New Comp Ratio

[PHILLYSUPRA]

Is there an easy way to figure out what my new comp ratio would be? got a math formula?
Im gonna keep my NA 9.2:1, but im between the GREDDY 3mm HG or the HKS 2mm Stopper HG. Tuning wise i gotta figure out the new comp. thanks in advance guys.

 

[TRACKMKIII]

I would be very interested to know if there is a math formula too!!

 

[IJ.]

Yes there's a formula but you need to know a few base dimensions such as piston dish and combustion chamber volume to work it out!

Give me a few minutes I'll see what I can figure out using an online calculator.

 

[IJ.]

Assuming no block or deck work I get

Greddy=8.15:1

HKS Stopper=8.74:1

 

[Afunk]

Assuming no block or deck work I get

Greddy=8.15:1

HKS Stopper=8.74:1

That is also not factoring in the "squish" of the head gasket, especially the stocker since it is hard to know what it is after it has been squished.

Or am I horrendously wrong on that...

 

[IJ.]

Stock gasket compresses to 1.37mm and yep I took that into account

 

[PHILLYSUPRA]

wow, thanks that sure does give me less to worry about. some numbnuts told me to just subtract the two HG thicknesses then subtract that from my orig. comp. At least now i got a better ballpark, thanks!

Anyway you could post that formula, im sure everyone could use it.

 

[IJ.]

It's just Cylinder volume + Head volume divided by Head volume.

The "trick" is know the numbers

Our motors are a little difficult to calculate as the pistons hang out a touch at TDC and are dished.

Those figures I posted are based on uncut head and deck, you'd usually allow for machine work to be 100% accurate.

I also reverse calculated the NA piston dish volume so may not be 100% accurate but at least you have a basis for comparison.

 

[SupraDerk]

Ok this may be a stupid question...but it's early, I've had a rough night...and things are a little hard to comprehend right now, haha.

IJ: So does that mean that if you were going to go say, NA-T, you could lower the CR from the NA's to the CR of what the turbos run by using a thicker HG rather than swapping out the pistons??

 

[JAWS]

Yes a thicker HG will lower the compression ratio...
If you look at the numbers above you'll see that 1mm thicker only changes it a little though.
I'd really only consider a thicker HG if both the head and block have been milled more than a few thousands.

 

[PHILLYSUPRA]

supraderk: thats exactly my train of thought. back in the 80's Cartech and HKS used this method to boost their NA-T's. nowadays, if we use a good enough IC, have enough fuel, and use the updated MAFT-T, we'll be cool going NA-T safely.

 

[IJ.]

Derk: Yep people have been doing it this way for many years!

In the old days is was a decomp plate and 2 stock head gaskets as fuel and ignition control wasn't as sophisticated!

Going for a thicker HG isn't the optimal fix as you lose the Quench/Squish effect so may not achieve the same HP as a motor built correctly but this loss is minimal.