Whats MY COMPRESSION ??

[2002_Viper_GTS_ACR]

My heads are NOT stock. So please save the 9.6 to 1 answer. I know stock compression.

My cylinder heads are 80 cc's. All OTHER stuff is stock 96-97 (forged pistons and cam) although all that would matter is possibly the pistons. And the deck is STOCK, it has not been cut. I mentioned the pistons, as I do NOT know if the forged pistons had any MORE or LESS cc's in the pistons head for valve reliefs or what not.


I know its a simple equation, as all variables are know, they just have to be plugged in. Thanks.

Jon

 

[txlen]

jon...try this link..and put in your specs...works great....

http://www.brcpistons.com/calculations2001.html

 

[2002_Viper_GTS_ACR]

great link, here is what I know, and dont know. Any answers would help.


Compression Ratio =

bore (2.000 - 5.500)= 4.0 inches
stroke (2.000 - 5.000)= 3.88 inches
Cylinder Head cc's (10 - 200)= 80 cc
Deck (-0.100 - +0.100)= inches = ?????????????
Head Gasket Thickness (0.000 - 0.150)=????????? inches
Piston net volume (-50 - +75)= ????????? cc

 

[99 R/T 10]

Hey Jon, first the heads, now this. Is there anything I don't do for ya! Stock is right around 150-155 PSI

 

[BruceW]

Here's a question that has been bugging me. Why can't I get my compression ratio by just measuring the cylinder pressure (assuming I have tight rings and valves)? Or can you do this? I have never seen this approach being used. You would have to make a correction for the difference between the spark plug and the pressure gauge fitting but I don't see that to be a big problem.

 

[2002_Viper_GTS_ACR]

Well, I did worry that you were going to be completely useless with your reply, when I noticed you had replied. While it was still useless in this case, at least you attempted to reply with real information.

Notice I am not asking at all what the STATIC compression of the cyclinders is. As I am looking for purely "mechanical" numbers here, to plug into this formula and get the right answer.

So I need stock head gasket thickness, piston net volume, and stock deck.

Thanks for trying, but unless you have that exact info, its going to help much.

I am trying to calculate my exact compression ratio for my car, given my new ported heads, that have been cc'ed BUT not cut back down, so my heads are 80 cc now, UNLIKE stock.

Jon

 

[2002_Viper_GTS_ACR]

Hey Bruce,

You CAN do that. But different cams and over lap, can change your effective compression ratio. So sometimes that system works, and othertimes it does not. I will use it later, as a bases (assuming a linear relationship) that 150-155 = 9.6 to 1 (stock compression)

Then when I find my new psi I will do 9.6 div by 155 = .0619354, then take .0619354 and multiply it against my NEW compression checked psi and find out roughtly what it equates to.

This will work in theory, I just want to "mathmatically" calculate my answer for right now.

Jon

 

[WCKDVPR]

Here you go:

Static compression ratio equals
[Chamber Volume(CV) + Gasket Volume(GV) + Deck height Volume(DV) + Piston Displacement Volume(PDV)] / [Chamber Volume(CV) + Gasket Volume(GV) + Deck height Volume(DV)]

With your variables and substituting in stock values for what you don't know:
bore = 4.000"
CV = 80cc (your spec) = 4.8819 cu in
GV (0.054" thick compressed) = 0.054"*(4.000/2)(4.000/2)*Pi = 0.6786 cu in
DV = Piston is 0.035" down in the bore = 0.035"*(4.000/2)(4.000/2)*Pi = 0.4398 cu in
PDV (stock 3.88" stroke) = 3.88"*(4.000/2)(4.000/2)*Pi = 48.7575 cu in

Plugging in:
(4.8819 + 0.6786 + 0.4398 + 48.7575) / (4.8819 + 0.6786 + 0.4398) = 9.1:1

For reference, stock chamber volume is nominally 73.5 cc (5.6036 cu in)

Sounds like you had someone put in bigger valves and open up the chambers to unshroud them. If you swap pistons and put them at zero deck (which helps prevent against detonation as well) you would get 9.8:1. If stock pistons, you would need to deck (mill) your heads somewhere between 0.030" and 0.040" depending on you exact chamber shape to get back to 9.6:1. You would also want to have your intake milled the same amount so the ports line up correctly.

