[67drake]

Hi guys,
As a few of you know,from my posts here lately,I've been trying to pick a cam for my 400 engine.While searching for the "perfect" cam for my application I've come across one unanswered question-dynamic compression.The articles that I've read say it should be 7.5-8.5 (Pat Kelley) or keep it below 8.2 (Visard).
The problem I'm having is that while running my cam choices through the DCR calculators with my specs,I'm not even close to these #'s or the recommened cranking pressures.For example,my top pick right now is the Crower 60241. I show a 7.1 DCR at 9.0:1 SCR, I would have to bump the SCR up to 9.5 just to get to the minimum 7.5 DCR,thats with a mild Crower 60241 with a IC of 66 degrees. The very mild Comp Cams 260H (260 212 446 110 LSA IC 56 degrees@.006")that is in the engine now shows that I would need SCR of 9.0 just to get to the 7.5 DCR. Wouldn't you think that cam would detonate at 9.0? And remember,the SCR is supposed to be minimum of 7.5.
Thats the problem,by the #'s of the SCR calculators I'm "overcammed" with very mild grinds like the Summit 2801, and Crower 60241.So I'm asking "whats going on"?
Anyone else run into this?
Thanks,Drake
BTW I've run all my cams #'s multiple times,and on a few different calculators,so it's not an input error.

[Z Code 400]

What is your current SCR???

Crower grinds in general, with their generous advertised duration, are best suited to higher SCR applications.

Try this grind in your virtual 400 and see what it does to your DCR. It is my favorite mild street cam in a low compression 400:

.467"/.467" - 270°/276° Advertised - 220°/226° @ .050" - 130°/135° @ .200" - 111.5° LSA.

[67drake]

The engine is out for a rebuild.It was at 8.0 with the 260H.I'm milling a set of donar heads to get up into the 9.0-9.3 SCR range.

[Z Code 400]

At a true 9.00:1 SCR with the camshaft I mentioned, installed with an intake centerline of 109.5°, you would have 8.23:1 DCR with approximately 165 psi cranking compression...Robert

P.S. The IC @ .006" with this camshaft would be 64.5°

[67drake]

Robert,using those #'s I'm showing a SCR of 7.17 on 2 different calculators.
http://kb-silvolite.com/calc.php?action=comp

http://www.projectpontiac.com/ppsite...nt/view/16/30/
Here are my specs. BTW
head 88cc (for 9.0 SCR)
piston relief 6cc
gasket .040"
bore 4.12"
block 0 decked
stroke 3.75"
rod 6.625"

[Z Code 400]

I just input your numbers into the KB calcultaor and derived 8.955 SCR and 8.27 DCR.

88cc heads
4.121" Std Bore
6.0cc valve reliefs
4.200" gasket bore
.040" thickness
0.00" decked
3.750" stroke
6.625" rod length
39.5° IC value

[Hillard]

Originally Posted by 67drake

Robert,using those #'s I'm showing a SCR of 7.17 on 2 different calculators.
http://kb-silvolite.com/calc.php?action=comp

http://www.projectpontiac.com/ppsite...nt/view/16/30/
Here are my specs. BTW
head 88cc (for 9.0 SCR)
piston relief 6cc
gasket .040"
bore 4.12"
block 0 decked
stroke 3.75"
rod 6.625"

The calculator I have showed 7.17 DCR with those specs. I plugged in a Comp XE262H and came up with 7.54 DCR. The XE268H had 7.39 DCR.

[67drake]

How is this possible?
I just went and did it again and I got 7.17!? (with you're cam specs)This has been driving me nuts! As I said ALL the cams I've been running #'s on come out low.I've ran the Summit 2801,Crower 60241,60916,60242,60241 and all come out between,6.7-7.2 at 9.0 SCR? I've run these no less then about 5 or 6 times each,on a few different calculators?

[Z Code 400]

Funny....I keep getting 8.90:1 SCR and 8.22:1 DCR...

[67drake]

Originally Posted by 1050goat

The calculator I have showed 7.17 DCR with those specs. I plugged in a Comp XE262H and came up with 7.54 DCR. The XE268H had 7.39 DCR.

