where it was). Early in the car's life, fuel doesn't matter, later on in the engine's life, IF it starts to knock, then it DOES matter.
My question though is, what happens to the Sol years from now? On a car that I intend to ALWAYS put premium in... what happens when it starts to knock? I'm already using the high grade fuel, so how do I 'get back' my good performance? Using low grade and going to high is one thing, but if you already use the good stuff then what do you go to after that? Race fuel?
 

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dori-san said:
My question though is, what happens to the Sol years from now? On a car that I intend to ALWAYS put premium in... what happens when it starts to knock? I'm already using the high grade fuel, so how do I 'get back' my good performance? Using low grade and going to high is one thing, but if you already use the good stuff then what do you go to after that? Race fuel?
You use one of the cleaning systems or get your engine profesionally cleaned. Higher octane fuel doesn't realy get back the performance you used to have, it lets your car compensate for mechanical issues that are robbing you of your normal performance levels. Eventaully even higher octane fuel wont compensate for the issues your engine will continue devlope over time.
 

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Discussion Starter #15
On the vehicles I have put premium in the whole ownership time, I've not had power reduction, nor knock, and the performance of the car seems to get slightly better as time goes on.
 

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SkyCaptain said:
One more thing - I'd check your catalytic convertor. Exhaust obstructions can increase backpressure, causing knock.
Yeah, I thought my olds firenza had a problem when it's power was off, but I didn't do anything about it ($$$). Then it started dying when I would drive long distances and then slow down. Problem was the catalytic converter was filled with carbon. After replacing it, the car accelerated like new. don't know about gas mileage.
 

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Discussion Starter #17
Yeah, had same thing happen with Fiero - mileage did suffer, but the fact that it felt like someone had stuffed a potato in the exhaust was the big clue :lol:

Just popped the 'verter out and replaced it with straight pipe, and PRESTO, instant power.

Bought it used in mid-90's, Red 1988, with the iron duke. Now I run premium through it and it seems to be fine (had a bit of knock, apparently the iron duke doesn't have a knock sensor).
 

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Sorry I missed out on this earlier discussion, but I was out driving my Sol (yeah, right, few more weeks). Anyway, I'll add my $.02 to the knock issue. Timing advance in cars is from the factory is not always the best for all conditions. The balance of power and economy is a major factor in factory tuning. Now, I have had several GM v6 with knock sensors, 3 of them 3800 with s/c. Best performance on these and what they were tuned for is not just premium, because around here 89 is now being called premium, but pure hi-test 93-95 octane. The cars have and do run the best on highest octane and run okay on 87 without knock because the knock sensor then tells the pcm to retard the timing until the knock goes away. Farthest advance, however is factory set and higher octane such as racing or av gas won't give apower boost, UNLESS you perform tricks inside the pcm and advance the timing to match the gas. Some of the gadgets on the market will let you adjust the chip on the go, some will just give you a chart of what your changes make. JC Whitney used to (and probably still does) sell kits for old-fashioned distributor ignitions that would put a knob or dial on the dash and you could adjust the advance while driving, although there was no feedback other than external knock sound and feel in seat of pants as to difference it makes.
 

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S Man, on your old 60s and 70s rides, the timing may have been off due to worn distributor points. This could cause knock due to the spark being timed incorrectly. Taking deposits off your combustion chambers will slightly lower your compression ratio, resulting in reduced knock. Our race car has a rules limit of 11:1 CR, removing what little carbon accumulates actually measurably reduces the ratio so you can only imagine what globs of black crud that you clean off will do...

The 4th gen F bodies had knock sensors for sure from 98-2002. These cars had LS1s like the Corvette and the knock sensor was/is located under the intake manifold in the lifter galley. I'm not sure about the previous LT1 cars, but I bet they had them too.

Current cars can sense the knock and will cut back iginition timing to accomodate the cheap fuel you have put in which cuts power. When you switch back to premium, the timing will go back to where it should be and the power will come back.

Engines will produce more power after running 20-50k miles due to reduced friction between metal parts, just check a magazine's long term tests and usually their acceleration times drop over time. At 100k, I bet you would experience less power due to ring blow-by and poor-sealing valves.

Let's Motor.
 

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Discussion Starter #20
SCCA HP Racer said:
S Man, on your old 60s and 70s rides, the timing may have been off due to worn distributor points. This could cause knock due to the spark being timed incorrectly. Taking deposits off your combustion chambers will slightly lower your compression ratio, resulting in reduced knock. Our race car has a rules limit of 11:1 CR, removing what little carbon accumulates actually measurably reduces the ratio so you can only imagine what globs of black crud that you clean off will do...

The 4th gen F bodies had knock sensors for sure from 98-2002. These cars had LS1s like the Corvette and the knock sensor was/is located under the intake manifold in the lifter galley. I'm not sure about the previous LT1 cars, but I bet they had them too.

Current cars can sense the knock and will cut back iginition timing to accomodate the cheap fuel you have put in which cuts power. When you switch back to premium, the timing will go back to where it should be and the power will come back.

Engines will produce more power after running 20-50k miles due to reduced friction between metal parts, just check a magazine's long term tests and usually their acceleration times drop over time. At 100k, I bet you would experience less power due to ring blow-by and poor-sealing valves.

Let's Motor.
Points/cap I used to be anal about. Compression check on the engine that I was talking about yielded 115-125 PSI, actually above the spec of 100 PSI.

Hadn't thought of the reduced volume due to carbon buildup - on one of the motors I took apart, that certainly had to be a contributor.
 
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