# What type of Gas?



## 65GTO1of1 (Jan 25, 2021)

So what are you guys putting in your vintage GTO's. Regular (89ish octane), super/premium (93 octane)? Anybody going with racing fuel? I'd love to put ethanol free in mine, but no stations near me that carry it.
Anybody using a fuel additive octane booster?


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## IceBolt (Jul 28, 2020)

I'm using regular 89ish, with STP octane boost and Hapco lead substitute - https://smile.amazon.com/gp/product/B01MTN30JG/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Runs pretty well with my Sniper EFI  

Would love to hear other suggestions.


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## Old Man Taylor (May 9, 2011)

91, but I lowered the compression when I built it. It's now 9.7:1.


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## O52 (Jan 27, 2019)

OMT
Thats good to hear. Is the 91 gas you're using also have 10% ethanol added? 
My new engine's compression works out to 9.5:1 and has a Melling 068 clone cam.


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## Old Man Taylor (May 9, 2011)

Yes, 10% Ethanol. One of the reasons mine is more tolerant is that I've got a very small quench. If I remember correctly it is 0.038". It might be as close as 0.035, I'd have to look it up. The danger with it that close is that if you lose a rod bearing then the piston can hit the head.


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## BearGFR (Aug 25, 2008)

65GTO1of1 said:


> So what are you guys putting in your vintage GTO's. Regular (89ish octane), super/premium (93 octane)? Anybody going with racing fuel? I'd love to put ethanol free in mine, but no stations near me that carry it.
> Anybody using a fuel additive octane booster?


I'm running 93 octane pump gas, but I built the engine specifically for it. (10.5:1 static compression with Edelbrock RPM aluminum round port heads, hardened valve seats, 'relatively/moderately' long duration cam, "good" quench, etc.)

Bear


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## rockdoc (Mar 16, 2009)

I'm running alcohol-free gas in mine, I believe it's 89 octane. My 67 runs great on it. But...I have a 2BBL car (which I love, BTW). It's pretty easy to find no-alcohol gas in central Indiana.


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## michaelfind (Dec 14, 2018)

I'm on the other end of the spectrum I guess. I was using Octane supreme, but recently switched to a mixture of race gas and 93 pump gas. I don't live too far from Bear but I have 10.5:1 with iron heads, so my motor is more prone to detonation. I also have the XE274H cam, which I'm now learning may be more prone to detonation. I'm still getting it dialed in as well, I just don't want to accidentally destroy the motor as I figure out exactly what works best. I suggest you listen carefully as you drive and try to find the most economical fuel available that does not destroy your motor.


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## 1967pontiac400 (Dec 10, 2020)

Ethanol free 101 from the pump since my car sits happiest at above standard timing.


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## IceBolt (Jul 28, 2020)

My GTO is my daily driver so couldn't afford 101 Octane at $8.99 per gallon in California!


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## 65Teal (Jun 10, 2020)

Here in B.C. I use Chevron 94 octane (no ethanol) and a lead substitute in my '65 GTO and in my '70 Camaro L34. Both are stock engines.


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## SolarRed68 (Jan 28, 2021)

My first post! Checked this site on and off for years, but finally joined and happy to be here. Glad to see this topic as well. I only drive my GTO 3-4 times a year (sadly), enough to make sure it doesn't sit for too long. I've long worried about what to do about gas, wondering what's best for the engine to burn, what will store best and not go stale, and what the ethanol is doing to the lines and other components. Engine is bone stock. For the past 15 years or so, I've bought the best pump gas I can (in my case, it's Costco TopTier 93) and then added lead substitute. I've tried to keep the tank at half to 2/3 for fear that there's crud at the bottom of the tank. This summer, I changed strategy and decided to switch to Sunoco Standard 110 leaded race gas. There's a drag strip close by with Sunoco pumps, so it's pretty convenient. I'm resting easier knowing that the engine is getting the octane it needs (and then some), the fuel has an extended shelf life (2 years +), it has proper lead, and there's no ethanol. The downside is that it costs $9/gallon.

Is it possible to run fuel that is "too high" in octane for our engines? I don't think so, but want to make sure the 110 won't cause any harm. The engine seems to love it, and I sure do like the smell of that race gas exhaust.


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## Old Man Taylor (May 9, 2011)

The 110 will not cause any harm since you don't drive it much.


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## michaelfind (Dec 14, 2018)

1967pontiac400 said:


> Ethanol free 101 from the pump since my car sits happiest at above standard timing


I never thought there was such a thing as too much octane, but now that OMT gave that somewhat cautious reply, I'm curious. Is there a risk of running too much race gas too often, other than lightening my wallet too much?


