# What kind of gas do you use?



## extinctmake (Feb 8, 2011)

I have been chasing a compression ratio number since I started on my 1965 GTO 389 build.

I found a similar engine to mine on You tube built by a shop called DCI Motors. What they did was expand the combustion chambers of the heads to 80cc by cutting them into a heart-shape. They used flat top pistons and the engine ran nicely on the video. The cam they used had a much higher lift than the grinds I have thought about using.

I went ahead and forwarded the video to my machine shop owner/operator who has my engine and I asked what he thought about taking this approach. I haven’t heard back yet. 

My biggest obstacle with this engine project is lowering the compression. But maybe I should think more of an octane number instead of a compression ratio. I would like to run on 91 octane BP (Amoco) Ultimate premium. I can buy 100 octane race gas for $8 a gallon. As little as I drive the car, I wonder if I should just bite the bullet and use that. Maybe mix it with the Ultimate. 

However, maybe I’d drive my car more frequently if I could use pedestrian grade gasoline. Besides, if I take it out of town, I might not find 100 octane racing gas.

I’d like to hear back from the machine shop about enlarging the chambers on my heads. If I can drop the compression by cutting the chambers, I can use Sealed Power flat top pistons (#288P). And right now Northern Auto Parts has them for $40 each. The Ross pistons and pins from Butler will run about $850 and I really don’t need racing pistons in a street driven stocker. 

I’d also like to hear from other GTO owners. What kind of gas do you use? Do you have to add octane booster or set your timing back to prevent the engine from pinging? How much trouble is it for you to get your old goat to run on today’s gas?


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## john23 (Mar 6, 2016)

you can do various things to facilitate running on today's gas with a "high" compression engine...first, since it's apart, be sure the combustion chamber is smoothed, smooth the tops of the pistons also......any sharp edges cause hot spots which lead to detonation...second, zero deck the block., this reduces quench which will also reduce detonation, i don't know your elevation in nebraska but rules of thumb you hear about how much compression requires what octane are based at sea level with iron heads....the higher the elevation the less dense the air and the more compression you can run on a given octane...aluminum heads will allow a full point of higher compression with the same octane.. if you are cruising around town and don't demand high load of the engine lower octane is fine...put in higher octane when you take it to the track...also, if you want, you can dial back the timing to use lower octane....be careful you don't get into an overheating issue with retarded timing....overheating can also cause detonation which will mimic insufficient octane for the engine....finally, look at your car as a package...higher compression is used to support cams with more duration and overlap, which in turn require steeper gears which help unload the motor which reduces detonation...unless you are running a cam that requires more compression, don't build in compression for the wow factor...use compression to help your overall combo...my 455 with 7k3 heads here in colorado has 10.2 compression and uses 91 octane at the track and 87 around town...i have no detonation issues...on 91 octane with a properly built engine, in nebraska, (3000 ft?) i think 9.5 or a bit more compression ratio is very doable...good luck...


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## bigD (Jul 21, 2016)

I think you've covered all your options. You either need to lower your compression or raise the octane level. You'll just have to weigh your options, and decide which way to go. 

If you don't wanna mess with your heads, you'll have to use dish pistons. I think you can buy Auto Tec/Race Tec brand custom pistons for about $200 less than the Ross brand pistons. Another way to go is to bore .060 over, if your block will safely bore that much, then use std bore SP L2262F forged 400 pistons, and have a competent machine shop cut a dish in the piston top. There is plenty of meat, and this has been done lots of times. 

I would not consider building with CAST pistons at all !!!! If you do get a little detonation, FORGED pistons are more likely to survive. 

Icon makes a 400 piston with a 14cc dish, for just over $500 shipped. If your block will go .060 safely, and 14cc will lower you CR enuff, then these pistons could be a solution. The valve reliefs are not in the same place as in 389 pistons. But, unless you run a really high lift cam, the dish should provide plenty of valve clearance. If you know all the specs you can figure out how much dish you need to get the CR you want, using this CR calculator.

