Matching Displacement, Cyl Heads, SCR & DCR-Duration &am

[Kevin Shasteen]

The following was my attempt at making sence of the mysteries of how a performance engine works and why.

 

I believe the biggest mistake we hot rodders can make is the fact that many of us never realize there is a distinction between certain levels of performance...and as a result of not knowing these distinctions, we end up mismatching components and this leads to an inefficient engine at that specific level of performance.

 

Another point to the distinctive levels of performance is that each level has its own RPM range where in the peak Tq/Hp will occur.

 

It appears there is and always will be on going questions about which displacement, cyl.heads, scr to dcr-duration, and peak tq/hp rpms is best for an engine.

 

The quick answer is that there isnt a quick answer unless you first ask yourself a few questions. Each one of those variables are inter-related and can not be addressed separate from one another: this is the "which came first - the chicken or the egg" scenario that confuses most beginners. At first glance it appears to be a circular approach but once you understand each variable on their own will you see that they are not that difficult.

 

This is the approach I have formed after much reading and analyzing more engine builds & dyno runs than I can count.

 

Questions you have to ask before even touching a wrench:

 

What is:

1) The intended purpose of the Engine-Car combo?

2) Cylinder Pressures required to reach a specific level of Performance

3) RPM in which the required Cylinder Pressure (Tq) is needed

4) RPM in which Peak Port Pressure (HP) occurs

4) Metallurgy of Block/Cylinder Heads

5) Cylinder Head Intake Port Volume Sizing

6) Intake / Exhaust Valve Size & Lift

7) Intake / Exhaust Port cfm Airflow (E/I Ratio...or E/I%)

8) Cylinder Head Combustion Chamber Volume

9) Engine to Cylinder Head Compatibility (DUH!)

10 Intake Manifold that will compliment Peak Tq/HP RPMs

 

The different levels of performance that appear to be separat from one another IMHO, are:

 

1) Pure Street Engine: Turbo, SuperCharged, Nitrous

2) Pure Street Engine: Normally Aspirated

3) H/O Street - Mild Race Engine

4) Weekend Warrior - Moderate Race Engine

5) Dedicated Race - Normally Aspirated Engine

6) Dedicated Race: Turbo, SuperCharged, Nitrous

 

Next is the cylinder pressure measured as BMEP (Brake Mean Effective Pressure). Peak BMEP occurs simultaneously with Peak Tq while peak Port Pressure occurs simultaneously with Peak HP.

 

Here are the typical distinctions in Peak BMEP:

1) 185-220 psi = Pure Street Engine: Turbo, SuperCharged, Nitrous

2) 130-150 psi = Pure Street Engine: Normally Aspirated

3) 150-165 psi = H/O Street-Mild Race Engine

4) 165-185 psi = Weekend Warrior-Moderate Race Engine

5) 185-210 psi = Dedicated Race: Normally Aspirated

6) 185-300 psi = Dedicated Race: Turbo, SuperCharged, Nitrous

 

To calculate BMEP you can use this equation:

 

(HP x 792,000) / (Engine Displacment x HP RPM) = BMEP

 

Regarding Cylinder Heads the E/I% is important to ensure the exchange of airflow from the intake to the exhaust is not choked or hindered. To obtain the E/I% simply divide the Intake Port's cfm Airflow into the Exhaust Port's cfm Airflow.

 

Something I call the IP/CV% involves dividing the Cylinder Volume of one cylinder into the Cylinder Head's Intake Port Volume...which also gives us a percentage. This IP/CV ratio can be further applied to the different levels of performance already given earlier. This IP/CV ratio will keep you from ever installing cylinder head's whose Intake Ports are too large for the level of performance you have intended for your Engine-Car Combo.

 

The IP/CV ratio requires the end user to convert both the Cyl.Head Intake Port Volume and Cylinder Volume into the same measurement. Usually Cylinder Volume, in the states is Cubic Inches, while the Cylinder Head's Ports will be in a cc measurment.

 

Here are the distinctive levels of IP/CV% that I have found repetative in the engine build ups and dyno runs I've researched.

 

IP/CV Ratio & Levels Of Performance Desired

1) .24-.27 IP/CV%: Pure Street Engine: Turbo, Supercharged, Nitrous

2) .20-.24 IP/CV%: Pure Street Engine: Normally Aspirated

3) .25-.27 IP/CV%: H/O Street-Mild Race Engine

4) .28-.30 IP/CV%: Weekend Warrior-Moderate Race Engine

5) .30-.33 IP/CV%: Dedicated Race: Turbo, SuperCharged, Nitrous

6) .35-.38 IP/CV%: Dedicated Race: Normally Aspirated

 

Because we are dealing with ratios (Compression Ratios, Cylinder Displacement to Cyl.Head Combustion Chambers, and the DCR to SCR relationship), we must understand that each level of performance per IP/CV% will have a Low Scale to High Scale...so how do you know which one you should use?

