DCR, SCR and Cam choices

[dladow]

I am putting my engine together for my 73z and am puzzling over how to pick the optimum compression ratio. My goal is to build a mild performance street motor for the Z, that will spend most of its time in the 2000 – 4500 rpm range. Here is what the engine/drivetrain looks like on paper.

 

SBC 350 block bored 30 over; 3.48 stroke; 5.7 rod length

Vortec 64cc heads machined for higher lift, with stock valve sizes

Performer RPM manifold

Carter 600 cfm carb

Quench .040

1 ½ shorty headers into 2 ¼ single exhaust turbo muffler

2004r auto tranny with 2000 stall

 

I can vary the SCR using KB dished pistons, and vary the DCR by the cam timing of the intake closing degree. I am not sure what the practical effects are.

 

For example, look at these two Crower cams, which appear to be pretty close in specs, but have very different overlap and DCR numbers.

 

267HDP – adv 267/272 .050 210/216 112 lca 4 deg advanced; 38 deg overlap; intake closes at 56 deg.

Using P Kelley’s calculator, I get the following CR’s by varying the dish size in the pistons.

12cc: scr 9.6/ dcr 8.2

18cc: scr 9.0/dcr 7.7

22cc: scr 8.7/ dcr 7.4

 

270HDP – adv 270/276 .050 214/218 112 lca 4 deg advanced; 49 deg overlap; intake closes at 63 deg

12cc: scr 9.6/ dcr 7.8

18cc: scr 9.0/ dcr 7.3

22cc: scr 8.7/ dcr 7.1

 

I am not sure how to translate these numbers into drivability and power characteristics. I assume that higher SCR equals more HP, and the 270 cam will shift the power curve upward and presumably make more power at higher rpm.

 

Any thoughts?

 

Dave

[Kevin Shasteen]

Dave,

 

Before attempting to answer your question I would like to know if your car-engine combo will be a daily driver or will it be a weekend warrior / dragstrip car.

 

In other words, is maximum performance your goal or is street driveability with a punch your goal?

 

Kevin,

(Yea,Still an Inliner)

[dladow]

Hi, Kevin -

 

My goal is street driveability with a punch. I plan on making this my summer car and driving it almost daily during the summer. I will take it to the drag strip to tune it, but am not building it to race.

 

Thanks,

Dave

[grumpyvette]

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

 

 

 

270HDP – adv 270/276 .050 214/218 112 lca 4 deg advanced; 49 deg overlap; intake closes at 63 deg

 

12cc: scr 9.6/ dcr 7.8

 

18cc: scr 9.0/ dcr 7.3

22cc: scr 8.7/ dcr 7.1

 

 

would be my choice and heres why

 

first theres several factors at work

 

the vortec heads are most effective in the 2000-5000rpm range, above that the small port size matched to a 350 displacement tends to limit flow, youll need to match the other parts to that upper edge of about 3000rpm -5000 rpm

 

your headers exhaust scavaging will effect the rpm range where the best powers made but since most headers use primaries that are shorter than ideal, the rpm range they are most effective in is higher than the ports ideally run in, this also tend to put you in the 3000-5000rpm range

 

detonation with iron heads and pump gas under high loads is USUALLY not a problem if you stay just under 8:1 DCR

 

the 7.8:1 dcr matches the cams most effective rpm range (270HDP – adv 270/276 .050 214/218 112 lca 4 deg advanced)BTW its already 4 degrees advanced installed dot to dot)

 

MAXIMIZING DCR to just under detonation range increases TORQUE over the whole rpm range

 

youll want a DUAL PLANE INTAKE NOT A SINGLE PLANE INTAKE!!

