Compression Ratio Help!

[tdstuart]

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?)

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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[bauern]

... I can pull the heads off in the car then I could replace the head gasket for a thicker head gasket later to increase compression ratio.

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

You mean lower.
Use 0.012" for deck clearance. Milling the heads will change the C.R. less than gasket thickness as the HG diameter is wider than the chamber in addition to whatever taper the chamber has. For comparison purposes, it's fine though.

 

[tdstuart]

You mean lower.
Use 0.012" for deck clearance. Milling the heads will change the C.R. less than gasket thickness as the HG diameter is wider than the chamber in addition to whatever taper the chamber has. For comparison purposes, it's fine though.

Thank you for the correction, I did mean lower.

Also, that deck clearance raises the stock gen 3 compression ratio to 12.14:1. If anything I think a deck clearance of 0.016 would make more sense.

And yes I understand milling the heads will change the CR less than the gasket thickness and will be more inaccurate the more you take off but thank you for pointing it out.

 

[GregO]

Time to CC the heads if you’re splitting compression ratio hairs.
Best you’ll get with the calculator is a WAG without exact measurements.

 

[Angrey]

I personally wouldn't go higher than 11:1 with 91 on gen 2 heads. One of the reasons Ford can get away with it on the gen 3 is the knock reduction from the DI.

Ford made a mistake in fielding the Voodoo with 12:1 and no DI. It's just begging for knock when questionable fuel is involved.

On E85, I'd say knock it out, but I wouldn't voluntarily run 12:1 on 93, let alone 91.

Unless you're going to run fuel additive constantly (which has it's own drawbacks) the stuff that comes out of the pump is just too unpredictable. It only takes one shady station owner or a delivery driver not paying attention and you end up with lower grade fuel than you're supposed to get. Most of the state inspectors focus on flow and volumetric tests (ensuring you're not getting screwed on how much the pump claims you've purchased). Fuel quality is very difficult for them to police because it changes everytime the station gets resupplied. It's supposed to be managed on the front end (certifications from the refiner before it rolls out) but all it takes is for an error or intentional shorting of the mix/blend.

12:1 on pump gas doesn't have much forgiveness in it. The tune can adjust and it is dynamic compression, but still. I'm betting a bunch of blown motors is from bad fuel. E85 not only gives you additional knock resistance, at least you have the ability to test the fuel and determine whether or not you should flog the car. If you can't run E85, then be wary of high compression (without DI).

 

[GregO]

12:1 on pump gas doesn't have much forgiveness in it.

LUND gave the OP the 12.0:1 blessing so here’s opening another can of worms.
11.0:1 is remarkable for 91/93 reformulated gasoline, 12.0:1 is mind numbing to guys over the half century mark but we know it’s DI magic.

 

[tdstuart]

I personally wouldn't go higher than 11:1 with 91 on gen 2 heads. One of the reasons Ford can get away with it on the gen 3 is the knock reduction from the DI.

Ford made a mistake in fielding the Voodoo with 12:1 and no DI. It's just begging for knock when questionable fuel is involved.

On E85, I'd say knock it out, but I wouldn't voluntarily run 12:1 on 93, let alone 91.

Unless you're going to run fuel additive constantly (which has it's own drawbacks) the stuff that comes out of the pump is just too unpredictable. It only takes one shady station owner or a delivery driver not paying attention and you end up with lower grade fuel than you're supposed to get. Most of the state inspectors focus on flow and volumetric tests (ensuring you're not getting screwed on how much the pump claims you've purchased). Fuel quality is very difficult for them to police because it changes everytime the station gets resupplied. It's supposed to be managed on the front end (certifications from the refiner before it rolls out) but all it takes is for an error or intentional shorting of the mix/blend.

12:1 on pump gas doesn't have much forgiveness in it. The tune can adjust and it is dynamic compression, but still. I'm betting a bunch of blown motors is from bad fuel. E85 not only gives you additional knock resistance, at least you have the ability to test the fuel and determine whether or not you should flog the car. If you can't run E85, then be wary of high compression (without DI).

So if I only run e85 and just have Lund turn down the timing on the 91 tune you think I will be fine for cruising on 91?

I don’t think it’s that big of an issue for me to run e85 constantly, just will need 91 when traveling far distances.

 

[K4fxd]

LUND gave the OP the 12.0:1 blessing

I think there are at least 3 threads of Lund tunes popping #8.

11.0:1 is remarkable for 91/93 reformulated gasoline, 12.0:1 is mind numbing to guys over the half century mark but we know it’s DI magic.

This 100%

 

[K4fxd]

This calculator is usually pretty accurate. Don't know why it is reading low. I'll have to pull out the slide rule......

 

[tdstuart]

This calculator is usually pretty accurate. Don't know why it is reading low. I'll have to pull out the slide rule......

Change dome so it is not negative. (8.4 not -8.4, the calculator I was using wants negative to represent dome)

 

[tdstuart]

I think there are at least 3 threads of Lund tunes popping #8.

This 100%

So what’s your recommendation?

 

[K4fxd]

Yea, I'm not trusting this calculator. 1.24 to 1

 

[tdstuart]

Yea, I'm not trusting this calculator. 1.24 to 1 View attachment 704951

?

 

[K4fxd]

OK used positive for dome, same 1.24 to 1. LOL

 

[K4fxd]

?

Check again. I cropped the first one

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