1971 351C-2V Piston type

[klinton994]

I have a 1971 Mach 1 with a 351C 2V I recently removed the heads to replace an improperly installed head gasket. Being that I am this far into it I was considering a cam change.

Looking at the pistons I can see that the lower end has had some work on it. The pistons are .030 over, and are a dished type piston.

Is a dished type piston correct for a 1971 351C 2V?

From what I have found, the 1971 351C 2V is rated at 9:1 compression. Is this ratio with the dished type piston that is in my car? Or, is this compression ratio with a flat top piston?

Thanks

 

[Mister 4x4]

I believe 9.5:1 Compression Ration (CR) is pretty standard for flat-tops with the 2V heads. I installed a set of .060" Keith Black Hypereutectic flat-top pistons in mine, and at TDC, they are flush with the deck with the standard valve depressions. The dished pistons would lower your CR by offering more volume to the combustion chamber, in the way of less piston taking up the area.

The old pistons I pulled out were most likely the stock pistons, and I didn't really notice any 'dishing' - they seemed like flat-tops as well... but they might've had a lower-than-deck-height profile, and still had the 9.0:1 CR.

If you want the recipe for my engine, click the 'Visit My Garage" button below my signature. The engine is running like a beast, supposedly is putting out somewhere around 400hp, and everything seems to be working really well together.

It ain't cheap, though. You can't just go in with a cam change and not expect to have a lot of other things that need to be improved upon along with it. The more cam you put into it, the more you'll need to improve the rest of the valve-train, along with induction and exhaust to take advantage of the increased capacity to move air & fuel through the engine. Depending on what kind of power you're looking for, you might visit CompCams and look through some of their offerings and maybe use their CamQuest utility - to get an idea of what the cam specs and numbers all mean and what they can do for your car. Then pick the manufacturer you prefer and look at similar products.

Hope this helps.

 

[red351]

Sometime just before 73 model yr, dish pistons were the normal for 2v and 4v. Sad times for performance

 

[Don C]

'71 2Vs should have flat top pistons with 9.0:1 compression. A PO may have replaced/swapped engines with one that is 8.0:1.

 

[barnett468]

there is often a part number on the top of the pistons . if you there is you can post that and we may be able to determine the piston volume . you can then measure how far below the block surface the pistons are at tdc . also post the numbers on your heads and with all this info we can give you a fairly accurate idea of what your static compression is.

 

[basstrix]

I made up a spreadsheet for calculating CR (primarily for stroker builds). As with any calculation, garbage in = garbage out. That being said, for flat top pistons assuming a 4cc valve relief I get the following max/min CR values for open chamber heads:

4.03 bore, 9.206 deck, 1.67 comp ht, & 77cc heads: 9.05

4.03 bore, 9.215 deck, 1.65 comp ht, & 80cc heads: 8.32

Notes: Volume of piston dish will have a major impact on CR. Knowing specific deck, and compression height is also necessary to get an CR accurate number.

 

[Don C]

Here's another spreadsheet you can play with. If you run your numbers in it please let me know if it matches yours.

http://www.7173mustangs.com/thread-engine-and-compression-calculations

 

[basstrix]

Hi Don, I don't see an attachment.

 

[Don C]

Yeah, I messed up, it should be there now.

 

[klinton994]

'71 2Vs should have flat top pistons with 9.0:1 compression. A PO may have replaced/swapped engines with one that is 8.0:1.

Don,

That is what I believed to be type of piston and cr. thanks

 

[klinton994]

there is often a part number on the top of the pistons . if you there is you can post that and we may be able to determine the piston volume . you can then measure how far below the block surface the pistons are at tdc . also post the numbers on your heads and with all this info we can give you a fairly accurate idea of what your static compression is.

I cleaned off the top of the piston. The only marking is .030 over

I believe Don is correct that the cr should be 9 to 1, and a flat top piston would have given that cr.

It appears to me that when the lower end was rebuilt they installed a lower compression fitting.

 

[barnett468]

there is often a part number on the top of the pistons . if you there is you can post that and we may be able to determine the piston volume . you can then measure how far below the block surface the pistons are at tdc . also post the numbers on your heads and with all this info we can give you a fairly accurate idea of what your static compression is.

I cleaned off the top of the piston. The only marking is .030 over

I believe Don is correct that the cr should be 9 to 1, and a flat top piston would have given that cr.

It appears to me that when the lower end was rebuilt they installed a lower compression fitting.

can you post a photo of the top of your piston?

you need to know what cc your heads are.

351 cleveland .30 over with flat top with no valve relief and 64 cc heads and .041 thick gasket is around 10.5.

74 cc heads would give the same combo around 9.4

any sort of dish is usually more than 10 cc and that would give you around 8.5 with 74 cc heads and around 9.4 with 64 cc heads.

.

.

 

[Don C]

There was a lot of pinging going on back in the late 70's and 80's due to gas quality and reduced octane ratings. It was common to see rebuilders using lower compression heads and pistons.

 

[basstrix]

Here's another spreadsheet you can play with. If you run your numbers in it please let me know if it matches yours.

http://www.7173mustangs.com/thread-engine-and-compression-calculations

Don,

We match to within 1 decimal on CR. I adjusted the head gasket diameter to 4.1" and used 9.206 deck height. Mine also used 1.65 compression height on piston.

Mine: 8.59

Yours: 8.54

 

[klinton994]

there is often a part number on the top of the pistons . if you there is you can post that and we may be able to determine the piston volume . you can then measure how far below the block surface the pistons are at tdc . also post the numbers on your heads and with all this info we can give you a fairly accurate idea of what your static compression is.

I looked for numbers stamped on the heads.

the only numbers I could find on the heads, are located near the valve springs. I located two different sets of numbers on each head.

Each head had a tag that appeared to be held in place by two screws.

1E19 on one head. 1E13 on the other head.

Each head also head a series of numbers in a circular pattern. The numbers are 19753. I do not know which number is the beginning figure.

I did not see any casting number on the outside of the heads.

 

[Don C]

Thanks much, I had run a lot of scenarios but had no way of verifying them. Given the variables possible in most of the calculations that's pretty close. The difference is probably in rounding of some figure, like pi.

 

[klinton994]

there is often a part number on the top of the pistons . if you there is you can post that and we may be able to determine the piston volume . you can then measure how far below the block surface the pistons are at tdc . also post the numbers on your heads and with all this info we can give you a fairly accurate idea of what your static compression is.

I cleaned off the top of the piston. The only marking is .030 over

I believe Don is correct that the cr should be 9 to 1, and a flat top piston would have given that cr.

It appears to me that when the lower end was rebuilt they installed a lower compression fitting.

can you post a photo of the top of your piston?

you need to know what cc your heads are.

351 cleveland .30 over with flat top with no valve relief and 64 cc heads and .041 thick gasket is around 10.5.

74 cc heads would give the same combo around 9.4

any sort of dish is usually more than 10 cc and that would give you around 8.5 with 74 cc heads and around 9.4 with 64 cc heads.

.

.

Barnett,

attached is a photo of the top of the piston.

 

[barnett468]

.

ok, those are at least 10 cc guaranteed and probably closer to 12 and they look exactly like badger cast pistons.

 

[klinton994]

ok, those are at least 10 cc guaranteed.

so, when I get feed back from another member as to what heads I actually have, I will have a good idea of my compression ratio?