 

best rear gearing will be in the 3.3-3.7 range

 

keep in mind that the milder cam will build low rpm torque slightly faster but drop off faster over 3500rpm, you don,t require much hp or tq to cruise around on half throttle or less, and theres not much sence in maximizing your combo for 1/4-1/2 trottle operation torque, but you can use all you can get between 3500rpm and 5000rpm when your passing or racing so the choice needs be the slightly wilder cam here! the slightly wilder cam with its greater overlap will pull harder once your above 3500rpm, either cam works here but when your foots on the floor the better breathing combo will show an advantage

[Kevin Shasteen]

..,2004r auto tranny with 2000 stall.., Dont skimp on your 2004r. I had a trany shop convince me into not buying a B&M 2004r because they claimed they could do the same for less. I spent $1600 for the perf.trans and it still couldnt quite keep up with the engine; Just FYI.

 

Dave,

 

When you are making your calculations be sure and doublecheck your math. As grump suggested - the 270 cam is 4 degrees advanced...so to is the 267 cam, as you already stated. Your calculatios were correct for the 270 but not on the 267 cam.

 

This means your Intake Lobe's Centeline will be 116, not 112. As a result your IVC you input in D.Kelley's DCR calculator is incorrect as well as the 38 degrees overlap: providing the cam is installed straight up. The incorrections are minor-yet if maximizing your engine-car combo is an issue then our degree of accuracy is mandatory. I'm not nit-picking...just something I noticed.

 

(Intake Duration / 2) - LCA = Intake Valve Opening

(Intake Duration - IVO) - 180 = IVC

 

Example: Using your 267 cam

 

(267 / 2) - 116 = 17.5*

(267 - 17.5) - 180 = 69.5*

 

(Int Duration + Exh Duration) / 4) - LCA) x 2 = Overlap

 

Overlap Example: Using your 267 cam

 

(267 + 272) / 4) - 112) x 2) = 45.5*

 

As grumpy said - there are a lot of issues at play here. To make it as simple as possible you can always look at a decent smooth running engine in categories of generalities. These are not vague generalities they are indicative of a smooth idle yet good performance for pure streetability.

 

SCR in the 9.25:1 to 9.7:1 SCR is a good rule of thumb. If you are using aluminum cyl.heads then you have nothing to worry about with the SCR ranges given. A good rule of thumb for a pure streetable DCR would be to choose a DCR in the 8.0:1 to 8.25:1 DCR.

 

If you want a streetable engine with a relatively smooth idle in the 700-750 rpm range you will need a cam that offers an IVC that leaves atleast 75-80% cylinder volume after the IVC.

 

If you had a Crank Angle Chart you would see that 75-80% Cylinder Volume would require an IVC somewhere in the 61-70* IVC ABDC range.

 

To ensure a peak rpm in the 5000rpm rang requires a cylinder head whose port volume is in the 170-185 cc range.

 

Overlap requirements for a good street engine with descent idle should be somewhere in the 30-40* range. Peak Cylinder Pressures in the lower rpm range 4500-5000 rpm, is your goal. So Cylinder Heads that allow a higher Airflow Velocity at lower rpms is a requirement: such as the Votec Heads grumpy suggested.

 

If you chose moderate valve lift in the .450-.470 range should net you a good performing street engine with a punch as long as you use a dual plane intake manifold whose peak power is in the 0-5000 rpm range.

 

The relationship between DCR and SCR is the remaining cylinder volume after the IVC based upon your desired peak cylinder pressures at a specific rpm. This is where understanding BMEP calculations are helpful.

 

It looks likes you are doing your homework based upon your questions-this is good because it will lead to educated choices. Any engine requires balancing DCR to SCR, Port Velocity, Valve Lift & Overlap: all these issues lead up to Cylinder Pressure. The trick is to apply your peak Cylinder Pressure at a specific rpm.

 

If you still have questions dont hesitate to ask.

 

BTW: I PM'd you. Check your HybridZ pm window.

 

Kevin,

(Yea,Still an Inliner)

[dladow]

Thanks Kevin and Grumpy for the thoughtful replies.

