Anyone know the volume of the stock piston domes on the 401 or 425?? Info on the Ross pistons would be appreciated as well. I'm trying to determine the true compression ratios of our 401-425's, as well as determining what effect deck clearance and head gasket thickness has.
Walt, I just dunked one of a new set of Badger 401 pistons I have had here for about 10 years. The dome displaced about 50 CCs. I did this with the Egge pistons that went in my 425 several years ago and they were about the same (53 CCs?). Cheryl
Thanks Cheryl:TU: I estimated 49cc...working backwards from the NHRA specs and a CR of 10.25:1 That puts the 425 I put together in 1979 at around 8.8:1 CR. It was a relatively stock rebuild with a 0.040" head gasket with the pistons 0.050" in the hole. With the big Isky cam, that puts the dynamic CR near 7.0! It's amazing the car et's as well as it does. In a way that's good news...it means there is allot of potential power to be gained!
if you went 30 over with 129cc heads, 50cc dome , .040 head gasket and 50 in the hole puts you at 9.76:1 9.67:1 if you kept the stock bore.. unless I missed something. a stock 425 was about 10.1:1 if you take away the 30 over. add the .015 stock steel head gasket, and .055 in the whole.
Here are the calculations for a 0.030 over 425: values are in cu in Swept volume Bore= 4.3425 Stroke= 3.64 Volume= 53.883 Compressed Volume Cylinder head(129cc)= 7.872 Deck (0.050")= 0.74 Head Gasket (0.045 thick, 0.415" dia)= 0.689 Ring land= 0.0046 Total compressed volume= 6.2556 C.R.= 8.61 Substituting a 0.015" head gasket(4.42" dia)= 0.23 total compressed=5.7966 C.R.= 9.295 Decking the block to NHRA min spec of 0.010"= 0.148 total compressed= 5.1846 C.R= 10 Reduce head to NHRA min of 123.7cc total compressed= 4.8616 C.R.= 11.08 Changing piston dome volume by 5cc (0.305 cu in)will change C.R. by 0.6 to 0.8 C.R. So that measurement is critical. Based on my calculations, I'd bet most rebuilt Nailheads with allot of deck clearance(stock) and thick head gaskets have actual CR's below 9.0! And those that use low-compression pistons may have CR's in the low 8's or worse! Please check my math if you have time. I'd be interested in what others come up with.:TU: P.S. are there any spread sheet wizards out there?
OUTSTANDING info, thanks! there's easy calculators online also.. save the "embarrassment" (i wouldn't get embarrassed, but some pansy's do) of making a mistake.
wkillgs- were getting closer you have 9.295cr I have 9.66cr. 30 over 425, 129cc heads, 50 in the whole and 50cc dome .045 head gasket. I must be doing something wrong. not tring to argue with you just passing time on a cold snowy day. what head gasket has .045 fel pro?. you guys and girls[Cheryl] have been a great help to me. thanks
Depending on which brand of composite head gasket your using the compressed thickness varies from about .036"-.040", with an average being between .038"-.039". Being only .004" difference, without doing any calculations, can't be any more than .1 in compression either way. Tom T.
Buickfarm also has the steel gaskets listed for $25 pair. I wish Carmen had a web site...I don't want to phone him just for price checks. I measured a Fel Pro head gasket. It's 0.045 uncompressed. And the actual ID of it is 4.415"...so it's a bit bigger than the actual bore. The steel 425 gasket has an ID of 4.42", and is 0.015" thick I also calculated the 'ring land' area...which is the space between the piston and cylinder wall above the rings. It's not much, only 0.0046 cu inches (0.075cc). but those small amounts add up. For pi, I used 3.14 Area of a circle= pi*radius(squared) I would assume that piston manufactures reduce the dome on oversize pistons to maintain the same CR as the std bore piston. I tried to be exact in my calculations, but maybe there's an error somewhere. I hope not! Anyway, it's interesting to see how a standard rebuild will result in much less than the 10.25:1 CR. That can account for some power loss! Time to Google 'measuring piston domes'. How did we survive without the internet???
