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Ignition Timing?


Nash

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My Father in law has a 94 chevy silverado with the 4.3L V6 Manual transmission. He believes he is having a timing problem. The truck will not accelerate when he goes up hill, or trys to pass. The engine will either mis, or bog down, or not change RPM's at all.

 

All of the sensors check out within range (Oxygen, MAP, EGR, etc.). He has replaced the timing chain and distributor cap and rotor, as well as new wires and plugs. In addition, he has attempted to check the timing, by disconnect the timing bypass connector, but the timing seems to oscillate even with it disconnected (No steady timing mark). Any ideas?

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"87 octane is not enough at time in a 9 to 1 compression"

 

 

 

89 "K" vin 350  = 7.9 to 1 compression ratio

 

 

 

 

No it is not. The only 7.9 to one CR GM made in last 20 years was on a 1995 454. The 89 350 was 9.1 to 1. Blow is a link to GM engine data I have complied from 86 till present.

 

http://forum.snoman.com/viewtopic.php?t=68

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"87 octane is not enough at time in a 9 to 1 compression"

 

 

 

89 "K" vin 350  = 7.9 to 1 compression ratio

 

 

 

 

No it is not. The only 7.9 to one CR GM made in last 20 years was on a 1995 454. The 89 350 was 9.1 to 1. Blow is a link to GM engine data I have complied from 86 till present.

 

http://forum.snoman.com/viewtopic.php?t=68

 

 

 

 

 

Sorry Rocky, Your data is wrong. 1988 1989 and 1990 GM light duty truck service manual and the AERA places the compression @ 8.3 to 1, however the engines had 2 styles of pistons sometimes identified in the aftermarket by the depth of the oil control ring + or -.180. The one that came in your truck (1500) has a 3.250 X .130 + recess with 2 trench style valve reliefs. The head has a 76cc combustion chamber add that to a .045 head gasket. This was confirmed with my own 1989 C1500 "K". The engine would have to have a Flat top piston with a compression distance 1.565 or more to produce 9.1 to 1 with a 76CC head.

 

 

Piston part # 10105166 10159424 10159425 10159430 10159431 10181279 10181280 10181281 14093989 14098989 14088933 12509283 12520264

 

Head casting # 14096217 14101083 14102183 14102191 14102193 376445 93417369 93417389

 

It was maroon and grey two tone standard cab step side. It had a 350, a 5spd and a posi. It was ordered that way.

 

I loved that truck, RED

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I think you guys are chasing the wrong dog here. Try higher octane fuel because on a hill it is going to load engine more and it will need more octane than 87 has and the ECM with retard the spark a lot to prevent knock (up to 20 degrees) and this will greatly effect power output and engines operation as well. Regardless o0f what some may think, 87 octane is not enough at time in a 9 to 1 compression engine especailly on a warm day under load. The only reason the knock sensor is there is to limit consumer complaints about knock on low quality fuels, not to enhance performance or emissions. Try using some 89 or 93 octane and setting the base timing at 4 or 6 degress BTDC and it will feel like a different engine and have more responce and power and better MPG. I have a 89 4x4 burb that I have had since new and I have run only 93 in it for over 10 years with timing set at 8 BTDC and it runs great and even on hills under load too and alwawy gets atleast 17 MPG on long trips and has gotten close to 19 a couple of times too. It has a 40 gallon tank and I refill it after traveling 500 miles or 600 miles on a trip so I know the MPG is valid. It never did close to that on 87 and it was a slug on hills too with 87.

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... The only reason the knock sensor is there is to limit consumer complaints about knock on low quality fuels, not to enhance performance or emissions...

 

 

Actually, knock sensors allow your engine's computer to keep spark timing right at the limit of detonation. They play a critical role in both emissions and performance, and they permit higher compression ratios (i.e. higher efficiency) while still using 87 octane fuel.

 

 

 

 

The only critical role they play is to limit consumer complaints about knock with low octane fuel and little more. They are not there foe emissions and it can enhance performance and power too IF you do not use 87 octane because it will allow more spark agvance but most are really hung up and using 87 octane....

 

 

 

Prior to the existence of knock sensors and electronic spark controls, engine designers had to rely on purely mechanical ways of changing spark timing in resonse to various engine loading and rpm conditions. Distributors usually had both a vacuum advance mechanism and a centrifugal advance mechanism. These mechanical systems were designed experimentally; the engine test lab determined how much vacuum advance, and how much centrifugal advance, could be applied in any given rpm and load situation for maximum power output without detonation (spark knock). Often, worst case scenarios drove the design; the engine operating temperature would be at its worst, and the gasoline quality would be at its lowest, humidity, intake air temperature, etc., were all in their worst case when the vacuum and centrifugal advance curves were drawn. Though it was suboptimal, the design was finalized and the engines were built and put into vehicles. No changes were made after that point. (The owner could adjust his base-line timing, but the vacuum and centrifugal advance was always layered on top of that and those were not easy for the owner to adjust.)

 

Once knock sensors and electronic spark controls became feasable, the amount of spark advance could now be increased, because now it became possible to adjust spark advance in real time. No longer was it necessary to design vacuum advance and centrifugal advance for the worst case; instead actual conditions (rpm, vacuum, coolant temperature, etc.) could be sensed and spark advance could be set based on actual conditions. As a result, with spark timing advanced relative to where it once was, improved efficiency, and improved flame propogation through the charge, could be realized. This both lowers hydrocarbon emissions and raises fuel economy.

 

It doesn't "not affect it."

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... The only reason the knock sensor is there is to limit consumer complaints about knock on low quality fuels, not to enhance performance or emissions...

 

 

Actually, knock sensors allow your engine's computer to keep spark timing right at the limit of detonation. They play a critical role in both emissions and performance, and they permit higher compression ratios (i.e. higher efficiency) while still using 87 octane fuel.
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... The only reason the knock sensor is there is to limit consumer complaints about knock on low quality fuels, not to enhance performance or emissions...

 

 

Actually, knock sensors allow your engine's computer to keep spark timing right at the limit of detonation. They play a critical role in both emissions and performance, and they permit higher compression ratios (i.e. higher efficiency) while still using 87 octane fuel.

 

 

 

 

 

 

The only critical role they play is to limit consumer complaints about knock with low octane fuel and little more. They are not there foe emissions and it can enhance performance and power too IF you do not use 87 octane because it will allow more spark agvance but most are really hung up and using 87 octane.

 

BTW, if you use a real time ECM data monitor you can see first hand how much 87 is crippling your timing and performance at times and on a side note, I had a eratic idle on my 2000 K3500 at times that never thru a code and GM said it was "normal" and fine. It turn out it was erratic data from crank position sesenor that did not through a code because it was with spec but live data clearly showed the problem.

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A classic distributor equipped engine will have two types of timing advance:

 

1. Centrifugal advance, which is the function of the engine speed and the timing advances as speed is increased.

 

2. Vacuum advance, which is greatest upon light acceleration(throttle barely cracked open) and practically zero with the fully opened throttle.

 

The distributor centrifugal advance spring in the distributor could be sticking or otherwise malfunctioning. A '94 Silverado probably does not have a vacuum ignition advance. If it does, check the vacuum advance unit diaphragm with a vacuum pump. If it does not hold vacuum, replace.

 

In this case however, if the timing mark jumps around, it is likely that the distributor drive gear is damaged or loose. Suggest pulling the distributor and inspecting the gear. You may need a new distributor. If only gear is defective, it can be purchased separately and replaced. In most applications, it slips over the distributor shaft and is held in place with a roll pin.

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