High Octane Fuel Use- Please shed some light

[joelhern2]

I recently read in one the posts about PCM discouraging the use of high octane fuel. So, I emailed them to get some answers about my particular engine which is the 454 (330 HP) big block from 1991.

Here is the response:
This information is applicable to 2002-current engines only. For your engine you can use 89 octane unless your total timing is < 30^ (non Pro-Tec models) then 87 should be used. Though this is considered a high-performance engine the ignition and fuel delivery systems are not set up (from the factory) to benefit from higher octane fuels and doing so may even damage your engine.

Can someone with engine knowledge shed some light on "total timing is < 30^" because I'm not quite sure I understand? I am definitely not knowledgeable about engines...

Thanks in advance,

Joel

[Hollywood]

RE: High Octane Fuel Use- Please shed some light

Take a timing light and clip it on the #1 spark plug wire. At idle you will see something around ~10 ("base") on the harmonic balancer cover. Between 2.5-3k rpm (won't change much) you will see something around 20-25 ("advance"). Add these 2 numbers together for your "total". The more timing you can safely get away with the better performance you will get. Knocking or "pinging" is from too much timing, old school motorheads usually set their timing by simply listening to the engine.

[TRBenj]

Re: RE: High Octane Fuel Use- Please shed some light

At idle you will see something around ~10 ("base") on the harmonic balancer cover. Between 2.5-3k rpm (won't change much) you will see something around 20-25 ("advance"). Add these 2 numbers together for your "total".

HW is correct but I think his wording is confusing. ~10 degrees at idle is correct. The stock distributor should advance 20-25 degrees, and will get to full advance by 3000 RPM. When you put the timing light on it at 3000 RPM, you will see the total advance (30-35 degrees).

High compression motors and/or very advanced timing will require the use of higher octane fuel. The stock 240-255hp PCM 351w only has a CR of 8.5:1, so 89 octane is plenty (possibly overkill depending on where the initial timing is set). I believe the ProBoss and GT40 motors have different pistons and slightly higher compression- but its still below 9:1 and 89 octane is fine on those motors as well.

[joelhern2]

Thanks guys...

[campbelljo]

I just reset my timing on the ProTec. I was instructed by DIM to disconnect the knock sensor and set to 17 deg. at 600 RPMs (idle), then reconnect sensor. The fin on the harmonic balancer only goes to like 10 or something so you will need a timing light with an adjustable advance (knob on the back). Dial it to 17 and set to timing to zero on the fin.

I just didnt want you to check you timing and think it was way advanced because it wasnt showing up on your fin. Once you have your timing exact it will start much easier and the starter will not drag.

I always use 89?

[core-rider]

The part where they say a higher octane fuel might damage your engine is highly unlikely. I've never heard of too much octane causing damage. Not enought however will. The fact of it is, the lower octane you can run without pinging the better. Lower octanes burn faster and more efficent. If your engine has too much timing or comppression it might start to "ping" which over time can cause catostrophic engine failure. One of two ways to fix pinging is higher octane fuels because their flashpoint is higher causing the preignition or "ping" to go away. Backing down on total timing is the other way.

You shouldn't have any problems running 87 unless your timing is advanced too much. In that case, either lower the timing or run higher octane fuel.

[photoguy]

I got this yesterday in refrance to my 2000 GT40....

Hi Steve,



The GT-40 is (as most modern fuel-injected engines are) equipped with a knock-sensor. If the knock sensor senses spark knock from a low-grade fuel it will automatically retard timing up to 8^ to compensate and if you have been running 87 octane then this would have preventing any damage from occurring. 87 octane that was pumped and maintained its integrity before it was burned is probably not low enough to even create a knocking condition even if your timing was set to 10^. I would run 89 from now until you have your next service and then you can have your mechanic verify your timing setting and even change it if you wish. While 10^ and 89 octane will probably give you better performance if you would like to run 87 octane exclusively then have him set the timing to 5^ and you’ll be on your way. I apologize for the confusion and please contact me if you have any further questions.



Thank you,



Max Bazen

Pleasurecraft Engine Group

[AirJeff]

Hmm, so technically we could run higher octane and advance the timing? In theory we would get more power then. Not worth blowing up an engine for a few extra HP though.

[core-rider]

Hmm, so technically we could run higher octane and advance the timing? In theory we would get more power then. Not worth blowing up an engine for a few extra HP though.

Yes, but it's not enough HP for you to really notice first of all... Secondly, bumping up the timing with make you loose low end torque, which is what we want anyways. The knock sensor will just take out timing if it's senses pinging to protect the engine, which kind of negates the whole purpose of raising the timing anyway.

Basically, just leave the timing where it is and you'll be safe.