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| compression ratio help https://www.slantsix.org/forum/viewtopic.php?t=34564 |
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| Author: | zorg [ Sat Mar 28, 2009 1:54 pm ] |
| Post subject: | compression ratio help |
Some quick help figuring out how much to deck/shave the block/head would be greatly appreciated. '76 225/A833OD Currently 3/16 in the hole, 60 cc head, bone stock. Going to Engnbldr valves, and some minor unshrounding. Not sure how much that will affect things yet... Also, cam is 280/270. Super six/long tube headers. I know there is much debate as to how high you can go and still run 87 octane, but I'd like to stay on the slightly conservative side (9:1?). Thoughts/advice/criticism? |
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| Author: | sandy in BC [ Sat Mar 28, 2009 2:04 pm ] |
| Post subject: | |
You can run up to 9.5 with that cam.....9.25 is prolly optimum. Unshrouding the valves should be done before measuring or milling. milling the block on big valve heads is preferred. Use an online calculater to get the dynamic CR you want with that cam/timing....then mill to get the static ratio. |
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| Author: | DusterIdiot [ Sat Mar 28, 2009 4:43 pm ] |
| Post subject: | Intake valve closing event... |
Quote: Also, cam is 280/270
What is the lobe separation angle on the cam? 106/108/110...114?I ran some 'typical' numbers and you're gonna have to do lots of milling to get anything respectable... 60 cc head with pistons in the hole .185 with a fel-pro gasket come up as a 7.6-ish SCR with a 6.7 or DCR with the stock cam (in other words...you can use that crappy ARCO stuff and still be safe...)... Just to give you an idea where to head.... If you were doing a .030 overbore and using a felpro gasket, and assuming that you went with a 108 LSA with 4 degrees advance ground into it... you are going to need to scrape the head to get a 53 cc chamber, and mill the block .100 to get your slugs .085 under the deck...which would net you a 9.6 SCR...and about a 7.76 DCR (my current two mills with shorter cams in them run in that range and use 87 regular...) Looking over this cam profile it's going to like to play in the midrange a bit so hopefully you have decided on rear gearing in the 3.55-3.91 range with your overdrive... Good Luck, -D.Idiot |
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| Author: | sandy in BC [ Sat Mar 28, 2009 7:06 pm ] |
| Post subject: | |
Thanks Rob....you just described my build with that cam. I have 3.55 gears with OD..... Total mill was 140. |
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| Author: | DusterIdiot [ Sun Mar 29, 2009 1:16 am ] |
| Post subject: | Lol... |
Quote: Thanks Rob....you just described my build with that cam. I have 3.55 gears with OD.....
...and next the idiot will attempt to read your mind...."More Red Ale...please..."  One of these days we're going to have to meet up...it's getting ridiculous in Oregon, that we can buy most any beer from any country...but they can't seem to order Rickards Red for some reason.....  -D.Idiot |
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| Author: | zorg [ Sun Mar 29, 2009 1:40 am ] |
| Post subject: | Re: Intake valve closing event... |
Quote: What is the lobe separation angle on the cam? 106/108/110...114?
I knew someone would ask... I was in a hurry when I picked up the cam, and forgot to get a cam card. -I'm working on it.Quote: Unshrouding the valves should be done before measuring or milling.
Right, I did think that through. Just giving my stock baseline here, so I'm not at the mercy of my machine shop. Quote: If you were doing a .030 overbore and using a felpro gasket, and assuming that you went with a 108 LSA with 4 degrees advance ground into it... you are going to need to scrape the head to get a 53 cc chamber, and mill the block .100 to get your slugs .085 under the deck...which would net you a 9.6 SCR...and about a 7.76 DCR (my current two mills with shorter cams in them run in that range and use 87 regular...)
Thanks, that's the kind of info I was looking for. I was having a little trouble with the online calculators, I'm no good with numbers.Quote: ...so hopefully you have decided on rear gearing in the 3.55-3.91 range with your overdrive...
I just happen to have a 3.91 geared 8 3/4" to be narrowed someday... Until then, I have no idea what ratio my BBP 7 1/4" axle is! The tag is illegible, and I haven't checked the good old fashioned way yet.
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| Author: | zorg [ Wed Apr 15, 2009 9:03 pm ] |
| Post subject: | |
I don't know what I don't know... This is all they gave me when I had my stock cam reground. So cam lobe separation angle and advance are probably still stock? Duration deg. .280/.270 @.50 .230/.224 Valve lift .470/.460 Valve lash .012/.012 I dropped the motor off at the machine shop yesterday, so I need to figure out how much we're decking the block/plaining the head. Thanks again for any help. |
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| Author: | WoodyB [ Thu Apr 16, 2009 12:26 am ] |
| Post subject: | SCR and street performance |
If you're looking at utilizing the cheap (quality but not price) gas we're stuck with, 9:1 SCR would be good. If you'd like to play with the compression ratio figures, here's a link for just that at Wallace Racing, enjoy...wb http://www.wallaceracing.com/cr_test2.php |
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| Author: | Dart270 [ Thu Apr 16, 2009 6:19 am ] |
| Post subject: | |
That is probably the cam I designed for them. Should have a 108 LSA. I'd shoot for 9.5:1 if you want 87 octane, or around 10:1 for premium. Lou |
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