Ctott70

My stock heads list 64cc for the combustion chamber size. I am having the heads rebuilt and wanted to install BBO valves. Is it necessary or beneficial to have the heads opened up to say 72cc or bigger? It seems to make sense to open up the combustion chamber if you increasing flow by adding larger valves. But I also don't want to lose too much compression. Any thoughts?

Thanks,
Chad

J-(Chicago)

Take all the compression you can get.
If you want to unshroud the valves, I'd send the block out for an overbore.

captjim

We have covered this a gazzillion times, but although the chambers "list" at 64 they are probably 68 +/-. What I did was go 2.02" on the intake and left the exhaust valve stock. I think the opinion is that the heads in stock form can't flow enough to justify the BBO valves, not to mention the bore shrouding issue J alluded to. I do think a larger intake and a bowl blend is a cost effective upgrade on a street car.

joe_padavano

I'm still struggling with the whole "bore shrouding" issue. The 2.07/1.625 valves work fine on an early 400 with 4.000 bore. They even work on a G-block 400 with 3.890 bore. Why will this be a problem on a 350 with a 4.057 bore? The 2.000/1.625 combo seemed to work pretty well in the W-31s, also.

cutlassefi

I agree Joe. General rule of thumb for an appropriate valve size is at least bore size divided by 2. 4.057/2=2.0285. Seems logical to me.

captjim

I was told by several Olds gurus (which certainly does not make it correct) that a 2.07, while maybe not bad, was more trouble than it was worth due to the design of the SBO head. Also, the guy who did my heads said that there were a lot of fast cars with 1.560 exhaust valves. I went on this advice, right or wrong. In a perfect world, someone would build an engine with 2 sets of heads and compare.

joe_padavano

I don't disagree that the 2.07 might be large for the stock SBO ports, but that's not a valve shrouding issue, that's a port issue.

HolidayCoupe

street or strip?? just a crusier? really how fast do you need to go?

captjim

I'll agree with that although my original post did state,
"I think the opinion is that the heads in stock form can't flow enough to justify the BBO valves, not to mention the bore shrouding issue J alluded to."

joe_padavano

Yes, I glossed over that.

On the other hand, I'm still of the opinion that larger valves (up to the point of shrouding) are still a benefit. Yes, the port may be the bottleneck at full lift, but the valve spends a lot of time with the lifter on the ramps of the lobe. There's a point on every cam lobe where the curtain area of the valve becomes the limit, not the port. At that point (and below), the larger valve provides more curtain area and thus should improve flow. I don't see that a large valve would hurt flow even if the port were the limiting factor.

At a minimum, the larger valve would allow you to run a cam with less radical ramps, lowering valvetrain loads for the same total airflow per intake cycle. Yes, this assumes you have an optimized cam profile, etc, etc. Just a thought experiment.

J-(Chicago)

Also keep in mind that his 7a headed motor probably has horrible pistons.
He could probably go smaller with the valves having 7.8:1 compression.

cutlassefi

Or stay with bigger valves and less cam for more cylinder pressure and consequently better performance.

Ctott70

I am looking to bore it 0.030" over, zero deck, and either flat top or 6cc dished pistons. Does this info change any opinions on this? Thanks for all of the info.

J-(Chicago)

Yes it does.
You can fit the 2.07 intake valves in, and a 1.625" exhaust valve.
Try a 30 degree intake seat, and a 45 degree exhaust seat.

I'd mill the heads down to CC between 65 and 70 if you need. That should put you between 9.5 and 10:1 compression with flat tops.

Recommended camshafts: Engle 2720H, Lunati 318A3, JM-20-22

Rickman48

And please don't forget to have the machine shop do a little 'bowl blending' to maximize the effect of the larger valves.
Great percentages to be gained there!

cutlassefi

Or an Erson TQ40

I disagree on the seat angles, if anything they should be reversed imo.

cutlassefi

Not picking on you but that won't make any difference at lower lifts if the side of the combustion chamber is the issue and not the bore.

