when motor is built for a certain compression How is it done

[Jkstang78]

Hi I bought a 331 stoker kit about a year ago. well I have still not built it. when I orgianl bought it I wanted it for 11:1 compression. Now when the Motor guy goes, and put this motor together does he need to do anything else to get the 11:1 compression. Or is that all set in the piston already. Or will it be a regular 9:1 comperssion if he was to just put it all together. I don't know if you guys need this info but here is the stroker kit setup.


Scat 4340 Crankshaft "standard style,3.250 stroke/1.680pin
Scat H-Beam Rod 5.400 rod length/.912 wrist pin
SRP Forged Piston flat top/.912 pin dia
11 to 1 compression.

 

[mustangrfast88]

it all has to do with the heads and the pistions you are running like the engine i just built is a 11 to 1 engine with a 64 cc head but i went with edelbrock victor jr heads with a 60 cc head so it upped me compresion alittle bit but those are you 2 factors on your compresion and the head gasket will mess with it alittle too

 

[90Notch]

Compresion ratio is figured by total cyl (all eight) vol at BDC divided by compressed cyl vol (eight again) at TDC so gasket compressed volume, cyl vol, head chamber volume, piston dome/dish vol all come into play.

I ran the same pistons as you in my 331ci and running 11:1cr is not going to that easy.

The easiest way to get there is have pistons sitting at zero deck and running a 54cc chamber head along with a FelPro 1011-1 gasket (0.039" comp hgt).

If you can't get a 54cc chamber head it's going to be diffcult to do with your parts and still run a decent cam.

Forgot to mention if you're running NA then you should be shooting a quench hgt of 0.035"-0.040" so keep this in mind when assembling motor.

 

[Rick 91GT]

Here is what I come up with as it sits right now...using a 58cc head, o deck block, standard 4.08" bore head gasket and, .039" thick headgasket. I also assumed they were the standard 5cc dish flat tops from SRP.

Bore 4.03 (inches)
Stroke 3.25 (inches)
Rod Length 5.4 (inches)
Cyl. Head Vol. 58 (cc)
Deck Height 0.0 (inches)
Head Gasket Bore 4.08 (inches)
Head Gasket Thickness 0.039 (inches)
Piston to Cyl. Wall Clearance 0.003 (inches)
Top Ring Land Height 0.25 (inches)
Piston Dome Vol. -5.0 (cc)


Compression Ratio:
10.513 : 1

Bore / Stroke Ratio:
1.24 : 1

Rod / Stroke Ratio:
1.662 : 1

Total volume:
750.879 cc's

Compressed volume:
71.422 cc's

As stated above a 54cc will get you a hair over 11:1, 60cc will get you 10.25"


Why not stay 10.5:1 and run pump gas all day long with no worries?

 

[Jkstang78]

wow I did not realize how compicated this really is. I am glad I am taking it to some for all of this. The heads I am sing are the victor Jr. heads. I bought these used and not sure on the cc size On the eddey website they say 60cc so I guess that is it then. SO now what do you guys think. I should have put this in before. I did not relize that the heads were a factory sorry guys.

 

[Jkstang78]

I should mention this to. I am using a d.s.s level 20 block and I am not sure what the deck height is. Looking on there website and I can't find it. Please No flack about the block. I did not have the money at the time to get a better block Would have love to get a 4-bolt. Just did not have the funds at the time.

 

[rd]

Most guys get to a compression ratio by a combination of milling the heads and decking the block. I have tried both, on different engines, and will do both on the next go around.

As my machine shop suggested to me.... Mock up the crank in the block, and install one rod and piston assembly, and using a straightedge (carpenters square here) and a feeler gage, measure how far the piston is from the deck surface at top dead center. Do this with the same piston and rod in each of the corner cylinders. Then you can figure how much to mill off the deck, at each end on each bank. If the block has not been machined before, you may be amazed at the variation, I had nearly .01 or more variation and a lot to remove to get to zero deck height.

Then have the heads milled. With some geometry you can figure how much volume you lose with each increment of milling. Or measure chamber volume with some valves installed and some water and a cheap drugstore syringe...it will get you close.

I did this with TRW flat top pistons with the standard four valve reliefs. With flat top pistons, zero or .005 in positive deck height is the practical max, and 58 cc is a good min chamber volume.

If you get the engine to near zero deck height and in the 58 to 60 cc chamber volume, you will be above 9 to 1.

Most people that want to go above 10.5 or so, will need domed pistons.

 

[90Notch]

Run those heads (good choice for stroker) but have them milled to 58cc then if at all still possible have deck machined so that pistons sit 0.005" out of hole. This with FelPro gasket that has 4.100" bore/.041" comp hgt will give you 10.59:1 with a quench of 0.036". I'm not aware of the gasket from above post that has a 4.08" bore but that is even better if you can get it.

If you can't get pistons 0.005" out of hole and they're zero deck you're looking at 10.45:1 with 58cc head & 10.2:1 with 60cc & 11.01 with 54cc.

 

[Rick 91GT]

I'd keep the pistons at zero deck, it is better for flame travel from what I know.