matching the cpr to the cam

[grumpyvette]

[lets look at a 350 with either 8:1 like you would normally run in a supercharged engine or 10:1 like youll want to run in a non-supercharged engine,with a cam that matches that 10:1 compression ratio in both combos

 

will it run? can I use it? will I be able to drive around town?

 

yes!

 

will it make decent torque?

no! theres not a chance in $%^$% that it will make good tq, at 8:`1 compression,and heres why

the work your engine can do or the rate at which torque can be applied is directly related

the both the average rpm range and average torque applied, and torque is basically the result of cylinder pressure.

lets look at the differance between an engine set up to run the correct dynamic compression and a low static compression ratio, both with a average performance cam.

now like I stated,lets pick a 350 chevy and a comp. cams xe274 cam.

heres the cam

 

http://www.compcams.com/Technical/Search/CamDetails.asp?PartNumber=12-246-3

 

heres a chart with piston location in crank degrees

 

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

 

heres a compression STATIC ratio calc.

 

http://cochise.uia.net/pkelley2/crc.htm

 

heres a DYNAMIC CPR CALC.

http://cochise.uia.net/pkelley2/DynamicCR.html

heres things to read carefully

 

http://victorylibrary.com/mopar/cam-tech-c.htm

 

http://www.crossedflags.com/community/viewtopic.php?t=6490

 

 

now I could get technical in the extreme and work it out for you to the exact psi for a given cylinderhead and gasket, etc. but the basic concept here is that if you compare the differance in dynamic compressed volume vs expanded cylinder volume youll find aproximately this

 

an 8:1 STATIC compression engine with that cam should have aproximately a 6.36:1 dynamic compression ratio while a correctly matched 10:1 STATIC compression ratio with that cam would have approximately a 7.9 dynamic compression ratio or the 8:1 cpr engine have about 25% less cylinder pressure. now on the surface youll think thats a 25% lower cylinder pressure but because the cylinder volume is significantly less in the higher compression engine when the ignition fires the true differance between the two engines effective working cylinder pressure is more than 30%.

so its not at all unlikely that a 10:1 compression engine that made 350 ft lbs of tq would make a minimum of 30% less at about 245 ft lbs with that cam thats badly matched to the compression available in the non-supercharged 8:1 compression engine.

remember the cylinder pressure in a supercharged engine is significantly increased with the super charger boost.and a 8:1 compression engine needs that boost to effectively make the correct cylinder pressure and tq, yet without that boost it has little potential to build cylinder pressure effectively. but once supercharged,it gains extra power due to the greater volume of fuel/air mix burned.

remember that formula fo hp?

torque x rpm /5252= horsepower

well given the 30% PLUS loss in torque that lower compression is a significant handicap to making the necessary cylinder pressure(torque)

 

and yes Im well aware that you should only gain or lose about 8%-10% if the cam is a reasonable match to the lower compression ratio due solely to the loss in cylinder pressure alone! but if you miss match the cam and it bleeds off significant cylinder pressure due to a delayed intake valve closeing in relation to the piston possition the engine performance falls far faster that the loss of compression alone would normally cause!

[Guest zfan]

Grumpy,

 

I just recently built a sbc 383 stroker that I believe if I did the dcr calculations corectly is on the borderline for street ability.

 

I run 93 octane and 34 degrees total timing with no detenation so am not to worried. Did I go to far over the edge? What's your thoughts on my build:

 

350 four bolt block 40 over

Eagle steel 400 crank

Eagle 5.7 rods

Forged flat top pistons with 4 valve relief, 4cc valve relief size with zero deck. 40 thousandths felpro gaskets and 65cc chambers give 11.17 to 1 compression.

Comp cams cam parts number # 12-433-8, I retarded cam 4 degrees to go in at "0".

236/242 duration, 550/540 lift with 1.6's on the intake and 1.5's on the exhaust side.

Canfield 200cc, 65cc chamber heads, full radius valve job with blended bowls and minor port work.

Weiand stealth dual plain intake..yeah I know I need a Accelerator or Victor jr. intake!

Holley/Proform 750 dp carb

 

Well Grumpy, did I overdue it? I know you have built many 383's and would value your opinion. The 240z ran mid 11's with this combo and with a better coverter, lets say a Vigalante 3500 stall and some tuning could go low 11's.

 

Thanks, Mike

[grumpyvette]

"did I overdue it? "

no ! in fact it it appears to be about perfectly matched to the cam and aluminum heads, if your running temp is normally under 200 degrees

 

http://www.compcams.com/Technical/Search/CamDetails.asp?PartNumber=12-433-8

heres your cam

with that cam retarded 4 degrees you should be almost able to run lower than 93 octane if your ignition timing comes in slow enough, because my computer shows about a 7.9 dynamic compression from that cam and a static cpr of 11.15:1

btw I normally run a crower 00471 hydrolic roller in similar engine builds look at the specs

 

http://www.crower.com/misc/cam_spec/cam_finder.php?part_num=00471&x=35&y=13

 

and yes I also tend to install it retarded a few degrees

[A. G. Olphart]

Thanks for the post Grumpy...

 

Pat Kelley's web site explaining DCR (with his excellent free calculator),

http://members.uia.net/pkelley2/DynamicCR.html

lists a range of DCR's for street engines of 7.5 to 8.5 on 91 or better octane gas, and suggests that

"Staying below 8.25 DCR is probably best for trouble free motoring."

 

What DCR have you found to work with a given octane gasoline in engines with iron Vortec heads? I'd like to consider your experience with these engines as well as Pat's calculator since combustion chamber design must play a part in real world results.