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Username Post: understanding DCR (dynamic compression )        (Topic#92966)
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
03-03-05 11:25 AM - Post#639780    

its obvious some of the guys on this site need to understand the differance between static and dynamic compression ratios, and thats understandable as its a difficult concept to grasp at first
but you need to understand it before selecting a combos components

the differance between STATIC COMPRESSION RATIO AND DYNAMIC COMPRESSION RATIO is where the piston is in the cylinder when the valves close and the piston can accually start compressing the REMAINING VOLUUM IN THE CYLINDER VS the STATIC COMPRESSION THAT ASSUMES THE PISTON STARTS COMPRESSING THE INSTANT IT LEAVES BOTTOM DEAD CENTER AND STARTS UPWARD ON THE COMPRESSION STROKE!


let me try and explain, the short version is that the PISTON COMPRESSES NOTHING untill BOTH VALVES ARE CLOSED, .......thats the only compression ratio that matters,.... since its the only compression ratio the engine ever sees.

static compression is simply the differance between the cylinder volume at BOTTOM DEAD CENTER(BDC) and its compressed volume at TOP DEAD CENTER (TDC), into the combustion chambers,... dynamic compression takes into account that on the pistons upward compression stroke the valves have not yet closed and nothing gets compressed by the piston untill they do, that of course depends on the cam and rockers, pistons and connecting rods, the cylinder voluum, the rod/stroke ratio, ETC.,used, in the combo, and the rpm levels to some extent BTW, ALUMINUM HEADS can usually operate at a higher dynamic compression simply because ALUMINUM releases heat to the coolant much faster than iron, its the lower heat levels that remain in the cylinder that help prevent detonation..when you increase the dynamic compression the heat levels in the heads combustion chamber rise , the differance in the RATE heat leaves the cylinder allows a slightly higher dynamic compression level from aluminum before the same HEAT levels are REACHED & MAINTAINED in the combustion chambers

heres a calculator for static cpr, which you need to figure first

http://www.rbracing-rsr.com/compstaticcalc.html

or
http://users.erols.com/srweiss/calccr.htm

or
http://not2fast.wryday.com/turbo/compression/compression.shtml
let me point out a few things
first look at this chart

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

then lets assume your 350 sbc engine has a static compression ratio of 11:1 but youve installed this cam

http://www.cranecams.com/?show=browsePar...lvl=2&prt=5

looking at the cam specs we see that the effective stroke is not the 3.48" that the static compression ratio is measured from ,at BDC, BUT from about 2.6 inches from tdc where the valves close as the piston moves upward, so your true working compression is closer to 8.1:1 NOT 11:1

heres a longer more detailed explanation and access to the software to figure dynamic cpr with the cam your useing in your engine

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

http://www.diamondracing.net/cocalc.htm

heres some differant calculators

http://www.kb-silvolite.com/calc.php?action=comp2

http://www.wallaceracing.com/dynamic-cr.php

http://www.smokemup.com/auto_math/compression_ratio.php

http://not2fast.wryday.com/turbo/compression/cranking_pressure.shtml
average the results

keep in mind that you can easilly run a stattic compressio of 11:1 with aluminum heads if you keep the cam timing in a range so that the DYNAMIC COMPRESSION is CLOSE TO 8:1
take the time to understand the concept,it VERY IMPORTANT


read this

http://www.diamondracing.net/cocalc.htm



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

http://victorylibrary.com/mopar/otto-c.htm

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

http://kb-silvolite.com/article.php?acti...3117842f4eb4c49

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

http://victorylibrary.com/mopar/piston_position-c.htm

http://www.iskycams.com/techtips.html#2003

http://victorylibrary.com/mopar/rod-tech.htm

http://www.zhome.com/ZCMnL/PICS/detonation/detonation.html

http://www.chevytalk.org/threads/showfla...true#Post397334

if cams are a mystery please take the time to read these, it will get you a good start

http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm
(read LESSONs 1-8)


