Use of 110 Octane Leaded in 327

[Roger Legge]

Re: Use of 110 Octane Leaded in 327

My '68 L79 definitely runs better with about 3 or 4 gallons of 110 octane unleaded, with the rest being pump gas 91 - best one can find in Phoenix. Same timing, same everything. Big difference? No. But I've done it four times and every time the car pulls better and, odd as this may sound, idles slightly smoother. Granted, there's always the chance of one thinking this or that because it has 'the good stuff' in the tank, but I honestly believe there is a difference.

Roger
#36316

[John C.]

How about that Jack Podell additive?

Does anyone have any experience with that additive Jack Podell sells? I think it's called "Real Lead" or something like that.

[Verle R.]

Re: there is enought lead on the valve and valve s

Octane rating is a measure that indicates a resistance to pre-ignition. Therefore, higher octane fuels are harder to ignite. They also burn slower, which is why more total advance can be used.

If as specific engine is pinging a higher octane fuel may help. If an engine is not pinging and no other changes are made, higher octane fule will provide no measurable benefit.

An engine in good condition, with proper fuel mixture and ignition curves will run best with fuel octane that matches. For example, a 327 with 9.5 compression ratio may make most power with 91 octane. A lower octane fuel may reduce power due to pre-ignition. A higher octane will produce no more power and may actually lose power due to slower combustion unless the ignition curve is adjusted.

Verle

[Clem Z.]

Re: How about that Jack Podell additive?

as posted above high lead racing fuel has only 4 ccs of TEL and TEL is very,very poisonous and you have to wear rubber gloves when you handle it. the stuff you can buy over the counter must not contain very much TEL as it would very dangerous for the public to have. if it was pure TEL i do not think that he would be able to sell it over the counter because the federal goverment has taken it out of gasoline all over the country except for avgas and racing fuel.

[Dave F.]

Re: there is enought lead on the valve and valve s

Clem - your response raises another issue that contradicts what I have been hearing outside of this forum - Hardened valve seats are required in performance engines manufactured before the advent of mass produced unlead fuel. This is a new issue for me, as I have been absent from ownership of such an engine for almost 25 years, back in the days when leaded fuel was still readily available. My '68 L79 is completely tore down right now, and I was going to have the shop proceed with hardened seats. My intended use is for weekend street use only. I would appreciate everyone's thoughts on this.

[John H.]

Re: there is enought lead on the valve and valve s

Dave -

Unless you're going to tow a loaded trailer at 100mph all day long or road-race the car, hardened valve seats are unnecessary, and installing seats in early heads with 2.20/1.60 valves can be risky - not much material between the valve seats and the water jackets. With the kind of duty cycle our cars see, there's no reason to worry about it.

[Clem Z.]

you do NOT need hardened seat

for you application. remember back in the 30s and 40s amoco sold "white gas" which was lead free and thousands of cars used it. my dad bought it all the time because it kept the spark plugs "cleaner"

[Duke W.]

Re: there is enought lead on the valve and valve s

Octane is a measure of a fuel's resistance to detonation, not preignition. They are not the same thing and the spark energy required to ignite a fuel-air mixture does not vary significantly with octane rating. Higher octane fuels burn at about the same rate as lower octane fuels. "High octane fuel burns slower" is a myth! If detonation is present total combustion time is "shorter" because the end gas reacts spontanteously before being consumed by the normal flame front, but the rate of flame front propagation is essentially the same for all octane fuels in normal detonation free combustion.

Sixties vintage high compression engines may not be able to run best torque timing at all revs before they get into detonation. By raising the octane level the timing can be advanced to the best torque level, and since detonation is more prevelant at low revs, a quicker advance curve can usually be run, which improves low and mid range power. Engines can experience light detonation, which may be inaudible to the driver, but it will reduce power.

Duke

[Clare Carpenter]

9.5 vs 11.25 in a 327/340 Desk Top Dyno Run

Dennis, What engine do you have? What cam are you running? Your comment, "It is good to get real with what your real long term intent is.", makes sense. However, I am still debating what to do in my case.

I have a 327/340 which was rebuilt with 9.5 to 1 forged aluminum pistons, the heads were CC'd and mildly ported and polished. It has a blue print copy of the original spec cam. The rotating assembly was balanced and everything else is stock except for roller tip rocker arms. I haven't installed the engine yet and it hasn't been fired. I will be changing the original cam to the LT1 on Duke's and John Hinckley's recommendation, which I understand is a better compromise for overall performance on my engine, especially at the lower CR.

