CajunCutlass

I am looking to rebuild either a #6 or #8 heads for my 1970 cutlass 350, plan is to put a cam in also, along with new pistons. which set of heads would be better?

coppercutlass

The #8's have appx. 79 cc's +/- the #6's can range between 68-72 cc's. It comes down to what cam you plan on running. On a zero deck .030 over flat top piston 350 with #8 heads after clean up appx 77cc's give or take you will have 9.5 to 1 compression with a .040 head gasket. Perfect for a nice cam and pump gas. Now if you want to avoid zero decking the block run the #6's. Both heads will need work the #8's will need a little more in the bowl area under the valves . It all comes down to your approach and how you want to build it cam compression and what the build calls for.


Edit : fwiw The #8 heads have bigger ex. valves that's one good thing if you have a nice core you can just go to a big intake valve . The #6's have less cc's but a smaller ex. valve.

johnnyjaws

1970 #6 heads have 64cc there the best u will need to put hardend valveseats in them
also YOU CAN PUT BIGGER VALVES IN THEM TO bigger is not alaways better

captjim

Gotta disagree with several of these statements. While the advertised chamber is 64cc, in reality most are closer to 68. Quite a few builders have debunked the hardened seat dealio. And, while the exhaust valve is fine, I think there are benefits to a larger intake valve. IMHO.

cutlassefi

Here are the facts;
#6 heads will average 68cc.
#8 heads will average 79cc.
The pistons that are in your 1970 350, unless it's a W31, should work fine with the #6 heads. Typically you'll have around a 9.0-9.25:1 compression ratio. You can mill them if you want more.
Stick the 2.00 intake in it, leave the exhaust alone, and go from there. That's a combination that's been proven to work well over and over.

CajunCutlass

both sets of heads will need to be redone, i am looking into probably bigger valves. Would bigger valves be better on the #6 or #8?

cutlassefi

Yes

coppercutlass

Mark he did mention he is going to new pistons so using his stock piston dish cc would probably not give an accurate compression number. I assume he would opt to go with a flat top speed pro's or 3cc probes etc. Cajuncutlass avoid the stock replacemnt cast units.

CajunCutlass

Thanks for the heads up, would the speed pro flat top work with bigger valves in a #6 head?

coppercutlass

I'm running speed pros in my current set up with #6 heads . The pistons are .025 in the cylinders and my heads where milled .024. Appx 66 cc's big valves etc. . I didn't need to mill the heads but I intended to run stock 14 cc pistons but all that changed at the time I had those heads done. I got some probes for the current under construction build. I would go with the probes they are a little more that the speed pros but they are lighter . There are many compression calculators which can help you play with the numbers changing deck height , head cc's etc.

cutlassefi

Gotcha.
Cajun - as mentioned do the Probes, lighter, stronger and a better ring pack.
Shoot for a 0 deck, with #8 heads you'll have approx. 8.9:1, with the #6 heads about 10.0:1. That's based on 79 and 68cc respectively.

Hope this helps.

CajunCutlass

ok probably a completely retarded question: do the chevy 350 sb pistons fit? there seem to be more options that route?

coppercutlass

Yes a chevy piston of proper size will fit if you use the correct length rod and have the crank machined for them. If you have the money its the way to go since you have better options for rods compared to the stock sbo rod.

Kennybill

What is your goal with this car. Street only driver with a little pep (pump gas), street/strip(high test+ gas) or mainly strip? I would decide use then build according. jmo, these builds can/do get costly. Ken

CajunCutlass

Goal is to have a street driver with some pep, nothing big just a little more get up and go.

coppercutlass

Keep it simple then. A nice 9 to 1 to 9.5 to 1 compression 350 with a good cam and properly set up drivetrain will be more than enough pep.

CajunCutlass

the guy I got the car from says it has a 4.11 rear end...i might be scaling that back a little

billmerbach

Well if you can find the high comp pistons from a 70 350 run it with the number 6 heads youll get somewhere around 300 HP basically stock which imo is pretty peppy that's what I'm doing and is it a 4 barbell or a 2 barel I have the 4

coppercutlass

High compression pistons would be flat tops I ran the cast flat tops on my car and after about 50 or so 1/4 mile passes and 3k hard street miles the skirts had take all the abuse they could take. If you don't plan on abusing the snot out of it if your shortblock is in good shape you could freshen up the short block deck the block a little and mill the heads and get the compression around 9.5 to 1 and have a nice runner. But freshening up a short block will run you around 1000 appx. all said and done with new cam and rings and all the other parts.

captjim

Just to clarify, the FACTORY high compression 350 pistons had a 6 cc dish while the low compression 350 had a 14 cc dish.

billmerbach

yea I'm talking about factory lol

coppercutlass

Jim w31's had flat tops. 325 hp 350 had flat tops. I have some in the basement that are left over.