 

On the advance issue, I might be misinterpreting Kelley's program, but he asks for the lobe separation and the intake lobe centerline, and then the program calculates the opening and closing points plus overlap and cam advance. In order to make the opening and closing points correct (i.e. match the cam card), I had to enter the lobe center as stated in the catalog or on the card, and then enter 4* less for the intake lobe centerline. You added 4* to go from 112 to 116.

 

I can't get the crower or comp cam sites to come up, so I'll use the crane cam site. Crane cam #113931 has adv duration of 262/268. The card doesn't show the lobe separation, but the PAW catalog lists it as 114*

 

Putting those figures into the Kelley program, I have to set the intake lobe centerline at 111* to get the opening and closing degrees to match the card. Intake open and close is 20/62 and ex open and close is 71/17 with an overlap of 37* The dynamic stroke length is 2.76782 and the DCR is 7.8490:1 (with an SCR of 9.6:1 with 12cc dished pistons).

 

It is interesting to compare this to the Crower 270HDP. Both have the about the same intake closing degree (62 vs 63), but the Crower has 49* of overlap, while the Crane, (due, I think, to the 114* lsa) has an overlap of only 37* The Crane has about .06 more DCR than the Crower (7.85 to 7.79).

 

Another interesting comparison is the Crane 113941 to the Crane 113931.

cam # 113931 113941

Adv dur 262/268 268/280

overlap 37* 50*

lsa 114* 112*

intake close 62* 63*

DCR 7.85 7.8

 

Based on grumpy's comment, the 113941 would be the better choice for me, because it would make more power in the 3500 - 5000 rpm range. I don't need much low rpm power because the Z is such a light car.

 

It's kind of fun to build on paper, because I can change my mind. One of these days I am going to have to make a decision and stick with it.

 

Thanks again for all your help.

 

Dave

[Kevin Shasteen]

I didnt know at the time you listed your cam if you had adjusted your Lobe Centerlines for the 4* advance that the mfg's have ground into their cam.

 

The point I wanted to express is to be sure and doublecheck your math, tripplecheck your math, and then check it again. Kind of like the carpenter's rule...."Measure Twice and Cut Once".

 

Just to make sure we are talking about the same thing here: sometimes the Lobe Separation Angle is called Lobe Center Angle or Lobe Displacement Angle...not to be confused with the Lobe Centerline which is also called Lobe Centers.

 

If the mfg has ground the cam 4* advanced then the Lobe Separation Angle (LSA), will be 4* advanced when the cam is installed straight up. You cant alter how the cam was ground. You can only alter how you install the cam relative to the piston's TDC. This means the timing events occur 4* earlier when compaired to that same cam without the factory 4* advance.

 

All Cam fundamentals can be lumped into 3 categories:

1) Cam Profile (opening-closing, duration, lift)

2) Lobe Separation Angle (overlap)

3) Cam Installation (advanced, straight up, or retarded)

 

If a cam was ground by the mfg w/a 4* advance-your cam will be 4* advanced when it is installed straight up (does this make sense?). So when you decide to calculate the timing events you will have to move the Intake Lobe Centerline 4* as well as the Exhaust Lobe Centerline, in your calculations to obtain the correct tiiming events.

 

So if a cam had a 112* LSA and the end user were to install the cam straight up (this is a reference to the Crank Rotation to Piston position based upon TDC), then your Lobe Centerlines would be

 

Intake Lobe Centerline = 116*

Exhaust Lobe Cenerline = 108*

 

This is all I was emphasizing when I mentioned to double check the math.

 

Either of these cams will make great torque in a light car, as you suggested. The question is do you want peak torque in the 2500-3500 rpm range or do you want your peak torque to be in the 3000-4500 rpm range. There are two ways of reaching peak power. Either manner you choose you have to realize we are still managing airflow velocity in the ports, and this effects cylinder pressures for a given rpm.

 

The old school, muscle car engine days, manner of building a performance engine was to utilize a cam with excessive duration , mild to moderate lift, and a high SCR coupled with cylinder heads that used smaller ports. This allowed the engine's horsepower to peak in the 4700-5000 rpm range. This occurred because airflow velocity was high at lower rpms.