See what I mean though,the XE 262 is supposed to be for lower SCR engines,as far as I know.It's usually recomended for guys running SCR in the 8's,and it just shows the minimum of 7.5 DCR at 9.0 SCR.And the 268 lower yet.

[67drake]

Anyway I hope someone can chime in here.I have to get to bed.I have to get up this afternoon and drop my heads off at the machine shop,so that he can start on them.Thats why I need to decide on a cam.I want the machine shop to have valve springs by the time he's done with them!

[Hillard]

What octane are you wanting to run? Unless the fuel quality where you live sucks, I would use a Comp Xtreme energy cam. You also have to remember there are more factors involved with DCR than just cam specs. It also involves intake, carb, and air quality where you live.

[67drake]

Dave,I was going to run 92. From what I understand DCR does not change,it's a constant. Cylinder pressure will vary with RPM and other outside changes.

[badlefthand]

Try this web site,will give you a quick idea.I went with the Summet 2802, higher lift,114 sep,mines a 4.12 bore ,ported ,double dish, about 72 cc . Http://www.projectpontiac.comoops:Web site already posted,OH well I was born in 40 ,need a little slack.

[Hillard]

There was another thread on here at one time that has a lot of information. Try this:
http://forums.maxperformanceinc.com/...ic+compression

Jim Hand is among those that posted in this thread and he can explain it better than I.

[badlefthand]

Cant make a pontiac into a chevy.For street keep static around 9.7:1 than get highest dynamic with correct cam as possible. Be more conserned with detonation than Dynamic pressure. That is what a pontiac is. Jim Hand runs 11-1 static but he is Jim Hand!!

[screamingchief]

I think the issue your running into here has a lot to do with the calculators used and the specs your using.

Most DCR calculators need the seat intake closing numbers for accurate results,and the crower cams dont give those numbers,the only way to get those is by doing some math on your own.

The KB DCR calculator tries to accomplish this by using the .050" IC event and "adding 15°" to approximate the seat IC event,but that at best is a crapshoot as a lot of cams will have the difference between the .050" IC event and the seat IC event at more or less than that approximate 15° will be correct for.

I like the DCR calculator at wallace racing:
http://www.wallaceracing.com/dynamic-cr.php

I gave you the formulas to get the seat events in that other thread Drake.

Now use the seat IC event on this calculator and report back to us.

HTH.

[bobzdar]

Originally Posted by 67drake

Dave,I was going to run 92. From what I understand DCR does not change,it's a constant. Cylinder pressure will vary with RPM and other outside changes.

DCR won't really tell you what fuel you can use, if you create peak cylinder pressures at higher rpm you will be more detonation resistant due to turbulence. You can have 2 engines with the same DCR but different detonation characteristics due to the rpm at which they make peak cylinder pressures...Along with a bunch of other things. Jim Hand typically uses wide lsa's which reduce peak cylinder pressure but spread the power out...I see a lot of people trending towards narrow lsa's which gives high cylinder pressures in a shorter range of rpm. I personally don't like them, you get poor manners and a narrow power band with less top end, regardless of SCR, particularly in longer stroke motors. Look at the LS7's cam specs keeping in mind that it will pass emissions and pull to 7k rpm while running well over 11:1 on pump gas with a flat torque curve.

[Jim Hand]

Correction on the static compression ratio in my 473 cube engine - it is 10.10. That fact and many others about my engine and car can be reviewed at:
http://www.pontiacstreetperformance....455jh2006.html

I don't pay too much attention to dynamic CR since my car runs fine at 10.14 on 92 octane pump gas with a variety of cams. However, all have been in a similar duration range so the dynamic CR has not been out of a reasonable range with all.

And 1050GOAT provided a very good reference Topic on this subject.

Jim Hand

[grandville455]

I tried mine out at wallace and came up with 8.63 dynamic.. with my 9.3 :1 static.. I like that!.. Runs hard on 90 octane.. which is 10% ethanol even