Compression Ratio Calculator - Wallace Racing

Another piston option is to bore .060 over, and use the Icon FHR IC9946-std pistons. They have 8 reliefs, for either early or late heads. And if these reliefs don't reduce compression enuff, a dish can be machined in the relief area. These are just over $400 + shipping. So, depending on machining cost, the total should be just over $500. Or, if you are handy with a hand grinder, you can dish 'em yourself. Hey, it's just an option.

https://www.rpmmachine.com/shop/index.php?l=product_detail&p=6276

If you are going to rebuild your shortblock anyhow, and are not set on using the block you have, or don't wanna bore it .060, you might consider switching to a 400 block. That way, you can just bore .030 or .040, use dish pistons, and have more cubic inches too. Hey, there are several ways to do it. Just depends on your budget & how you wanna go about it. 

If you wanna keep your block, you can also buy DSS pistons, with 5cc reliefs, and either have a dish cut in 'em, or have your head chambers enlarged. 

https://www.summitracing.com/parts/...BfaLkO20sVUcUJXHruDHAIQWJgBFIujDyMaAkU08P8HAQ

Hey, if you are not set on using your 389 heads, you can go with later heads, which have hardened seats, screw-in studs, and will reduce your compression with flat top pistons. DSS also makes a forged flat top for this app. DSS 389 pistons come in 2 bore sizes--4.080 & 4.100

https://www.summitracing.com/parts/...BRRZhwZaKg082kqdPIucX3LemSKdkYUvtQaAqZl8P8HAQ

https://dssracing.cld.bz/DSS-RACING171/38-39

Late '73 & '74 #46 small valve heads are the 1st late heads that come to mind, for this app. With these you can use an Edelbrock Performer intake, a good rebuilt Q-jet, and a small Voodoo cam. But hey, if you wanna keep your engine looking original, I understand that. 

I use Torco Accelerator octane booster in my 455 bracket engine. It is one of the boosters that actually works, unlike most of the parts store boosters. It's a little over $20 a can. 

The Best Fuel Additive Octane Booster Torco Accelerator | eBay


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## extinctmake (Feb 8, 2011)

Yeah I saw those Icon pistons listed on Spotts’ web site for $400. To use those pistons my block would have to be bored .060 and the compression ratio would be 9.8:1. I am resigned to having to bore my 389 .060 and use some brand of standard-size dished 400 pistons. Otherwise it’s $850 for Ross to make some pistons for my engine. I’m not sure if I really want to do that.

Before I assume my heads have the published 65cc chambers, I am going to take Bear’s advice and have them checked to find the true volume. With my luck they will turn out to be smaller yet…..I hope to hear from my machinist this week and find out what I have to work with. 

Spotts also offers hypereutectic pistons with a 17cc dish. Again, my block would have to be bored .060 to use these pistons and the compression ratio would be 9.5:1. Would hypereutectic pistons be as fragile as cast pistons against any detonation? Should I only use forged pistons?

I would have to figure whether 9.5:1 or 9.8:1 is low enough for 91 octane Amoco Ultimate. I am running the small AFB carburetor and a 3.23:1 gear, so it won’t be a stressful running gear. This engine will be strictly for street use, and my idea for a cam is either a Melling 068 clone or the Summit #2801 that has about 10-15 percent more lift than the 068.

And yes, I want to use my original 389 block/heads to keep my car the real deal. If I didn’t have the numbers-matching engine to my car, I’d already have plans to build a 1970s vintage 400.


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## bigD (Jul 21, 2016)

"...Would hypereutectic pistons be as fragile as cast pistons against any detonation? Should I only use forged pistons?..."


Stay away from ANY cast piston, including the "hypercast" type !!!


"...Otherwise it’s $850 for Ross to make some pistons for my engine. I’m not sure if I really want to do that..."

I'm almost positive that the Auto Tec/Race Tec brand customs, is a cheaper option. I priced a set of 350P customs. The price was just under $500, shipped. What they do is use a piston blank, in a common size, in this case, probably a sbc piston, then machine the top as needed, and put the correct size pin in the location you choose. That's another advantage of a custom. You can increase the pin height(compression distance), to reduce deck height, rather than cutting so much off the decks. That way all you need is to square up & clean up the deck surfaces, then have the machine shop measure to see what pin height you need, in order to put the piston top near zero deck height. 

So, considering all the advantages, I'd say if you can get these customs for around $600 shipped, it will be the best deal to achieve all your goals. But, if I'm wrong, it won't be the 1st time.