 

Then a long came the aftermarket who built better breathing cyl.heads and cams more suitable for those heads on the street and strip: coupled with the removal of lead in our pump gas. You have to decide do you want a street performaer or do you want a drag strip performer? You have already stated you wanted streetability with a punch. 270-280cams are borderline Hot Street Engines which is one level higher than a streetable engien with a punch.

 

The new approach to building a good street performer is to utilize a cam with higher lift, moderate duration (compared to the musclecar cams) that take advantage of the better breathing cyl.heads which have larger ports, moderate compression ratios, and do so on unleaded gas.

 

Volumetric Efficiencies have improved with the aftermarket cams, cylinder heads, intake manifolds, and reverse dome bathtub dished pistons. So the higher cam durations are not always needed; especially if you want a smooth idle.

 

The differences between the two approaches to building an engine is basically in how the airflow velocity is managed. The early musclecar heads utilized smaller ports while the newer aftermarket cyl.heads utilize larger ports on average. You can still get aftermarket cyl.heads with smaller ports-you just have to know what port sizing is going to give you the peak rpm range and idle quality you desire.

 

This brings me to the next question...when you take your car to the strip are you going to rev your engine beyond 5000 rpm or is 5000 rpm going to be your limit?

 

I'm only asking because you stated earlier in your first post that the majority of your driving would be in the lower rpm range and that racing was not your intention. When you set your peak hp rpm you have also set your mid peak torque rpm range as well. Usually a 270-280* cam will put your peak rpm range in the 5500-6500 rpm range depending on the cyl.heads chosen. This also moves your mid peak torque power to the 3500-4500 rpm range...mismatched to daily street driving.

 

IMHO if you were going to drive this daily then you would want a 2500-3500 peak torque range. This way your engine would come on the cam quicker and run more efficient during hwy cruise (better gas mileage).

 

Just some issues to think about.

 

Kevin,

(Yea,Still an Inliner)

[dladow]

Lots of food for thought.

 

I definitely want peak HP to be below the 5500-6000 range - probably closer to the 4700-5000 range. I don't have a feel right now for what different combinations do to the HP and torque curves. Sounds like I should get myself some computer dyno software and model a few combinations just to map out the relative torque and HP curves. Any suggestions on what software is the best?

 

At the moment, it seems like the constants will be the Vortec heads, dual plane intake manifold, 600 cfm carb, and 12cc dished pistons, with a .040 quench (.025 deck plus .015 gasket). This gives me an SCR of around 9.6:1. I live in the cool Pacific NW, and that together with the optimum .040 quench should let me run on 92 octane pump gas.

 

Now I need to make some cam selections that will give me a DCR of between 7.5:1 and 8:1, then model them on dyno software to select the right RPM range for peak HP and torque. Am I missing anything?

 

Dave

[grumpyvette]

thats a good question that comes up very often

the short answer is that ALL the components must match the EFFECTIVE rpm range and your DURRATION,DYNAMIC COMPRESSION and RAM TUNING on both the EXHAUST SCAVAGING AND INTAKE RUNNER CROSS SECTION AND LENGTH must complement each other

 

your ports cross sectional area and length from the back of the intake valve to stack enterance distance will have a major effect on the most effective rpm range provided the engines cam and compression also match the combo correctly

 

here this may help

 

http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm

 

http://www.newcovenant.com/speedcrafter/calculators/intake.htm

 

http://www.bgsoflex.com/intakeln.html

 

http://www.victorylibrary.com/mopar/intake-tech.htm

 

http://www.sdsefi.com/tech.html

 

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

 

http://www.howtodoitvideos.com/html/cross_ram_manifold.html

 

its a difficult concept to grasp at first

but you need to understand the differance between STATIC COMPRESSION RATIO AND DYNAMIC COMPRESSION RATIO

