Running a high compression engine on pump gas

[elonblock]

Educate me on the subject of running a high compression engine on today's pump gas.

The engine I am considering is a bone stock 11:1, L-72.

Here's the background. I live in a rural area and finding high test gas can be difficult at times. Many gas stations only have two choices, either regular or premium. And there are many occasions when the premium grade is sold out.

The car will be driven occasionally for cruises and similar events. Maximum distance may be 150-200 miles round trip. There will be no drag racing, or stop light antics. Maybe an occasional running it through the gears,but no high rpm operation.

I have a number of questions....

1) What is the reality of running an 11:1 compression engine on pump gas with the given scenario and not have it detonating and pinging?

2) Is this something which I could run on premium and add an octane booster?

3) Aside from race fuel, or av gas, are there any other products which work well in a 11:1 engine?

4) If you currently, or have had a similar engine/set up, what has been your experience driving with pump gas?

School is in session...let the education begin!

Elon

[Salvatore]

You can keep your timing retarded a little and run 93 octane or you can run a fuel/led additive. I would run 93 octane and try to buy some Sunoco racing gas or even aviation fuel. Don't run your motor real lean either. VP is very good racing gas also. I have around a true 10.75 to 1 in my 69 Z/28 and I used 93 octane and some of Jack Podell's additive "Max lead 2000." It may or may not have helped but never had any pinging or detonation. I now run 5 gals. of Sunoco 112(all that I can get around here with lead) with about 6-7 gallons of 93 with about 36 degrees timing. Have your distributor recurved also for power and better timing adjustments.

[TimG]

I've owned my share of L71 Corvettes, some very original low mile versions. I've found nothing works like a splash of race gas. Jack's stuff works for 10 to 1 big blocks or high compression small blocks, but L72 and L71 need race gas to keep them happy. I use one gallon to three or four gallons of pump fuel.

[PxTx]

Tim, do you think the gains from the race gas is due to thr stock distributor configs from back when good gas was common? Or more to do with the nature of big blocks simply not responding to timing mods? Were the distributors modified on the low mile original cars?

[MosportGreen66]

With the cost $ of leaded race fuel in positive inverse proportion to the quality of pump gas, unless you're willing to spend copious amounts of money on race fuel, compression must decrease. There is no substitute.

[Salvatore]

I like that answer young man!

[Tracker1]

My LS6 Chevelle is dead-nuts on 11:1 compression - I use VP Racing Fuel - 5 gallons for very tank of 92 octane unleaded. Happy car. Without it, she's a dieseling b#*tch.

[Big Block Bill]

I agree. Anything from back in "The Day" with points, a carb, and over 10:00 to 1 compression will eventually get the rings rattled out of it if you don't give it a shot of real, honest to goodness, 100 octane leaded racing fuel in it once in awhile. In the real scope of things, with what we all have invested in our cars, what does it really cost for a 5 gallon can of Cam2 leaded racing fuel any way? Long live the Big Block!

Bill

[Salvatore]

occasional driving the racing gas won't kill ya in price. Don't really need alot just a decent mixture.

[MosportGreen66]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: [email protected]</div><div class="ubbcode-body">I like that answer young man! </div></div>

I crafted that with you in mind Sammy... lol...

Dave and I are frustrated with this very notion. We're building the motor to our '70 LT1 Corvette now and are hovering around 8.5:1 compression with a more radical than stock cam. We also had the heads ported for even flow throughout the rpm band. We'll be identical power without the headache or financial drain of always running race fuel.

[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.

[Hemicolt]

Have the valve seats been changed to hardened versions?

[VintageMusclecar]

No, no need to.

[Hemicolt]

I was asking the author of the thread.

[67 Nova Boy]

There's that racing fuel smell that gets me going! <<GRAEMLIN_URL>>/3gears.gif

I have Sunoco 260 running through my vains from birth...thanks to my Dad being a racer and my brother owning a Sunoco station. <<GRAEMLIN_URL>>/headbang.gif I run VP 112 straight now in my 302...like the way it runs...will see if it likes the Cross ram. Was putting in octain booster at $8.65 for 32 oz. with Sunoco 93,it made more sence to just run 112 for the amount of time it gets driven.

Dave
67 Nova Boy

[miket1]

I agree with Eric, I have ran 11.1 engines for many years on 93 octane Shell gas setup same as his,
use mechanical advance , make sure the carb. is not running lean,

[ss427copo]

In 'ol Yeller, it's 12.7 to 1 and I've run AV gas since 1984. For every 5 gals., she gets 3 1/2 ounces of each lead additive and Lucas Octane Booster. Runs great and all looks good with a recent removal of the heads for a long overdue sets of valve springs....

[elonblock]

<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.

[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.

[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 (http://www.yenko.net/ubbthreads/ubbthreads.php/topics/429671/Re_Actual_HP_for_an_L_78#Post429671)). 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??

[Salvatore]

Don't know if the octane is greater just some of the additives are better and maybe more of them Mark.

[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.