Technical What's the practical limit for compression ratio on pump gas?

My son-in-law just inherited a car powered by a '69 Z28 motor with an advertised 11 to 1 compression ratio. Supposedly the motor is stock with low miles, it hasn't been on the road since the mid-'70s. Will he be able to run this on pump gas successfully, or will a piston change be needed? Generally, you can retard timing to help with this and more-or-less get it to run but whenever I tried this (on other 60's 'premium fuel' motors) the results weren't good...

 

The only sure way to tell is to try it....there are too many variables....

 

GENERALLY ,9.5 -10 with iron heads , 10-11 with aluminum

 

Depends on cam as well. Worse case you throw a bottle of octane boost in each tank

 

Plus advertised compression ratios are for a blueprinted engine, who knows what real CR is, probably low 10’s.

 

1969 engines,don't have FI/Knock Sensors/computor control= You can not get away with low octane at 10 to 1 ,,max.>> At 10 to 1,it has to have min. of 100 octane or more. 12 to 1 > 105 oct min. Any less=detonation= killed the engine............................... <Info for fatfooters. If you drive like Grandma,you may getaway with crap gas.

 

If it's a real '69 DZ 302 with correct markings, and not in a '69 Camaro. You might want to check on what it's worth to a restorer or someone building a clone. Gene.

 

Found this a while ago. Just a guide mind you but gives you an idea.

 

Again, this is a '69 Z28 motor. High compression with pop-up pistons, solid cam (the famous '30-30' Duntov). I figured this should be a known quantity to the Chevy guys. And I have mentioned its value to a restorer, but he's determined to try to keep the car 'as-is'. But he does want to drive it, so some compromises may be in the future....

 

And I thought about race gas, but he may have issues getting it where he lives. There is an airport however...

 

A cam with a later intake closing , (ie) more duration would knock the compression down without changing anything else. Lippy

 

If you don't have an aircraft tail number is is pretty much illegal to buy av gas...

 

Think about piston crown, combustion chamber shape, 'Squish' and 'Quench'.
Lots of factors here.
Av Gas (the old Airport hi-octane fuel) was changed around 1967. The earlier 105 octane rate was
'superseded' to a government spec, and no longer 'worked like it had'!
Late bud Jack Belick found out with his faast '58 Chev 348.

 

It probably is on the early Z28 forum. This is the HAMB

 

100 octane minimum with 10:1? I'm not sure where you got that from. The real answer is that it depends on cam choice and a variety of other factors and isn't an arbitrary line in the sand. There are many people successfully running pump gas with 10:1.

https://www.onallcylinders.com/2020...1-compression-compatible-with-91-octane-fuel/

 

Exactly. It's not something most Chevy fans have had in their life.

 

My Olds is 11:1 but it runs aluminum heads and you can get away with it. Been built for 20 years and no pinging on 92 octane with no additives (octane boost). That DZ may require a bit or retard in the timing but like squirrel said, run and it find out.

 

Thats why water injection was popular back in the day cleans carbon out of the cylinders and helps down detonation. Olds did it on the jetfires big issue was not keeping the tank filled

 

I used to run with a guy that had exactly that animal .
He ran the living daylights out of that car,,,it truly caught it ,,,,,,, it never rattled or pinged at all .
He used Amoco white gas (premium),,,,,and added about a half a can of 104 octane boost .
It was truly one of the strongest small block cars here .
He experimented with different cams,,,,,intakes,,,,,everything .
It always was perfect,,,,in every way .
Yes,,,,,he had money,,,,and lavished it in that car,,,,,,but it never broke .
40 years later it is still immaculate.

So yes,,,,,factory style Z28 pistons are fine,,,with a little added attention to the fuel .

Tommy

 

True, but guys were building 301s with 30-30 cams long before the Z28 was built. GM corporate didn't invent this combo, they stole it from the hot rodders...

 

Yup...and my guess is that everyone who was doing it, had a slightly different combination, and different fuel requirements.

Really...put 91 or 93 in it and go for a drive. Bring along a distributor wrench in case you have to back off the timing a bit.

 

I run my 327/365 horse on premium pump gas, with 104 boost. Stock compression ratio is 11.25 on it. Stock solid lifter cam at factory timing. No pinging, no problems.

 

"Pump gas" is also a term of art. I've often heard that term used synonymously with 87 octane, but if we're being really technical about it, you can get 93 at a lot of pumps too.

My 10.25:1 engine will ping on anything less than 93 with iron heads, especially in the summer. The rest of my low compression junk will run on 87 just fine. My whooped Olds 324 will devour pretty much anything flammable.

 

In the 80's I ran a stock 350 LT1 that vintage with the factory 11:1 and factory cam on unleaded premium 10% ethanol pump gas. As long as you didn't lug the motor it seemed to be ok. Watch the timing and inspect the plugs until you gain a comfort factor.

 

I’m not sure if this is a direct comparison or not, but 10.25 (factory spec) 454 with iron heads was fine on 91. In the early 80’s I was coming back about 2 hours from home, and needed fuel, only place open had 87 octane. The lightest blip of the throttle and the engine rattled. I’m sure leaded gas was phased out by then and no clue if octane ratings changed either.
But, unless he wants the wow factor of having an OEM DZ engine, World Products, etc, have 75cc? heads that probably flow better than the DZ heads do, at under a grand. Less if bought bare and he wants to assemble them himself.
Oh how I like spending another’s hard earned dollar

 

Buy octane booster by the case. Every once in a while splash some race fuel in it. Stir in a little Marvel once in a while for lubricity.

 

What squirrel said.
What cam design, weight of car, gear ratio, manual or automatic trans.
The cam is the biggest question. What's the overlap and what's the duration ?

The "dynamic" compression will tell you what gas you can use, MUCH more than the "static" compression ratio.

Mike

 

I recall seeing some discussions here about extra thick head gaskets to get a little bit of compression relief. If that engine has some value as it exists now for the right kind of restoration project, a gasket fudge shouldn't devalue the engine because it's easily reversable.
I can get 93 hereabouts but others can only get 91 in some other places, I believe. What can the son-in-law get?

 

Just curious, would E85 help with the compression and ping? Isn't it supposed to run cooler than standard gas?

Of course probably have to change carbs but it might be an option to look into.

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