408 compression ratio

[canadian94gt]

I am building a 408. Have a 69 block with girdle. Scat 4" forged crank and Scat 6.200 forged H-beam rods. I am going N/A for now but may be adding a vortech in the near future. What is a safe compression ratio to shoot for that will run good for N/A but not be too much for a blower? Need to know before I buy my pistons. Thanks in advance.

 

[soontobefastfox]

you want to stay under 10:1 for sure. 8:5:1-9:5:1 is optimal.

And this should help you
http://www.fordmuscle.com/calculators/compression.shtml

dished pistons and a thicker head gasket will drop the compression.

 

[my289eatsrice]

I'd say it also depends on how much boost you're planning to run, but 9.0:1 is as high as I'd run for compression, but then again, you might be able to get away with a little higher if you're only running 6 or so pounds of boost.

 

[SmockDoiley]

I'd shoot for 9.0-9.3. Thats a good ratio that makes good NA power and can even function with regular unleaded when NA if you have a good tune. If its a street car I wouldnt go over 12 psi. Just make sure you use a boost cooler kit or intercooler.

 

[bluevenom867]

Depends...do you like buying race fuel?

Than stay under 10:1 unless your runing a blow thou carb or a big intercooler or both.

p.s. if you dont got heads yet,get heads with a larger CC volume,much rather have more CC volume than a thicker head gasket.

 

[canadian94gt]

p.s. if you dont got heads yet,get heads with a larger CC volume,much rather have more CC volume than a thicker head gasket.

Already have afr 205's with 58cc

 

[Rick 91GT]

Well your block height is 9.48" (or should be, have you had it checked) so with that 4" stroke crank, 6.2" rod your going to need a piston with a CH of 1.28".

JE makes a shelf piston with a -30cc inverted dome, they use a .927" pin so as long as your rods are .927 you should fine....only thing I could find in my catalogs.

Here is what I come up with..

Bore Size: 4.030
Gasket Bore: 4.100
Piston Top Land Dia: 3.995
Stroke: 4.000
Connecting Rod Length 6.200
Compression Height: 1.280
Deck Height: 9.480
Ring to Top of Piston 0.250
Gasket Thickness: 0.040
Valve Notches Volume: 30.00
Dome Volume: 0.00
Volume Head: 58.00
-------------------------------------
:COMPRESSION RATIO: 9.570
-------------------------------------



calculation variables:

Piston to Deck: 0.000
Crank Throw: 2.000
Vol. Above Top Ring: 0.90 cc
Piston to Dec Vol.: 0.00 cc
Gasket Volume: 8.65 cc
Volume Clearance: 97.56 cc
Cylinder Volume: 836.10 cc
CID: 408.018

Switching to a .050" headgasket will get you 9.38:1, if you want less then that then you'll need to go Custom as far as I know. I can't find a larger shelf stock dish to lower the compression. By using a .050 you still have suffucient quench area.

What blower model will you be adding, you'll need atleast a T -trim preferrably a YS-trim (if you want to stay Vortech) to feed that large cubic inch motor, a S-trim just won't cut it.

 

[canadian94gt]

Well your block height is 9.48" (or should be, have you had it checked) so with that 4" stroke crank, 6.2" rod your going to need a piston with a CH of 1.28".

JE makes a shelf piston with a -30cc inverted dome, they use a .927" pin so as long as your rods are .927 you should fine....only thing I could find in my catalogs.

Here is what I come up with..

Bore Size: 4.030
Gasket Bore: 4.100
Piston Top Land Dia: 3.995
Stroke: 4.000
Connecting Rod Length 6.200
Compression Height: 1.280
Deck Height: 9.480
Ring to Top of Piston 0.250
Gasket Thickness: 0.040
Valve Notches Volume: 30.00
Dome Volume: 0.00
Volume Head: 58.00
-------------------------------------
:COMPRESSION RATIO: 9.570
-------------------------------------



calculation variables:

Piston to Deck: 0.000
Crank Throw: 2.000
Vol. Above Top Ring: 0.90 cc
Piston to Dec Vol.: 0.00 cc
Gasket Volume: 8.65 cc
Volume Clearance: 97.56 cc
Cylinder Volume: 836.10 cc
CID: 408.018

Switching to a .050" headgasket will get you 9.38:1, if you want less then that then you'll need to go Custom as far as I know. I can't find a larger shelf stock dish to lower the compression. By using a .050 you still have suffucient quench area.

What blower model will you be adding, you'll need atleast a T -trim preferrably a YS-trim (if you want to stay Vortech) to feed that large cubic inch motor, a S-trim just won't cut it.

That is the largest dish I could find also. And yes it would be a vortech ys trim.

 

[65ShelbyClone]

You could always build it 10.5:1 N/A and not use the blower.

9.6:1 is a little high, but still usable with boost. I deally it would be 8.5 or 9:1 for a future with the Vortech. You'll have the ability to split that block anyway. It wont be a real threat until around 700+fwhp, though.

 

[IntenseBlue]

9-9.5:1

 

[vristang]

I asked myself the same question when I built my 408, as I plan on boost in the future as well.

I went about as low as I could get away with N/A. The motor is pretty healthy now, without boost. Lower compression ratios mean you can run more boost later.

CR is in my sig.

 

[canadian94gt]

I asked myself the same question when I built my 408, as I plan on boost in the future as well.

I went about as low as I could get away with N/A. The motor is pretty healthy now, without boost. Lower compression ratios mean you can run more boost later.

CR is in my sig.

So did you have to go with a custom piston? or did you just have larger cc heads

 

[vristang]

So did you have to go with a custom piston? or did you just have larger cc heads

Going from memory...
The pistons are from Probe and have a 20cc dish.

The heads had a fairly large 73cc chamber. I have not been able to find these heads listed anywhere, but this is what the machine shop measured them as. I bought them used, and have no way of contacting the previous owner. They do not appear to have been machined.

 

[86bluecobra]

why not bump compression to 10.5 to 1 and go with a nitrous hit. you can make 500+ hp N/A and run a little juice if you ever need the extra to beat whoever.