Octane/Detonation Discussion
 

So for discussion I wanted to post something that I read on one of the FB Pontiac groups. I find it interesting because it goes against very common knowledge on this forum & is closer to what most folks think or say when it comes to high compression engines. Note, I am not agreeing or disagreeing with this persons post, I am merely looking to educate myself & have a discussion that hopefully helps others as well. Here is the post in quotations below:

“FYI: Just a notice or reminder to all of you 1970 and ealier 455, 400 engine owners that those engines were made for 98 octane minimum. If you're trying to get away with 91 or 92, even 93, you're looking down the barrel of future problems that most guys will try to blame it on something else. Know this, inaudible detonation can break a crank in two. Seen it. Because the detonation isn't so severe to where you hear a "ping" most guys don't think they're having a problem. All the while they're chasing heat problems or increased oil consumption. I bring this up because there's no education anywhere about this problem. And I want everyone to be enjoying their Pontiacs, not pouring money into them. You do not want to rebuild your engine stock. That's the worst idea. If you're going to run 93 octane or less you must drop the compression to be compatible with that octane. So listen up - the first reply which no doubt is coming will be from some guy trying to defend his high compression engine by saying "I've been running mine for ten years on 92 octane and it runs great!!!" As if that means his combo is safe. Understand that because of the plus and minus allowable factory tolerances I've measured a stock factory rated 10.75Cr. 400 with # 13 heads at 9.75Cr. Still too high for 93 but will last a lot longer before the signs manifest themselves. Here's a pic of inaudible detonation. Good luck everyone and if you have questions“

There are more factors at play as to whether a certain compression number is a go or no go. That post barely even touched the surface.

Not an apples to apples comparison. I don't know when the calculated Octane Rating changed, but in the muscle car heydays, octane was calculated as RON. Our fuels now are calculated by the anti-knock index which is (RON+MON)/2. Fuel octane measured with RON alone will be higher "Octane" than fuel measured as (R+M)/2. Its the same situation we see when talking to the Aussies for instance. Their "98 Octane" fuels are close to our 93 octane fuels. It's just a difference in how the octane rating is calculated.

What IS a known issue is the existence of anti-knock and lubricative agents such as tetra-ethyl lead. Many vintage cars (including but not limited to Pontiac) did not use hardened valve seats. There wasn't as much of a need because the TEL cooled and lubricated the seats and kept them from wearing.

Its not black and white like that.
You can run a 400 with 13s or 62s all day long with the right cam. You can build a 400 that runs PERFECTLY well on 93 with zero detonation.

I agree, it’s not that cut & dry.

Seems like some grand statements from someone that doesn’t have there facts all that correct. That’s likely one of those people that thinks all that era’s 72cc heads are actually 72 cc, which they weren’t. Plus I can’t recall a 400 with 13s ever having 10.75 SCR from the factory. The 9.75 SCR 400 13 head example was probably even high, some 400s with 13s have low 9s compression, the 13 head ran 75-78 cc, cylinder scallops on one side add 1 or 2 cc’s, pistons down about .020”, and on top of that, probably were sporting a .045” to .050” thick head gasket from the factory. Even with a small 067 cam, they would run on 91 octane pretty easy.

Lots of that eras stock grocery getter 400 engines have been rebuilt once are running around with 8VR pistons now too, compression is in the 8s. Not many engines even are configured much like how they were from the factory. Different pistons, surfaces trued up, valve jobs, different cams.

I am trying to remember the last pump gas performance engine I have put together that had less than 9.5 compression. Actual compression, not what the factory soft ball rating. I think I would have to go back into the 1990s.

You have to take it where it is coming from..someone trying to still be current ..

67Lemons do you have a link to fb you could share?

I think his opinion is the same as most folks think when you talk high compression for the street. What I see here is that you all discuss/explain how cam & ignition timing (as well as other factors) come into play for high compression motors not only surviving but performing with the current fuel formula’s we are stuck with.

The author of the FB post is Bruce Fulper

Whoda ever thunk it ?

:pound: :pound: :pound: :pound: :pound:

Bruce was kicked off this forum before I even joined, over 21 years ago so that might tell you something.

This octane/compression subject has been beat to death here multiple times already.

Rock 'N' Roll baby!!

One of the problems I see with race motors today is that many do not take advantage of todays programmable ignition systems, and even the ones that have it do not make full use of it.

At idle many race Cam’ed motors can take and want a lot of advance, even up to 55 degrees (and especially if your trying to get the motor to idle clean and as lean as possible ) but once the throttle gets cracked open that needs to drop back to 30 to 35 degrees .
At about half throttle with a good tune and fast burn chambers that may need to then drop back to 25 degrees.

Once at full throttle then timing needs to advance in the conventional manor.

Your not getting this with springs and weights, although at idle you might come close with vacuum advance.

Fulper, the same guy who said a 400 block could hang with a IA2 in the reliable big power dept.
Whatever you say Flopper.

Why was Bruce chucked off the forum? Over here in UK we have to read and learn because there are no pontiac specialists. I often read his posts on FB and know that he isn't a liked person but, he must still get work. It can't all be wrong surely.

I've known Bruce a long time going back to the first drag race days we had at the firebird nats. So I guess 30 years or so.

I think most peoples issue with Bruce is his arrogance making him hard to talk to for some. Being a body builder and power lifter for most of my youthful life I grew up around arrogant people so that doesn't bother me as much as it bothers others. I tend to find humor in it. So once you get past that you realize he's not a bad guy and has some interesting theories. I don't always agree with him but that doesn't make him a bad guy. Personality aside he's ok. I always did appreciate the effort he put in for the drag races and the give aways were always stellar. Cost him a small fortune to do that every year and he never complained. I never did go back to the nats when they stopped the drags. Bruce had a good thing going there. That should have been continued after his accident but that's a different discussion. Such is life.

Your right, the electronic timing control makes this so much easier.
Ive been able to manipulate mechanical timing control and higher idle timing with vacuum advance on engines like this and have good success that way as you mentioned, but electronic control has the ability for so much more.

Modern water injection will break the octane compression rules.

Otherwise my 472 ran the 1/4 to 12.2s on 89 Octane, no ping, because 9.00:1 CR was too low.

You all are safe to design for 9.5:1 - 10.5:1 depending on details. Then 10.5+ higher compr needs a real plan from real experiences.

Revisit water injection to run super high CR on 93 octane pump gas, for high HP.

Modern water injection will break the octane compression rules.

Otherwise my 472 ran the 1/4 to 12.2s on 89 Octane, no ping, because 9.00:1 CR was too low.

You all are safe to design for 9.5:1 - 10.5:1 depending on details. Then 10.5+ higher compr needs a real plan from real experiences.

Revisit water injection to run super high CR on 93 octane pump gas, for high HP.

"I think most peoples issue with Bruce is his arrogance making him hard to talk to for some."

Most folks had issues with him because he'd take their money for parts, or engine builds, etc.....and NOT deliver.

I've seen this many times over the years but am NOT going to waste any of the valuable time I have left discussing it.......