http://www.wighat.com/fcr3/confusion.htm

http://www.chevyhiperformance.com/techarticles/95298/

http://www.idavette.net/hib/camcon.htm

http://www.centuryperformance.com/valveadjustment.htm

http://www.totalengineairflow.com/tech/valvelashing.htm

http://www.chevytalk.com/tech/engine/Cam_Selection.html

http://www.chevytalk.com/tech/101/Cam_Theory.html

http://www.babcox.com/editorial/ar/ar119736.htm



http://www.symuli.com/vw/camp1.html

http://www.symuli.com/vw/camp2.html

http://home.wxs.nl/~meine119/tech/camqa.html

http://www.chevytalk.org/threads/showfla...true#Post200511

http://www.crower.com/misc/valve_timing_chart.html

http://www.speedomotive.com/Building%20Tips.htm

]

http://chevyhiperformance.com/techarticles/94138/

http://www.aera.org/Members/EngineTech/engine.htm

http://www.zhome.com/ZCMnL/PICS/detonation/detonation.html



 




Bahama Dan 
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Posts: 1734
Bahama Dan
Loc: suburbs of Chicago IL, US...
Reg: 05-25-04
Re: understanding DCR (dynamic compression )
03-03-05 06:18 PM - Post#639781    
    In response to grumpyvette

Thank you!!!!! Just what I needed.

Bought in 2002 with 38,200 original miles
My little old lady find


 
57WAGGN 
Senior Member
Posts: 697
57WAGGN
Age: 45
Loc: Morro Bay, CA.
Reg: 09-25-03
Re: understanding DCR (dynamic compression )
03-03-05 06:52 PM - Post#639782    
    In response to grumpyvette

Hey Grump, Thanks for this post. I have a much better understanding of how a camshaft affects the dynamic comp ratio of an engine. For instance, the cam I chose for my 10.5:1 (SCR) 396 is a Comp 280 magnum hyd. But since the int valve closes at 66 ABDC, it will allow me to run on 91, possibly even 89 octane. The DCR for my engine is 8.16, perfect for pump gas. I was thinking I was going to have to run octane boost or mix a little race gas in my tank.

Thanks again!

Troy - 57 Handyman Wagon




 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
Re: understanding DCR (dynamic compression )
03-04-05 02:41 AM - Post#639783    
    In response to 57WAGGN

http://kb-silvolite.com/article.php?action=read&A_id=36

you might want to read this also guys

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
MikeB 
Ultra Senior Member
Posts: 10058
MikeB
Loc: Plano, TX
Reg: 08-28-03
Re: understanding DCR (dynamic compression )
03-13-05 04:48 AM - Post#639784    
    In response to grumpyvette

Hey Grumpy -- isn't DCR still just an estimate, since the different cam manufacturers specify advertised duration at different lifts? For example, I think Comp Cams uses .006" lift, whereas others like GM use .002". The old 204/214 cam sold under many different brands is made in one factory (Camshaft Machine Co.?) but is sold as a 278/288 by Edelbrock and 264/274 by Speed Pro, and who knows what by Blue Racer, Summit and others? So which one of those seat-to-seat numbers would you use to calculate DCR? And why isn't there a standard lift (like .006") for advertised duration when we all know the cam's Intake Closing Point is such a critical spec?

Thanks, Mike


1982 C10 SWB pickup: Unmolested base truck, original paint. Originally had 250 six and 3-on-the-tree
Now has 355 with Vortec heads, RamJet roller cam, LS6 beehive springs, TH350
Retired, but working part-time on 50s-70s cars & trucks.


