# Building A Mystery 400



## dialH4hero (Jan 27, 2020)

I'm having concerns with compression ratio limits to '69 #62 heads off a 428. So I bought a '70 400 to build and throw into a '68 Tempest. I was told the motor was stuck, but have found it's actually .30 over with new pistons, cam, timing chain, and crank. The heads are press in stud #16(I have freshly machined #62s on stand by) and a stout cam. According to the Wallace CR calculator I'm looking at an 11.58 CR. I'm fine with using octane booster for this setup and dealing with driveability, but it hold together or detonate?


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## Lemans guy (Oct 14, 2014)

I would use some different heads,....why deal,with all that mess. Build a nice 9.5 compression engine and love it.....


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## BearGFR (Aug 25, 2008)

Before you use any of the calculators (I have an Excel spreadsheet I built myself that also will compute real compression ratio), you have to make sure you know the actual chamber volumes in your heads. Pontiac heads are known to vary at least some from the factory nominal specs, and vary enough to make a difference. Plus when dealing with used heads, you never know if they may have been 'cut' at some point in the past. Always measure. It's not all that difficult to do. I made a "cc tester" myself using a tall graduated burette I got from Amazon and a sheet of plexiglas large enough to cover the chamber.

Bear


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## PontiacJim (Dec 29, 2012)

Read this:









FAQ - Engine Building: The relationship between Octane...


I grabbed several sources from both the internet and a few books to piece this together. The question of what octane should I use for my "X" compression engine seems to be an ongoing mystery. You can build your engine to any compression you want, but be aware of what octane you will need to...




www.gtoforum.com





If you have that high of compression, you may be using up to 3 oz per gallon of gas as you will want at least 100 octane, possibly more - so may be cheaper to go with racing gas?

You also want to have a cam that will work with that compression and not aggrevate it further - long duration, high lift, and 114-116 LSA which means your engine will pull best power at higher RPM's and be not so friendly at lower RPM's. So probably go with 4.11-4.56 gears and high stall converter if automatic.

Watch your valve to piston clearances and valve spring coil bind.


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