Final Dynamic Compression Ration Check Assistance

Matthew68

Well-known member
Hi Folks,

Finalizing the camshaft for my motor and just got off the phone with Jerry from Schneider Cams. Below are the specs for my build. Am I in the "safe zone" for compression ratios? Based on the tech articles and other calculators I seem to be, but wanted another set of eyes on it.

Engine Build Specs:

Aluminum Head/Intake
Either 350cfm 2100 or 480cfm 4100 (undecided, will see which runs better I have both)
Piston Dish = 0
Deck Clearance = 0
Combustion Chamber Size = 53cc
Stroke = 3.126"
Rod Length = 4.715"
Gasket Thickness = .044" (Victor)
Gasket Bore = 3.81"
Cylinder Bore = EITHER 3.70" or 3.71" (awaiting feedback form machinist)

Camshaft Specs (Hydraulic):

Intake Duration = 262 (at .050" = 208)
Exhaust Duration = 270 (at .050" = 214)
Intake Valve Lift (1.65 rockers) = .462"
Exhaust Valve Lift (1.65 rockers) = .480"
Lobe Separation = 110
Intake Valve Close ABDC = 59

Static compression is coming out to be right at 10:1. I run exclusively premium 91-93 octane. I have 91 ethanol free around me, I plan to use that.

Dynamic compression I'm finding multiple calculators that are giving me different results. One says I'm right at 8.37:1 which is in range per the tech articles on static vs dynamic. Others I've been in the 7s. Does anyone have a tool to calculate dynamic CR based on my build?

Cheers,
Matthew
 
Double check what I put in for the inputs, but this is what my handy-dandy little homebrew tool came up with. Oh, I ran it the first time with the ground-in advance set to 0deg. Assuming the cam card gave you the intake closing value, it would appear that there is 2 deg of advance ground in. I'll assume this means it's a Schnieder cam?

Anyway.....
================================================
INPUT - BUILD PARAMETERS
================================================
Block Specifications:
Cylinder Bore = 3.71 in
Centerline to Deck Height = 7.808 in
Rod Length = 4.715 in
Stroke = 3.126 in
Piston Dish Volume = 0 cc
Piston Compression Height = 1.51 in
Cylinder Head Specifications:
Combustion Chamber Volume = 53 cc
Intake Valve Diameter = 1.8 in
Exhaust Valve Diameter = 1.5 in
Rocker Arm Ratio = 1.65:1
Cylinder Head Gasket Dimensions:
Gasket Diameter = 3.81 in
Gasket Crush Height = 0.044 in
Camshaft Specifications and Events:
Intake Duration = 262 deg
Exhaust Duration = 270 deg
Lobe Separation Angle = 110 deg
Cam Intake Lift = 0.28 in
Cam Exhaust Lift = 0.291 in
Ground-in Advance = 2 deg
Camshaft Installation Advance = 0 deg
Intake Opening Angle = 23 deg BTDC
Intake Closing Angle = 59 deg ABDC
Exhaust Opening Angle = 63 deg BBDC
Exhaust Closing Angle = 23 deg ATDC
Intake Valve Lift = 0.462 in
Exhaust Valve Lift = 0.48015 in
Valve Overlap = 46 deg
================================================
OUTPUT
================================================
Engine Displacement = 202.76 in^3
Rod to Stroke Ratio = 1.51:1
Rod Angle = 19.36 deg
Resulting Deck Height = 2.394e-16 in
Squish Distance = 0.044 in
Static Compression Ratio = 10.0:1
Dynamic Compression Ratio = 8.41:1
 
Minor correction. I forgot to adjust for the piston compression height. Most the new pistons come with a 1.500" compression height. The numbers don't actually change, but here is the output for the corrected calculation. This is with the same 0.030" overbore that I forgot to mention previously as well.

================================================
INPUT - BUILD PARAMETERS
================================================
Block Specifications:
Cylinder Bore = 3.71 in
Centerline to Deck Height = 7.808 in
Rod Length = 4.715 in
Stroke = 3.126 in
Piston Dish Volume = 0 cc
Piston Compression Height = 1.5 in
Cylinder Head Specifications:
Combustion Chamber Volume = 53 cc
Intake Valve Diameter = 1.8 in
Exhaust Valve Diameter = 1.5 in
Rocker Arm Ratio = 1.65:1
Cylinder Head Gasket Dimensions:
Gasket Diameter = 3.81 in
Gasket Crush Height = 0.044 in
Camshaft Specifications and Events:
Intake Duration = 262 deg
Exhaust Duration = 270 deg
Lobe Separation Angle = 110 deg
Cam Intake Lift = 0.28 in
Cam Exhaust Lift = 0.291 in
Ground-in Advance = 2 deg
Camshaft Installation Advance = 0 deg
Intake Opening Angle = 23 deg BTDC
Intake Closing Angle = 59 deg ABDC
Exhaust Opening Angle = 63 deg BBDC
Exhaust Closing Angle = 23 deg ATDC
Intake Valve Lift = 0.462 in
Exhaust Valve Lift = 0.48015 in
Valve Overlap = 46 deg
================================================
OUTPUT
================================================
Engine Displacement = 202.76 in^3
Rod to Stroke Ratio = 1.51:1
Rod Angle = 19.36 deg
Resulting Deck Height = 2.498e-16 in
Squish Distance = 0.044 in
Static Compression Ratio = 10.0:1
Dynamic Compression Ratio = 8.41:1
 
