How much compression for XR276HR? What about DCR ?

bobinbc · #1 (**permalink**) 08-20-2010, 03:11 AM

I have a XR276HR going into a 400 SBC with a couple choices of heads. The block is decked to .005" and bored .030", pistons are 4.155 Speed Pro L2489F flats w/2 valve reliefs (about 4 cc?), rods are 5.7, crank stroke is stock 3.75. With my head gasket 4.200 , .039 (9cc) and 64cc heads I get about 11.6 SCR and 76cc heads about 10.2

3500 lbs, 3.73 gears, 2300 stall, 700r4, victor jr intake, 750cfm carb, excellent cooling system with shroud, street&strip.

My question is how much static compression is ideal for this cam on 91 or better octane? Can anyone calculate the dynamic compression ratio for this combo? IC is 61* ABDC Which one of these Static CR's would get me in the area of 7.5-8.25 DCR ?

Thanks for any help.

iwannagofaster · #2 (**permalink**) 08-20-2010, 11:18 AM

The best I can tell, the 10.2:1static would give you about 8.25:1 dynamic. the 11.6 would be up around 9.35:1 dynamic. I don't have the exact science of it down, but I know the equation, so I can get pretty close (hopefully).

bobinbc · #3 (**permalink**) 08-20-2010, 02:23 PM

Thanks William

So if I understand this correctly then 10.2 :1 CR is about the max I could run with this cam on pump gas? I can't go any lower without changing pistons.The smaller heads are better flowing so I'd prefer to use them if the CR was safe/compatible with this cam and pump gas....

iwannagofaster · #4 (**permalink**) 08-20-2010, 02:51 PM

That is correct, but I have seen as high as 11.5:1 run on pump gas with aluminum heads. If it is a cast iron head, I woouldn't want to see anymore than 10.2:1 on pump gas, but you can run well above 9:1 dynamic with aluminum heads. I would not foresee any issues with this cam and 11:1, but 11.6:1, you would probably need a mixed fuel. If you could find a 69cc or 72cc aluminum head, that would be the ideal solution, but where it stands now, the 76cc is probably going to be the way to go. RHS does offer a 72vcc head, if you wanted to take a look at them.

bobinbc · #5 (**permalink**) 08-20-2010, 03:13 PM

Thanks, I will probably use my 76cc iron heads for now, the 64's are iron also. Will probably look into getting a set of alum72's in the future.

amick · #6 (**permalink**) 08-20-2010, 05:35 PM

Ok just let us know if you need anything else.