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
Re: understanding DCR (dynamic compression )
03-13-05 10:49 AM - Post#639785    
    In response to MikeB

"Hey Grumpy -- isn't DCR still just an estimate, since the different cam manufacturers specify advertised duration at different lifts?"

yes and no! it depends on whether your refering to the exact instant the valve leaves the seat(in which case yes, its an estimate in that it might be a degree or two off exact)
or if your refering too, when the valve opens enought to flow air in significant voluum to effect the true working compression, in which case the answers closer to NO1 in that one or two degrees makes very little effective differance.
think about it
http://www.cranecams.com/?show=browsePar...lvl=2&prt=5
heres a cam card from a cam I use frequently
look over the differance in the opening degrees between .020 and .050 lift, then look over this piston degree chart
http://www.iskycams.com/ART/techinfo/ncrank1.pdf
we quickly see that the piston only moved about .4" durring that time and the engine idleing at lets say 900rpm has that valve move that .030 inches of lift during the 15 or so degrees and that happends 15 times a second and takes aproximately .00275 seconds per intake/compression stroke
if your thinking theres much effective flow thru a valve thats only a few thousands more open or closed in .00275 seconds,you wrong, and don,t forget that your not dealing with that whole 15 degrees but perhaps 1/5 of that time....and as rpms increase the time drops significantly. so in the real world your dealing with changes OF LESS THAN .01 LIFT and in WELL UNDER .002 SECONDS for a change of a few degrees in the opening point

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
GOSFAST 
Member
Posts: 15

Loc: Long Island, NY
Reg: 04-10-05
Re: understanding DCR (dynamic compression )
04-14-05 02:51 AM - Post#639786    
    In response to grumpyvette

Anyone out there who picks a large(r) cam to lower the dynamic C.R. will no doubt be down in the end with both horsepower and torque unless he tends to run the RPM's off the scale. THERE IS NO SUBSTITUTE FOR THE RIGHT COMPONENTS no matter how you slice it and dice it. We're in the dyno room all day long up here for customers off the street, and our own, and only a handful from outside have the proper C.R.'s and cams installed in the engine. Some have no clue what's goin' on!
PS. I just want to add a little to the above and maybe some will understand it a little easier. We have at this time a 540 BB with 9.75:1 C.R. & alloy heads. The cam (a Comp Retro-fit Hyd. Roller is a 270/276 degree @.050 with a .725 lift at the valve. It'll run on 89 (we think, we ran 93 for the test.)This one came off at 760 HP and 720 Ft.Lbs. torque and all this happened under 6150 RPM and keeping with a 23 degree pair of heads and carbureted. Which is a standard BB configuration. If you were to put the same cam (spec-wise except for the lift) in a 9.75:1 350 SBC you would probably not be able to ever get it started. You just simply need the right parts. There are no "tricks or big secrets"



 
GOSFAST 
Member
Posts: 15

Loc: Long Island, NY
Reg: 04-10-05
Re: understanding DCR (dynamic compression )
04-14-05 09:19 AM - Post#639787    
    In response to GOSFAST

Apology for wrong cam specs on Comp Cams BB retro had 270/276 posted was supposed to 252/262 (I think these right now are one of the biggest custom grinds available from Comp) on that blank.



 
waterwalker 
Member
Posts: 6

Loc: portland
Reg: 02-28-05
Re: understanding DCR (dynamic compression )
05-11-05 06:23 AM - Post#639788    
    In response to grumpyvette

With all that said, is there a correlation (or approximation) of what a compression ratio is, based on the readings you get from a compression gauge??? 130 psi ????



 
sgian 
Valued Contributor
Posts: 4563
sgian
Loc: MO
Reg: 12-25-03
Re: understanding DCR (dynamic compression )
05-11-05 09:55 AM - Post#639789    
    In response to waterwalker

The higher the DCR, the higher the compression pressure should be, but there are also other factors like the condition of the engine and altitude at which the test was done. There are calculators which give cranking pressure along with DCR, but they differ significantly from each other (just like DCR calculators) depending on what duration intake closing point is used.

