A question of compression

[holorinhal]

I have been trying to run the math to figure the compression ratio of a particular set of pistons,and cant seem to get it right.So i turn to the ones with knowledge.
454 4 bolt block bored 30 over for a 4.280 bore size
scatt 4.25" forged crank(489c.i.d.)
block has been 0 decked
6.135" rods
gasket thickness .039(using a 4.280 gasket bore)
"049" iorn heads with 122cc open chamber( per Mortech,chambers have not been cc'd)
SRP forged pistons/w 17 cc dome-prt#212149
I an trying to figure out what static comp ratio I should be at wit these particular pistons.Thank's for any help....hal

[YeLLowBoaT]

do get it perfect I would need to know what your deck height is
using .020 you would be at 9.42

[SmokinLowriderSS]

Yep, Yellowboat is right. I get 9.43:1 asssuming .020" below deck piston height TDC. Need to know that too. At "0" (flush) it becomes 9.78:1

[steelcomp]

Look again454 4 bolt block bored 30 over for a 4.280 bore size
scatt 4.25" forged crank(489c.i.d.)
block has been 0 decked
6.135" rods
gasket thickness .039(using a 4.280 gasket bore)
"049" iorn heads with 122cc open chamber( per Mortech,chambers have not been cc'd)
SRP forged pistons/w 17 cc dome-prt#212149
I an trying to figure out what static comp ratio I should be at wit these particular pistons.Thank's for any help....hal

[Moneypitt]

OK guys, I have been over this with some piston manufacturers. When figuring the volume of the cylinder at BDC, do you subtract the dome volume? And then when figuring the combustion chamber volume at TDC do you subtract that same dome volume? In my mind you must do both to accuratly figure the compression ratio. So the CCs of the cylinder would be reduced by the dome, and the chamber volume would be added, then the dome would be subtracted from the quench area..........I argued with a couple of manufacturers because the math didn't play out to what they were claiming for compression ratio. How did you guys figure it????.......Ray

[Daytona100]

I got 9.4 comp.

[Daytona100]

Go to gearhead section check tech links lots of good free info there. :idea:

[steelcomp]

OK guys, I have been over this with some piston manufacturers. When figuring the volume of the cylinder at BDC, do you subtract the dome volume? And then when figuring the combustion chamber volume at TDC do you subtract that same dome volume? In my mind you must do both to accuratly figure the compression ratio. So the CCs of the cylinder would be reduced by the dome, and the chamber volume would be added, then the dome would be subtracted from the quench area..........I argued with a couple of manufacturers because the math didn't play out to what they were claiming for compression ratio. How did you guys figure it????.......Ray
Start with cyl Volume (*Bore X Bore X stroke X .7854 = c.i., X 16.39 + cc.)
*Cyl vol. (V) =1002.18cc
You add:
*Dome vol (Vd) -17cc ...with a bump, you subtract, since you're reducing the vol. above the piston, with a dish, you add.
*Valve notch vol (Vn) -unknown
*Area above top ring (Vr) -unknown
*Deck clearance (Vp) =0.00
*Gasked volume (Vg) =9.19cc
*Chamber vol. (Vh) =122.0cc
this gives you the volume clearance (Vcl)
then the formula is:
V + Vcl/ Vcl
(V) cyl vol = 1002.18
(Vd) dome = -17
(Vp) deck = 0.0
(Vg) gasket = 9.19
(Vh) chamber = 122
Vcl = (114.19) + V (1002.18) = 1116.37cc divide by Vcl = 9.776:1 cr

[holorinhal]

Thanks every One!
Steel , That formula is alot clearer than the one that I was working with.How important is knowing the valve notch and area above the top ring? would knowing these two parrameters likely lower the CR a little bit? The compression ratio That You came up with,Is pretty well below where I was hoping to end up.I am looking for closer to 10.5:1 or a smidge higher.I Gues I was really more disapointed in the Chamber size of the "049's".I guess i was expecting somewhere at least 118-116cc's.Maybe I will have to consider a larger domed piston!
According to the book that I am reading("how to build Max performance Chevy Rat Motors") the formula goes like this-
"1+(.7854 x bore x bore x stroke) divided by [( chamber vol +head gasket vol + piston to deck vol ) - piston dome vol] = CR"
All values are converted to cubic inches in volume.
I do not understand where the "1' at the beginning of the formula comes from!...Thank's...Hal

[SmokinLowriderSS]

454 4 bolt block bored 30 over for a 4.280 bore size
scatt 4.25" forged crank(489c.i.d.)
block has been 0 decked
6.135" rods
gasket thickness .039(using a 4.280 gasket bore)
"049" iorn heads with 122cc open chamber( per Mortech,chambers have not been cc'd)
SRP forged pistons/w 17 cc dome-prt#212149
I wasn't sure exactly what he meant by this line, which is why I gave the 2 figures I gave, the one being with the TDC piston flush with the block face. 0.000" deck height. 9.78:1

[SmokinLowriderSS]

I didn't have a formula to use, but a couple of programs to plug numbers into. Lazy baaahstaaahd I am.
If you want 10.5:1 with that setup, locate a 26cc domed piston. 10.53:1
10.44:1 with a 25cc dome.

[steelcomp]

Hal, that's the formula I've always gone by...there's lots of different formulas out there. Some confusing, some not so. I always compute to cc's, since that's what most vol's are given in, except cyl. displacement. Cubic inches X 16.39 will give you cc's.
Yes, the valve reliefs and area above the ring can add a few cc's, and lower the compression. I always do a liquid pour with the piston in the cyl, which gives an actual exact dome volume. In most cases like yours, it's really not critical.
You could get a bigger dome, which means a heavier piston, or find smaller chamber heads.

