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So how much dynamic compression are you guys getting away with on street pump gas?
It seems like 8.3 to 1 may be a good limit for an aluminum head engine with ceramic chamber coating (?).......
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I assume static compression is the theoritcal C.R. calculated from piston compression height, deck height, combustion size, head gasket thickness, etc.?
Dynamic, it seems, varies with the camshaft profile? How do you calculate your dynamic compression? |
I have 11.5 CR and alluminum heads .Alluminum heads let you get away with at least .5 CR higher. I figure 10.5 with iron heads or 11 with alluminum heads I could live with 91 octane pump gas. I'm tickeled thatI just found the local SafeWay has 93 octane. Racing gas costs allmost $6 per gal $2.45 is a better deal.
8.3, isn't that ready for a supercharger? |
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Now THAT was a good read and finally explains WHY cam manufacturers recommend a particular static pressure C.R. for a given cam. Like if you buy Cam X it suggests a C.R. of 10 to 11. While Cam Y may only reguire a C.R. of 9 to 10. The correct C.R. for the cam profile gaurentees that the Dyanmic C.R. will remain in an acceptable range.
8.3 appears to be an acceptable range. Armed with this new information I can now begin to calculate my Dynamic C.R. I'll get back to you... |
Click on Pat Kelley's home page and look at his take on high gas prices.
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I have a 351 with aluminum heads (425 HP) with 9.5 to 1 compression and it runs without pinging on medium grade (89 octane).
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allowable compression
nm .
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Pat Kelly’s program calculates mine at 8.4 (slightly over) DCR (10.6 Static) Its an iron head 390 and it is very touchy about timing, but does run on pump premium with a fairly slow curve in the dist and 36 degrees total.
I was hoping for a bit lower compression, but that is how the numbers worked out. I hear that the KB piston calculator comes up with different numbers though, so be careful comparing numbers if they are not all calculated on the same basis. |
great
nm .
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Tricky stuff nailing down the NUMBERS!
9.8 static C.R. with a 282S Comp Cam.
7.9 Dynamic C.R., more or less. The numbers are HARD to nail down! I'm not sure about the size of the head gasket "hole", I rated it at 4.4" 'cause it's not actually "round" you know! I'm also not sure of the head gasket thickness which I rated at .050 based on measuring my OLD head gasket. 428 rod length 6.490? 428 crank stroke 3.980. MY 427 bore worked out to 4.252 (.019 over). Piston Deck height, as I recall it was .005? -17 cc dish on the piston and 72cc chamber (I CC'd that ). Cam lash? Dang, thats a tough one to calculate! Cranking Comrpression (gauge) is 210 psi. Runs great on 92 octane and finishes the 1/4 in the 11 second range. According to to PKellys site 7.5 to 8.5 is acceptalbe for a street engine. |
Compression calcs
nm .
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Fel pro (and victor) head gaskets are rated at .041-042 compressed 4.400 bore for an efective cc of ~10.2
My block is zero decked and the quench is high, which may be one thing that is letting me get away with it as well as I am. Colder plugs helped too (124 autolites) and I still have the option of going to the 32 racing plugs (for about 3 times the cost) |
I'm surprised some of you guys are getting away with such high static compression ratios (11-to-1 and up). From what I've read, FE's with high SCR's tend to detonate easily because of the fact these motors need to use high domed pistons, versus 429's/460's which don't. Is it all in the choice of cams (read: plenty of duration and overlap) that lets your engines live at these high CR's?
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Good observation Byots.
The 427 piston height spec is in fact .030 below deck. In "blue printing" my engine I was specifically looking to introduce "quench", thus the pistons were designed for .005 below deck. Iron high riser heads, by the way. You know, I thought I had covered every possible base in blue printing my motor, but I totally over looked the importance of Dynamic C.R. Very interesting stuff, this thread. I can see why some motors get away with 11 to 1 static C.R. What REALLY counts is the Dynamic C.R. That explains SO much! Change the cam and get the wrong profile on your 11 to 1 monster and you could be in BIG trouble! |
Its not the overlap but actually the intake valve timing. Here are couple of links where people way smarter than me explained it:
http://www.fordmuscle.com/phpBB/view...c=1588&forum=1 http://www.network54.com/Forum/threa...eid=1088015266 |
Simply put, in a performance cam, the intake valve is still open while the piston is on the compression stroke. While the intake valve is open, the engine cannot compress the fuel/air mix. The wilder the cam, the longer the intake valve is held open, lowering the "dynamic" compression ratio and thereby allowing more static compression, which in turn raises the dynamic compression. Iron heads get by on around 8:1 dynamic, and aluminum heads up to somewhere around 8.4:1 using premium pump gas. More quench (or squish) you have helps, as does rounding any sharp corners on your heads (combustion chamber to deck area) and pistons (valve reliefs, dish to flat areas and/or domes).
Hope this helps, Dan |
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