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## Old Man Taylor (May 9, 2011)

My reply wasn't about the Octane per se, but the lead in it. It's no worse than the gas of old. The unleaded gas is very easy on cylinder walls and plugs, it's just the valve seats that are a worry. I really shouldn't have added that cautionary statement, I was just remembering what the lead did to the cylinder walls.


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## michaelfind (Dec 14, 2018)

Old Man Taylor said:


> My reply wasn't about the Octane per se, but the lead in it. It's no worse than the gas of old. The unleaded gas is very easy on cylinder walls and plugs, it's just the valve seats that are a worry. I really shouldn't have added that cautionary statement, I was just remembering what the lead did to the cylinder walls.


Thank you. I was afraid I had yet another factor to worry about, "too much octane." Still, I just learned from you that the leaded fuels, and I suppose the Octane Supreme additive, can be bad for cylinder walls. I never knew that before. I thought the lead was a little extra lubricant throughout the whole combustion process. Y'all are years ahead of me in knowledge and expertise!


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## Old Man Taylor (May 9, 2011)

Don't worry. The lead was a lubricant for the valves, but it was actually an abrasive for the cylinder walls. When I ran an engine with 70K or so miles on it that ran only leaded gas, the taper in the cylinders was several thousandths, and there was a significant ridge at the top of the cylinder. However, the engines were still running fine. My first engine that ran unleaded exclusively came out at something over 25K miles. The machine shop asked if I had put any miles on it at all as there was no measurable taper or ridge. Remember, up until the 80's all engines ran leaded gas exclusively.


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## Old Man Taylor (May 9, 2011)

"Y'all are years ahead of me in knowledge and expertise! " - That's what happens when one gets really old.


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## PontiacJim (Dec 29, 2012)

Old Man Taylor said:


> 91, but I lowered the compression when I built it. It's now 9.7:1.



Iron heads? Stock cam?


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## PontiacJim (Dec 29, 2012)

rockdoc said:


> I'm running alcohol-free gas in mine, I believe it's 89 octane. My 67 runs great on it. But...I have a 2BBL car (which I love, BTW). It's pretty easy to find no-alcohol gas in central Indiana.



1967 400CI 2 bbl has 8.6 compression. Probably could pee in the tank and still run.


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## PontiacJim (Dec 29, 2012)

BearGFR said:


> I'm running 93 octane pump gas, but I built the engine specifically for it. (10.5:1 static compression with Edelbrock RPM aluminum round port heads, hardened valve seats, 'relatively/moderately' long duration cam, "good" quench, etc.)
> 
> Bear


Note *Bear*'s build is well thought out and his parts are matched. The aluminum heads allow for higher compression and his cam choice keeps cylinder pressures down at the lower RPM's which he pics back up in the higher RPM's - so 93 octane works due to a well balanced build.


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## Old Man Taylor (May 9, 2011)

PontiacJim said:


> Iron heads? Stock cam?


670 Iron heads, hydraulic roller - 242/248 at 0.050" on a 110 LSA.


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## PontiacJim (Dec 29, 2012)

Keep in mind, altitude/air density has a big factor on compression and is often overlooked or not taken into consideration. This is why 1 guy can run 11.0 compression on 91octane and 1 guy with 9.3 compression cannot and neither can figure out why.

Here is a little more info:

*PRE-IGNITION VS DETONATION*

Pre-ignition and detonation are not one and the same. Pre-ignition is exactly that, ignition previous to the desired moment while detonation is an explosion (same as when you detonate dynamite, B-A-N-G) of the fuel-air mixture charge instead of the normal smooth and progressive (productive) burning of the air/fuel mixture. However, one condition can progress into the other.

The normal and desired combustion process within the cylinder is rapid but it is not instantaneous. The air/fuel mixture burns evenly and smoothly with the flame front advancing at a controlled and measurable rate, about 35 feet per second as combustion begins and increasing to roughly 150 f.p.s. and then slowing down as the combustion process nears completion.

*Detonation*. If sufficiently heated and compressed, any combustible mixture of gasoline vapor and air will catch fire. If the temperature and pressure of the unburned portion of the air/fuel mixture within the cylinder reaches a critical value, a spontaneous and simultaneous explosion of all the remaining unburned charge occurs. This violent process is called detonation. You very likely have heard it coming from your engine, especially upon acceleration. It's typically an audible sound and it is often referred to it as "pinging" or "knocking". But it can also be masked by the loud sound of your exhaust. Make no mistake, just because you can't hear it doesn't mean it isn't there. It's important to realize that this spontaneous combustion occurs AFTER normal ignition and AFTER some portion of the charge has burned. The engine is unable to convert this explosive energy into useful work and horsepower is lost. Supersonic pressure waves are set into motion that can produce harmful and destructive effects on engine & combustion chamber parts such as damaging rod bearings, pistons, rings, valves, and head gaskets.

