Piston Change

[twodko]

The fella I bought our 69 R/T from, the OO, pulled the original 10.5:1 pistons out of the 440 and replaced them with 9:1's about 10 years after he bought the car new. He said there was nothing wrong with them but the 9:1 pistons worked better with unleaded gas. What's the deal here guys? What am I missing? Thanks.

[FLG]

Depends on the rest of the build, 10.5:1 could be run at 9:1 or 12:1 it all depends on where they sit in the cylinder, and head chamber...etc

[flyinlow]

Until the 1970's tetra Ethyl lead was used as a gasoline additive to increase the anti-knock rating of the fuel and it also helped lube the valves and seats. As the manufactures' had to lower the compression ratios to help reduce oxides of nitrogen output ,reduce the amount of lead being released into the atmosphere and with catalytic converters coming the Oil company's started manufacturing unleaded gas that we have today. To get premium gas you use more iso-octane and less iso-heptane. In theory 100 octane gasoline is pure iso-octane. If you add enough tetra Ethyl lead to this you can get fuel up to the Aviation fuel  called Purple Passion (115 octane rating) used in P51 Mustangs during WW II. When they removed the lead from the fuel they had to start using hardened valve seats in the head and they ran lower compression ratios.

How much compression you can get away with , depends on many factors: fuel available, compression ratio, cam duration and overlap, quench design of the head/piston, ignition timing curve ,air/fuel ratio and the amount of deposits built up in the engine, just to name a few.  A stock headed late 60's 440 Magnum with a 10:1 CR  will probably knock a little on to days 91-92 premium unleaded and you may not be able to use as much ignition timing as you would like.  The 9:1 CR pistons would help with the knocking and you could run more ignition timing, but less HP and MPG.

Craig

[twodko]

Thanks guys. He did have hardened valve seats installed at the same time as the 9:1's were and put in a mild cam too although I don't know it's specs. The car runs very well but it would be a bit quicker with the original 10.5:1 pistons. Prolly more trouble than it's worth.........it's my delimma. I want more hp but want to keep it original too. Can't have both.   

[BSB67]

The fella I bought our 69 R/T from, the OO, pulled the original 10.5:1 pistons out of the 440 and replaced them with 9:1's about 10 years after he bought the car new. He said there was nothing wrong with them but the 9:1 pistons worked better with unleaded gas. What's the deal here guys? What am I missing? Thanks.

I don't think 69 440 had 10.5 CR.  And without knowing the details of what he did, there is no telling what the CR is now.  Unless it is a reverse dome, most off the shelf pistons are pretty far down in the hole.  Have you done a cylinder cranking pressure test?

[twodko]

No I haven't. The car runs very strong so I was just curious. I have all the receipts for what he had done. I'll dig them out and run it by youse guys.

[John_Kunkel]

The advertised compression for '69 was 10.1-1 but they were actually a half-point lower than that at around 9.6. With proper tuning they will run fine on 91 octane unleaded.