pump gas question

This is about as close to a good question I can muster. I have been told that my mercedes with the eurospec DOHC I6 m110 engine should use premium fuel, 91 her in california. The compression ratio is about 9:1. But given the expensive gas during the summer and the fact I have smog tune(runs rich) switch to midgrade without any ill effects. If I have someone lean out the mixture I'd probably want to switch back to premium, but want to ask a question of you folks.

I read too many of the GRM and CM magazines and they've influenced my decisions. The reason to use premium is to prevent detonation due to hot spots. I figure that because I have the DOHC the engine breathes better than a comparable SOHC ie the m103 engine I might not have to use the expensive gas. The only reason I ask is another fellow runs the same engine with a manual and says regular gas works fine. I figure if a guy with a manual can get away with it so can I. If you lug a manual the engine will knock, thats my perspective. I've never heard any funny sounds out of my engine and would like to keep it that way so wanted to see what you guys think. Thanks for the help folks!

Well I think that the need for higer octane rated fuel has as much to do with actual running cyl pressure as it does static compression ratios, so by that reasoning a DOHC might have measurably higer cyl pressure than a nearly identical SOHC engine with the same static compression ratio, and thus could legitimatly need better fuel. That having been said though, I would try regular for a while and see how it works out. Do you know what your engine sounds like when it knocks? If you do then you know what to look out for.

BTW 9.0:1 static compression for an NA engine doesn't sound that high that high to me, that is what is found on alot of low performance industrial engines.

benzbaron wrote: This is about as close to a good question I can muster. I have been told that my mercedes with the eurospec DOHC I6 m110 engine should use premium fuel, 91 her in california. The compression ratio is about 9:1. But given the expensive gas during the summer and the fact I have smog tune(runs rich) switch to midgrade without any ill effects. If I have someone lean out the mixture I'd probably want to switch back to premium, but want to ask a question of you folks. I read too many of the GRM and CM magazines and they've influenced my decisions. The reason to use premium is to prevent detonation due to hot spots. I figure that because I have the DOHC the engine breathes better than a comparable SOHC ie the m103 engine I might not have to use the expensive gas. The only reason I ask is another fellow runs the same engine with a manual and says regular gas works fine. I figure if a guy with a manual can get away with it so can I. If you lug a manual the engine will knock, thats my perspective. I've never heard any funny sounds out of my engine and would like to keep it that way so wanted to see what you guys think. Thanks for the help folks!

I know this is going to sound like I am splitting hairs, but you are mixing up a few terms.

Pre-ignition (aka dieseling) can be caused by hot spots. Pre-ignition is where the fuel self ignites without the spark igniting it. It is ignited by something else- hot spots, carbon deposits, excessive pressure, etc. Pre-ignition occurs normally in diesel engines.

Detonation (aka knocking) is an explosive ignition of fuel outside of the flame front. Detonation can be prevented by:

-Using a higher octane fuel, which increases the combustion temperature and reduces the likelihood of detonation

-Running richer fuel mixes, which cools the temperature in the cylinder

-Reducing cylinder pressure

-Decreasing the manifold pressure by reducing the throttle opening

-Reducing load on engine

Both pre-ignition and detonation are bad for your motor. Both increase the temperatures, and both conditions can cause the other condition.

I know nothing about MB motors, so don't listen to me too carefully. But 9:1 compression ration is generally considered pretty low, and would normally be fine with lower octane fuels. I can't imagine much you can do to an MB of that vintage that would kill it- they run forever.

If you try it and it knocks, upgrade.

Happy Andy wrote: Well I think that the need for higher octane rated fuel has as much to do with actual running cyl pressure as it does static compression ratios, so by that reasoning a DOHC might have measurably higher cyl pressure than a nearly identical SOHC engine with the same static compression ratio, and thus could legitimately need better fuel. That having been said though, I would try regular for a while and see how it works out. Do you know what your engine sounds like when it knocks? If you do then you know what to look out for. BTW 9.0:1 static compression for an NA engine doesn't sound that high that high to me, that is what is found on a lot of low performance industrial engines.

On Normally aspirated motors. The dynamic compression will never exceed the static compression. technically it could if your valves were closing exactly at BDC and if you optimised the intake pulse but t my knowledge every car has valve lap with the intake closing as the piston is rising so no matter what you do you will never exceed the mathimatical maximum compression that static compression represents.

At 9:1 you have no worries. I would run 87 with out giving it a second thought. Where I would be careful is to make sure that you get good quality gas. I believe that the newer MB's actually recommend specific brands of fuel due to the additive packages they use with the base stock to make there fuel.

Another thing I have noticed about octane is that some motors will run smoother on the 91/93 due to the timing being set for the slower burning higher octane fuel. My Porsche is like that. I have the older pre 85.5 (1983) motor that has about a half a point lower compression ratio as compared to the post 85.5 NA motors. I use the older motor due to it having forged internals that were only in the 83 motors but due to me using a later cam and the late DME my car likes 93. It just runs smoother and has better power.. It does not knock with the lower octane. The lesser octane does not hurt my car but I can tell that it runs better with the higher octane. And yet my static compression is that of a pre 85.5 car (around 9:1). I have since changed over to a strand alone ignition system so the issue is no longer a concern.

Thanks for the replys, I'll reread what was written again to make sure I understand what is being said. My pops used regular before and didn't ever have any trouble. Would you believe that this was a high compression engine for mercedes, the US versions are around 8. I think I'll just use midgrade or premium when I'm gonna take off on the freeway.