I thought if the DCR went up, so would the cranking comp. ??
Here's my lastest experience with this stuff and the results, which i still don't understand.

1st combo 9.3-1
355 piston.020 ( -7cc) in the hole, .039 gasket 72cc Iron Eagle head, cam adv. 290-300- 218-230 @.050 112 LSA Adv. 4 deg.installed 106 ICL 71 deg of overlap weiand stealth. The DCR on this was horrible 7.43..crankin comp was 180.

2nd combo ( to raise DCR )9.8-1
355 piston .020 in the hole ( same piston) .026 gasket, 65cc Alum canfield ADV. 262-286- 218-230 @.050 113LSA installed 108 ICL. 48 deg overlap. The DCR on this was 8.4..cranking comp. after cam break in and 150 miles, 180 ???

I sure wish i knew why it didn't change ? I thought it would go up. The bright side is with 180 cranking comp. i can probably run the car on 87 octane.

Mike

BTW the car runs excellent and drives great, as streetable as can be, plus ran the fastest ever 2 weeks ago.

You are dealling with too many variables for a fair comparison. Advertised durations vary widely. The second cam, though shorter on seat timing was installed 2 degrees retarded. Battery charge and cranking RPM come into play also.

The cams are two different manufacturers, correct? No two companies use the same advertised duration specs, so calculating the DCR with these numbers has no relevance. The numbers at 0.050" show them to very similar cams.

when you did the compresion test did you remove all the plugs and open the throttle fully or just pull a plug and get a reading? makes a bit ot a difference in the readings. Dave

that's kind of the whole point to the DCR, to see which cam is best,i thought anyway. I didn't know that it was dependent of the manf. ?

For the test, all plauge were removed. There was one diff. I had NO carb on for the larger cam, just the intake, and for the 2nd one the throttle was help wide open.

I should clarify, i'm not upset, this is just one of those HUH questions, where you thought by all rights something should change.

Mike

A couple things. If the first cam has 112 LSA and installed a 106, that is 6º advanced not 4º. The second cam has 113 LSA installed at 108 ICL, it is 5º advanced not 2º retarded.

This is one of the reasons cranking compression is not good for much other than checking ring and valve seal. Engine needs to be hot and all other factors need to be the same between test.

I get 6.8 (106 ICL) or 6.68 DCR (4º advance) for the first cam's DCR. I get 8.24 DCR for the second cam. Recheck your numbers.

I get 8.9 SCR for the first combo and 9.89 for the second.

Not all companies use the same lift for advertised duration. Most use .006 or .004. To say that no two companies use the same spec is just wrong. While the two cams have the same .050 spec the second cam is clearly the more aggressive. If we had the .200" lift specs, I'm sure the second cam would have several degrees more duration than the first.

Well I beg to differ re; advertised duration numbers.
You just have to look at how the following companies advertise total duration;
GM, Comp, Crane, etc..

GM cams look impossibly huge and if you plug these numbers into the DCR calc, they will give a totally false impression on how they will run.

aahh, that clears it up PAT. I'm sure you remember doing this for me, and again the car is just awesome now.

I gave my engine builder your web address and shared this info with him as you calculated it. He's going to look into using it. Should be fun calculating DCR's on 15-1 632's with 12deg. heads.

Mike

Ron @ Isky seemed to feel that DCR is not an exact science, due to the fact that you will still build cylinder pressure when the intake valve is still open at very low lift, as you have a very small area for leaks. However, I would think a drastic change like you have made would show something in a cranking compression test. Of course, barometer, temperature, etc., etc., etc. all skew guage readings with this test, so this may not prove anything.

Pat,

I was only referring to the change in cam ICL from the first cam to the second. The 1st was 106ICL, the second 108ICL - per his posting. Even though both cams were advanced, the second cam was installed 2 degrees retarded as compared to the first cam.

To accurately measure how changes will effect the outcome, all other variables should remain constant. In this case, the ICL was 2 degrees later. I am assuming thses numbers are attained by actually degreeing the cam, BTW. There are just too many variables given. The best way to verify the effect of the changes is ET/MPH or Dyno with only engine changes only.

GM cams look impossibly huge... Not unless they use 0.0000" lift as their checking point.

Originally posted by 70GS455:
</font><blockquote>quote:</font><hr />GM cams look impossibly huge... Not unless they use 0.0000" lift as their checking point. </font>[/QUOTE]That is exactly what they usually do. For example the original LT1 cam has an advertised duration of 300/312. But at 0.050" it's 242/254. That would confuse alot of people, when the compare it to a Comp grind for example.

Yes, the old 30-30 or -346 solid "fuelie" cam was rated by GM at 346!!

W/B/R rates their version of it at 314, and

PatK rates it at 304!

Plus, Clevite rates their version of it at 295 so go figure???

pdq67

Originally posted by Pat Kelley:
This is one of the reasons cranking compression is not good for much other than checking ring and valve seal. I have to agree with this.

I changed starters and the cranking compression in my 7.9:1 DCR motor went from 180 psi to 150 psi.

I was degreeing a cam in a friend's 289 once and advanced the timing chain one full tooth just to see how it affected cranking compression. Went from 145 psi to 150 psi.

No, we did not leave it that way. smile.gif

I suspect that you would have seen a gain if the test was done similarly, i.e., both tests through either a carb, or open manifold. The less restriction there must have affected resutls in some way.

Quizzical none-the-less.

Quizzical none-the-less.

definatly. I know i went the right way , as the little hyd flattapet 355 makes 421 RWTQ @ 2100 RPM.


I just learned some about the old GM cams, it is quite confusing.

I will test with the engine warm and the carb off, just to see where it falls, I'll let everyone know the results, if your interested.

Thanks to everyone for the many explanations.

The adv dur of the old GM cams is the main reason we have .050" dur now. When lash is introduced into the equation it gets even more bizzare.

Originally posted by mike1985:
Quizzical none-the-less.

definatly. I know i went the right way , as the little hyd flattapet 355 makes 421 RWTQ @ 2100 RPM.

The high rear wheel torque number is due to torque multiplication by the torque converter. Measured rear wheel torque in a manual transmission car would be considerably less.