Zac28

So I was talking with a buddy who decided to calculate my DCR. Its something I hadnt done, because its a custom cam I figured it wouldnt be an issue. Cam is from one of the most well know cam guys out there so I trusted him 100%. Told all my specs and what I wanted and thought I was good to go. Now I'm starting to think maybe I wasnt clear or there was a miscommunication with him somewhere, or maybe I am concerned for nothing....

​​​​My request was for a smooth idle with low to mid power (its in a truck) and run off 93 pump. Its a stock bottom end LQ9 with stock (except springs) 799 heads, stock exh manifolds. So basically a iron block ls2 with TBSS intake, nothing crazy.

DCR came up as 9.39
bore ~ 4
stroke ~ 3.622
chamber cc ~ 64.5
piston dish/dome ~ +1.2 cc (area between the piston and cylinder wall)
HG thickness ~ .051 (stock mls. I've seen .052 and .051. I went with worst case scenario)
HG bore ~ 4.020
deck clearance ~ -.008 (piston out the hole)
CR length ~ 6.098
IVC @ .050 ~ 39.6

What are peoples thoughts on this? The heads are off (waiting on new lifters) so although I already have new head gaskets I can swap them out and possibly some light bowl work but I'm no pro and dont want to screw anything up.

gagliano7

What are the cam specs?

Kawboom

You need to calculate it with the 0.006" duration. That is approximately when the valve will close.

Zac28

216/224 604/604 114 LSA

I used this calculator which says IVC @ .050 The static and dynamic compression ratio calculator (gofastmath.com)

G Atsma

You need to know the valve events at .006, not .050, to properly calculate dynamic compression ratio

Zac28

I just updated my previous post to include the link I forgot earlier. That calculator specifically says to use IVC @ .050. Not doubting you guys at all but I figured that calculator performed whatever math was needed to convert @ .050 to .006.

I'm looking into getting .006 figure now

Jake Wade

You will be fine. I don’t think your DCR is that high. Anyway, I have a friend with the exact engine combo but, with an earlier IVC and his runs great on 93 with the proper tune.

Kawboom

Assemble it and tune so it doesnt ping. Keep a bottle of octane booster on hand.

Fast355

Use the Pat Kelley calculator with 0.006 values.

But I do agree, build it, tune it so that it is detonation free on the high octane tables, pull some timing out of the low octane tables.

I have had GM vehicles as old as 1983 that had a knock sensor and they do work. My 83 G20 was 9.5:1 from GM and had an HEI with a piggy back knock system on it from GM. Even the old analog knock system never allowed for pinging that you could hear and that engine lived its life with no signs of damage from detonation. At 250K miles that engine had plenty of bore wear; both out of round and taper from running all those miles with a carb and ultimately failed from oil starvation on #1 & #2 rod bearings. Pistons still looked good though. It never ran on anything but cheap 87 either.

The SBC I tow a heavy travel trailer with is 11:1 static and 9:1 dynamic. I do see the occasional bit of knock retard but IMO if you never see anything your tune is too conservative or your knock sensor is dead.

If you have E85 in your area you can always set it up for flex fuel.

bthomas

iTrader: (1)

As mentioned .006 is required for proper calculation.

The .050 calculators usually are just adding 15 to the closing spec and calling it good. It’s a guess at when the lobe actually seats the valve fully.

every cam will have a different closing ramp, and .006 is more accurate.

Zac28

I've never seen octane boosters that work. I've only seen the "safe" way is mixing in higher octane. This also defeats the purpose of this truck. Its more of a dd/weekend cruiser. I dont want to have to worry about fuel so long as its not crap 93.


Not sure how I can get the .006 values. Cam card only lists .050. Looked into how to convert from .050 to .006 and it seems it cant be done due to the different ramp rates of the lobe.

G Atsma

Darken the print on this post. It is unreadable this way; too light a print...

bthomas

iTrader: (1)

Only other ways to get .006 specs would be to send cam in to get a cam doctor sheet, or install cam and degree wheel when the valve closes.

Fast355

I have found vehicles consistently cheaper to operate on 91 or 93 while offering more power everywhere.

Zac28

I didnt change the color of the print, but recently started using the "dark mode" on the forum so for me its light grey/white. Sounds like you are using the regular version and for some reason its still showing up light grey/white for you. Not sure why only that post was affected though.

I've had the same problem reading a few others posts since going "dark"; theirs comes up almost invisible to me. I'll change it to something hopefully everyone can see.

Zac28

That sucks. I really dont want to tear any deeper into it right now. I got other projects backing up because this thing has eaten up way more of my time than anticipated. I'll probably just run it as is. Hopefully when cam got spec'd nothing in my build got overlooked.

Hopefully everyone can read this. Not sure why this just now started.

RonSSNova

iTrader: (3)

Since you trust your cam guy 100%, why are you worrying about this?
And, whatever mode you are using, I can't read your last few posts.

G Atsma

I'm glad I'm not the only one!

Zac28

I dont know why all of a sudden you guys cant see my posts. Hopefully this color works. Let me know if not.

I trust that he can spec a great cam. If the accurate measurement is off .006 (not .050) then I'm guessing thats what he's using. When my buddy used the previously linked calculator and got 9.4 DCR, I started wondering if I didn't make clear I had 799 heads. If I was using the original 317's the DCR would be somewhere around 8.75 iirc.

G Atsma

It shows as red now. Why not let it print in black, as it should be??