Here are a couple:
http://www.wallaceracing.com/dynamic-cr.php
http://not2fast.wryday.com/turbo/compression/cranking_pressure.shtml




 
enigma57 
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Posts: 9745
enigma57
Age: 70
Loc: Texas
Reg: 10-28-00
Re: understanding DCR (dynamic compression )
05-22-05 07:09 AM - Post#639790    
    In response to grumpyvette

Sigh...... Guess the days of leaded high test pump gas, 11:1 static compression and 30/30 Duntov solid lifter cams are just fond memories, Grumpy. So I am attempting to learn about and factor DCR into the equation when considering cam choices for my project engine, as I must run what passes for pump gas now of days.

I have read through the highly informative links that you have posted regarding DCR and have learned a lot from them. What is confusing to me is that whilst each DCR calculator that I find uses differing input figures regarding intake closing angle and results in differing calculated DCR results...... I can find no adjustment factors to equate each differing DCR result to a set value of what is considered a "safe range" to make good power whilst avoiding detonation. As far as I am aware, a good rule of thumb is to stay between 7.5 and 8.5 DCR with 8.0 as a good target range for pump gas (primarily street driven vehicle) and to allow a half point more for aluminum heads.

Now I have been playing around with a computer dyno program to see what works well with my combo, looking at variations of off the shelf grinds as well as custom grinds utilizing cam lobe profiles from several cam manufacturers. I am rather partial to solid lifter flat tappet cams and am not so sure that I really need to spend the extra bucks to retrofit either a solid or hydraulic roller setup into my street driven '57, as I am looking for peak torque around 3,800 - 4,000 RPM and peak power around 5,500 - 5,700 RPM...... So radical cam timing should not be necessary with the decent flowing heads I picked up at Joe Mondello's porting school a while back. Also, my engine is carburetted, so I don't need to be concerned about EFI knock sensors picking up noise from solid tappets.

As I am unsure of how to equate all the differing DCR results that I get from the various DCR calculators to the 8.0 DCR target, I have taken 3 examples and have averaged them as you suggested to BowtieFreak.

http://www.chevytalk.org/threads/showflat.php?Cat=0&Number=1216772&an=0&page=0#1216772

Here are the results utilizing my engine specs with static comp. ratio of 9.44:1. I cc'd the chambers and piston valve reliefs/dished area to ensure accuracy. I did not cc the heads with spark plugs installed, but sealed the holes with tape flush with the chamber surface, so the plugs might decrease chamber volume and increase compression slightly when installed. I might add that quench is 0.039 in. and my heads are aluminum.

Intake Closing Angle for the solid lifter short track cam I chose for these DCR calcs is 61 degrees After Bottom Dead Center (advertised) and 47 degrees ABDC @ 0.050 in. lift......

http://www.iskycams.com/timingchart.php?product_number=201532

My computer dyno program indicates that I will achieve my power goals with this cam at the RPM that I want along with a broad, flat torque curve if I have Isky grind this cam on 109 degree LSA rather than the spec'd 106 degree LSA and install it at 106 degree ICA. It was interesting to note that this change in LSA resulted in a worthwhile boost in both power and torque in the 4,000 RPM and under range with my combo whilst losses at targeted peak RPMs for power and torque were minimal. I plugged in both narrower and wider LSA values with several variations of ICA for each, but 109 LSA and 106 ICA worked best with my combo.

DCR Calculator Results are as follows......

7.83 DCR - Kelley (uses adv. ICA)
7.77 DCR - Silvolite (uses 0.050 in. ICA plus 15 degrees)
8.49 DCR - Wallace at sea level (uses 0.050 in. ICA)

http://members.uia.net/pkelley2/DynamicCR.html
http://www.kb-silvolite.com/calc.php?action=comp2
http://www.wallaceracing.com/dynamic-cr.php

Average of these 3 DCR results is 8.03 DCR using sea level for the Wallace calculator.

Now if I factor an altitude of 600 ft. above mean sea level into the Wallace calcs to simulate a place we are thinking of relocating to when I retire, DCR drops to 8.37 and that drops the average of the three to 7.99 DCR. Also, cranking pressure drops from 171.68 at sea level to 168.55 PSI at 600 ft. altitude.