I'd recommend that you talk to a the guy I got my custom quote from, and see what he can come up with. 

https://shanonsengineering.com/coll...ll-block-pontiac-400-428-455-flat-top-pistons

Then also talk to Paul Knippen. He's a Pontiac engine builder, who I think can order the Auto Tec pistons.

https://www.facebook.com/PaulKnippensMuscleMotors/

I'd also definitely talk to Paul Carter, at Koerner Racing Engine. He is a well respected engine builder, who may have experience with exactly what your trying to do.

https://www.facebook.com/pages/Koerner-Racing-Engines/158663244168447

I'll inquire over on the PY site, and see if I can find any more good info on the subject.


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## Pinion head (Jan 3, 2015)

I'd drop an email to Dave @ SD Performance & see if he has performed the CNC open chamber mod to any "77"s or "093"s. Could save a lot of custom piston choice trouble, & end up with cyl heads that not only provide the C/R you desire, allow more port flow, & would not hurt the value of the heads in the least.


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## PontiacJim (Dec 29, 2012)

"Many late model engines today are factory-equipped with *hypereutectic pistons*. Hypereutectic pistons have a higher silicon content (typically 16 to 20 percent versus 8 to 11 percent for a standard piston), which makes them harder, more wear resistant and much stronger than ordinary cast alloys. Consequently, hypereutectic pistons are better able to resist ring pound out and scuffing. They also expand less than standard cast alloys, which allows them to be assembled with closer tolerances (helps reduce noise). For high performance, severe service or heavy-duty applications, hypereutectic or forged pistons are usually required."


"It is important to realize that the engine sees three different compression ratios. One is the *static ratio* which we are all familiar with: clearance volume + swept volume, divided by the clearance volume. A number like 9:1 is a common static compression ratio.
The second is the *effective compression ratio*, which the engine sees when the intake valve closes against the valve seat. A number like 7:1 is common. This is determined by the interactions of the static compression ratio, the rod ratio, and cam timing for closing the intake valve. (Wrist-pin offset has an additional but minor effect.)
Third is the *dynamic compression ratio* which is when the engine is in the peak power range and the volumetric efficiency is above 100% then the cylinder pressure-compression when the intake valve closes is at its highest, example above 8:1.
Building an engine for more performance often means raising the static compression ratio close to
10:1, but keeping the effective compression ratio not much over 7:1. Anything lower gives up power. Anything much higher will not run at low speed with WOT on pump gas without detonating and destroying itself.

Copied from an article written by D. Elgin"

Good quench, cam overlap, heat control, timing, and fuel ratio will lead to succes with high compression on pump gas. A step or two colder on the plugs can sometimes help.


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## 1968gto421 (Mar 28, 2014)

Ah, pump gas. There's a website that lists "gas station" that sell unleaded gas WITHOUT ethanol. I use this on my GTO as I haven't been able to go through and replace every hose and other things tat ethanol would eat up. Around here this highest octane on 10-15% ethanol gas is 91 octane. The places with non-ethanol gas have a rating of 93 octane. Of course they are more expensive than the ethanol-gas. Since my GTO with unknown compression ratio pings on 87, I went directly to 93. I also bought five gallons of race gas at my local track last year (5 gallons at $7 gal=$35 total) which is VP 110 octane. I have been mixing this in my tank to dilate the 87.

Hope this info is helpful.


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## bigD (Jul 21, 2016)

*Hypercast Pistons*

"Many late model engines today are factory-equipped with hypereutectic pistons. Hypereutectic pistons have a higher silicon content (typically 16 to 20 percent versus 8 to 11 percent for a standard piston), which makes them harder, more wear resistant and much stronger than ordinary cast alloys. Consequently, hypereutectic pistons are better able to resist ring pound out and scuffing. They also expand less than standard cast alloys, which allows them to be assembled with closer tolerances (helps reduce noise). For high performance, severe service or heavy-duty applications, hypereutectic or forged pistons are usually required."