[Moneypitt]

The formula you show doesn't account for the dome in the chamber. The 122 would be reduced by 17, so the quench area is actually 104. You are taking the cylinder volume and stuffing it in a considerably smaller area than 122. (104)........104 is around 4/5ths of the total 122, thats a 20% error............Isn't it??.......You also didn't add the chamber volume into the cylinder volume, which would add, in this case 122cc. As I said, there is alot missing for dead on figuring. Maybe I'm missing something here, or maybe not........Ray

[steelcomp]

You're right, but you're forgetting about the gasket thickness, which when added, gives the volume clearance of 114.9. It's all there in the formula. If there was any deck height, that would have to be added as well. Think of these things as basically increasing the size of the chamber.
Compression ratio is defined as the ratio between the total volume (cyl volume plus clearance volume) above the piston at BDC and the clearance volume above it at TDC. The clearance volume has the dome calculated into it. When added to the cyl volume, the cyl volume includes that calculation.
Hope that helps.

[Moneypitt]

The clearance volume has the dome calculated into it. When added to the cyl volume, the cyl volume includes that calculation.
Hope that helps.
This one kinda lost me.............MP

[steelcomp]

This one kinda lost me.............MP
OK...let's back up.
The formula you show doesn't account for the dome in the chamber. The 122 would be reduced by 17, so the quench area is actually 104. You are taking the cylinder volume and stuffing it in a considerably smaller area than 122. (104)........104 is around 4/5ths of the total 122, thats a 20% error............Isn't it??.......You also didn't add the chamber volume into the cylinder volume, which would add, in this case 122cc. As I said, there is alot missing for dead on figuring. Maybe I'm missing something here, or maybe not........Ray
The formula does account for the dome in the chamber...it's part of the clearance volume calc. The 122 was reduced by 17, but also increased by the gasket volume, which was 9.19. That's where we get the 114.9 cc's. There's no error there.
The chamber volume was added to the cyl volume as part of the calculation by adding the cyl vol. plus the clearance vol. (which includes the chamber), then deviding by the clearance vol. There's nothing missing, except for the factors we don't know like valve relief, area above the ring, spark plug volume, piston corner radius, etc.

[Moneypitt]

OK...let's back up.
The formula does account for the dome in the chamber...it's part of the clearance volume calc. The 122 was reduced by 17, but also increased by the gasket volume, which was 9.19. That's where we get the 114.9 cc's. There's no error there.
The chamber volume was added to the cyl volume as part of the calculation by adding the cyl vol. plus the clearance vol. (which includes the chamber), then deviding by the clearance vol. There's nothing missing, except for the factors we don't know like valve relief, area above the ring, spark plug volume, piston corner radius, etc.
So the clearance volume is 114.9 after all the factors are figured in. OK....Then you add the clearance volume, which includes the chamber, to the cylinder volume. But, the clearance volume, 114.9, should be calculated as 122 when adding to the cylinder volume, not the clearance volume of 114.9 This is 7.1 CCs error. Not alot, but not dead on either.....Ray

[steelcomp]

So the clearance volume is 114.9 after all the factors are figured in. OK....Then you add the clearance volume, which includes the chamber, to the cylinder volume. But, the clearance volume, 114.9, should be calculated as 122 when adding to the cylinder volume, not the clearance volume of 114.9 This is 7.1 CCs error. Not alot, but not dead on either.....RayThe clearance volume is just that...the clearance volume. If the ratio called for just the chamber volume, then you'd be right, but it calls for the clearance volume, which has the dome figured in. If you remove the dome factor from the clearance volume, you'd just have to factor it in to the cyl volume. The piston dosen't disappear, which is essentially what you're saying.
It's a ratio of what you start with at BDC, devided by what you end with at TDC.

[holorinhal]

well i guess i will have to look at purchasing a diffrent set of pistons,with a larger dome.It does'nt look like I am going to be satisfied with the CR that these will get Me.

[Moneypitt]

The clearance volume is just that...the clearance volume. If the ratio called for just the chamber volume, then you'd be right, but it calls for the clearance volume, which has the dome figured in. If you remove the dome factor from the clearance volume, you'd just have to factor it in to the cyl volume. The piston dosen't disappear, which is essentially what you're saying.
It's a ratio of what you start with at BDC, devided by what you end with at TDC.
I think we're saying the same thing, just a different way of getting there, I think? The entire chamber, 122, must be factored in to the cylinder volume when computing how much volume will be compressed into how much area. That volume looses the dome volume. When compressed into the 122 it also looses the dome volume, so its actually 105 that everything is pushed into. I think what you're saying is to factor the dome at both ends and it all comes out the same, or something like that. There are other varibles as you mentioned, but this clearance volume can't be one value when figuring the combustion chamber "net" volume, and something else when figuring "gross" cylinder volume. Cylinder + chamber(122) - dome(17) = gross cylinder volume. Compressed chamber volume, 122 - 17 = 105 (net chamber volume). The additional volumes of deck, gasket, area above rings, valve reliefs, plug areas etc. would also have to factored in for an accurate number. But all of that must start with cylinder "gross" and end with chamber "net". Cylinder gross must enclude the entire chamber, not the net chamber, (clearance volume) ..............Ray

[Roaddogg 4040]

Whew! Glad that I was absent the day that they tought Math! :rollside: :rollside: :rollside:
Steve

[SmokinLowriderSS]

Whew! Glad that I was absent the day that they tought Math! :rollside: :rollside: :rollside:
Steve
Why do you think I plugged it into Compression Ratio Calculator v2.0 LOL :)