And here is what I come up with if the machinist only does a 0.020" overbore for a 3.70" bore. Not significantly different, but here they are anyway :beer:

================================================
INPUT - BUILD PARAMETERS
================================================
Block Specifications:
Cylinder Bore = 3.7 in
Centerline to Deck Height = 7.808 in
Rod Length = 4.715 in
Stroke = 3.126 in
Piston Dish Volume = 0 cc
Piston Compression Height = 1.5 in
Cylinder Head Specifications:
Combustion Chamber Volume = 53 cc
Intake Valve Diameter = 1.8 in
Exhaust Valve Diameter = 1.5 in
Rocker Arm Ratio = 1.65:1
Cylinder Head Gasket Dimensions:
Gasket Diameter = 3.81 in
Gasket Crush Height = 0.044 in
Camshaft Specifications and Events:
Intake Duration = 262 deg
Exhaust Duration = 270 deg
Lobe Separation Angle = 110 deg
Cam Intake Lift = 0.28 in
Cam Exhaust Lift = 0.291 in
Ground-in Advance = 2 deg
Camshaft Installation Advance = 0 deg
Intake Opening Angle = 23 deg BTDC
Intake Closing Angle = 59 deg ABDC
Exhaust Opening Angle = 63 deg BBDC
Exhaust Closing Angle = 23 deg ATDC
Intake Valve Lift = 0.462 in
Exhaust Valve Lift = 0.48015 in
Valve Overlap = 46 deg
================================================
OUTPUT
================================================
Engine Displacement = 201.67 in^3
Rod to Stroke Ratio = 1.51:1
Rod Angle = 19.36 deg
Resulting Deck Height = 2.498e-16 in
Squish Distance = 0.044 in
Static Compression Ratio = 9.99:1
Dynamic Compression Ratio = 8.37:1
 
Steve - THANK YOU! Looks like we’re spot on.

In terms of the aluminum head, does the DCR ranges still apply as linked for the iron heads? Or do the ranges shift (up, I would assume)?

ci/CompressionR.html
 
Looks like he may be using 2.5 pistons. If so they have a compression height of 1.556"

Edit: Never mind, I was thinking this was a 250, not a 200. It is 1.5 for the 2.3 flat tops.
 
Matthew68":270ryo4j said:
Steve - THANK YOU! Looks like we’re spot on.

In terms of the aluminum head, does the DCR ranges still apply as linked for the iron heads? Or do the ranges shift (up, I would assume)?

https://fordsix.com/ci/CompressionR.html
The aluminium head can handle 1/2 higher compression than an iron head.
The best thing you can do is get rocker arms at least 1.6 or higher. The CI -VI head flows so much batter than an iron head.
You are leaving at least 15HP on the table with 1.5 rocker arms.
 
wsa111":2aowmki0 said:
Matthew68":2aowmki0 said:
Steve - THANK YOU! Looks like we’re spot on.

In terms of the aluminum head, does the DCR ranges still apply as linked for the iron heads? Or do the ranges shift (up, I would assume)?

ci/CompressionR.html
The aluminium head can handle 1/2 higher compression than an iron head.
The best thing you can do is get rocker arms at least 1.6 or higher. The CI -VI head flows so much batter than an iron head.
You are leaving at least 15HP on the table with 1.5 rocker arms.

Bill, I have 1.65 YT rockers I’ll be using.
 
Take advantage of the heads breathing capability and run a larger cam.
Move up to the Schneider 270-80H
It will also drop the DCR to just under 8.2
 
pmuller9":wymb6ils said:
Take advantage of the heads breathing capability and run a larger cam.
Move up to the Schneider 270-80H
It will also drop the DCR to just under 8.2
I agree & get the chamber cc's closer to 50.
 
Jerry and I decided on that cam since I am mostly using this car as a cruiser and not really a racer. RPM range matches my street driving with limited highways.

If I take the head down to 50cc my SCR will jump. I’m already at 10:1 with 53cc and flat top pistons.
 
almost seems like tryin to choke dwn the advantages ofa alu head'n that intake.

