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

So there’s a correlation between the compression ratio/ octane needed and the numbers you see during a cranking compression test. Aluminum heads is another big factor/game changer because of the heat dissipation.
When you see your cranking compression numbers very near 200 you’re going to have issues with pump gas and iron heads. You can tune around some and use booster additives.
There’s aluminum head out that are dead ringer copy of the iron head for the semi-purists Chevy guy. Then there’s a couple dozen others that can be painted orange and fool 90% of everybody.
Water/methanol injection also works and could be mostly hidden.

You’ve got a 4” bore, but smaller bores can handle more compression on pump gas, hemisphere heads also do better

 

They make a lot of fuels, here is a link https://www.sunocoracefuels.com/fuel-selector

 

Convert it to propane....

 

Put some thicker head gaskets on it to make it driveable, and put a pack of the correct head gaskets on a shelf in case someone ever wants to return it to "original".
ttps://www.hotrodders.com/threads/thick-head-gasket-to-lower-compression.98377/

 

Illegal maybe, but depending where you live, the airport owner may allow it. We have a place in NJ where you can literally drive the car up to the pump. Swipe your card. No questions asked.

 

Anyone considering adding thicker head gaskets to lower compression in an attempt to successfully run pump gas should research quench distance and detonation resistance. It may prove enlightening.

Terry

 

This^
It may get worse with a thicker gasket and loss of quench .

Ideally you want about .040" ( so they claim )
If the heads are original then they do not have hardened seats and unleaded is not your friend .

I had a real DZ block 302 in a Chevelle years ago and the 30-30 cam . I had to back off the timing on 91 octane fuel .
If it was mine id find some 68 CC heads for it or even bigger chambers with hardened seats .
Maybe the heads were already done .
Not many original z28 302's in non Cameros that have never been apart a few time or more running the original cam ?? hMMMM. It has value to the right guy and screwing around and hurting a piston can wreck the block pretty easy

I have a 327 that is a true 11.5 to one motor with 64CC heads . It has a 30-30 cam in it I built it in the 80's I can not run it on pump gas unless the timng is way to far back .
Its sittng on the stand here and I was going to put it in my 27 T project maybe and was thinking of going e85 as we have it around here . other wise what do i really do with it ?? race gas is not something Im going to deal with .
I suspect I could make it run OK by blending in e85 with unleaded to say about 25% or so and use a standard carb jetted up for it with the right stuff for ethanol not to eat it ( had to replace a holly needel vale last night on the vette after sitting ) but then Im not filling up at the pump unless im testing and crap and mixing from two pumps PITA . I considered some cheap Aluminum heads 70 cc or so but Is it worth dumping a grand into a 327 that I have redone at least 5 times in the last 40 year. It should have blown up 35 years ago .

The juice of the bragging rights to say you have a original 302 chevy might not be worth the squeeze if you drive it much. As other said octane booster . race gas mixed , water injection might get you buy but its a pain unless you need every last HP

I thought I saw one company was making a off the shelf 302 flat top piston for these when I looked a couple years ago . I have a 302 crank here and was half ass considering a turbo on it for a non HAMB friendly car . lot of screwing around and money to get less than with a bigger engine .

 

70 Cobra ( Torino ) 429 SCJ advertised Ratio 11.75 :1 . Pull down the license plate was a sticker , “ Use Minimum 105 Octane or serious engine damage will result voiding all factory warranty “ It would laugh at you , on 93 and a qt of octane boost ! My Buddy worked on tanker pumps at a local air strip , when tank had fuel and needed help I would get free fill up of Blue Gas .

 

Pretty much all the small block Chevys I build are between 327 and 406 inches. I only use aluminum heads anymore unless it's a restoration. The quench is always .040 or tighter. I have zero issues running pump supreme. I forgot to mention I shoot for 10.5 to 11.0

 

I never thought much about rod Leight and dentation until i built a SBC 348 chevy (400 block 327 crank) and 6.25 rods the combo was mild Comp roller Edelbrock heads and rpm intake zero deck and .040 head gaskets car ran on 89 octane. but a time or 2 87 was all available on audible pinging.

 

Yes, it may be called their Ultra line, you can do a search by zip code on the Sunoco site. I looked it up a few years back and the good stuff is out there.

I forget what the OK Petroleum by me is, likely 100 octane, they have a pump off to the side.

 

OR "gas"lighting? I'll see myself out...