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Old 12-29-2010, 08:57 PM
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dynamic compression?

who can help me figure this out? trying to decide what i should aim for, will be 383 running 93 octane, AFR Aluminum heads, 4.030 bore, 3.75 stroke, cam is a comp cam 270, 40 degrees IVC @.050, 226 intake duration @.050 any suggestions on what i can get away with. like everyone else i just want the most power for my setup without killing it

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Old 12-30-2010, 08:36 AM
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With that cam, I would shoot for 10:1cr with aluminum heads
9.5:1 with iron.

could go up a little more depending on the engine load (weight of car, gears, trans type, ........).
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Old 12-30-2010, 11:04 AM
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Manual 4 speed, in a full size 70's 4x4. It's a must bog racer but used to get to work and back as well. So with aluminum heads was that 10:1 static or dynamic? I would assume static and with my setup I think that gives me around 8.7 - 8.8 dynamic. And around 180 cylinder pressure. Is that doable on 93 octane?
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Old 12-30-2010, 12:13 PM
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yes, 10:1 static should be ok.
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Old 12-30-2010, 12:16 PM
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I'm getting about 8.2 DCR with your combo. intake closes at 61 degrees.
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Old 12-30-2010, 12:23 PM
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Is that seat to seat? Comp Cams site says 60 degrees seat to seat closing and 40 at .050 with aluminum heads could I bump up the dcr a little?
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Old 12-30-2010, 01:15 PM
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closing point also is determined where you install the intake lobe center. 60 or 61 will not make much difference.

I would stay at 10:1cr in a big heavy truck. (lots of load)

what gears and tire diameter?
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Old 12-30-2010, 01:32 PM
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Quote:
Originally Posted by 383_Gladiator
Is that seat to seat? Comp Cams site says 60 degrees seat to seat closing and 40 at .050 with aluminum heads could I bump up the dcr a little?
I would use caution as you approach the point of detonation. The small amount of power gained by hugging the danger line is not worth the cost of grenading the mill, IMO.

You just never know if someday you'll be stranded in Chappedass, AZ w/nothing but smuggled Pemex to put in the tank. Or the engine gets real, real hot because you don't realize the radiator fins are plugged w/mud- but you've GOT to go! All the DCR figuring, compression ratio fudging in the world will not bail you out of unforeseen situations that can occur in the real world.
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Old 12-30-2010, 01:42 PM
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4.56 gears and 35-38 inch tires. Cobalt with all the reading I've done on here I do tend to take yours with a lot of respect. I had set my limit at 10:1 maximum for this motor but it had been lurking in the back of my mind if I could safely get away with anymore. How close to a danger point am I with 8.2:1 dcr?
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Old 12-30-2010, 02:13 PM
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Quote:
Originally Posted by 383_Gladiator
How close to a danger point am I with 8.2:1 dcr?
In all honesty, I am not one to ask that question to- I don't rely on DCR figures when choosing the compression ratio.

That said, there shouldn't be a problem w/around 10:1, I wouldn't say to go over that, and possibly less depending on the entire build and use of the truck.

My MAIN point was, at something like ~4-5% per point of CR, it just isn't worth pushing it for so little gain. Adding 0.5:1 CR will not change the output very much at all- but can KILL you w/detonation.

BTW- that percentage (~4-5%) is open to debate and depends on whether the engine was optimized for that CR, BEFORE the added CR or not- along w/other variables.
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Old 12-30-2010, 02:33 PM
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here is a nice article on dcr.

http://www.popularhotrodding.com/eng...tio/index.html

going from 10 to 11:1 is about a 1.5% increase (as a minimum) but other factors need to be considered.
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Old 12-30-2010, 05:27 PM
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Quote:
Originally Posted by 383_Gladiator
who can help me figure this out? trying to decide what i should aim for, will be 383 running 93 octane, AFR Aluminum heads, 4.030 bore, 3.75 stroke, cam is a comp cam 270, 40 degrees IVC @.050, 226 intake duration @.050 any suggestions on what i can get away with. like everyone else i just want the most power for my setup without killing it
This is a place where one has to exercise care and judgement. The AFR head uses a highly efficient combustion chamber so rules of thumb that are appied to the Vortec will track pretty closely here. Those ROT's would say the cast iron Vortec will take to about 9.5 against a Comp 270 without issue. The rules, also, generally agree that aluminum can be pushed from .5 to 1.0 ratio higher. We're talking premium unleaded fuel here in the range of 91 to 93 octane. When talking Dynamic Compression Ratio (DCR) we're looking at a range from about 8 to 9:1. Some folks like to derate that by about .25 of a ratio. But what we're really looking for is what Static Compression Ratio do we need to arrive at for a livable DCR as the DCR is the SCR adjusted for stroke loss at intake closing.

So I humped what data you provided and assumed what you didn't which is some really important stuff like the piston you're using, head gasket and deck clearance.

I used:
- 64 ccs for the head volume. I think the AFR is reall 62-63.
- A 4.1 inch diameter head gasket that's .051 inch compressed.
- A bore of 4.03 inches and a stroke of 3.75 inches.
- A flat top piston with 4.8 ccs worth of valve reliefs.
- Being lazy I didn't look up a Comp 270's real intake closing point but added 18 degrees to your 40 degrees at .050 inch lift.

This drives out an SCR of 10.65:1 with the DCR of 9.54:1. I'd say for a street engine, this is probably a good half ratio to high and that the total squish/quench space of .076 inch is too wide. So if my assumptions about the innards of this engine are correct, I say the squish/quench distance needs to be tightened up to .040 to .060 inch as well as configuration adjustments need to be done to get to an SCR of 10 to 10.2:1.

If you can supply actual pistons used and what the deck height is, I can take another look. If you haven't built this yet, I'd say that a slice off the deck is a better choice than a thinner head gasket, although there are some solutions to using a multilayered steel gasket that can be employed. The gasket I used is a typical GM for the ZZ4 engine, it is a bit thick and is of composite metal and organic construction. A great gasket for minimizing the fretting between the iron block and aluminum head, but is a real PIA for idealizing the squish/quench gap dimension.

Bogie
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Old 12-30-2010, 06:03 PM
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No this engine isn't built yet. block hasn't been machined yet either. I haven't decided on which heads to order the AFR 1034's which list a 65cc combustion chamber or the 1036 with a 75cc chamber which is another dilema of mine. Not sure what pistons my builder will use, but i think he said something about hypereutectic. Ill list the specs of the cam as per comp cams' camquest program

X4270H
Duration @.050 Intake 226 Exhaust 234

Seat to seat intake 270 Exhaust 278

Timing @.050 IVO: 6 IVC: 40 EVO: 52 EVC: 2

Seat to seat IVO: 28 IVC: 62 EVO: 74 EVC: 24

Anything more i might be able to answer for a better answer?
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