Running a high compression engine on pump gas

[Salvatore]

the 93 octane is better than it used to be. I have my advance in at around 2,800 rpm's also. No vacuum advance hooked up.

[Lynn]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: VintageMusclecar</div><div class="ubbcode-body">The 496 in my Chevelle has 11.2-1 compression with square port closed chamber heads, and runs perfectly on 92 octane. The key(s) to making it work are 1) cam timing (needs to bleed off some cylinder pressure @ lower rpm), keeping the engine coolant temps ~160°-170°, and a nice, conservative timing curve--in this instance, 14° initial and 36° total, but it doesn't come in fully until ~4000 rpm. (no vacuum advance either)

FWIW, YMMV. </div></div>

I agree with all of the above, but would add one thing. Tight quench. I have it down close to .030 (don't think you want it that close on big block) on my turbo motor, and it never pings even with 15 psi boost.

Of course, this doesn't help you if the engine is already together. If you are still in the building stage, you can choose your headgasket accordingly.

I rarely see this discussed as a weapon with which to combat detonation, but believe it is a vital piece of the puzzle.

[VintageMusclecar]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: elonblock</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: VintageMusclecar</div><div class="ubbcode-body">The 496 in my Chevelle has 11.2-1 compression with square port closed chamber heads, and runs perfectly on 92 octane. The key(s) to making it work are 1) cam timing (needs to bleed off some cylinder pressure @ lower rpm), keeping the engine coolant temps ~160°-170°, and a nice, conservative timing curve--in this instance, 14° initial and 36° total, but it doesn't come in fully until ~4000 rpm. (no vacuum advance either)

FWIW, YMMV. </div></div>

Tell me more....

1) Can you elaborate on the cam timing?

2) Please provide more detail about your experience with the cam.

Thanks! </div></div>

In a nutshell, it's all about managing cylinder pressure. The two things to consider are 1) static (&quot;mechanical&quot compression ratio and 2) intake valve closing point. Large cams typically close the intake valve later than milder cams, which has the effect of bleeding off cylinder pressure at low rpm. This reduces the engine's tendency to detonate.

A high mechanical compression ratio builds more cylinder pressure than a low one, which requires either running higher octane fuel, or using some other means to adjust the cylinder pressure.

Lowering cylinder pressure can be accomplished either with a larger duration camshaft (noting the intake valve closing point) or retarding the existing camshaft in order to delay intake valve closing point.

The latter is a common method among pure stock racers who are required to run an OEM camshaft as this also serves to raise the rpm power band slightly.

A general rule of thumb is advancing or retarding a cam 4° will lower or raise the powerband ~200 rpm, respectively.

Hopefully that all made sense.

*EDIT* Lynn's response above this one re: quench is dead-on, I failed to mention it in my earlier post.

[Salvatore]

you guys may be getting to technical for this application I believe. Its a stock motor that is getting driven occasionally. I think you are going to confuse or lose our original poster.

[VintageMusclecar]

Sam, you're right...I think I was having a &quot;Sheldon Cooper&quot; moment there.

A *truly* pure stock L72 won't measure an honest 11-1 compression, it will probably be closer to 10-1 (see Rob Clary's comments on the 2nd page of this thread). As such, you should be able to run pump premium (possibly with a splash of race gas) as long as the temp and timing are kept in check, and the carb is set up correctly.

Sorry for the data dump.

[TDW]

I run my bone stock 69, L78 car with 93. No problems at all.

[69 Post Sedan]

My motor is around 10.4:1. We have one gas station in Janesville that has 93 octane with no ethanol. It runs great on the street but at the strip I add NOS octane boost and have had great results. According to studies it bumps the octane up to 98.

Kurt

[m22mike]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: TDW</div><div class="ubbcode-body">I run my bone stock 69, L78 car with 93. No problems at all. </div></div>

Me too, and I suspect the compression is not close to 11:1 like Eric suggested.

Mike

[Hemicolt]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: elonblock</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Hemicolt</div><div class="ubbcode-body">Have the valve seats been changed to hardened versions? </div></div>

The seats have not been replaced yet. </div></div>


The reason I asked about the seats is because based on your first post, I wasn't sure if it was a used engine or not.
If I missed that point, sorry for my oversight.

If it is used, and you do know for a fact that the seats havn't been replaced with hardened pieces, then there is an unknown, (concerning valve seat condition), that may need to be figured into your decision.
If that is the case, I probably wouldn't take a chance on not running some type of lead additive. This way you will not give any possible problem a chance to become bigger.

If it's a stock type rebuild that you are doing or have done with no miles, then this is a moot point. But I believe it's well worth mentioning.

Kudos for the point being made again about camshaft specs affecting cyl. pressure bleed off. Another point that was worth mentioning again, it's not always about the dist. mechanics.

[Mr.Nickey Nova]

I heard that todays fuels are like two to three points higher than what they are rated,like 93 octane is actually 95 to 96 octane.Is there any truth to this??