help ident these heads
- Thread starter spstacman
- Start date
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77-82 318 heads with 1.78 and 1.50 valves. Are they good? i reckon it depends on what you're doin with um.
63dartman
Well-Known Member
Good for a daily driver. If your looking for performance just go with the iron RHS heads to start. More money up front but alot less in the long run.
I'm not familiar enough with 273s to really make that comparison. what are the casting numbers on your heads now? Those heads you're talking about may be open chamber heads. Your 273 probably has closed chamber heads. If so and you make the swap, you may end up regretting it. Maybe somebody can chime in who has more 273 experience than I. Why not just upgrade the whole engine? that would leave your 273 original and unmodded. Just a thought.
64physhy
Well-Known Member
Seems like the bolt pattern for the intake on '66 and later 273's is different than pre-'66 273's. If you change the heads, you'll might have to change the intake. It's been at least 10 years since I've done anything on my 273 other than oils changes and a battery, so maybe someone else can be more help.
"Those heads you're talking about may be open chamber heads. Your 273 probably has closed chamber heads." StrokerScamp is correct. Performance will probably be down but you should be able to run full advance with regular gas. You now have the standard intake bolt pattern and a better Intake manifold selection. You can get a good valve job and they have hardened exhaust seats. Have them checked for cracks. Keep your eye on the money involved as it may be better in the long run, as others have suggested, to start with other heads. I would not go bigger than 1.88 Intake valves myself. Too scary, notching the cylinders in the block for me.
cjh
Well-Known Member
I remember something vaguely about angles of the heads & intake differing at a particular point in time with the 273.....you know....prior this engine number and after this engine number.....somewhere around 67/68.
you don't think the drop in compression would be too much? I'd hate to see him end up with a 7 to 1 motor.....but like i said, I'm not real fimiliar with the 273.
273's were never smog motors. We actually had to put 318 open chamber heads on my brothers 67 Cuda to be able to run gas less than premium. You guys are too obsessed with compression. It is flow that makes you go fast. Not that you want 7:1 unless you are huffing a motor, but I don't think you could get to 7:1 with any factory combination on a 273.
I'm afraid you can get less than 7 to 1 with a 273...
bore..3.625.. stroke...3.31.. chamber vol...74cc(a 318 head) Head gasket .040 FelPro...Head gasket bore...4.100...piston to deck....040...flat top pistons with 4cc valve reliefs=6.48 to 1 static
Pretty much any open chamber head and that felPro gasket is going to cause the cr to come crashing down...
bore..3.625.. stroke...3.31.. chamber vol...74cc(a 318 head) Head gasket .040 FelPro...Head gasket bore...4.100...piston to deck....040...flat top pistons with 4cc valve reliefs=6.48 to 1 static
Pretty much any open chamber head and that felPro gasket is going to cause the cr to come crashing down...
1wild&crazyguy
Banned
what psi does an engine crank at 5000 rpm...50-70 psi.[example] a lot less than cranking dynamic.
more compression keeps that # high/er, which pulls on the heads=flow
with rpms comes less time for high psi seek the low psi in the cylinder, which is how you end up going from 140 cranking psi to 50-70.[example]
what pulls and pushes harder, 50 or 70?[example]
response comes from high compression , amongst other things, thats due to stronger signal to the carb which aids fuel mixing which aids torque which is multiplied by rpm which is HP...
you want as much compression as you can reasonably get away with on the octane you have available or are limited to.
jmo
heres a good read
http://circletrack.automotive.com/11016/piston-tech-air-fuel-optimization/index.html
and a lil quip from the artical..
Sir Harry R. Ricardo made certain statements about turbulence in his academically classic High Speed Internal Combustion Engine books of the early 1900s. Among them was the following: The rate of burning depends primarily upon the degree of turbulence and may be expressed in terms of increase of pressure per degree of crank angle. This approach is currently used in the method of Engine Cycle Analysis (ECA) being employed by leading Winston Cup teams. His statement clearly defines the fact that mixture completeness (homogeneity) and burn rate are related.
Ricardo also noted that the need for turbulence decreases in proportion to an increase in mechanical compression ratio (c.r.). Stated another way, as compression ratio is lowered, the need for and benefits from mixture motion increase.
For example, gains from turbulence in a 9:1 c.r. engine gasoline will be more substantial than in one of 13:1 c.r., although some gains are possible at the higher c.r. In addition, Ricardo suggested that a turbulent vs. non- turbulent engine tends to experience detonation earlier by comparison. This provides an opportunity to remove a measure of spark timing, allowing for an increase in net cylinder pressure after TDC and subsequent gain in torque (positive vs. negative work).
I beg to differ, but I'm not obsessed with anything. All I was trying to do was help someone. I said from the onset I was unfamiliar with the 273. I am fimiliar with them enough though, that I know they were not smog motors....however, they did have PCV valves, which is a form of emmisions control, so that point could be argued as well. You need to remember, that bore size has a serious effect on comperssion ratio as well. The 318 heads have open chambers, and they are sized for the 318 bore. The smaller the piston, the lower it's compression ratio because there is less surface area in a given bore. This is why bore size is an important factor when calculating compression ratio. I think it will be relatively easy to come down into the 7 to 1 range with open chamber heads on a 273. I've done enough machine work to take a real good guess. If I am obsessed with anything, it is trying to help someone. I would try to help you in the same way, if you needed it. So remember that next time before you throw around accusations.
The lowest compression 273 I have seen was 9.0 in 68 with the open chamber 318 heads. I have only really checked to the max my "64" and "66" 273's. The deck height was very close to 0, but they were TRW 10.5 pop ups. Assuming 0 deck height flat tops, 74cc heads, the .040 Felpro 4.100 head gasket (that does not leak) and a standard bore, I calculate CR as 7.8. I agree you have to check deck heights and valve notches and these will lower CR from there. The trick is you have to check these things. What I mean't about compression was that all small block heads run from about 63cc to 74cc. This also includes magnums at about 64.5cc. You can easily, and relatively cheaply, mill any small block head to go from the largest to smallest. Cutting .040 on 73cc "72" 340 heads will get you to 64.7cc. Early unmilled closed chamber 273's measure about 64.8cc, If I remember correctly. 318's went from 9.2 CR in 1968 to 8.6 CR by 1974.
Please take no offense, I did not mean to accuse, see my post above about compression. I assume we are all tring to help here, whoever needs it. The above heads were used in 68 and 69 on 273's and can be used to lower compression if you find your engine is in tune and it still pings, Like my brothers 67 Barracuda. Not hypothetical... It has 273 2 barrel shortblock and Direct Connection Vacuum advance Distributer. This 273 required 93-92 Octane to be happy at 35 degrees total advance. I have decades of experience building and trying different combinations on Daily driver 273's starting with a 64 4 speed Barracuda in the 70's. I feel those heads would be fine for his purpose and allow him to use the standard intakes in my opinion.
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