Tips on determining piston compression ratio?

[CN Spots]

Got nearly all the stuff ready to begin the 390 FE rebuild but I have a concern... pop sed the pistons that were installed in the last rebuild (which was a wash due to improperly installed exaust seats) were 7 to 1 compression pistons. I ain't for that. Problem is, I have no way of telling as these pistons have no markings other than the 60 over stamp on the face. They are in good shape but if I KNEW they were 7/1 I'd cough up the $300 for a set of forged 9.5/1s.

Is there some old school trick to determining what the compression ratio would be this side of a full rebuild? They're not dished and everybody that has seen them thinks they're higher than 7/1 but if the wrist pin were cast higher in the piston it would lower the piston's height in the cylinder, affecting compression with little regard to top design (flat /dished) no?

Thanks
spots

 

[QC]

Re: Tips on determining piston compression ratio?

if the wrist pin were cast higher in the piston it would lower the piston's height in the cylinder, affecting compression with little regard to top design (flat /dished) no?

I don't have any tricks, but that's a true statement. Si.

 

[Reel Poor]

Re: Tips on determining piston compression ratio?

if the wrist pin were cast higher in the piston it would lower the piston's height in the cylinder, affecting compression with little regard to top design (flat /dished) no?

I don't have any tricks, but that's a true statement. Si.

Although it is true it's not a very good way to reduce compression. By reducing piston hight in the cylinder you increase the quinch area which is a bad thing. To much quinch area can cause detonation.

The only way to truely know what you have is to know what pistons (brand and part #) you have or assemble the engine and install one of the pistons and check the deck hight, know the compressed head gasket thickness, and the head combustion chamber volume, and do a little math work. :%

 

[dolluper]

Re: Tips on determining piston compression ratio?

Basic Compression Ratio is Bore+stroke+volume
OR
The ratio is calculated by the following formula:
,,,,,,, ". 2
CR =4 b S+ Vc
_____________
Vc

, where
b = cylinder bore (diameter)

s = piston stroke length

Vc = volume of the combustion chamber (including head gasket). This is the minimum volume of the space into which the fuel and air is compressed, prior to ignition. Because of the complex shape of this space, it usually is measured directly rather than calculated

 

[Bondo]

Re: Tips on determining piston compression ratio?

Identifying the Heads,(how Big the Chamber is).......
Measuring How Far Down the bore the Pistons are at TDC......
Along with the formula posted by dolluper will give you the Exact Compression Ratio........

 

[CN Spots]

Re: Tips on determining piston compression ratio?

Thanks guys. I'll try to head over to the shop in the next few days and try to figure this out. Being the mathmatical mastermind that I am, I'm sure I'll have just a few hundred more questions when I return .8)

There are three types of people in this world.
Those who understand math and those who don't.

spots

 

[CN Spots]

Re: Tips on determining piston compression ratio?

Went over to the shop today and made some quick measurements. If you're a Ford gearhead, the following might shed some light on things:

The heads are C8AE-H and the distance from the piston top to the top of the wrist pin bore is 1.172, the bore is .980 (according to my Chinese calipers) so I guess the pin height is 1.662. Neither the heads nor the deck have been resurfaced.

One member on the FordTruckEnthusiasts site confirmed that these are low comp pistons based on this info but I'd like a second opinion before I barf up $300 for a new set. Thanks.


spots

 

[Bondo]

Re: Tips on determining piston compression ratio?

and the distance from the piston top to the top of the wrist pin bore is 1.172, the bore is .980 (according to my Chinese calipers) so I guess the pin height is 1.662. Neither the heads nor the deck have been resurfaced.

It would be Much Easier to have just Measured How Deep the Piston Top is Down into the bore.......At TDC.....

 

[CN Spots]

Re: Tips on determining piston compression ratio?

That would have involved installing the crank, bearings,1 rod and piston and the engine is in pieces... I would have done that had someone not suggested that they could ID the pistons by the measurements above. Thats why I posted them. Turns out that the last shop used 410/428 pistons in a 60 over 390. I appreciate everyone's help on this.
d

spots

 

[Boomyal]

Re: Tips on determining piston compression ratio?

Spots, here is a static compression calculator:

http://www.rbracing-rsr.com/compstaticcalc.html

I don't see anyway around mocking up a piston and crank to get an accurate calculation.

Two other things that you might have to do, one if you do not have MFG'R piston specs, is to actually measure the liquid volume of any depressions in the top of the piston. The other is to measure the liquid CC volume of the combustion chamber in your heads.

If you know anyone at an auto machine shop, they have the measuring equipment to do both these. It is simple enough.

 

[SuperNova]

Re: Tips on determining piston compression ratio?

To me it all depends on how exact you want to get or if you are willing to accept some reasonable assumptions. Generally speaking most manufacturers don't reduce compression by lowering the piston in the bore, so it is reasonable to assume the wrist pin is in a stock location. They do lower compression by either dishing the piston or increasing the combustion chamber volume in the cylinder head. You already said the pistons were flat tops, now you just need to determine which cylinder heads you have. The casting number on the head should give you enough information to i.d. the head or at least get the year the head was made. You can also "c.c." the head with a burette(sp.) and some water. My bet is the flattop pistons are fine and any forged pistons (waste of money unless using nitrous or forced induction with high boost) will be identical unless you go domed(inefficient). my $0.02.