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351C Compression Ratio
#1
Good Day All,

I had thought about making a "Fun Question" on this subject but decided to just start a thread on it.

I have noticed a couple of threads where it has been stated or otherwise inferred that the 4V Clevelands have higher compression ratios than the 2V Clevelands. I think it needs to be stated that this is true under certain circumstances but NOT true under others.

I am curious as to who among you believe you can identify the circumstances where a 4V Cleveland has a higher compression ratio than a 2V Cleveland (and vice versus) during the 1971 - 1973 time period.

Lets dialogue.

BT
Do the RIGHT thing.
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#2
1970-71 4v "M" code had higher compression

1971 4v Boss "R" code had higher comp

1972 4v "R" code had low comp

1971-74 4v "Q" code had low comp

1971-74 2v "H" code had low comp

The aussie 2v heads had high comp

The difference is in the cylinder heads, open chamber heads had i believe 76 cc chambers and closed chamber heads had 67 cc chambers.
[Image: 25rnz1y.jpg]

~Buddy
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#3
Very GOOD start, WIDOWMAKER but I would like to try and focus on the actual compression ratio numbers - - as identified from the Factory.

For example in 1971, the Boss 351had WW as the compression ratio, the 4V had XX while the 71 2V had YY and so on...

I think it will be a GREAT learning process for some of us as we compare the differences (AND similarities) over the three year period.

More comments?

BT
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#4
IIRC the closed chamber heads produced a 10.7:1 compression and open chamber heads yielded a 9.4:1 ratio.
The Boss had an 11+:1 ratio due to a small dome on the pistons, other 71,72 Clevelands used flat top pistons. The 73 used a dished piston with an open chamber head for a 8.7:1 ratio on both the 2v and 4v. The 73 4v heads had the large ports with 2v sized valves.
Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

[Image: 1_12_09_14_10_15_11.png]
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#5
You are definitely "on to something" Jeff!

However, I would like to take it just a bit further. What was the actual compression ratios on the 71 and 72 4V and 2V, respectively.

Both of you obviously know the answer but I want to make it very clear to those who for some reason believe there is a BIG difference between the compression ratios for REGULAR 4V open chamber heads and 2V open chamber heads for a given year as it relates to compression ratio.

I submit there is a difference in port volumes in the 4V and 2V heads and I believe that is where the confusion is coming from as it relates to the perceived impact on compression ratios.

BT
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#6
1971 2V 9.0 240HP
4V 10.7 285HP
BOSS 11.7 330HP

NET HP IN 1972
1972 2V 8.6 161HP
2V 8.6 164HP
2V 8.6 177HP
CJ4V 8.6 248HP
4V 8.6 262HP
4V 8.6 266HP
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#7
Well done, TNFASTBK!

To summarize:

1971 2V has @ 9.0 compression ratio
4V Has @ 10.7
BOSS @ 11.7

1972 2v has @ 8.6 compression ratio
4V has 8.6
HO has 8.6

1973 All has @ 8.0 (uses a slightly different head and piston from 71 and 72)

The thing to keep in mind is that 1971 (for our scenario) is the only year where the quench (closed) chambered heads were offered. The quench chambered heads had a smaller chamber volumes which made for higher compression ratio when compared with the open chamber heads.

After 1971, the open chamber heads were used exclusively within the U.S. BOTH the 2V and the 4V had the SAME approximate chamber volume which means they shared the SAME compression ratio.

Going to open chamber heads when replacing open chamber 2V heads will NOT (in of itself) give you a higher compression ratio. Of course, you could change the pistons to reach that goal (on any of the heads)Smile.

Hope this helps!

BT
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#8
Advertised compression ratios were generally considered inflated.
73 conv. 460, D0VE large valve heads, Performer RPM manifold, Voodoo 227/233 cam, Holley 950 HP carb, C6 trans, 3.25 trak-loc.
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#9
I can agree with Tommy on his statement relative to advertised compression ratios (and HP) sometimes being inflated.

However, that does not detract from the fact that 4V and 2V open chamber Cleveland heads (for a given year) have essentially the SAME compression ratio from the factory.

BT
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#10
A very good point was brought up on another thread (THANKS again, Totalled) which speaks to often "different" published data as to the clearance volumes and resultant compression ratios of the different 351C engines.

Lets discuss "Real World" evidence, specifically I would like to address situations where you may have had your heads cc'd to determine combustion chamber volumes.

I will start it off by noting that when I had my original 1973 351CJ heads cc'd, there were variances in the heads which I had the machine shop equalize. Unfortunately, I don't think I have the original "numbers" anymore.

Lets dialogue furtherSmile!

BT
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