While it is true that hypercast pistons are used in many apps, and they have been used very successfully in a lot of sbc builds, the KB BRAND hypercast pistons have failed in lots of PONTIAC builds. Most shops use SP brand hypercast pistons. But SP does not make hypercast pistons for Pontiac engines. The KB brand are the only ones I know of for Pontiacs. From what I've read, the main problem is the top ring gap. The gap must be larger than the 2nd ring, and above a certain number. If the gap is any smaller, it can & in many cases HAS caused the top of the piston to fail. For this reason MOST competent Pontiac engine builders refuse to use the KB hypercast pistons in Pontiac engines.

http://forums.maxperformanceinc.com/forums/showthread.php?t=800911&highlight=kb+hypereutectic

http://forums.maxperformanceinc.com/forums/showpost.php?p=5618552&postcount=57

http://forums.maxperformanceinc.com/forums/showpost.php?p=5424056&postcount=36

http://forums.maxperformanceinc.com/forums/showpost.php?p=5424147&postcount=38

Some have gotten by with 'em. But others weren't so lucky. Why take a chance ?

http://forums.maxperformanceinc.com/forums/showthread.php?t=727161&highlight=kb+hypereutectic


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## 1968gto421 (Mar 28, 2014)

Pinion head said:


> I'd drop an email to Dave @ SD Performance & see if he has performed the CNC open chamber mod to any "77"s or "093"s. Could save a lot of custom piston choice trouble, & end up with cyl heads that not only provide the C/R you desire, allow more port flow, & would not hurt the value of the heads in the least.


I think this is excellent advice as it would give the OP the ability to use off the shelf forged pistons and have a useful compression ratio, efficient combustion chamber, and better flowing heads. Also Dave at SD is a very knowledgeable and helpful guy. Well worth looking into.

SD Performance- Pontiac Performance Specialists

Phone: (604) 490-2211

[email protected]

Also big D is right about avoiding hypereutectic pistons---very susceptible to detonation damage.


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## PontiacJim (Dec 29, 2012)

bigD said:


> "Many late model engines today are factory-equipped with hypereutectic pistons. Hypereutectic pistons have a higher silicon content (typically 16 to 20 percent versus 8 to 11 percent for a standard piston), which makes them harder, more wear resistant and much stronger than ordinary cast alloys. Consequently, hypereutectic pistons are better able to resist ring pound out and scuffing. They also expand less than standard cast alloys, which allows them to be assembled with closer tolerances (helps reduce noise). For high performance, severe service or heavy-duty applications, hypereutectic or forged pistons are usually required."
> 
> 
> While it is true that hypercast pistons are used in many apps, and they have been used very successfully in a lot of sbc builds, the KB BRAND hypercast pistons have failed in lots of PONTIAC builds. Most shops use SP brand hypercast pistons. But SP does not make hypercast pistons for Pontiac engines. The KB brand are the only ones I know of for Pontiacs. From what I've read, the main problem is the top ring gap. The gap must be larger than the 2nd ring, and above a certain number. If the gap is any smaller, it can & in many cases HAS caused the top of the piston to fail. For this reason MOST competent Pontiac engine builders refuse to use the KB hypercast pistons in Pontiac engines.
> ...



Well, like anything else, everyone has an opinion AND machine shops/people don't pay attention to manufacturers recommendations nor double check all clearances - so there are going to be screw-ups and engine damages as a result. Here is what KB says about their Hyper pistons.

Special Notice on Top Ring End Gap

The Keith Black pistons unique thermal conductivity, ring location and varied end use requires special attention be paid to top ring end gap. KB pistons make more HP by reflecting heat energy back into the combustion process and, as a result, the top ring runs hotter and requires additional end clearance. Increasing ring end gap does not affect performance or oil control because normal end gaps are realized at operating temperatures. Failure to provide sufficient top ring end gap will cause a portion of the top ring land to break as the ring ends butt and lock tight in the cylinder. The broken piece may cause further piston or engine damage. Safe top ring end gaps can be found by multiplying the bore diameter by the appropriate ring end gap factor from the clearance chart below.

Example: 4" bore "Street Normally Aspirated" = 4" bore x .0065 = .026 top ring end gap. NOTE: Second ring end gaps do not need extra clearance. Dyno and track testing has shown that 34 degrees or less total ignition timing makes the best HP and time. Excessive spark advance, lean fuel mixture or too much compression for the fuel and cam used will make heat sufficient to butt piston rings with as much as .060" ring end gap. The entire top land can expand enough to contact the cylinder walls, when close to melt down temperatures are reached.

