Calculating my 7mgte compression ratio.

[kemicalx]

Hi all,

Am in the process of figuring out what thickness head gasket i need. I'm pretty sure I've worked out the compression ratio correctly but it'd be great if someone here with more experience could give me the all clear before I order the gasket.

Basic measurements (Formulas used below)
Bore Diameter = 83.5mm
Stroke = 91mm
Bore Radius = 41.75mm
Piston Volume = 19cc
Deck Clearance = -2.8cc
Combustion Chamber Volume = 37.5cc
2.0mm x 84mm Head Gasket Volume = 11.08cc

Cylinder Volume
Bore size = 83.5mm
Stroke = 91mm
Bore Radius = 41.75mm
Volume of Cylinder = (pi*radius*radius*height)/1000
(3.1416*41.75*41.75*91)/1000 = 498.316cc
Total Bottom End Capacity = (498.316cc*6 = 2989.896cc)

Gasket Volume
Gasket thickness = 2.00mm
Bore size = 84mm
Volume of head gasket = (pi*radius*radius*height)/1000
(3.1416*42*42*2)/1000 = 11.08cc

Deck Clearance
Piston height = 0.5mm
Bore Size = 83.5mm
Volume of Exposed Piston = (pi*radius*radius*height)/1000
(3.1416*41.75*41.75*0.5)/1000 = 2.74cc

Compression Ratio
CR = Compression Ratio
D = Displacement
PV = Piston Volume
DC = Deck Clearance Volume
G = Gasket Volume
CC = Combustion Chamber Volume

CR = (D + PV + DC + G + CC) / (PV + DC + G + CC)
8.685:1 = (498.316cc+19cc-2.74cc+11.08cc+37.5cc)/(19cc-2.74cc+11.08cc+37.5cc) = (563.156)/(64.84)

Do my calculations look correct?

Now, Stock compression ratio for a 7mgte is 8.4:1, so mine is high. though I didn't calculate the volume between the top ring land and the bore, so there is probably another cc or 2 there as well. As I'm using a copper head gasket and o-rings I don't really want to go any larger than 2.0mm for the gasket to ensure the o-rings bite in and seal well. Boost pressure will be around 16-17psi, Engine has forged pistons and I'll also be using aftermarket fuel and ignition control. So taking all that into consideration that do you think this higher compression ratio will be ok?

Thanks for any input guys,
Ashlin.

edit - will also be using forged pistons.

[SillyCarS]

edit: my bad

[kemicalx]

Should i drop my compression ratio any lower?

[Kedderz]

to be honest its not THAT much higher than stock. Since you are using standalone aftermarket management providing the tune is good I dont see the problem with your CR.

[kemicalx]

Cheers mate, thats sort of what i figured. I'm guessing ignition timing just won't have to be as aggressive.

Before I order the gasket does anybody else have anything to add?

[abently]

I personally wouldn't recommend a Head Gasket thickness of over 1.5mm at the extreme end of Boost builds......... Don't be surprised if the Engine doesn't make the power it should at the relative Boost pressure. Its okay if your never going to tune it on VP Import / C16 or even Shell 100 but if you are, then consider getting a 1.2mm Head gasket.

Also depending on your camshafts timing, your static CR is actually something not to worry about. You can seriously go as high as 10:1 on a 100Hp per cyl setup running 98 or 100 Ron Fuel.

Buy the 1.2mm Copper Head gasket and be happy.

[kemicalx]

ahhh I don't really follow your logic sorry.. Isn't a 1.2mm gasket going to make my static CR go over 9:2:1 i can't see how that could really be a good thing for a motor running about double factory boost when the factory static ratio is 8.4:1, I've heard of people running 3mm gaskets on 7m's to drop the ratio to about 7.5:1 when running near 2bar, which i personally consider the extreme end of realistic boost levels. If i was going to be using race grade fuels then I'd feel more confident running a high CR. As my car is only ever really going to be run on 98 ron i think I'd feel happier with a head room a lower compression ratio will provide. Maybe my understanding of how a higher compression ratio effects a motor is incorrect As for the cam timing I'm not adjusting it from factory spec yet.

[abently]

What is boost ? Its the back-pressure in your Intake manifold, nothing more nothing less.

What Turbo you using ? If its not bigger then a GT35R then trust me when I say to run a 1.2mm Head gasket.

I couldn't care what others do with their own setups, what I'm telling you is that 10.1 static CR on a 100Hp per cyl using 98 Ron Fuel is considered an 'advanced level' build. So 9.## :1 CR is something that most tuners will have no problems dialling all knock out of it and then some just for fuel quality safety margin.

The only time a professional even considers a 2mm Head gasket is when they are restricted to a particular Fuel but not Boost level (ie 40+ psi).

[kemicalx]

mmk, i'll take what your saying on board.. Obviously I'm going to have to do a little more research. Just out of curiosity, what have you built up in the past?

[abently]

150-180Hp per Cyl FI Engines. Haven't pushed for more power as traction and things tend to break hourly after then..... not to mention lag especially with 2L Engines.

If someone tells you that static CR is too high for the "boost" your running, without even asking what Turbo and cams your using, then quietly make a mental note to yourself about that persons true experience and knowledge.

[q.c.autosports]

150-180Hp per Cyl FI Engines. Haven't pushed for more power as traction and things tend to break hourly after then..... not to mention lag especially with 2L Engines.

If someone tells you that static CR is too high for the "boost" your running, without even asking what Turbo and cams your using, then quietly make a mental note to yourself about that persons true experience and knowledge.

I second that. Youll be safe with what you have. Just stay away from the boost controller if you are using pump gas 'coz there is always the urge to raise the boost. Work with the safest boost for your set up.

I use a .065 metal head gasket for our race car and we are running an 11:1 CR, VP C16 race fuel, 30 lbs. of boost...650 HP

[kemicalx]

I really do appreciate the advice guys, but I've gone the other way and stuck with a 2.0mm I'm not chasing huge power figures. And I feel more comfortable keeping everything closer to stock figures.