Hope this helps.

Regards,

 

[WCKDVPR]

Me again, I just looked at the calculator linked above, here is what you are missing:

Deck = -0.035"
Head gasket thickness = 0.054"
Piston net volume = 0
The pistons are flat tops

Gives you 9.13:1 with their calculator.

I guess that would have been easier

Regards,

 

[2002_Viper_GTS_ACR]

YOU ROCK !! Thanks man !!

Course, are you positive they are flat tops ? No Valve relief cuts ?

Also, I DONT want more compression !! Boooooosted..

I asked him NOT to cut them, so I could have less compression, and run more boost with less risk of detonation.

And yes, these are a set of stage 3 heads, with bigger vales, and unshrouding.

Jon

 

[WCKDVPR]

Jon,

Glad I could help. Who did your heads?

 

[2002_Viper_GTS_ACR]

Jeff Mories (also can you answer the question about wether there are ANY valve reliefs in the stock 96-97 forged pistons ??

Jon

 

[txlen]

stock 97-99 pistons had no valve reliefs..stock head gasket thickness is .052 , you can also get thicker head gaskets made for you...thats what i'm doing on the motor i'm building..it came out to 10.3 compression after the build..i'm opening up the chambers to 80cc and using a .093 gasket to get it down to 9.1-1...was wanting a little lower but i think it will live like this...how much boost ya wanna run????...Len

 

[2002_Viper_GTS_ACR]

Hey Len,

I dont know yet, just figured the closer I could get it to 8.5 to 1, the better for its ability to run those higher boost pressures.

I am glad I am at 9.1 to 1, but I would have liked it lower. I want to avoid getting a bigger head gasket as that diminishes the squelch/squish area that you get somewhat with these motors. Sure zero deck would be nice, and tons of cc's in the head, but I'll take what I can get.

Guess ideal would be a positive deck height (piston out of the block a bit) and some big CC heads. That way you could get the squish area, and still have low enough compression. OF course dished pistons will help lower the compression if getting more cc's out of the head isnt possible, and you still want to get a tight squish area with close deck/head/gasket tolerances.

Jon

 

[utahviper]

I have two questions. Isn't the deck height volume different from motor to motor? If all things being equal wouldn't boring and stroking your motor create lower compression?

Thanks

 

[joe117]

Bigger bore or stroke would increase cylinder displacement without changing combustion chamber size.
This would cause higher compression.

 

[txlen]

yes....if you bore/stroke a motor you will increase comp....if you want lower comp...open the chamber or go thicker gaskets....here's the comp i'm using....they have gaskets avaiable from .021-.093 in .010 increments available......call them and ask for greg...he's a nice guy and very helpful...i'll have them by the end of next week...

http://www.scegaskets.com/

 

[WCKDVPR]

Jon,

I have a '96 GTS and the stock pistons are definately flat tops with no valve reliefs.

You should have fun, I hear Jeff does a great job on heads.

I checked a bit and I was told 80cc is just about the max you can open up a Gen II combustion chamber to (maybe another cc or two). I guess your easiest route would be the thicker head gasket route, otherwish dished or lower deck height on some new pistons to go with those new rods Best bet on the chamber volume would be just to call Jeff and ask him directly.

Regards,

 

[2002_Viper_GTS_ACR]

Jeff is the one who told me my heads were 80 cc's, he guessed I would be 8.9 to 1, but my numbers show 9.1 to 1

I was basically double checking Jeff's estimate.

Either way. 9.1 is cool by me.. its a little more boost buffer over 9.6

Jon