tdstuart
Well-Known Member
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- #1
Need help with figuring out what compression ratio I should go with, and if someone could double check my calculations that would be nice!
I have a gen 3 coyote shortblock and gen 2 heads (so port injection only). The gen 2 heads have a slightly bigger head volume than the gen 3 heads, so the compression ratio will be less than 12:1. According to my calculations it would be about 11.82:1 with the gen 3 shortblock and gen 2 heads. I am getting the heads machined flat, and am wondering if I should ask them to take off some more to raise the compression ratio. As of now I will be a FBO N/A car with stage 3 comp cams.
If they take off 0.010" then I should be around 12.15:1 CR. Is that too much for pump gas (91). I will be running e85 mostly but I still need the ability to run 91 as it is my daily car. If they take 0.020" off then I will be right around 12.5:1 CR. I may end up running mild boost eventually, if I can pull the heads off in the car then I could replace the head gasket for a thicker head gasket later to decrease compression ratio.
Lund says I should be fine with 12:1 CR on pump and will even be able to run 8-9lbs of boost. Asked my machine shop what they thought and they said it was way too much for pump gas. To be fair I don't really trust my machine shop when it comes to the specifics of the coyote platform. Let me know what you guys think.
Here are my calculations using https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator :
(I got most of these numbers from the forum and as you can see it may be 0.04 off, given its calculating the gen3 a little high. To simulate removing head material I just decreased the compressed gasket thickness, is this an okay approach?)
I have a gen 3 coyote shortblock and gen 2 heads (so port injection only). The gen 2 heads have a slightly bigger head volume than the gen 3 heads, so the compression ratio will be less than 12:1. According to my calculations it would be about 11.82:1 with the gen 3 shortblock and gen 2 heads. I am getting the heads machined flat, and am wondering if I should ask them to take off some more to raise the compression ratio. As of now I will be a FBO N/A car with stage 3 comp cams.
If they take off 0.010" then I should be around 12.15:1 CR. Is that too much for pump gas (91). I will be running e85 mostly but I still need the ability to run 91 as it is my daily car. If they take 0.020" off then I will be right around 12.5:1 CR. I may end up running mild boost eventually, if I can pull the heads off in the car then I could replace the head gasket for a thicker head gasket later to decrease compression ratio.
Lund says I should be fine with 12:1 CR on pump and will even be able to run 8-9lbs of boost. Asked my machine shop what they thought and they said it was way too much for pump gas. To be fair I don't really trust my machine shop when it comes to the specifics of the coyote platform. Let me know what you guys think.
Here are my calculations using https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator :
(I got most of these numbers from the forum and as you can see it may be 0.04 off, given its calculating the gen3 a little high. To simulate removing head material I just decreased the compressed gasket thickness, is this an okay approach?)
Stock Gen 3 Coyote: | Measurements: |
---|---|
Bore: (diameter) | 93 mm |
Stroke: | 92.7 mm |
Cylinder Head Volume: | 55.9 cc |
Effective Dome Volume: Use (-) for Dome and (+) for Dish. | -8.4 cc |
Deck Clearance: | 0.015 in. |
Compressed Gasket Thickness: | 0.040 in. |
Compression Ratio : | 12.04 : 1 |
Gen 3 Shortblock with Gen 2 Heads: | Measurements: |
---|---|
Bore: (diameter) | 93 mm |
Stroke: | 92.7 mm |
Cylinder Head Volume: | 57 cc |
Effective Dome Volume: Use (-) for Dome and (+) for Dish. | -8.4 cc |
Deck Clearance: | 0.015 in. |
Compressed Gasket Thickness: | 0.040 in. |
Compression Ratio : | 11.82 : 1 |
Gen 3 Shortblock with Gen 2 Heads (-0.010" Test): | Measurements: |
---|---|
Bore: (diameter) | 93 mm |
Stroke: | 92.7 mm |
Cylinder Head Volume: | 57 cc |
Effective Dome Volume: Use (-) for Dome and (+) for Dish. | -8.4 cc |
Deck Clearance: | 0.015 in. |
Compressed Gasket Thickness: | 0.030 in. |
Compression Ratio : | 12.15 : 1 |
Gen 3 Shortblock with Gen 2 Heads (-0.020" Test): | Measurements: |
---|---|
Bore: (diameter) | 93 mm |
Stroke: | 92.7 mm |
Cylinder Head Volume: | 57 cc |
Effective Dome Volume: Use (-) for Dome and (+) for Dish. | -8.4 cc |
Deck Clearance: | 0.015 in. |
Compressed Gasket Thickness: | 0.020 in. |
Compression Ratio : | 12.5 : 1 |
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