 

Simple. For smaller displacement engines you will utilize the smaller IP/CV% numbers. For the larger displacement engines you can utilize the larger IP/CV% numbers. Example: If you had a 283 SBC and wanted a "Pure Stree Engine: Normally Aspirated" then you would utilize the .20 to .22 IP/CV% numbers whereas if you had a 560 BBC and wanted a "Pure Street Engine: Normally Aspirated" with that engine displacement, then you would utilize the .23 to .24 IP/CV% numbers when choosing a Cylinder Head's Intake Port Volume that best matches that distinctive level of performance.

 

This brings up another question: how do you know what is considered a small displacement -vs- a large displacement? It appears this approach, after you have crunched the numbers, will include anything 454 cubic inches or less as a smaller displacement (I know a 454 is a Big Block - this is just how the numbers work out..so dont kill the messenger please).

 

Here is the 1st equation you will utilize when calculating the IP/CV%. It involves converting Cubic Inches into a cc measurement:

 

(Eng. Displacement / #of Cylinders) x 16.387 = Cyl.Displacement cc

 

Here are the three possible equations you will utilize when calculating which IP/CV% is best for your engine and desired level of performance:

 

1)

Required Eng.Displacement = ((Port Volume cc / IP/CV%) / 16.387) x #of Cylinders

 

2)

Required IP/CV% = Intake Port cc / ((Eng.Displacement / #of Cyl's) x 16.387)

 

3)

Required Intake Port cc = ((Eng.Displacement / #of Cyl's) x 16.387) x IP/CV%

 

*NOTE: How you know which IP/CV% equation to use will depend on which variable in your engine build you already have.

 

In other words, if you already have a short block "Bore x Stroke" then you would already know your Engine Displacement. This requires you to choose which level of performance your Engine-Car Combo is going to operate in: then you would use the 3nd formula. If you had a set of Cylinder Heads and know what their Port Volume cc measurment is but didnt have an engine you would choose a distinctive level of performance your Engine-Car combo would operate in and then use the 1st formula. Now if you already had a short block and cyl.head(s) and wanted to know which level this combination would best operate in then you would utilize the 2nd formula.

 

This list is what I believe completes the different levels of performance:

 

1) Pure Street Engine: Turbo, SuperCharged, Nitrous

a) Pre-Boost and Pre-Nitrous BMEP = 100-130 psi

Post-Boost and Post-Nitrous BMEP = 165-185 psi

c) Any Boost higher than 6-7 lbs is no lnger in the "Pure Street" Category

d) Smooth idle in the 600-750 rpm range

e) Intake Manifold Pressure at idle 18-21 (in.hg.)

f) 2500-4500 rpm = Peak Torque range

g) 4500-5500 rpm = Peak HP range

i) .20-.24 IP/CV%

j) 7.50:1 - 8.5:1 SCR

k) 8.25:1 - 8.5:1 DCR

l) Cam Profile Required: Each power adder has itso own approach to making power, therefore it is in your best interest to consult the experts before settling on one profile over another.

 

2) Pure Street Engine: Normally Aspirated

a) BMEP Cylinder Pressures = 130-150

Smooth Idle 600-750 rpm range

c) Intake Manifold Pressure 18-21 (in.hg)

d) 2500-3500 = Peak Tq range

e) 4500-5200 = Peak HP range

f) .20-.24 IP/CV%

g) 8.75:1 - 9.25:1 SCR range

h) 8.25:1 - 8.50:1 DCR range

Cam Profile Required:

i) Duration in the 240*-250* (Seat to Seat) range)

j) Valve Lift in the .400-.425" range

k) LSA in the 114-120* range

l) Overlap at 35* or less

 

3) H/O Street - Mild Race Engine

a) BMEP cylinder pressures = 155-165 psi

Semi-Smooth to Choppy idle in the 700-850 range

c) Intake Manifold Pressure at idle 15-18 (in.hg)

d) 3000-4500 = Peak Tq rpm range

e) 5000-6000 = Peak HP rpm range

f) .25-.27 IP/CV%

g) 9.25:1 - 9.75:1 SCR

h) 8.25:1 - 7.50:1 DCR

Cam Profile Required

i) Duration in the 260*-270* range

j) Valve Lift in the .440-.470" range

k) LSA in the 110*-114* range

l) Overlap in the 34*-55* range

 

