Hi, I'm trying to approximate my compression ratio. Its a 401 Nailhead bored 60 over, so I can calculate the increase in swept volume making the swept cylinder volume 15.4609 cubic inches. Can anyone give me the rest of the figures I would need to approximate a compression ratio. I don't need it super accurate. Its got standard 60 over pistons and a negligable skimmed head after a rebuild. Many thanks Gus
You really need to measure everything exactly. There are way more knowledgeable guys here when it comes to the Nailhead engines. Doing a little research here on V8, I read that the 401 pistons are domed, about 45cc. To figure out how much those pistons sit below deck requires just a bit of math. I relied on the table below posted on the TA Performance site, If you add half the stroke + rod length + piston compression height, and then subtract that from the deck height, 3.640/2 + 6.219 + 1.930 = 9.929. 10.025 - 9.969 = .056 in the hole. 4.187" + .060 = 4.247 bore, 124cc chambers, .040" gasket, I'm sure I probably made a mistake. Someone will correct me.
Larry, that is very helpful. Hopefully your figures are correct, but you are way ahead of me in the math department when it comes to the piston to deck clearance etc.
As Larry mentioned, you won't really know what's up until the parts are measured, including actual chamber cc's. On the '67 & up big blocks, we've come to know some of these numbers because they keep coming up after so many folks have already done the measurements, so it's a bit easier to come up with a compression ratio approximation without measuring. Hopefully some nailhead gurus will chime in and share what they're accustomed to seeing on things like chamber volume and deck height. It's good to know, because as we've already seen with the big blocks, the piston is much farther down the hole than what the "specs" say, resulting in much lower comp ratio's than what the math would show. Good luck! Devon
Larry, I found this on teambuick.com Blueprinting Specifications for Buick Nailheads Cubic Inches 401 Bore 4.1875 Stroke 3.640 Rod Length 6.220 Rod Width 0.967 Rod/Stroke Ratio 1.7088 Bore/Stroke Ratio 1.1504 Bore Spacing 4.750" Piston-Deck Clearance .025 Deck Height 10.0" Your figures are, if anything, more accurate looking than those, so I think you may be spot on. The only one I can't work out is piston compression height.
Larry has it close but the real unknown is the compression height of what they call a standard piston. 56 thousand in the hole can turn into 95 thousand in the hole with after market pistons.
1/2 the stroke + rod length + piston compression height. Then subtract that from the block deck height. I took all my figures from the table I posted. You'll have to measure your engine exactly to know for sure. That motor is over 50 years old, and I am sure you are not the original owner. Lots of things get changed in 50 years.
Sorry, just read all that again. The teambuick figures give the piston to deck clearance as 0.025 How does that sit with the calculation you made?
I'm putting 4xWeber 48 IDA carbs on my engine. I need to know the approximate jetting to begin tuning them and I asked Jim Inglese for help. He said the compression ratio was a critical factor in that, so that's why.
Excuse my ignorance, but is the relationship between compression test figures and compression ratio going to be helpful?
A cranking compression test will give you an indication of cylinder pressure which will be affected by static compression and cam timing. If you see pressures of 180 +, that can tell you something. If you want to be sure, you’ll need to remove a head and measure, no shortcut around that.
Agree, Larry's numbers are pretty close. And the available aftermarket pistons usually have a lower compression height than stock, What brand pistons are they? I'd adjust Larry's numbers a little based on the '64 401 I last did: - Chamber volume, 124cc, mine was 125-127 after a light 0.007" cut. - Head gasket bore, 4.250". If the gasket fits 401 and 425, it's larger. Fel-Pro's are 4.420" - Heda Gasket thickness, 0.045". An uncompressed FelPro is 0.050" so 0.045"? - Piston to deck clearance, 0.056". I was 0.045" with good pistons. Aftermarket could be 0.075"-ish. - Piston dome of 45cc is about right. I've measured 44.5 to 46.5 on Ross, TRW, and stock pistons. Here's the calculator: http://members.uia.net/pkelley2/crc.htm Increase the piston-to-deck clearance to 0.76" and cr drops to 8.755!
it's going to be an educated guess at best with out pulling the head cam specs would help, is it a stock cam ?
Piston compression height: Zollner are listed at 1.919", Silvolite at 1.912, Sealed Power at 1.905", and Egge are reported to be as bad as 1.90". That's a difference of 0.011", 0.18", 0.025", and 0.030". That can lower your compression by up to 0.5 .
Also, consider that if the head gaskets have been replaced any time in the recent past, they probably used newer composite gaskets. These are much thicker and drop you CR drastically.
