The solid lifter Lunatidyne cam I just bought is a 272/282(238/248 @.050) to replace the hydraulic UD 276/286(223/232 @.050). When I use the duration @ .050 in Pat Kelly's DCR calculator I have gone from a DCR of about 9.8 to about 9.4, with the total duration figures I get the same 8.5 DCR with both cams.

Stupid me is trying to solve a detonation problem but now that I do these calculations it looks like I messed up again!!! graemlins/clonk.gif

I was afraid to go any bigger with a solid cam for fear of drivability issues.

Am I looking at this wrong? :confused:

What engine and what is your static compression ratio?

You should always should use advertised duration when figuring DCR. "Lunatidyne" uses advertised duration at .020 which is quite a bit different than Comp's .006 or Crane's .004. Your Lunatidyne cam should act bigger when it comes to DCR so if you are borderline high don't sweat it.

Static is 10.5, it's a .030 over 454 with .133 dome pistons, 290(101cc)heads. It's a borderline issue in that when I can get to 95 octane instead of 93 the detonation decreases dramitcally but is still there to some degree. The Lunati figures are @.050 per the cam card.

I'm going with it anyway but I'll run it straight up instead of advanced 4 deg..

James,I'm running 10.5 to 1 with .100 domes that are .008 down in the bore with 101 cc's and a 223/231 @.050 with NO detonation..you'll be fine with your new cam graemlins/thumbsup.gif

BigJames,

With the 272* cam and 10.5:1 compression you are pushing it just a little IMO. Your DCR calculates out to 8.5:1, maybe slighly lower than that with the Lunatidyne acting a bit bigger.

I think you will be OK as long as your timing curve isn't too agressive and you don't go too lean with your AF mix. Keeping the coolant temperature low (like 165*F) will help too and so will a cold air induction setup.

I manage 11.3:1 static CR on 93 pump gas with aluminum heads . . . I'm kinda pushin it though.

James, I'm getting numbers in the 8.15-8.2 range if I cal'c you at 10.45 and then plug the 272/282 cam in with 112 lsa installed straight up ( 112 ICL).

Is that cam 112 lsa?

Originally posted by 427L88:
James, I'm getting numbers in the 8.15-8.2 range if I cal'c you at 10.45 and then plug the 272/282 cam in with 112 lsa installed straight up ( 112 ICL).

Is that cam 112 lsa? Yep, it's 112 LSA. The only variation from the published specs is the lobe centerline is at 108 deg instead of 106. I get 8.24 with it installed straight up so we must be figuring about the same.
I'm thinking that will solve the problem of detonation but I'm wondering about how much power that will cost me if I had gas good enough for detonation not to be an issue.

James, are you running vacuum advance? If so, try limiting the vacuum advance. It may kill your problem and it won't cost you any power since vacuum advance isn't active at WOT.

Originally posted by cjlandry:
James, are you running vacuum advance? If so, try limiting the vacuum advance. It may kill your problem and it won't cost you any power since vacuum advance isn't active at WOT. Nope,no vacuum advance, Mallory Comp 9000 with mech advance only. I have the most conservative advance springs installed now.

112 LSA is better - I was assuming a 110* LSA. I don't htink it will cost you much power . . .

James, Are you running Speed Pro pistons? If so, piston to valve clearance may have a determination in how much advance/retard you put the cam in at. I have the 272/272F12 cam and with it in at 107 CL I only have .112 exhaust valve clearance (.120 exh clear is usually the recommended number) with Speed Pro L2242 (396) forged pistons. Any less cam advance would make it less. My pistons are flycut .040 on the intake side so intake clearance is OK (.140) but that should be checked close too. Advancing the cam reduces intake valve clearance and retarding it reduces exh valve clearance. Neither minimum is right at TDC but a few degrees before or after.

Originally posted by von:
James, Are you running Speed Pro pistons? If so, piston to valve clearance may have a determination in how much advance/retard you put the cam in at. I'm using KB Silvolite pistons and I'm pretty sure I have adequate clearance but I will do a quick check once the cam is in.

This brings up a thought. According to KB, Silvolite pistons tend to retain more heat up top than other pistons, hence the need for wider ring gaps. Could this lead to the detonation issue?

Originally posted by cjlandry:
This brings up a thought. According to KB, Silvolite pistons tend to retain more heat up top than other pistons, hence the need for wider ring gaps. Could this lead to the detonation issue? You may have something there Chad! I knew I was at the max compression wise when I decided on what to build but I did not think about the pistons retaining heat. I do have the proper ring gaps but it must be the heat that puts me over the edge as far as the detonation goes.

As I said though, I'll put it in straight up and not advanced. If the problem persist I'll just bite the bullet and start saving for a set of those new Brodix oval port aluminum heads. This engine really does run good and the detonation is not at the point of being so bad it's going to kill the engine so I'm not going to change much if I can help it.