# Refreshing 1968 400, what pistons for better CR?



## brdngtn (Feb 1, 2020)

I pulled the 400 out of a 1968 4 speed I've had a couple years. It is original numbers matching with 96k miles and running ok but the usual leaking rear main, oil pools on the front lip of the valley pan and puffing out the breather enough for oil to run down and smoke off the exhaust. Coolant bubbles through a plug on front of passenger side head. Exhaust manifold gasket blown out on #8, and the clutch is slipping some (possibly caused by rear main leak), but the car needs a radiator support so all evidence pointing to - it's time...

During teardown I discovered the valley pan had a 2" section of gasket sagging down inside. Difficult to believe that was factory but obviously been that way a very long time. Explains the pooling mystery.

Rockers, rods and lifters cleaned up and looking good. "067" (P stamp) cam looked ok to me. The front bearing was flaking apart, can't see the rest but new set required anyway.

Pulling heads I found coolant had been seeping into #7 (rust film). Small ledge around top of all cylinders but walls look good. The section between the oil ring and compression ring was broken on pistons #1 & #7, about 1/3 the circumference at what would be rear of each piston can be pulled out in one piece (no shrapnel). Pistons otherwise in good shape, each worn clean in the same area but not through the shallow ridge texture that runs perpendicular to direction of travel. Rod bearings also look ok, worn through the silver layer uniformly starting just piston side of parting line, but continuing all the way around that half only on #4 and all odd cylinders. Crank bearings all look ok, no evidence of wear or damage to any area on any of them.

I am hoping to take the stripped block to my machinist today, and decided to post here for advice that may help others in a similar scenario. I only wish to refresh this motor with compression ratio more suitable for pump fuels available today. I will drive the GTO almost daily (I built a 68 Firebird for more "spirited" driving occasions).

I see kits from Butler with .030 over piston selections of -6 and -17 cc, which is stated as a decrease in piston volume but results in an equal increase in combustion chamber size. Assuming stock for 16 heads is 72cc, 17 more is 89cc for a *20% increase*. 400/8=50 cubic inches per cylinder or about 820 cc. This would also be increased by the same 17cc for 837 total, a 3% increase. The difference in the percentages is the change in compression ratio. I am stuck unable to determine which will provide the desired CR. I realize I am overlooking the .030 bore increase, but 820/72=11.4 (why is this not 10.75?), and 837/89=*9.4*
Please help,
B^)


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## BearGFR (Aug 25, 2008)

Check the FAQ section here. There's a post in it with a spreadsheet I built that you can use to plan compression. 
Mine includes all sources of clearance volume on the calculations, including the space above the top compression ring, between the piston and the cylinder, but below the top of the piston. 
Bear


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## O52 (Jan 27, 2019)

I used ICON forged pistons with a 14cc dish in my 68 400. 16 heads were milled resulting in a 71cc chamber. Bored 40 over
Estimated CR is 9.55:1.


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## geeteeohguy (Feb 2, 2008)

Have used Ross forged dished pistons in a 389 for about a 9:1 CR. Engine has been ripping along on 89 octane for about 15 years now. 
General rule of thumb is to remove the ':' from the CR and that is what octane you need. Examples---8.7-1 CR would need 87 octane. 9.0-1 would need 90 octane. 10.0-1 would need 100 octane. And so on. I have found this formula to work for me very consistently here in CA over the past 40+ years. This is with iron heads.


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## lust4speed (Jul 5, 2019)

Let's cheat and just get to some numbers. A .030" over 400 with 17cc dished pistons and 72cc heads with the pistons .010" in the hole will give you 9.27:1 ratio using BearGFR's Excel calculator. Those heads probably started out at closer to 76cc's when they were new - but we don't know how much material the cleanup mills have removed over the years and this rebuild might need another cut. You could very well be at 72cc's or even less but won't know until someone gets out the burette and does some measuring.

You really don't want to be over 9.3:1 with todays gas with a fair sized cam and should stay closer to 9.1:1 with a small tight cam that raises the dynamic compression ratio. The 17cc dish is right on the line but could work - and you definitely wouldn't want the small 6cc reliefs.

geeteohguy's rule of thumb works very well and I've stuck with that basic formula for our builds using California's excuse for gas.


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## geeteeohguy (Feb 2, 2008)

What lust said. My '67 GTO is right at 9.3 CR and it pings on 100 degree days pulling a load on 91 octane. Runs fine on it the rest of the time. Wish I could get 93 or at least pure gas. But what do you want for $6.50 a gallon? Also, since alcohol has 50% of the energy of gasoline, gasahol like E-10 will make your car run leaner, which can lead into detonation. If you have to run gasahol, you should richen up your carb jets a hair. I haven't touched my Q-jet in 35 years, so I stand guilty of procrastination!


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