Wallace DCR results (sea level)......

Quote:

Your engine summary is as follows:
Bore - 4.125 inches
Stroke - 4.00 inches
Rod c-c length - 5.85 inches
With a static compression ratio of 9.44:1.
Your camshaft specifications call for an inlet valve closing of 47 degrees ABDC (after bottom dead center).
Your chamber volume is 103.79 cc's. With this camshaft your dynamic, or effective stroke is 3.55 inches.
Your dynamic compression ratio is 8.49 :1 corrected for cam timing, altitude, and rod length.
Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 171.68 PSI.
Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 8.49 :1.




My goal is to fine tune my combo to make good power on pump gas without skating on the ragged edge of detonation whilst maintaining good driveability as well. Am I on the right track here?

Thanks in advance for your assistance,

Harry



 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
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grumpyvette
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Loc: FLORIDA USA
Reg: 03-16-01
05-22-05 07:37 AM - Post#639791    
    In response to enigma57

well your certainly doing a much better job of thinking the combo thru than most guys do, I would think by looking over your figures that your not only on the correct track but may even be very slightly less DCR than my combo, and that runs fine on 92 octane available here in florida.



IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 05-17-09 04:50 AM. Reason for edit: No reason given.

 
enigma57 
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Posts: 9745
enigma57
Age: 70
Loc: Texas
Reg: 10-28-00
Re: understanding DCR (dynamic compression )
05-22-05 08:30 AM - Post#639792    
    In response to grumpyvette

Many thanks, Grumpy! Really appreciate your help. I'll give Ron a call next week and work out the details with him.

This DCR stuff is something that we didn't have to pay as much attention to many moons ago. In those days, we just knew by rule of thumb that the 375 HP 327 Duntov 30/30 solid cam worked well with 11:1 static CR and the 350 HP "151" hydraulic cam worked well with 10:1 static CR on high test leaded gas if you geared your car appropriately for them...... And that aftermarket cams of similar spec could be expected to do the same.

Well, those days are gone forever and now I must learn to play by the new rules. Say...... Who says you can't teach an old dog new tricks, anyway?

Happy Motoring,

Harry

"Dark and difficult times lie ahead, Harry. Soon we must all face the choice between what is right...... And what is easy."
Albus Dumbledore


 
57WAGGN 
Senior Member
Posts: 697
57WAGGN
Age: 45
Loc: Morro Bay, CA.
Reg: 09-25-03
Re: understanding DCR (dynamic compression )
05-22-05 12:24 PM - Post#639793    
    In response to grumpyvette

Grump,
Why does the Wallace calculator always come up with a higher DCR than the other calculators? If you just take the average of the 3 calculators mentioned above, the Wallace calc brings the avg way up.

It just seems to me that if DCR means "effective compression", then the DCR should always come out the same, no matter what calc is used.

Is one calculator more accurate than the others?

Troy - 57 Handyman Wagon




 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
Re: understanding DCR (dynamic compression )
05-22-05 01:55 PM - Post#639794    
    In response to 57WAGGN

personally I think pat kellys calc
http://cochise.uia.net/pkelley2/DynamicCR.html

is one of the best

heres some differant calculators

http://www.kb-silvolite.com/calc.php?action=comp2

http://www.wallaceracing.com/dynamic-cr.php

http://www.smokemup.com/auto_math/compression_ratio.php

http://not2fast.wryday.com/turbo/compression/cranking_pressure.shtml
average the results

I think the Wallace calculator much less acurate
but i usually use all 5 and average the results KNOWING it may be slightly high but that tends to keep you out of serious detonation problems

keep in mind the DCR is not an EXACT CPR value that you reach and above that your into engine failure!,its a GUIDE that you use to evaluate your combo, use of things like aluminum heads, mpfi vs carbs,pollishing the combustion chanber and coating the piston surfaces, knock sensors, nitrous/alcohol injection,quench, tumble,swirl, ETC. all result in changes in detonation threshholds
you engine may or may not run without detonation at 8.5:1 dcr, but its a good guess that exceeding that level with randomly choosen parts while using 87 octane fuel is usually a bad idea.
Ive run 13.7:1 static cpr in a street engine, and without detonation, but there were several other factors in the engine combo that allowed it to work
its the total combo and the way all the parts work with each other that makes or breaks a combos effectiveness