J-(Chicago)

I don't take it that way. This engine stuff is fun and interesting to me, and I pay attention to valid opinions. You can feather that in, it's just a bit more work

BLUZIN

Hi Mark, I'm doing my research....this thread is 21 years old and just where I'm at today! Lol. Hoping to talk to the shop in the next couple weeks to pin down my numbers.. I'll call you for the cam, piston set up when I get there.
Jack

7314haywood

Not sure what your goal is, but stock valves in a 7a head 307 motor in a Cutlass has run 12.20 in the 1/4. That is pretty respectable for 3400lb car. On my 307 I'm. 004 below deck, .032 head gasket, 62cc combustion chamber and with stock dish pistons I'm 10.3 to 1 ratio.

joe_padavano

Are you talking about 7A heads from a 1985-90 307 or 7A heads from a 1972 350. This thread, which is now legally an adult, is about the 1972 heads.

7314haywood

Sorry I'm talking 88 Olds 7a head and motor I have. I was assuming that model head and not the early head. My friend has same motor and 7a head and has been 12.70 in a 88 Cutlass first Time out

olds 307 and 403

Nice. Cutlassefi flowed the 7A heads with just the bowls opened up with a cutter and 2"/1.625" valves, they were 210/170 I believe. Untouched the numbers are around 180/160. Yeah, 12.7 in a car that can weigh around 3800 pounds, is moving! Any other specs on the car? Include it in the time slip thread if you know the car well.

7314haywood

For class racing I can't touch bowls or runners. On another thread I stated my heads flow over 160. Your 180 number is actually closer to what mine flow. I didnt want to give exact numbers as it took a lot of 3 angle design time to get flow up. Stock diameter valves also required.

olds 307 and 403

Like I said, your car is impressive. You learn something new everyday. The 7A heads have a huge dog reputation, many complained about their Delta 88's being gutless, all 85 and newer swirl port motors, maybe because they were all OD trans with 2 something gearing. My 81 Delta 88 with 5A heads and a 3 spd with 2.41 gears was OK for power and a dam good car. The 7A Vin 9 cars were a 1/2 second slower in the 1/4 than earlier 5A head Vin 9 cars, stock. Not just magazine times, actual owners on this site and other sites. Aren't the 7A heads needed due to your car being a 85? There were some class racers who ran 5A heads, due to being pre 85 cars. Who would have thought the much bigger 5A intake ports were a disadvantage. Their exhaust port is horrible and the 7A probably have a good advantage due to the raised floor. That is just it, other than Dale Robinson showing what record chasing class racers do to 5A and also 7A heads, no one but a select few know or how to do the massive amount of work in the record chasing cars heads alone. I was shocked he could push 450 hp out a 307, it was a 85 roller block, wonder why he chose 5A or the 7A heads? Impressive just a good valve job can get those numbers from a 144cc port. I put 5A heads on a 350 Olds in a truck, they worked really well. My 94 Z71, idle to 4000 rpm out performed 5.3 LS trucks with the same 4L60E and gearing. Yes, 4000 rpm and over was a different story but who cares, it is a truck. Maybe the next Olds powered truck I build will have 7A 307 heads on an Olds 350😎.

77403

Well that depends on your compression ratio currently. And what you want to Accomplish. I would say keep the 64cc and port the intact and exhaust runners and match a intake for your cam grind. Would be better money spent than making larger combustion chamber

olds 307 and 403

This thread is ancient but recently revived. Many in this thread left the Olds World for greener pastures, a sign of the times. J-Chicago is a Challenger owner now. I do have a set of #6 heads with the 2.07" intake valve with a 1.56" factory exhaust valve. The bowls were opened with a cutter. It does fit and isn't right on the edge of the chamber. Throw them on a 403, which I just did and the bore opening shrouding is definitely not an issue.