Correlation between Octane and High Engine Temps affecting Exhaust Manifold?

[RAM Patriot]

In an effort to discover why we are having issues with the exhaust manifold, wanted to conduct a survey and discussion to see if there is any correlation between the octane used and the heat produced by the engine to cause the exhaust manifolds to crack.

Here is some information that led to this discussion:

The higher the compression ratio, the higher the octane rating and higher ignition point needed to get a proper fuel burn. If you cheap out and use a lower octane rating than recommended, you'll notice knocking and pinging from the engine, which is the fuel detonating before the proper moment. Other than the noise and possible reduced power caused by using a lower-than-recommended octane rating, you'll risk overheating your engine if the uncontrolled detonation is too great. If your vehicle requires high octane fuel, you should use the fuel grade recommended by your manufacturer. Some drivers can use a lower octane fuel with no problems, but it's no guarantee that will happen with you and your vehicle.

 

[Bt10]

Couple problems with your assumptions. Lower octane does not cause higher engine temps with "uncontrolled detonation". These engines, and really everything made for the last 30 years with computer controlled fuel injection, have knock sensors that the computer uses to pull back ignition timing to stop the knock. This will change the percentages of NOx vs hydroC, which makes Greta mad, and lowers the actual power output, but doesn't really hurt the engine. The owners manual only references emissions and economy/power for this reason.

That aside - in general, OEMs like to run everything as hot as possible to help with emissions, and most likely as thin a manifold as possible to save truck and material weight. They also have to reach convertor lightoff temp as fast as Greta wants, so the different materials are under stress as they change size rapidly. My $.02.

Drivers side done, waiting for passenger side availability, probably 9-10 months total.

 

[RAM Patriot]

Couple problems with your assumptions. Lower octane does not cause higher engine temps with "uncontrolled detonation". These engines, and really everything made for the last 30 years with computer controlled fuel injection, have knock sensors that the computer uses to pull back ignition timing to stop the knock. This will change the percentages of NOx vs hydroC, which makes Greta mad, and lowers the actual power output, but doesn't really hurt the engine. The owners manual only references emissions and economy/power for this reason.

That aside - in general, OEMs like to run everything as hot as possible to help with emissions, and most likely as thin a manifold as possible to save truck and material weight. They also have to reach convertor lightoff temp as fast as Greta wants, so the different materials are under stress as they change size rapidly. My $.02.

Drivers side done, waiting for passenger side availability, probably 9-10 months total.

You are correct, before EFI was used, knocking commonly occurred and could cause significant engine damage. as you have said, modern engines have sensors to detect knocking. When detected, the computer delays the initial spark, which causes the controlled combustion to take place at a point when compression is not at its highest point. Although this eliminates the knock, it can still cause the engine to run less efficiently with rising temperatures and pressure from the primary combustion chamber.

A similar undesirable condition is called pre-ignition, when the fuel ignites on its own before the spark ignites it. (the lower the octane the faster the burn rate) Modern engine computers minimize this condition by controlling the timing of valves and fuel injection; however, this control mechanism can also come with a fuel-efficiency decline and or emissions penalty with excess heat dissipated that is not used in the combustion chamber but exit the valves into the manifold as hot exhaust gasses.

That is why I have set this post up as a poll to test this theory.

 

[Idahoktm]

If you cheap out and use a lower octane rating than recommended, you'll notice knocking and pinging from the engine, which is the fuel detonating before the proper moment.

You made this statement as if it was a fact...it is not.

 

[RAM Patriot]

You made this statement as if it was a fact...it is not.

Just because you do not hear or detect the knock or ping on lower octane fuel doesn't mean that it is not occurring. (another fact)

 

[Idahoktm]

Just because you do not hear or detect the knock or ping on lower octane fuel doesn't mean that it is not occurring. (another fact)

I didn't say it couldn't knock, but you are saying it WILL knock. That is not true.

 

[mikeru82]

Actually he said "you'll notice knocking and pinging from the engine". This is definitely not true. It might occasionally knock, but I've never heard it. Blasting tunes doesn't help with hearing things like that though.

 

[theblet]

We’ll it’s 50/50

 

[Rick3478]

My take on some of that is that the driver is going to push as hard on the accelerator as is needed to achieve the desired push against his backside, thereby compensating for any differences in fuel energy content.