Running a high compression engine on pump gas

[VintageMusclecar]

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.

[elonblock]

<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!

[JRSully]

I religously run a mix of AVGAS and 93, no other additives. The L78 actually runs better on a 100LL and 93 mix then all Avgas. I keep 5-10 gallons of it on hand and dump some in before heading to Sunoco. I have all the timing in at 3k rpm and never have any problems

[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.

[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.

[Salvatore]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: [email protected]</div><div class="ubbcode-body">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. </div></div>As always Eric. That is a right on statement you made. I am sure Elon wants to start this car up run down to get a newspaper on Sunday morning or an occasional drive to the Dairy Queen cruise. I would be surprised if that motor ever sees 4,000rpm's or 300 miles a year. JMO though.

[elonblock]

I appreciate all of the insights and feedback. My &quot;take away&quot; from all of this discussion is it possible to drive the car with a bit of planning regarding the areas I will be driving to, and using the correct gas and or mixture of gas.

[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.