I remember U said a trailer'n hi altitude drivin, no? Also not-quite-DD but used like one (sits w/o use then driven), yes?
What is the rest of the picture?
 
chad":2nrpyn9x said:
almost seems like tryin to choke dwn the advantages ofa alu head'n that intake.

I remember U said a trailer'n hi altitude drivin, no? Also not-quite-DD but used like one (sits w/o use then driven), yes?
What is the rest of the picture?

What do you mean trying to choke down? 10:1 compression and 1.65 rockers with a decent cam...not sure what I’m choking down and still have it for the style of street driving I’m doing.

No trailer hauling and no altitude issues. I’m about 450’ of elevation with just stop and go traffic mostly. Not a daily driver, maybe take the car out 1-2 times a week depending on the weather in the Midwest (Ohio).
 
sorry, must needa read whole threads
looked like leavin some on the tabel mabel,
("... increased LSA's that bleed off cylinder pressure...")
back to my nap
:(
 
Matthew68":tchf2v8r said:
Jerry and I decided on that cam since I am mostly using this car as a cruiser and not really a racer. RPM range matches my street driving with limited highways.

If I take the head down to 50cc my SCR will jump. I’m already at 10:1 with 53cc and flat top pistons.

The DCR is more meaningful and what the engine actually sees based on the cam selection and other variables. The static compression is not really relevant in today's world because it doesn't take into account more meaningful engine parameters and data. Many newer factory performance V engines have static compressions of 11:1 or more and still use pump gas. Its all in the cam selection and cam timing that makes the engine more manageable. If you ever see in a cams description that increased compression is required, its because it usually has increased LSA's that bleed off cylinder pressure, and needs higher compression to help compensate, but can still has low enough DCR to be driveable on pump gas. So don't let the static compression number fool you as to the engines true rating.
 
Make sure you degree the cam. In your usage i would advance it at least 2 degrees.
 
CNC-Dude":ltnc5d34 said:
Matthew68":ltnc5d34 said:
Jerry and I decided on that cam since I am mostly using this car as a cruiser and not really a racer. RPM range matches my street driving with limited highways.

If I take the head down to 50cc my SCR will jump. I’m already at 10:1 with 53cc and flat top pistons.

The DCR is more meaningful and what the engine actually sees based on the cam selection and other variables. The static compression is not really relevant in today's world because it doesn't take into account more meaningful engine parameters and data. Many newer factory performance V engines have static compressions of 11:1 or more and still use pump gas. Its all in the cam selection and cam timing that makes the engine more manageable. If you ever see in a cams description that increased compression is required, its because it usually has increased LSA's that bleed off cylinder pressure, and needs higher compression to help compensate, but can still has low enough DCR to be driveable on pump gas. So don't let the static compression number fool you as to the engines true rating.


That’s fair, but today’s engines have things like variable valve timing and other features that can help deal with higher SCR that our little old sixes can’t replicate.

But if I’m going to go to 50cc chambers my SCR becomes 10.54:1 and my DCR becomes 8.81:1. Do I need that high for a cruiser on 91 pump gas? Do I risk pinging. I’m asking bc you guys are smarter than me and I’m looking for advice. This is going to be a grocery getter, date night, etc. I’m not looking to drag race or street race whatsoever.
 
With the extra bump in compression, the larger cam that was suggested can lower the DCR lower. So can advancing/regarding the current cam. Do you have the actual cam opening and closing events to calculate the DCR accurately?
 
Matthew68":10uh45fg said:
But if I’m going to go to 50cc chambers my SCR becomes 10.54:1 and my DCR becomes 8.81:1. Do I need that high for a cruiser on 91 pump gas? Do I risk pinging. I’m asking bc you guys are smarter than me and I’m looking for advice. This is going to be a grocery getter, date night, etc. I’m not looking to drag race or street race whatsoever.
You are correct in being concerned about increasing the SCR any higher than 10:1 with a resulting 8.4 DCR.
There comes a point where you have to back off ignition timing and richen the mixture to prevent detonation which decreases power over having a lower compression having and some margin for tuning.
If you have to detune it will also affect gas mileage.

I feel that an 8.4 DCR is on the high side.
I would polish the combustion chambers smooth and install the cam straight up for an 8.3 DCR.
 
For the last few years, I have been heavily involved with quite a few LS racers and have been performing a lot of custom mods for them. I thought I remembered you saying you were using both 91 and 93 octane gas, so if you pushed your DCR into the upper 8's, you would be fine with that. The LS guys are running 8.8-8.9 DCR with 93 octane and aluminum heads comfortably with their combos. They also are running as high as 8.5 DCR with 91 octane. So if you are considering only 91 octane, then I'd agree with pmuller in that you are pushing the high side of 91 octane and should stay where you are.
 
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