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
sgian 
Valued Contributor
Posts: 4563
sgian
Loc: MO
Reg: 12-25-03
Re: understanding DCR (dynamic compression )
05-23-05 07:29 AM - Post#639795    
    In response to 57WAGGN

DCR calculators come up with different values because they use different intake closing points. Wallace uses the industry standard of .050 and PK uses the advertised (varies at .004 or .006 depending on manufacturer).

90 C1500, 05 Tahoe 4WD


 
SC383Dream 
Member
Posts: 5
SC383Dream
Loc: Detroit
Reg: 05-29-05
Re: understanding DCR (dynamic compression )
06-01-05 01:50 AM - Post#639796    
    In response to Bahama Dan

ugh... i don't know how i ever thought building an engine would be easy trying to spec it out so i can run 91o fuel with a blower. at least i have everything i need in one post now to figure out what combo i should use

thanks grump



 
SMOKEmUP 
Member
Posts: 1

Reg: 07-10-05
Re: understanding DCR (dynamic compression )
07-10-05 03:17 PM - Post#639797    
    In response to grumpyvette

I've added a couple of calcs just for this subject.

DCR
Cam Timing

2002 Camaro Z28 - Mostly Stock 13.1 @ 105 1979 Camaro Z28 - Mostly non-stock 12.6 @ 115


 
EDDIE.H2 
Senior Member
Posts: 179

Loc: NJ
Reg: 01-31-05
Re: understanding DCR (dynamic compression )
09-27-05 01:32 PM - Post#639798    
    In response to grumpyvette

it S_CKS to be me because that all went over my head i think ill just go back to school i was gonna go this year but with 3 little ones at home its hard but any way one day ill get it the green one

the green one


 
74LagunaS3 
Valued Contributor
Posts: 3784
74LagunaS3
Loc: Winchester, MA , USA. H...
Reg: 02-23-00
Re: understanding DCR (dynamic compression )
01-20-06 05:49 AM - Post#639799    
    In response to EDDIE.H2

Some of the Static Comp ratio calc links are broken

Here is a real cool one I found

http://www.rbracing-rsr.com/compstaticcalc.html

njoy


74 Laguna S3 Project

2004 Merc Marauder driver


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
Re: understanding DCR (dynamic compression )
05-10-06 03:52 AM - Post#639800    
    In response to 74LagunaS3

Detonation vs power

some of the major factors in your engines potential power, is the volumetric efficiency (how efficiently you fill and empty the cylinders) and the octane of the fuel used, compression ratio and detonation limits,
With detonation, prevention the main factors are
Ignition timing
Quench
FUEL OCTANE
DCR
and cylinder heat level AND THAT HEAT LEVEL CHANGES RAPIDLY WITH RPM AND LOAD
youll be fairly safe if you stay under
8.5:1 dcr at 170f degrees
8.25 dcr at 180f degrees
7.8:1 dcr at 210f degrees
and keep the quench in the .036-.043 range
heres some differant calculators
http://www.kb-silvolite.com/calc.php?action=comp2
http://www.wallaceracing.com/dynamic-cr.php
http://www.smokemup.com/auto_math/compression_ratio.php
http://not2fast.wryday.com/turbo/compression/cranking_pressure.shtml
average the results

the numbers are for 92 octane premium gas run at a 14.7:1 a/f ratio

BTW heres a VERY SIMILAR GRAPH



richen up the mix to 12.5:1 for max power/tq vs low emmissions and you can cheat slightly as the cylinder temps go down slightly.
keep in mind ALUMINUM absorbs and allows the transfer of heat to the coolant at a much faster rate, so your less likely to have cylinder temps raise into the detonation range as quickly.
and it should be obvious that your igntion curve and spark strength will also effect results, personally Ive found the BETTER MSD multi strike ignitions keep the cylinders cleaner and less likely to detonate