Kennybill

Yes, 4:11 is too much gear for a street car or for a street/strip car even. I like a 3:23/3:36 even a 3:08 for just a 350 street car. Jmo, Ken

DoubleV

That may be true but those older horsepower ratings were not rated like they are today. That old 70 350 that puts out '300 hp' realy only puts out about 220 hp or so and 220 hp is fun to drive for all of about 10 seconds.

captjim

The W-31s were special, low production cars. For the masses, Oldsmobile make the low comp 350s with 14cc dishes at 9 to 1 and high comp 6 cc dishes at 10.25 to 1.

coppercutlass

I actually belive both are consider high compression. I forgot who posted the piston id for sbo chart but it had them there.

CajunCutlass

in the end I am looking to be around 300+ HP, I don't know what the stock rating is?

Fun71

Forget the stock ratings. As DoubleV posted, the early engines were rated differently and those ratings were quite generous compared to reality.

jag1886

I think you need to use what ever heads make your compression ratio come out to what will work best for you, I built a 10.2 engine with a small cam and it didn't work unless I could buy gas at one place here in Boise, but I use the car out one the highway and found the when you have to fill up with gas in nowhere Oregon it pinged so bad I was wondering if I was going to get home. I change from a set of #5 Heads to a set of #8 heads to get the compression where it's compatible with what ever gas I run across.

RockinRindy

I've been running the 6 heads but wonder if the 8 would be a better fit as it seems fuel rich with a 650 cfm and tried down jetting to see if I can cut the extra fuel smell but with no success, even upgraded the electronic ignitions distributor an still didn't cut the extra fuel smell from tail pipes, any thoughts?
thought about also going back to Qjet to see if its just the carb messing up as its an edelbrock 1407 on there now also here is the over view on that cam.
Also I'm running all the recommend longer push rods snd adjustable rocker arms not the stock ones.
Brand:
COMP Cams
Manufacturer's Part Number:
42-600-5
Part Type:
Camshafts
Product Line:
COMP Cams Thumpr Hydraulic Flat Tappet Camshafts
Summit Racing Part Number:
CCA-42-600-5
UPC:
036584213185
Cam Style:
Hydraulic flat tappet
Camshaft Use:
Street/Strip
Basic Operating RPM Range:
2,000-5,800
Intake Duration at 050 inch Lift:
226
Exhaust Duration at 050 inch Lift:
241
Duration at 050 inch Lift:
226 int./241 exh.
Advertised Intake Duration:
278
Advertised Exhaust Duration:
296
Advertised Duration:
278 int./296 exh.
Intake Valve Lift with Factory Rocker Arm Ratio:
0.491 in.
Exhaust Valve Lift with Factory Rocker Arm Ratio:
0.476 in.
Valve Lift with Factory Rocker Arm Ratio:
0.491 int./0.476 exh.
Lobe Separation (degrees):
107
Computer-Controlled Compatible:
No
Grind Number:
279TH7
Valve Springs Required:
Yes
Quantity:
Sold individually.COMP Cams Thumpr Hydraulic Flat Tappet Camshafts

COMP Cams is bringing the musclecar sound back, and it's nastier than ever! Few things grab our attention at a car show or cruise-in like a thumping, high performance idle. In fact, one of the most frequent questions from customers is, "What camshaft will give my car that mean-sounding idle?" For the performance and sound you demand, COMP Cams engineers introduce the Thumpr camshafts—a new series of hydraulic flat tappet cams. The lobe profile and grind characteristics of Thumprs use a combination of early exhaust valve opening, long exhaust duration, and a generous amount of intake and exhaust overlap—to maximize the rough-idling characteristics of the cams. They give you a seriously aggressive exhaust note without negatively impacting the power output or streetability of the engine. Thumpr camshafts are available in several grind profiles; each one will produce a rough, racy exhaust note and impressive amounts of horsepower and torque within a different rpm range.

Fun71

I can't imagine dropping the compression ratio by swapping the ~70cc #6 heads for ~80cc #8 heads would do anything at all for a "fuel rich" condition. You need to look at the carburetor, not the compression ratio.

RockinRindy

Thanks for kind of reassuring me that I was in the right mind thinking it was the carb messing up, also almost thought of trying 7A heads but didn't.

edzolz

Why did you not start your own thread instead of going back to 2014 to reply to that old thread?

olds 307 and 403

What are the specs of the rest of your motor?

joe_padavano

"Fuel rich" has nothing to do with the heads. Get an O2 sensor and verify the A/F ratio. Jet accordingly. Changing the heads is a great way to spend a lot of time and money introducing new and even more confusing variables.