DCR advice for my setup, with calculator printout
DCR advice for my setup, with calculator printout I made some changes since my last post after I found out the the cam I'm putting in the Firebird has a 2* advance ground in the intake lobes which boosted the DCR up several tenths of a point. This DCR is much higher than I was anticipating, honestly I didn't understand/and was ignorant, and I'm a bit concerned as to the outcome. I'd like to keep the cam if all possible, its a Comp XER287, I'd like to make it work by adjusting some different parameters of the engine. I really need some advice on the head gasket I should use since everything else has been purchased. I usually run Sunoco Ultra 94 octane but I would rather set this up for 93 octane since it is more readily available. If I were to pick up the 0.040" thick gasket how how close to the ragged edge am I? I've heard that people run high 8 DCR's without a problem on pump gas. Need a little advice. Thanks
Last edited by squealingtires; Sep 14, 2006 at 11:38 PM. Reason: Updating numbers
I get 9.16 with Piano Prodigy's calc. Either way it is higher than I would be comfortable with. If you really don't want to get another cam get an adjustable timing set and retard it 4 degrees otherwise It may be time for a bigger cam or a later IVC. On the other hand, if I already had this setup sitting in my garage I would be too eager to get it together to wait for a new cam. Maybe you can just put it together, pull out most of the timing, gradually put in more timing and see if you like the results.
TECH Senior Member
Joined: Mar 2004
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From: BFE
Squealingtires,
your calculations are correct.
But 8.82 on 93 octane is not a big issue, I've ran 9.2 before with no issues.
Just keep in mind your geographical location. If DA is high, better stay in the 8.5 area.
You can retard your cam to degree straight up (112 ICL) and you'll be 8.65 DCR.
IMO i would run it if your DA permits it, you have good quench, and all you need is to get the right plugs. At least 1 step colder (TR6) or similar.
your calculations are correct.
But 8.82 on 93 octane is not a big issue, I've ran 9.2 before with no issues.
Just keep in mind your geographical location. If DA is high, better stay in the 8.5 area.
You can retard your cam to degree straight up (112 ICL) and you'll be 8.65 DCR.
IMO i would run it if your DA permits it, you have good quench, and all you need is to get the right plugs. At least 1 step colder (TR6) or similar.
Originally Posted by PREDATOR-Z
Squealingtires,
your calculations are correct.
But 8.82 on 93 octane is not a big issue, I've ran 9.2 before with no issues.
Just keep in mind your geographical location. If DA is high, better stay in the 8.5 area.
You can retard your cam to degree straight up (112 ICL) and you'll be 8.65 DCR.
IMO i would run it if your DA permits it, you have good quench, and all you need is to get the right plugs. At least 1 step colder (TR6) or similar.
your calculations are correct.
But 8.82 on 93 octane is not a big issue, I've ran 9.2 before with no issues.
Just keep in mind your geographical location. If DA is high, better stay in the 8.5 area.
You can retard your cam to degree straight up (112 ICL) and you'll be 8.65 DCR.
IMO i would run it if your DA permits it, you have good quench, and all you need is to get the right plugs. At least 1 step colder (TR6) or similar.
What's is my DA? I've seen it before but I'm not sure what it means.
I do have the SLP double roller adjustable timing set and I think I'm pretty close to having piston/valve clearance issues, but I won't know for sure until I physically check. So I'm considereing retarding the cam a few degrees. What are the drawback of this?
What does everyone think about the 40 thousanths head gaskets? I'm unexperienced in optimizing quench or trading off quench for a lower compression ratio.
TECH Senior Member
Joined: Mar 2004
Posts: 14,620
Likes: 17
From: BFE
DA is density altitude (O2 amount at a certain altitude), basicaly the higher it is the less O2 your motor can injest, the less O2 the more prawn to detonation (Knock, pinging).
with 6cc reliefs, I would doubt you'll have PTV issues, but better measure always.
And did you measure the .007 out of hole piston deck height? Usualy built motors are more in the .010 area (at least that is where i like mine). That would effect your DCR as well.
The tighter the quench, the more you fight detonation. A .030>.040 is a good range.
Mine is at .040 with 8.65 DCR and 11.75 SCR with no issues running 26* advance timing (motor doesn't seem to make more power on more, but I experienced no ping at 28*)
with 6cc reliefs, I would doubt you'll have PTV issues, but better measure always.
And did you measure the .007 out of hole piston deck height? Usualy built motors are more in the .010 area (at least that is where i like mine). That would effect your DCR as well.
The tighter the quench, the more you fight detonation. A .030>.040 is a good range.
Mine is at .040 with 8.65 DCR and 11.75 SCR with no issues running 26* advance timing (motor doesn't seem to make more power on more, but I experienced no ping at 28*)
Originally Posted by PREDATOR-Z
DA is density altitude (O2 amount at a certain altitude), basicaly the higher it is the less O2 your motor can injest, the less O2 the more prawn to detonation (Knock, pinging).
with 6cc reliefs, I would doubt you'll have PTV issues, but better measure always.
And did you measure the .007 out of hole piston deck height? Usualy built motors are more in the .010 area (at least that is where i like mine). That would effect your DCR as well.
The tighter the quench, the more you fight detonation. A .030>.040 is a good range.
Mine is at .040 with 8.65 DCR and 11.75 SCR with no issues running 26* advance timing (motor doesn't seem to make more power on more, but I experienced no ping at 28*)
with 6cc reliefs, I would doubt you'll have PTV issues, but better measure always.
And did you measure the .007 out of hole piston deck height? Usualy built motors are more in the .010 area (at least that is where i like mine). That would effect your DCR as well.
The tighter the quench, the more you fight detonation. A .030>.040 is a good range.
Mine is at .040 with 8.65 DCR and 11.75 SCR with no issues running 26* advance timing (motor doesn't seem to make more power on more, but I experienced no ping at 28*)
The rotating assembly is an SLP spec'd build: Eagle 4" stroke, Eagle H-beams and Ross pistons. I just assumed the out of deck was the same as stock. The heads have been milled some; how much I'm not sure which raises fears of piston/valve interferance concerns. So I'm looking really good on quench at .030". I think I'm gonna go with the 40 thousanths Cometic head gaskets, install the cam without retarding, and play with the spark timing tables. I think I've heard enough advice that what I'm planning is safe. Here in Dayton, Ohio we are sitting at aproximatly 900 feet above sea level.
Last edited by squealingtires; Sep 15, 2006 at 04:05 PM.
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Originally Posted by PREDATOR-Z
DA is density altitude (O2 amount at a certain altitude), basicaly the higher it is the less O2 your motor can injest, the less O2 the more prawn to detonation (Knock, pinging).
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Verdad Gallardo TECH Senior Member
Joined: Mar 2004
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From: BFE
Higher DA=Less air=less oxygen=less power
Less oxygen=less efficient burn=more carbon deposit=more chance of hot spots=more detonation.
maybe i could be wrong, I'm at sea level too. some high DA guys may want to chime in.
Hmm you got me thinking, I'll do more research.
Less oxygen=less efficient burn=more carbon deposit=more chance of hot spots=more detonation.
maybe i could be wrong, I'm at sea level too. some high DA guys may want to chime in.
Hmm you got me thinking, I'll do more research.