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), 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 used

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

 

http://www.newcovenant.com/speedcrafter/calculators/compressionratio.htm

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://dab7.cranecams.com/SpecCard/DisplayCatalogCard.asp?PN=114681&B1=Display+Card

 

looking at the cam specs we see that the effective stroke is not the 3.48" that the static compression ratio is measured from about 2.6 inches from tdc 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

 

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

 

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

 

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

 

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/showflat.php?Cat=&Board=UBB64&Number=397334&Forum=UBB64&Words=LSA&Match=Entire%20Phrase&Searchpage=3&Limit=25&Old=allposts&Main=397176&Search=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.mercurycapri.com/technical/engine/cam/vtg.html

 

 

 

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/showflat.php?Cat=&Board=UBB64&Number=200511&Forum=UBB64&Words=your%20kidding&Match=Entire%20Phrase&Searchpage=0&Limit=25&Old=allposts&Main=200510&Search=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

 

 

 

 

BEFORE GOING ANY FURTHER, heres a few combos to look over

 

http://www.ryanscarpage.50megs.com/combos1.html

 

http://www.chevymania.com/dyno/

 

http://www.bracketmasters.com/small_block_stroker_383_cu.htm

 

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

 

keep in mind that the dynamic compression ratio,cam,timing heads port flow and intake type and flow rates, plus the header exhaust scavaging of the combo must match for the best results

 

heres some other things to look over

 

http://www.built4speed.com/

 

http://chevyhiperformance.com/techarticles/71298/index4.html

 

http://www.speedomotive.com/building%20tips.htm

 

http://www.strokerkits.com/build_your_own_383.htm

 

http://www.dumpit.org/grimreaper/reaper_engine.htm

 

http://www.carcraft.com/techarticles/77458/

 

 

if you decide to build a 383-406-427 youll want to buy an INTERNALLY BALLANCED ROTATING ASSEMBLY

 

http://www.scatcrankshafts.com/smblckrotatingkits02.html

 

and I WOULD STRONGLY SUGGEST (H) style cap screw connecting rods with 7/16" rod bolts

 

 

BUT you really should get these books FIRST and read them before going any further, it will help a good deal,AND SAVE YOU YEARS AND THOUSANDS OF WASTED DOLLARS, while only the basic info is presented it still makes for a good knowledge base, and referance[/color]

 

IF YOU SMART ENOUGHT TOO DO YOURSELF A HUGE FAVOR FIRST and

buy these books, it will be the best money you ever spent, read them, and you will be miles ahead of the average guy youll save thousands of dollars and thousands of hours once youve got a good basic understanding of what your trying to do! IF YOUR ON A VERY LIMITED BUDGET START WITH BOOKS 3,4,AND 5, ONLY BUT YOU NEED AT LEAST THOSE BEFORE DOING ANYTHING ELSE

 

[ http://www.amazon.com/exec/obidos/tg/detail/-/0912656069/ref=pd_sxp_elt/102-1234339-0571324?%5Fencoding=UTF8&v=glance

 

http://www.amazon.com/exec/obidos/tg/detail/-/1557880298/ref=pd_bxgy_img_2/102-1234339-0571324?v=glance&s=books

 

HOW TO BUILD THE SMALL BLOCK CHEVEROLET by LARRY ATHERTON&LARRY SCHREIB

.

HOW TO BUILD MAX PERFORMANCE CHEVY SMALL BLOCKS ON A BUDGET by DAVID VIZARD

.