I have also considered swapping back to the 11.25 to 1 domed pistons prior to the installation of the engine. I know this will require me to blend fuel or retard timing. I do plan to drive my car when finished and I'd like to maintain as much of the original character as is practical. I can't seem to convince myself which way to go. I know there will be compromises either way. I wonder what my give up will be if I stay with 9.5 to 1. I know I will lose some horsepower and low end torque but how much? Maybe a Desk Top Dyno run would help convince me which way to go.

[Dave F.]

Re: you do NOT need hardened seat

Yep! I remember the white gas - that's all my Dad would use in small 4 cycle engines. That was in the 50's. The "not much material' between the water jacket and seats issue sounds a little scary. What if the machine shop has good experience in this area. If installed properly, no harm-no foul then - right?
Wrong? I'm stuck with them, unless I can stop the process tomorrow morning as the heads are already at the machine shop. Should have asked before....I know

[Dave F.]

Re: there is enought lead on the valve and valve s

John - thanks for the info. I may already be pregnant though unless I call the shop tomorrow and can stop them. This particular shop has a great deal of experience of installing hardened seats however, and have been in business since the 40's doing nothing but engines etc. Another required leap of faith, and as I told Clem - I should of asked first before plowing ahead. Let this be a lesson to others - when in doubt, use this forum first !!!

[Clare Carpenter]

Re: you do NOT need hardened seat

I had myself convinced I needed hardened valve seats installed before my rebuild due to magazine articles I'd read, etc. I had no practical knowledge of the issue and thought it made sense due to non-leaded fuel issues. My builder, who was old school (now retired) and who did all of his own machine work in house, told me that it wasn't necessary and explained why. He also explained the down side of installing them. He didn't want to cut into a set of what he called "cherry double hump heads" to install them. Obviously, this is a service a machine shop can sell but my builder said it isn't needed unless the valve seats are already severely receded. This isn't usually seen unless the engine had been subjected to long term severe duty or a few hundred thousand miles of use. Save yourself the money and call your shop in the morning and tell them to just do a normal grind.

[Jack H.]

Octane vs. energy

Much confused is the concept of octane. Yes, as others have mentioned, octane is a rating system that measures the consistency of ignition. Think of it as the variance (standard deviation) associated with a sequence of 'identical' combustion trials. Two mixtures of gasoline with different octane ratings contain the same energy content, but how/when they ignite is the difference.

Now, with today's 'environmentally friendly' fuels, alcohol is blended (ethanol, MTBE).... There's less energy content in alcohol vs. gasoline. If memory serves gasoline= 25K-cal/mole while alchol= 18K-cal/mole. So, there IS something to the notion that race gas or av-gas can deliver better performance when compared to 'typical' blended pump gasoline. But, it's not a matter of octane difference--it's an apples to oranges issue; REAL gasoline vs. a gasoline + alcohol blend.

[Duke W.]

Re: 9.5 vs 11.25 in a 327/340 Desk Top Dyno Run

Whoa! You need to slow down and do some measurments. Also, the plant started assembling 340s with two head gaskets during the '62 model year and this continued through '63, so the actual CR was no more than about 10.75 with the two gaskets and would be a little less than this with a typical .040" compressed composition gasket. A "9.5:1" piston with a compostion gasket and production machined deck and head is going to probably be no more than 9.0:1. Do you what to run regular fuel? The '71 and '72 LT-1 were built with 9.5:1 pistons, so they could run REGULAR UNLEADED FUEL.

Dropping the CR to that nominal level is going to drop your torque curve 5-10 percent across the range - a big hit.

The LT-1 and Duntov cams have the about the same effective overlap, so they can both run about the same CR and it can be more than milder cam engines because the overlap reduces the dynamic compression ratio. Owners of many vintage SHP engines, both original and rebuilt with the OE type pistons report detonation free operation with unleaded premium, perhaps with a little timing adjustment.

Measure you piston to deck clearance, and with the compressed gasket thickness and head chamber volume compute the actual CR. Shoot for about 10.5:1. If it's too high due to block or head machining, you can have the SHP piston domes machined off. The have a volume to 5.3 cc.

Duke

[Duke W.]

Re: Octane vs. energy

Modern oxygenated fuels typically have about 3 percent less energy, however, since they also have about 3 percent oxygen a typical rich WOT mixture of 12.5:1 will produce about the same power due to the extra oxygen.

The difference is fuel economy!!! Because we spend most the the time driving
in the cruise mode where the mixture is near stoichiometric, we have to open the throttle a hair more to achieve the same power and the net result is about 3 percent higher fuel consumption or 3 percent less MPG, which is about 0.6 MPG on a vehicle that typially gets 20.

Duke