I have a 70 SS 396 (402). The advertised compression ration is 10.25. I rebuilt the engine a couple years ago and the machine shop talked me into lowering the CR to 9.5 to run better on pump gas. A year later and some head work and I wanted to bring it back to 10.25 so I had the heads milled 50 thousanths to give it an additional .75 or back to 10.25. Cam went flat so while the engine is out I had them check the CR. They are telling me it is more like 9.15.

Machine shop talks about dynamic and static CR. Also the word advertised is used a lot. Which is important and most meaningful?

Also, I plan on having them bring the CR up to 9.8. Anyone see a problem with this ?

Finally, I am trying to figure out a good cam to put in this car. Old one was a flat tappet hydralic. I want to put in a roller hydralic. I have talked to Comp 2 times and Crane one and have 3 answers. If I posted info about the car and engine does anyone have a good idea about what would work well ?

Thanks
Howard

the compression ratio is how much smaller the combustion space is at top dead center compared to what it is when the piston is at bottom dead center. this is the "static" number.
for example, if you have 1000cc of space at bottom dead center, and only 100 cc at top dead center, then the true mechanical compression ratio is 10:1 (1000 /100).
as for dynamic compression ratio, it is a number derived by putting cam specs and a few other variables into the equation. i really know nothing about that.
if you want some good answers from a reputable source, maybe look for one of the many threads called "cam question for UDHarold".
i think there are 500 threads with that name..

You need to use an Eff./Dynamic CR calculator that uses advertising timing numbers not .050" lift plus 15 numbers!!

Try Pat Kelley's http://cochise.uia.net/pkelley2/DynamicCR.html

or this one (Boost calculator).

http://www.rbracing-rsr.com/comprAdvHD.htm

AND I strongly recommend going over to Pat's site first and READING it at least THREE times to get all this straight, then have at it.

I did!!

pdq67

the compression ratio is how much smaller the combustion space is at top dead center compared to what it is when the piston is at bottom dead center. this is the "static" number.

If this is true, all I can say is that's about the most simplest way I've ever heard it explained.

Howard, good info here, do a lot of reading. It's all in here, just set aside some time and go for it. And one more thing....with 50 thou. being taken off your current heads, and wanting to go with a roller cam with (likely) more duration and lift, your piston/valve clearance may very well be a problem, with losing that .050 right off the top. It depends on what other parts you will use, but keep it in mind. Static c/r is just that, the c/r figured when you build the engine, based on space pretty much. Dynamic c/r is the 'running' c/r, determined mostly by the cam you use, as different cams bleed off (or dont bleed off) some of the cylinder pressure. A dynamic c/r of around 8/1 is about all you want for an engine that will run on today's 91 or 93 octane. Advertised duration is sort of an ambiguous figure, different cam co's measure from different points sometimes, so the duration at .050 has pretty much become a standard of measurement. You will notice that most cam specs give the advertised duration, then the dur. at .050. And in my opinion, most of the guys you talk to at the major cam co's are just reading off a screen or book, and will give you very general info, and that's also why it's usually all over the place. You need to speak with one of the smaller mfgr's, or even a specialty cam co to get more and better info. Again, there is lots of good info right here in TC. Yes, post all your info and questions here, and you will get all the help you need. John

Thanks for the replys. I am going to give all the car info I have to see if someone can confirm what we settled on or another idea before I start the rebuild.

Here is what I have.
-1970 Chevelle SS 396 (402)
-Bored .60 over (414)
-Stock heads have been ported and polished and port matched to exhaust and intake milled 040 - 98 cc
-Eldebrok Victor Jr. Intake with Frog in the Carb Intake opening
-Barry Grant Speed Deamon 750 CFM
-Headman Headers 1 3/4 in.
-Compression Ratio 9.15 (to my surpirse as I just had it checked at machine shop)
-Cam Doug Hubert special grind 226/236 and 534/553 w/1.8 I and 1.75 E rollers 565/569
-Head air flow Intake @.650 325 CFM - Exhaust 217@600 (70% of Intake)
Valve 215 Intake 177 Exhaust
- PSI Springs - 160# Seat 310# open @570 lift

- Street Tires 28" 275 60R15
- Strip Tires 26" Mickey Thompson ET Stree Slicks
- Rear End Gears 3.73
- Car Weight 3,830 lbs
- Transmission Tremec 5 spd 1st gear 2.87
- Exhaust 3" with Flowmaster Super 40's
- Starting line launch 3500 Rpms

I need an opion on a good cam to put in this car. I have dyno'd this car and consistently get 385 hp at the rear wheels. All of the non racer 454's get about the same so I am getting quite a bit from a 396. I don't want to loose horsepower and would like to get above 400 at rear wheels.

The cam I have had the most consesus on is a Comp Cam Xtreme Energy 11-432-8 which is 230/236 and 510/510 w/Rockers above 540/529

I see the Duration is about the same as my old cam 226/236 vs 230/236 but the lift is a lot different 565/569 vs 540/529. I am concerned about the lift. I know the tuner person who is working on the Cam specs with me said he did not like the Exhaust only being 70% of the Intake Air Flow on the heads and said we would compensate in the Cam.

Also, we plan on bring the engine Compression Ratio from 9.15 to 9.8. Again some concern about Detonation but my car tuner says we will keep the timing low (32 to 34) and run the car a little rich 12.6 to 12.8 to compensate.

I have also had 2 recommendations from Comp and Crane over the phone although they never ask anywhere the number of questions my tunner did.
Comp 1 - 11-443-8 = 242/248 and 574/577 w/rockers
Comp 2 - 11-433-8 = 236/242 and 552/558 w/rockers
Crane 1 - 139671 = 236/242 and 651/646 w/rockers

If anyone has some ideas please let me know your thoughts. I guess I always though the bigger the number the better so I am scared of the smaller cam numbers on the lift.

Thanks to all who respond.

--Howard