Engineering’s a funny thing: there’s no such thing as “Best”, only “Best for a Purpose”. We’ll take The Ranchero’s cylinder head as a case study: I’ve got a good 200ci block bored .030″ over and a mystery head. I need to determine how much to mill off that head to ensure straightness and the proper compression ratio.
I wanted to do a whole tech write up on this process, but to be honest it’d just be recreating this diagram and repeating much of this Classic Inlines article about static Vs dynamic compression ratios. To summarize the first: there are many things that contribute to the ratio of displaced volume and leftover space at top-dead center. The second: gnarly cams effectively drop your compression ratio.
In my case, it’s a pretty common theme you’ll see: early-to-mid-’60s motors tend to have lousy cylinder heads with small ports and valves. Later 60s-70s-80s motors have better flow, but also larger combustion chambers. If you drop a later model head on an early motor, you’re likely to end up with a pathetically low compression ratio in the 7s-to-low-8s:1 range, so you’ve got to mill it down.
In my case, I measured the existing chamber volume using kitchen oil and a freebie drugstore syringe, then created my own Google docs sheet to do the math for how much milling gives how much leftover space.
With that in mind, what compression ratio would you target to eek a few more ponies out of a gutless motor without creating something that’ll detonate to oblivion on lap two?
Calculating Head CCs, Compression Ratio and Risk Tolerance
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18 responses to “Calculating Head CCs, Compression Ratio and Risk Tolerance”
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So are you milling the head or the block? If you are milling the head are you assuming that the bottom of the combustion chamber is a cylinder?
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I caught that too. It’s not, in fact it’s significantly less volume per unit length milled off than a cylinder.
To compensate, I told the shop to shoot for 35thou, which put me at like 9.6-7, but will probably be under 9.5.
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To play devil’s advocate for a moment, 8:1 is a decent compression ration for a turbo motor…
But in all seriousness, with the sharp edges and irregular shape of the combustion chamber, I’d be worried about hotspots at anything over 9.5-10. I’m not an engine specialist, though. -
You have a block that has been overbored. What about pistons? A taller replacement will also increase static compression.
Also, if you wanted to be totally scientific about it, you would cc the heads after they have had a valve job, if you plan on one. The valves will sink into the head a little. And use a piece of lexan with a hole in it to cover the combustion chambers to get a more accurate reading. The way you did it can have a little variance due to meniscus, the angle being off,etc.
Regular gas, or something with more octane?-
The pistons are a 6.5cc dish and have a .025 deck height.
I tried the plexiglass way, but honestly it was harder to tell when “full” than just observing the meniscus. The head was pretty damn level.
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10:1 is a nice, reasonably conservative, round number and is a pretty safe balance between power and tolerance for low octane. I’m assuming that you’re looking to keep it pump-gas friendly…are you willing to go premium only? E85 only? Methanol/water injection? Any of those would raise the ratio that you could safely run.
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91 is premium out here. If I have to go with octane booster, so be it. No way to get an e85 compatible carb on this thing for cheap.
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Mongo no calculate. Mongo just buy parts. Parts break, Mongo buy more.
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He wore a hat, and he
Had a job, and he
Brought home the bacon, so that
No-one knew… -
Mongo overqualified, but Mongo want to wrench for our LeMons team?
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Mongo only tool in garage of life.
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“With that in mind, what compression ratio would you target to eek [sic] a few more ponies out of a gutless motor without creating something that’ll detonate to oblivion on lap two?”
Precisely whatever we were running at Thunderhill in 2013. We turned ten laps, blew up, then turned two more the next day after cleaning out most of the debris and bolting everything back together, so you might even get away with slightly higher compression than that.
http://www.murileemartin.com/UG/LTH13/0312-Thunderhill_24_Hours_of_LeMons_2013-UG.jpg-
Ah, the patented self-adjusting dynamic compression ratio.
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Cool. I can remember seeing an article in Hot Rod when I was a kid, on how to CC heads. I don’t think they used plexiglass.
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In the day when I was building the DSP MGB I had to replace the head. I had the choice of a smog replacement head, an early model head or for a few bucks more, a shaved head. The last 2 would have put my into the class that was full on tube racers at the autox. Ended up with the crappy smog head.
Looking back I should have used the shaved head. I was getting crushed by the MKI Sciroccos anyway.
The d-ldos at the crap shop misdiagnosed a blown head gasket on the Ranger and left me with a truck that can only use 93. The bright side is that it added pep. It didn’t have that before. -
Hmmm… I knew those machining classes had use for more than just extra credits….
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So to summarize the facts so far: 91 Octane, a 6.5cc dish piston, and your aiming for 9.5:1. and assuming that you are going to use a thinner head gasket since your milling it. I’d have them actually aim for 10.5:1 because the shop is going to be conservative so the reality will be closer to 10:1. Even if they miss and you end up closer to 11:1 you can still use octane boost and it won’t be as lazy as 9:1 or 8.*:1.
I slightly different root would be: http://sfbay.craigslist.org/sby/ptd/5055511105.html-
Except that’s not in the budget unless he drops it in his Falcon and trades the 260ci into the Ranchero, and it’s a bigger pain to deal with rest of an engine swap too.
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