BBC cyl head opinion wanted

ClassicComp · Post by **ClassicComp** » Sat Jul 14, 2007 7:18 pm

I'm building a .030 402BBC w/a set of trw L2287 pistons that have 39cc domes and a small roller cam .660 lift I&E 246°I dur 252°Edur grd on a 112 lobe sep which is installed at 107°Intake c//l.
car weighs 3000lbs race ready 5.14 rear 8"conv w/a glide and will be running on alky.

My questions are which gm cyl head to use (I have these already)
a set of closed chamber ret ports 291's w/100 cc chambers

a set of 206 oval ports 100cc chambers

or a set of 781's which will lower comp ratio to around 10.5
the comp ratio will be up around 12/1 or so with the small chamber heads
thanks guys

BLACK BART · Post by **BLACK BART** » Tue Jul 17, 2007 6:54 am

Hi ClassicComp, I'll go out on a limb and give you my $0.02 since nobody else has replied and there have been 150 views.

The 291 heads have ports that are really big for an engine of this size (around 320cc). Unless you plan to spin it to a pretty high rpm (8000 or so) you will not take advantage of this head and it will probably be soft on the bottom end. I would also be concerned about the weight of the TRW pistons with that rpm.

The 781 is a good head, but has large chambers that will drop your compression too low IMHO. These heads make some killer street strip engines in a 454 with some compression though.

The 206 head will keep your compression up there to match your other components and will give you a very usable port size that will not kill the engine with rpm. This is the head I would use if I had to choose one of the three sets you have.

I'm sure that others may have differing opinions, but there's mine. Good luck, CJ

jeff swisher · Post by **jeff swisher** » Tue Jul 17, 2007 10:53 am

my 781 with milodon 2.19 and1.88 valves installed and milled had 106cc chamber a little closer ,but alky likes compression ...why alky ? less compression ..possibly better overall flow , and less shrouding on gas might make more power ...I remember an article TRACO built a 427 chevy bb tested with open and closed chamber heads rectangular port ,even with race gas added to the higher comp engine it was still down 50 hp from the low comp engine on pump gas 540 hp level ...but then again with un shrouding of the closed head and mill back to the cc you need it might win the battle.if done right

rmcomprandy · Post by **rmcomprandy** » Tue Jul 17, 2007 11:16 am

The 291's aren't too bad when using alcohol but, would be terrible for this combination on gasoline.
The 781's with LARGE intake valves, (2.250"), would be my choice when limited to these three. Use stock size exhaust valve and get .030" offset head dowels to move the head toward the exhaust side.
The 781/240 is a mid size chamber, (not really open however, not closed either and the exhaust valve is certainly unshrouded), and they can easily be milled to 100cc's and lower. as 106/110 is normal production.

ClassicComp · Post by **ClassicComp** » Tue Jul 17, 2007 6:32 pm

I use Alky because I run a mech fi set up and also because it cost me a little of 2 bucks a gallon.
The 781's cc at 119cc chamber and I don't believe theirs enuf valve to cyl wall clearance to use a 2.25 Intake valve.
The closed chamber oval's will require grinding the comb chamber to clear the piston dome in spots.

needforspeed66gt · Post by **needforspeed66gt** » Tue Jul 17, 2007 9:53 pm

I would be using oval port - small chamber heads, not enough cubes to use the rectangle ports, and you need the compression with the alky, if you have to, just replace the pistons with a closed chamber version.

You don't need a 2.250" valve, use the 2.19"

ClassicComp · Post by **ClassicComp** » Tue Jul 17, 2007 11:03 pm

pistons are for closed chamber heads but will not fit a round port head comb chamber without grinding.

jeff swisher · Post by **jeff swisher** » Wed Jul 18, 2007 7:38 am

if your injection set up is for rectangular. then that is the way you should go for now....we raced a 396 425 hp years ago we had 2 -4 barrel carters on top a 246 duration at .050 solid cam in a 67 chevelle with 4 speed and 4.11 gears super gas class that car was no slouch ..the rectangular ports did not hurt the bottom end on that car. but we did twist a lot of rpm ,,i think it was 7900 ...that cam i may have the numbers crossed could be 264 too many years ago ...Things change

BLACK BART · Post by **BLACK BART** » Thu Jul 19, 2007 2:39 am

Hi ClassicComp, it sounds to me like it's time to break out the clay and a die grinder.

Fun times ahead.

As was stated earlier by needforspeed66gt, you don't need a 2.25" intake valve. For any of these heads, a 2.19"valve will work fine and give you more valve to bore clearance with that small bore and leave the valve unshrouded better in the chamber, especially in a closed chamber head.

jeff swisher has a good real world example of the rpm you will need to turn with a rectangluar port head like the 291 casting if you put them on a small engine like yours.

Jeff, not to argue or say your wrong, but the automatic trans is a little more sensitive to get set up right than your 4 speed would have been. If he misses on the converter, that is what will kill him on the bottom end. That's what you gotta love about a stick car.

Good luck, CJ

jeff swisher · Post by **jeff swisher** » Thu Jul 19, 2007 9:32 am

yes i completely agree ...i get all tingly inside , rembering the days of buddy racing auto vs stick ..you better have them both right ...I swapped out 3 converters one day and drove 24 miles across town to race a buddy each time to see which one was quicker . and what works in one car may not work in another ...same with a stick you better have a good clutch ,,and know how to use it ...I have seen over a second difference between drivers in a 4 speed stick car.