JOHN LINGENFELTER on modifying small-block chevy engines

 

how to build & modify CHEVROLET small-block V-8 CAMSHAFTS & VALVTRAINS BY DAVID VIZARD

 

SMOKEY YUNICK,S POWER SECRETS

 

SMALL BLOCK CHEVY ENGINE BUILDUPS

 

KNOWING HOW TO DO AND WHY YOUR DOING EACH MODIFICATION AND WHAT YOULL LIKELY GAIN FROM EACH PUTS YOU WAY AHEAD OF THE AVERAGE GUY WHOSE JUST BUYING PARTS ALMOST AT RANDOM AND HOPEING THE RESULTS WILL BE GOOD[/color]

 

picking a cam for your combo, lets say your building a hot street/strip 383 with a auto trans, alto the process will remain the same with any engine combo, the rpm ranges,etc. will change with the engine combo you chose to build

 

ok lets go thru the basics

first the reasonable rpm range

with a 383 youll want to stay to a max rpm thats not much more than 4000fpm, thats about 6500rpm with the 3.75 stroke

next youll want to find a cam that allows both a reasonable dynamic compression ratio and good volumetric efficiency up to that approximate 6500 rpm red line

 

next youll need to know your true static compression ratio

http://www.newcovenant.com/speedcrafter/calculators/compressionratio.htm

 

and then dynamic compression ratio

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

 

next youll want to find a cam with a reasonable overlap / LSA and lift for your application

 

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

 

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

 

these are the valve timeing overlap ranges that are most likely to work correctly

trucks/good mileage towing 10-35 degs overlap

daily driven low rpm performance 30-55degs overlap

hot street performance 50-75 degs overlap

oval track racing 70-95degs overlap

dragster/comp eliminator engines 90-115 degs overlap

The LSA, or lobe separation angle, is ground into the cam and cannot be changed. It is the angle that separates the intake and exhaust lobe for a particular cylinder, and is measured in camshaft degrees.

The intake lobe centerline is measured in crankshaft degrees. The #1 intake lobe centerline is usually between 100° to 110° ATDC and is what you use to degree the cam. The cam manufacturer will publish the specs for the cam based on a given intake lobe centerline. Comp Cams, for instance, produces a large number of cams with 110_° LSA ground 4° advanced, so they list the specs for the cam with a 106° intake lobe centerline. You can calculate the ILC by adding the intake opening angle in °BTDC, the intake closing angle in °ABDC, plus 180° for the distance from TDC to BDC. Divide by 2 and subtract the intake opening angle and you will have the ILC. For example a 12-430-8 Comp Cam lists IO at 34°BTDC, IC at 66° ATDC, so 34 + 66 + 180 = 280. 280/2 = 140. 140 - 34 = 106° ILC.

 

this is some of the best basic cam info youll find so read this first http://www.newcovenant.com/speedcrafter/tech/camshaft/1.htm (lessons 1-8)

 

then youll want to match the valve lift to the port flow charicteristics

http://www.chevyhiperformance.com/techarticles/41598/

 

now since your not giving all the info, Ill need to guess at some info.

I can tell you right now youll be looking at a cam with close to 230 deg@.o50 lift, wheither it should be slightly larger or smaller in duration, its LSA and wheither the cam needs a longer duration exhaust lobe will be determined by the other parts in your combo and if your going to run nitrous

without knowing your true compression ratio,your true cylinder head flow, and your rear gearing and trans stall speed you can,t get a true picture, your cars weight and tire height will also have a noticable effect on the results, so you need to factor in your probable shift points and rpm drop between gears

http://www.prestage.com/carmath/dynochart.asp

 

http://www.wallaceracing.com/reargear.htm

 

heres how to find the port size that close to ideal

http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm]http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm]http://www.newcovenant.com/speedcrafter/calculators/runnerarea.htm

heres how to find the port length thats close to ideal

 

http://www.bgsoflex.com/intakeln.html

 

heres how to get some idea on the header config

 

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

 

keep in mind that you don,t want to build an engine that makes 500hp at a narrow peak at 6500rpm if your stall speed and gearing and trans shift points limmit your engine to the 2000rpm-5500rpm range.

 

WELL YOU ASKED WHY!!! AND HOW TO FIGURE IT OUT!!

 

yeah ,I know your swamped! don,t worry, just wade into the info, it will make sence after awhile!