btw, reading material

http://www.kennedysdynotune.com/Dynamic%20Compression%20Tech.htm

http://www.misterfixit.com/deton.htm

http://www.federal-mogul.com/cda/content/front/0,2194,2442_7359_7525,00.html

http://www.diabolicalperformance.com/hotrodoctane.html

http://www.sdsefi.com/meltdown.htm

http://www.procharger.com/intercooled.shtml

http://www.kb-silvolite.com/article.php?action=read&A_id=36

http://www.chevyhiperformance.com/techarticles/94138/

http://racingarticles.com/article_racing-10.html

http://www.popularhotrodding.com/tech/0311em_power_squeeze/index.html

1985,1992,1996 vettes
keep the rubber side down and the fiberglass off the guard rails



 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
03-28-07 05:02 PM - Post#1130671    
    In response to grumpyvette



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

http://victorylibrary.com/mopar/otto-c.htm

http://garage.grumpysperformance.com/index.php?thr...

http://forum.grumpysperformance.com/viewtopic.php?...

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


Edited by grumpyvette on 12-29-16 04:55 PM. Reason for edit: No reason given.

 
artak 
Valued Contributor
Posts: 4008
artak
Reg: 05-22-05
04-28-07 10:13 PM - Post#1153140    
    In response to grumpyvette

I took this subject apart myself, and I noticed that rod length has an effect on DCR. I made the diagram below, and it would be great if someone could check that it is accurate.



If you take your everyday 350 Chevy with 5.7" rods, and give it this cam which I picked randomly, and a 9:1 SCR, you get an effective stroke of 2.83" and a DCR of 7.54:1. With the same engine using 6" rods, you get a stroke of 2.82", and a DCR of 7.51:1.

So it looks like a larger rod:stroke ratio makes a cam appear larger to the engine, although the effects are small.

Casting Numbers


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
04-29-07 05:45 AM - Post#1153216    
    In response to artak

pehaps the CHART LINK Id posted in the first post would help you visualize it better

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

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
snowstorm 
Forum Newbie
Posts: 43

Loc: Norway
Reg: 09-05-09
09-05-09 01:01 PM - Post#1766934    
    In response to grumpyvette

Hi Grumpy. Thanks for some really good and useful information. I used Wallaceracing’s calc to line out the numbers before I purchased the rotating assembly and got the machine work done. It was calculated to be CR 10.48 and with the AFR 180’s / hydr. roller the DCR come out to be 8.31.

Now I have problems to get the engine to run “proper”. I measured the cranking pressure to 209psi – 220psi! Wallaceracing scheduled it to be 176psi. Obviously I did something wrong, but I did degree the cam on installation and I measured the cylinder and cylinder head volume by using liquid.

Here in Norway we have 98pump gas. My setup is 385 small block, Holley Stealth Ram induction, Holley Commander 950 MPFI, electronically controlled ignition, MSD all over, knock sensor, Wideband sensor, single pattern CompCam 280HR (224@0.05).

What do you consider to be the limit reg. cranking pressure (measured with warm engine, and throttle blade wide open)?

What do you thingk the actual idling timing should be (seen on the vibration damper)?

What spark gap would you use?

Cheers from Norway


-86 Z28, -73 sbc 4bolt, -385cid 3.75"x4.04",-CompCam HR 280 224@0.050" .565" w/1.6 rocker arms, -14in/Hg@idle,
-AFR 180cc/68cc, -9.6:1 SCR, -HSR induction, -950 ComPro,-MSD electronic ignition, -TH 700 Raptor stage III, -Strange 12bolt 3.42:1


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
09-05-09 01:19 PM - Post#1766945    
    In response to snowstorm

if your getting anything over about 170psi on a compression test then trying to run the car on pump octane gas is usually a potential problem
read this

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

210 / 14.7=about 14:1 cpr

I generally run a .043-.045 plug gap

whats your engine specs?, combustion chamber volume
piston type (DOME) or (dish)
gasket thickness etc.



IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
snowstorm 
Forum Newbie
Posts: 43

Loc: Norway
Reg: 09-05-09
09-05-09 01:47 PM - Post#1766956    
    In response to grumpyvette

My combo: -385cid (4.040”*3.75”) 4-bolted 1972 block, AFR 180cc w/66cc, -CR 10.48:1, -DCR 8.31:1, -quench distance 0.039”, 6cc dish piston ( I thing to remember), piston to deck distance 0.027” -CompCam hyd. roller .560” lift W/1.6 rocker arms 280 advertised 224@0.05 close 66ABDC (0.006”), felpro #1094 0.015” heads gasket.

-86 Z28, -73 sbc 4bolt, -385cid 3.75"x4.04",-CompCam HR 280 224@0.050" .565" w/1.6 rocker arms, -14in/Hg@idle,
-AFR 180cc/68cc, -9.6:1 SCR, -HSR induction, -950 ComPro,-MSD electronic ignition, -TH 700 Raptor stage III, -Strange 12bolt 3.42:1


 
snowstorm 
Forum Newbie
Posts: 43

Loc: Norway
Reg: 09-05-09
09-05-09 01:52 PM - Post#1766957    
    In response to snowstorm

Ohh.. and rod length is 6"

-86 Z28, -73 sbc 4bolt, -385cid 3.75"x4.04",-CompCam HR 280 224@0.050" .565" w/1.6 rocker arms, -14in/Hg@idle,
-AFR 180cc/68cc, -9.6:1 SCR, -HSR induction, -950 ComPro,-MSD electronic ignition, -TH 700 Raptor stage III, -Strange 12bolt 3.42:1


 
grumpyvette 
Senior Chevytalk Moderator -- Performance Subject Matter Expert --
Posts: 17111
grumpyvette
Age: 69
Loc: FLORIDA USA
Reg: 03-16-01
09-05-09 02:55 PM - Post#1766971    
    In response to snowstorm

  • snowstorm Said:
My combo: -385cid (4.040”*3.75”) 4-bolted 1972 block, AFR 180cc w/66cc, -CR 10.48:1, -DCR 8.31:1, -quench distance 0.039”, 6cc dish piston ( I thing to remember), piston to deck distance 0.027” -CompCam hyd. roller .560” lift W/1.6 rocker arms 280 advertised 224@0.05 close 66ABDC (0.006”), felpro #1094 0.015” heads gasket.




I get roughly 10.7:1 static and about 8.4 dcr which should result in between 135-160psi

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!


 
snowstorm 
Forum Newbie
Posts: 43

Loc: Norway
Reg: 09-05-09
09-06-09 02:14 AM - Post#1767109    
    In response to grumpyvette

Hmm…. There’s surely some loose threads here.

Could my compression tester be that much off?

My Rotation kit is a Scat 1-90460BI-040-X.

Before I installed the scat rotation kit the engine was working ok as a 355 with the same camshaft and heads. The CR was 9.8:1 ish.

Now after the rebuilt it runs worse, especial at idle. It use to idle at 16 – 19 degree actual timing. But now it idle best at 28degree.

The interest for the particular camshaft came after reading an article in chevy high performance, http://www.chevyhiperformance.com/techarticles/148...

-86 Z28, -73 sbc 4bolt, -385cid 3.75"x4.04",-CompCam HR 280 224@0.050" .565" w/1.6 rocker arms, -14in/Hg@idle,
-AFR 180cc/68cc, -9.6:1 SCR, -HSR induction, -950 ComPro,-MSD electronic ignition, -TH 700 Raptor stage III, -Strange 12bolt 3.42:1


 




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