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SpaceFrank
PostPosted: Tue Apr 14, 2015 1:43 pm 
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I just want to make sure I'm not going overboard here. This is a relatively mild motor, and I still want to be able to run 87 octane.

Current measurements:
Bore: 3.430" (.030" over)
Deck height: 0.185"
Head Chamber Volume: 58-60 cc (59 cc avg.)

I don't remember where I got these numbers, but this is what I'm using for the stock Fel-pro head gasket. Please let me know if these are wrong.
Bore: 3.5"
Compressed thickness: 0.040"

Using these numbers, I calculate my SCR at 7.69 (7.62-7.76 max variance between cylinders).

So using Doc's figure of 0.0067"/cc, I can shave off 0.080" to bring my average chamber volume from 59 to 47, giving me a SCR of 8.68 (8.59-8.78 max variance)

Taking this one step further, I found the following magical formula on the Corvette forums:
Quote:
Figuring Intake Valve Closing Point for Cam selection and DCR calculation.
This seems to come up quite frequently so I thought I would post this.
To figure this you take the advertised intake duration and divide by 2.
Then you add the LSA to this number.
Then you subtract out the ground in advance.
Then you subtract 180
We will assume the cam we are installing is installed straight up.
This gives you an actual intake valve closing point to use in the DCR calculator.
Before someone jumps on this, I do realize that some manufacturers uses different lift points for advertised duration but almost all are figured at .004 or .006 lift. This is close enough to get you within a degree or two of the actual intake closing point.
My cam specs (264 duration, 109 LSA +0) give me an intake closing angle of 61 degrees ABDC. This gives a DCR of ~6.37 before cutting and ~7.16 after cutting 0.080". I have to assume the actual closing angle will be a bit smaller due to valve lash, making the actual DCR a bit higher. Just to be safe, I used 55 degrees instead of 61 and got a final DCR of 7.44.

Still OK to run 87 octane in endurance racing conditions, or should I set my sights a bit lower and only cut 0.060" or so?



What if I decide to run 8-10 psi boost later? Too much compression then? Could the engine survive with less total timing and 93 octane fuel?

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sandy in BC
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PostPosted: Tue Apr 14, 2015 5:26 pm 
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It all makes sense.

I would deck the block instead of the head .

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Dart270
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PostPosted: Tue Apr 14, 2015 7:14 pm 
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Yes, all that sounds good.

Lou

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SpaceFrank
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PostPosted: Wed Apr 15, 2015 7:25 am 
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Thanks, guys. I would deck the block if it weren't already in the car.

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sandy in BC
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PostPosted: Wed Apr 15, 2015 4:34 pm 
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.......do you have an angle grinder?

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Dart270
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PostPosted: Wed Apr 15, 2015 7:20 pm 
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Have you moved up from hand files, Sandy?

Lou

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sandy in BC
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PostPosted: Thu Apr 16, 2015 6:22 am 
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,,,,,,,,no......I like the hand files.

....the angle grinder is for those in a hurry. 80 thou is a lot of drawfiling....

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SpaceFrank
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PostPosted: Thu Apr 16, 2015 7:51 am 
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Good idea on the angle grinder. Since the bottom end of the engine is assembled, I'll just coat the grinding disc in axle grease to trap the shavings.

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sandy in BC
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PostPosted: Thu Apr 16, 2015 11:36 am 
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Make sure you shut it off before adding the grease......

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