Since light detonation cannot usually be detected from the driver's seat, any effective protection from its occurrence must be provided in other ways i.e., prevention. Detonation is caused by either excessive temperature or excessive pressure of the fuel-air charge. Control of these two factors is what constitutes avoidance of detonation. Controlling these two factors can be broken down into two parts; (1) the design of the engine and selection of the engine parts - especially when increasing HP & TQ and (2) proper tuning/servicing/maintenance of the engine. The design of the engine and selection of the parts can include engine cooling design, fuel grade, compression ratio, ignition timing & ignition curve, spark plug selection, and the intake charge temperature due to a turbo or supercharger, all of which are some of the major factors that can promote detonation tendencies. They all influence the temperature and pressure of the fuel-air charge just prior to combustion. Proper tuning/servicing/maintenance of the engine can be seen as "operational control" of detonation by its owner/driver by adjusting and maintaining the best timing & timing curves for the engine, selecting the proper grade of octane gasoline for the compression used and outside air temperatures & altitudes, avoiding too lean of an air/fuel ratio, maintaining the cooling system in peak condition to keep the engine & air/fuel mixture temperatures down to a "best running" condition.

The most immediate and sure counteraction for detonation is a prompt reduction of engine power by letting off the gas pedal. Then adress the problem and make adjustments to eliminate the detonation.

*Pre-ignition.* Pre-ignition is the uncontrolled ignition by an object heated to an incandescence (glowing red hot) state within the combustion chamber of the air/fuel mixture before the normal ignition spark occurs. This premature combustion results in excessive pressure being exerted on the piston during the final portion of its upward travel on the compression stroke which leads to destructive tendencies. The same excess heat/pressure conditions that result in detonation are present when pre-ignition is encountered. But unlike most detonation, pre-ignition will usually be detected by engine roughness, backfiring, and possibly a rapid increase in engine temperatures. Any sustained operation, even for a brief period, in this condition can result in broken, cracked, or burned pistons, a broken cylinder heads, scuffed cylinder walls, and damage to the valves and sparkplugs. Pre-ignition can be caused by several possibilities, such as localized hot spots, carbon deposits, machining irregularities, sharp edges & pieces of metal that were not deburred or removed, , valves ground with too sharp an edge, and glowing spark plug electrodes.

Detonation can progress into pre-ignition, and vice versa. If you let detonation continue long enough to cause damage, this damaged area (in the form of broken parts or anything that causes a sharp edge) can become heated to an incandescence state - much like a knife or chisel's edge held against a grinding stone, pretty soon it glows. The same thing can happen depending on how the internal part within the cylinder has broken and now you've created an ignition device inside the cylinder(s) and the whole situation will get worse very rapidly.
Conversely, once the pre-ignition process begins, the rapid rise in combustion chamber temperatures can raise the air/fuel mixture temperature to its critical value, thereby initiating the detonation process.

Pic #1 Shows what the "Squish" area is the *OMT* mentioned. Very important.
Pic#2 General Compression/Octane requirement chart when using an octane booster.
Pic #3 Shows how cam choice can affect cylinder pressure Static (actual) Dynamic (cam choice)


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## PontiacJim (Dec 29, 2012)

Old Man Taylor said:


> 670 Iron heads, hydraulic roller - 242/248 at 0.050" on a 110 LSA.



Thanks, it should be noted that the "670" heads are closed chamber, not the open chamber heads from 1968 and up. The closed chamber heads will tolerate more compression. Tight squish as noted can help to ward off detonation by forcing the air/fuel mixture into the chamber for a more complete burn. Cam specs are also a plus. So the engine will work on 91 octane, but would be a problem with an open chamber iron head at 9.7 compression.


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## Old Man Taylor (May 9, 2011)

I used to run the same combination with cast 16 heads. The closed chamber heads have a slower burn and require more timing. It was an accident that open chamber heads “worked better”. They were designed to improve emissions. I’ve run 16’s and 670’s for years with the only notable difference that the 670’s require more advance. Typically about 4 degrees, but a lot of racers went further. The most I heard of was 42 degrees, but I never used that much.


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## gtojoe68 (Jan 4, 2019)

I run a mix of 93 unleaded from boat dock, no ethanol and 3 gals of 100LL Aviation fuel per fill up from the Airplane marina next door. Usually a 4 to 1 ratio depending on fill up. I’m lucky to live about 2 mile from Lake Washington though. Got 10:1 compression so in order to run the advance I like, I don’t have much of choice. Seems to work well. Cost is roughly $1.50 more than regular pump gas and since I only drive about 1000 mi annually, not too bad. Iron heads, mild build, Ram Air 2 grind cam. Had it rebuilt that way so it is fun to drive but still streetable.


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