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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

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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.

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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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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.

[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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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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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):).

 

Hope this helps!

 

BT

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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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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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  • 2 years later...

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 further:)!

 

BT

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Also the valve size change in 73 4V motors [ I think ] it had to be a very late change

Out of the 15 or so 73 4V motors I have had, never found a set yet yet.

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Only data I have, that I could find, for original, unmodified, unmilled, 71 CC 4V head shows 66.3 CC.

 

This is a subject worth discussing especially if a person is building an engine from scratch and optimal performance is the goal. As for any mass produced part of the time there is a tolerance range for chamber volumes, deck heights, rod center to center distance, piston compression distance. piston dish/dome volumes, etc.. As an example I've never seen a production block of this era that was the same deck height front to back or left to right. The the question of what has been done to the components before you owned them. Was the block decked, heads milled, valve set too high or too low in the head? Now factor in aftermarket parts that may or may not be what you need. Example: Summit lists 4 different piston compression distances for a 5.778 rod length, 351C +.030 bore. What are the dish or dome volumes? How far down the piston is the first ring land? What is the compressed height of the head gasket? What is the bore diameter of the head gasket? Optimal results require optimal research and effort. There are numerous other issues to consider. Assume nothing and do the homework if you want great results. Chuck

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I believe Chuck makes a number of REALLY important points (in the post above).

 

Due to the tolerances often found in these motors "back in the day", we usually "blueprinted" them as part of our "builds" for performance. Some "source documents" actually show a range of clearances instead of a set number which is probably more accurate for motors from the Factory.

 

Several years ago, some of the "prime movers" on the 460 Ford Site had a "Block Summit" whereby they brought in several versions of the 429/460 blocks and it was noted there were many variances with the same blocks. I would suggest that we will reach a similar conclusion with our 351C heads.

 

Again, I believe this thread could serve as an authoritative source if we plug in the data (real world experience) as it relates to what we see with the heads on our 351C motors.

 

Please don't be shy:)!

 

BT

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My 73 4V OC heads have standard size valves. The CC heads I replaced them with have no externally observable differences until the valve covers are pulled, then it is only casting numbers.

 

 

I have nothing more to contribute of value at this time

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Here is a little more information on what I mean when is say "cc the heads" for those who may be a bit confused:).

 

Most performance oriented engine builders probably know that compression increases as head volume decreases and the more varied the combustion chambers are the more likely the engine will not reach its full performance potential. This is the reason for blueprinting the heads to ensure each combustion chamber is the same size as its neighbor.

 

To do this, the heads are "cc'd" by filling each chamber with a liquid and measuring the results. To do this, you will need a burette (probably a 200cc model for the Cleveland chambers), a see-through plate with a hole drilled in the middle and a large dab of white brake grease. You can use automatic transmission fluid or 10 weight motor oil as the liquid.

 

It is important to ensure the chamber is kept sealed as you do not want any liquid to seep past the valves or spark plugs. You also do not want the liquid to slide under the clear plate. It is important to keep the head absolutely level.

 

Start by spreading the brake grease on the machined surface around the chamber you are checking. Place the plate over the chamber hard enough to effect a liquid retaining seal. Fill the burette until the level is on "0" for the calibration, then carefully slide the package over the filler hole. Open the spigot and allow the liquid to COMPLETELY fill the chamber, making sure no liquid is spilled. Once filled, there should be NO bubbles. Write down the amount of liquid ejected and move on to the next chamber.

 

Once you have checked all eight chambers, you can then start equalizing the chambers. Of course, you would try to match the smaller chambers to the volume of the largest chamber as it is easier to remove metal than to add metal:).

 

After the chambers are equalized, you can then mill the heads to get the desired compression ratio.

 

Yes, it is an "intense" process but it pays off when you are trying to get the best from your mass produced heads and you might be lucky and have a set that are "spot on" as it relates to shared chamber volume - - it can happen:).

 

BT

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Well if it helps the cause, I happen to have my '71 Boss 351 engine sitting on an engine stand. It'll take a while because of my schedule, but I'll CC the chambers and report back.

 

Steve

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Well if it helps the cause, I happen to have my '71 Boss 351 engine sitting on an engine stand. It'll take a while because of my schedule, but I'll CC the chambers and report back.

 

Steve

 

Well we can confirm if you fall into the 8-9 compression range you are in the low-po range! But gas is cheap for those that run it in that range. Pros and cons for everything.

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Does anyone have an NHRA spec sheet for the Cleveland? That would list chamber volumes.

[align=left]Jeff T.

 

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passengers. [/align]

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Yes, I believe this chart probably captures the "advertised" chamber volumes pretty well. I still believe there are probably real variances on individual heads.

 

Over the next week or so, I am going to try and have the two sets (Quench and Open) that I have cc'd to see what they have.

 

BT

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This was originally posted by another member. I've forgotten who it was or I'd acknowledge them. It should answer the chamber volume question, at least for 1973, as well as gasket volumes (different for 2V and 4V, difference in compressed height) Look on original document page 5, pdf page 8. Chuck

AMA specs mustang_1973.pdf

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This was originally posted by another member. I've forgotten who it was or I'd acknowledge them. It should answer the chamber volume question, at least for 1973, as well as gasket volumes (different for 2V and 4V, difference in compressed height) Look on original document page 5, pdf page 8. Chuck

GREAT posting, Chuck!

 

That explains a LOT as it relates to the 1973. I would say that (76.9cc - 79.9cc) is quite a variance on the 351CJ and explains a LOT relative to those different specifications brought forward by the "usual" sources.

 

Over the years, I have often wondered about the different compression ratios often shown for the stock 1973 351C engines. I wonder if there really were two different 351C 4V (4V and CJ) produced in 1973 as the document in the above post shows different chamber volumes. According to the "Hollander Interchange Manual", there was not but I have a "Ford Engines Book" (put out by Peterson Publishing) that shows four (4) different versions of the 351C 2V for 1973 and two (2) different 351CJ versions.

 

Very GOOD thread to bring back up, in my opinion.

 

BT

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