IV) Weekend Warrior-Moderate Race Engine

a) BMEP cylinder pressure 165-185 psi

Choppy to Wavy idle in the 850-1000 rpm range

c) Intake Manifold Pressure in the 6-12 (in.hg) range

d) 3500-4500 = Peak Tq rpm range

e) 5000-6500 = Peak HP rpm range

f) .28-.30 IP/CV%

g) 9.75:1 - 11.0:1 SCR range

h) 7.50:1 - 7.00:1 DCR range

Cam Profile Required

i) Duration in the 280*-290* range

j) Valve Lift in the .480"-560" range

k) LSA in the 108*-114* range

l) Overlap in the 50*-76* range

 

5) Dedicated Racer (Sportsman Drag or Pro-AutoX)

a) BMEP Cyl,Pressures 185-210 (Normally Aspirated)

BMEP Cyl.Pressures 240-300+ (Turbo, SuperCharged, or Nitrous)

c) Sporadic Choppy Idle at 1000+ rpms (idle is not an issue in this class)

d) Intake Manifold Pressures (no data for me to form an opinion with)

e) 5500+ = Peak Torque range

f) 7000+ = Peak HP rpm range

g) .30-.33 IP/CV% for Normally Aspirated Race Engines

h) .35-.38 IP/CV% for Turbo, SuperCharged, Nitrous Race Engines

i) 12.5:1 SCR or higher

j) 8.8:1 - 9.1:1 DCR

Cam Profile Requied

k) Duration in the 290*-325* range

l) Lift in the .540"-.870" range

m) LSA in the 106*-114* range

n) Overlap in the 75*-110* range

 

6) Top Fuel Dragsters

NOTE: I have no data to offer an opinion on this kind of an engine-car combo

 

Valve Sizing

 

As far as Intake and Exhaust Valve Sizing goes, the difference in sizes depends on the size of the BORE of that engine's cylinder and the different level of performance: such as Small Bore and Mild Performance or Small Bore and Moderate or Maximum Performance...same goes with the Larger Bore-you will still have to determine if the performance of a larger bore is Mild, Moderate, or Maximum.

 

For a smaller bore engine or a mild performance engine you can multiply .505 to the bore of that cylinder to determine its Intake Valve Size while multiplying .41 times that bore to obtain its Exhaust Valve Size.

 

For larger bores or engines whose performance will be maximum you can utilize the same principle but instead of the .505 & .41 you will need to use the .53 & .43 constants.

 

Hope this helps to keep you within your proper level of performance. Adhering to this list will ensure you never over-cam your engine or mismatch your Cyl.Head's Intake Ports to the level of performance you desire with the displacement you have chosen: within the rpm range needed.

 

One word of advice when crunching the numbers based upon an engine build up/dyno run you are reading in some magazine...I have discovered that those who write magaizine articles and attempt to build an engine for that article dont always know what they are doing and will at times over-build that engine with cylinder heads whose Intake Ports are actually too large for their intended purpose & SCR-DCR ratios.

 

So, only mimic the engine builds where the engine was build by some pro...to which the magazine writers then chronicled.

 

Kevin,

(Yea,Still an Inliner)

[Kevin Shasteen]

Sorry for the typo in the title.

 

Where it reads "&am" in the title of the post, "Matching Displacement, Cyl Heads, SCR & DCR-Duration &am" it should read "& RPM" as in, "Matching Displacement, Cyl Heads, SCR & DCR-Duration & RPM".

 

Kevin,

(Yea,Still an Inliner)

[2126]

Wow! That was a great techincal presentation. This should be of great value to the beginner or as far as that goes, to most everyone involved building an engine that has always wondered about the how and why of performance engine building.

 

Nice work and thanks for sharing your knowledge with us all.

[Pyro]

thanks for the write up!

 

Don't forget to write on the Rod/Stroke ratios and how it effects piston speed and how it changes ideal cam timing. Lower maximum piston speed good for higher rpm use and longer dwell time at TDC with higher ratios which works better with smaller Lobe Seperation Angles. Higher ratios are also less prone to detonation, and so on.....

[Kevin Shasteen]

Thanks guys.

 

Pyro,

 

I thought about the stroke to rod ratios but it has been extemely documented already: with the exception of the piston acceleration issue...which should be talked about more often..nonetheless,

 

What I believe has not been thouroghly discussed is the DCR to SCR and the IP/CV relationships w/in a specified rpm range...so that is what I concentrated on.

 

Glad to hear you guys appreicated it.

 

Kevin,

(Yea,Still an Inliner)