Which BBC heads should I use?? [Archive] - Chevy Nova Forum

: Which BBC heads should I use??


mrrepodude
25th-January-2007, 04:37 PM
here is my dilema..
I just built some 336781 heads dated H10 76/ O25 74 with new stainless valves, new compcams 924-16 springs,741-16 retainers and new 10 degree locks #612-16 to go with a comp cams H284 which has .574 intake and .578 exh.

I have since bought a date correct CTW 70 396 350hp engine. These heads are #3964290 dated C18 70/ D16 70

Now my question is..... do I spend the $ and swap my valves,springs,etc to the 3964290 heads or just run the later model 336781 heads?? and why?

thanks in advance, Jim

Professor Fate
25th-January-2007, 04:46 PM
I think there is an increase in combustion chamber size from the 290's to the 781's, so there is the loss of compression to consider also. The 781's will flow better though.

LUCAS BREMER
25th-January-2007, 04:53 PM
7 8 1 :beer:

jason snyder
25th-January-2007, 05:57 PM
the 781 is open chamber , and i think the 290 is closed.. therfore around a point in compression.SO IT DEPENDS on alot of other factors. cam specs, existing compression ,and goals..

veno
25th-January-2007, 06:01 PM
781 heads are 120 to 122 cc chambers, the 290 are 107 to 110 chambers.

350 hp 396 will not be able to tell the diff. you will loose about 2 points on compression with the 781 head, however you will be less prone to detention with the 781, with the 290 head will make around 10.25 comp with steel shim gaskets, with .039 felpro gaskets it will be around 9.6. either head will do. use the steel shim gasket with the 781 head less you wind up with a very low comp ratio. make sure you clay your pistons. you did not say what size valves are in the 781 head. if they are the stock 2.06 intakes then you will not have cylinder problems, if 2.19 intakes check.. check.... check...

just my o2.

NovaRich
25th-January-2007, 06:27 PM
7 8 1 :beer:

I agree with your choice, and the beer. :beer:

mrrepodude
25th-January-2007, 08:25 PM
781 heads are 120 to 122 cc chambers, the 290 are 107 to 110 chambers.

350 hp 396 will not be able to tell the diff. you will loose about 2 points on compression with the 781 head, however you will be less prone to detention with the 781, with the 290 head will make around 10.25 comp with steel shim gaskets, with .039 felpro gaskets it will be around 9.6. either head will do. use the steel shim gasket with the 781 head less you wind up with a very low comp ratio. make sure you clay your pistons. you did not say what size valves are in the 781 head. if they are the stock 2.06 intakes then you will not have cylinder problems, if 2.19 intakes check.. check.... check...

just my o2.

ok then, I ordered this cam with 10:25 to 11:00 c/r in mind. Will this drop in c/r using th 781 heads have any ILL effect on my performance? And yes, I went with stock size 2.06/1.72 stainless (this is for street fun) and clearance should NOT be an issue with the .574/.578 lifts. In past exeriences, 96 octane has been fine for my "mouse" motors running 10:50 c/r so again I do not think detonation will be an issue either.
Please keep the info and opinions coming...I am soaking it ALL in.

Thanks again, Jim

WILMASBOYL78
25th-January-2007, 08:40 PM
Need some more info: what pistons are in the motor now?? what are the other specs on the cam?? duration?? etc.... I like the 290 heads, especially if you have the right pistons and cam.....they are a closed chamber head but they make great power with the right setup...those open chamber heads will flow a little better, but the loss in compression is big.....there is real "seat of the pants" power when you have some "squeeze"....don't sell it short!

wilma

mrrepodude
25th-January-2007, 09:15 PM
The pistons are .30 GM replacement domes 10:25:1 (what ever the stock dome height was)
the block is #3969854 pad #T0420CTW (if this helps you get an idea what I am working with which is a 1970 396/402 2 bolt 350hp motor.)
The cam straight off the card is as follows:
comp cams grind #CB XE284H-10
groos max lift .574/.578
duration @.006 tappet lift 284 296
@.006 int:36 BTDC 68 ABDC
exh:82 BBDC 34 ATDC

cam installed specs
@.006 intake center line
Duration @.50 int. 250 Exh 246
lobe lift int. .3390 exh 3410
lobe seperation 110.0

veno
25th-January-2007, 09:45 PM
the 290 head is the semi open chamber head, not the 96cc closed but a better or good comprise between the 96cc and the 122cc head. the ports on the 290 are as large as the 781 ports, The chamber provides a better flame front than the 96cc. If I had to chose... I would run the 290. all it will cost you is the machine work for the transfer of parts... If you are willing to go that route. I prefer the 290 head .. did I say that before?...

To me it is the better head..... given the present combo. And will be very hard hittin lil rat

BTW where are you getting 96 octane? HERE is TEXAS the best I can get is 93

Mike Feudo
25th-January-2007, 11:08 PM
Set the deck heigth correctly and use the 290 head. For a street car a closed chamber head with the correct deck heigth will always work better because of the increased torque and detonation control.

mrrepodude
26th-January-2007, 09:26 AM
ok, so the 290's appear to be the choice.. Now, any real need to swap the valves or just spings,retainers and locks??? The 290 heads were just freshened.

veno
26th-January-2007, 10:42 AM
WERE THE FRESHENED 290 heads rebuilt to stock specs? if so you will run into valve float issues with you NEW cam. The springs will not hold up to the ramp speed of the new cam, Plus could run into coil bind issues with the stock springs and that would be bad. If the 290 heads are stock rebuilds with original valves then metal fatigue could be an issue using 30+ year old valves. You just acquired the correct engine and heads for your car, why skimp and take a chance on destroying what you looked so long for and paid money for ?

BE safe. put the good stuff in

mrrepodude
26th-January-2007, 10:47 AM
point well taken.. Man, I just HATE that I waisted $225. @ the machine shop on the later model heads...oh well!! also, I have a NEW set of 1.72 roller rockers (compcams) that I "acquired". Can I consider using them or stick with the factory rockers for now?

thanks again for all the input, Jim

veno
26th-January-2007, 11:06 AM
Yeah I can relate to that. Hate to spend hard earned cash then get that OOPS feeling.

the 781 casting will sell for about 200.00 bare most mid level performance bbc strip guys run them.

YES run the Rollers. Its worth a few ponies. Save the original rockers for if or when you sell the car. Besides the stock rockers had a tendency to gall with aggressive cams.

mrrepodude
26th-January-2007, 11:19 AM
Cool beans, as Steppen Wolf sang "get my motor runn'n" will be @ the machine shop this afternoon.

anyone intrested in the 781'S ??? hint,hint