Recommended gaps are .026" for a 400 CI normal street use, .032" for towing, nitrous, supercharged use. Many of us don't read the directions and go with the factory ring gaps that we know or are used to, that being .020" top ring (from factory manual) or what ever we are used to gapping them with previously used pistons. 

Read this online about Pontiac engines, This Tech Tip is From the Full Book, HOW TO BUILD MAX-PERFORMANCE PONTIAC V-8S: " Generally speaking, modern ring packs are designed to operate with about .004 inch of top compression ring end gap for each inch of bore diameter." If we used this formula to go by, the top ring gap on our Pontiac 400 would be .016". With KB Hypers needing a minimum of .026" from the above numbers, you can see that we already have a .010" too tight a difference and if we got out pistons really hot, well the rings would be butting with the possibility of busting rings or ring lands.


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## extinctmake (Feb 8, 2011)

The detonation factor is what I’m concerned about. The 389 in my GTO seems to be Ok. I have the timing set to keep it at bay and I don’t pound on it much, if at all, to put it at risk. I rarely open the secondaries on the AFB.

The only forged pistons I can find is for a 400, which means I will have to bore the 389 cylinders .060 to fit these pistons. Spott’s has forged head relief KB pistons for around $400 and they are standard size 400 pistons. These net a still high 9.8:1 CR. My estranged machinist told me he uses Diamond pistons and he says a .060 bore will be required to use those as well.

I don't want to have to modify any pistons I find. I'd rather use something off the shelf if possible.

My alternative is the Sealed Power cast paper weights. It’s too bad those are not up to the task. I can buy them from Northern Auto for $40 each. But if they can’t hold up to any detonation, then it is not a good deal. I hope for no detonation, but I can’t chance using inferior pistons if detonation ends up occurring by chance.

I have waited long enough to hear back from my machinist. I am going to call him tomorrow and find out what is going on. I took my engine to him for a “clean and mag” on Feb. 11.


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## geeteeohguy (Feb 2, 2008)

Here is my experience, with both of my GTO's: I rebuilt the 389 in my '65 over 35 years ago, and it is an 11:1 engine. Big cam, headers, forged pistons, heads slightly milled. Over the years, gas dropped in quality and lead vanished. I have to run race gas when I drive it, which is not very often simply because of the gas requirements. My options are aluminum heads, or tearing down a perfectly sound 50k mile engine and installing dished pistons. Or simply leave it and run race gas. Right now, I'm leaving it alone. My '67 GTO with stock 400 started pinging in the late '80's as gas quality dropped. I tried water injection. No go. I bolted on a set of huge chambered heads which dropped the compression a bunch but allowed the car to run on regular. I finally got fed up with crappy performance and economy and installed a set of re-worked 87cc 455 heads. That fixed it. It's about 9.3 CR, needs premium, but gets great mileage and has good performance. Because I can run straight pump gas, I drive it all the time, and it's my 'go to' vehicle for long road trips. I've put about 6000 miles on my '65 in the past 25 years, and about 90,000 miles on my '67 in that same time frame. If you build it with high compression and settle for 'just running race gas', I honestly believe that you will not drive the car nearly as much as you thought you would or would like to. I personally may very well end up installing aluminum heads on my '65 so that I can drive it on pump gas again!


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## geeteeohguy (Feb 2, 2008)

Just read your last post. I have done the '400 pistons in a 389' before, with so-so results. At '060" over, the block can not be bored again or used again without sleeving. In my case, the engine ran hotter than optimum. Also, the pistons were not optimal for the valve angle of the 389 heads. No contact, but not optimal. A band-aid at best. Two friends of mine had a set of Ross dished pistons made for their 389's. These worked out excellent: plenty of power on pump gas with no pinging anywhere. Not super cheap, but absolutely worth it. If I were building a 389 today, I would go this route. Also, be advised that an engine can ping inaudibly. I had a 428 in the '80's that I destroyed by detonation I never hears. Pounded out bearings, blue crankshaft, and several broken pistons. I would seriously consider a proper set of custom pistons for your engine: done right, it'll probably outlast you.


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