Tuesday, August 16, 2016

Questions Answered About Chevy Cylinder Heads and Camshafts

Big Block Chevy With Trick Heads Pulling a Hole Shot

Alan’s 1957 ChevyEngine Building Performance Questions Answered About Chevy Cylinder Heads and CamshaftsIn An One-On-One Bench Racing Session With My Old Friend, Bud Root

-Alan Arnell

AFR Big Block Chevy 357cc Rectangle Port Magnum Aluminum Cylinder Heads

Alan;  What would be the deciding factor when deciding on the proper-sized intake runners or aftermarket cylinder heads?

BR;  Of course you must take in consideration the cubic-inch displacement of the engine.

Alan; Well then, besides an engine’s cubic inch displacement what other factors should be considered when choosing intake port sizes?

BR;  I got you thinking boy now don't I?  Let me make it hard for you which you know makes me feel superior?  Well then, how you plan to use the mill in your car of choice has to be though of by one and all.  Let’s say we want to put a 540-ci big block in your 1957 Chevy, that will be primarily driven on the street.  With your fat butt behind the wheel she would weigh close to 4,000 pound.  1955. 56 and 57 Chevy I would suggest buying the highest-flowing, largest ported head on the Market.  I built such a car back in 2006 and I choose a 357 Magnum Cylinder Head ‘cause of the engine’s huge displacement.  However, I found out that this set of heads would have run better with as 315cc set.  For that build, that  I did for an old buddy of mine, the idea was to make as much low-end torque as possible, ‘cause he wanted to run a 3.36:1 rear gears.  He is also a lead foot and drove routinely 45 miles to work one way in his hopped-up Triv-Five.  Lucky guy, right!  He wanted to drive his car on the street but still be a street/strip hot rod.  The 315cc head, like I said, in retrospect might have been the better choice.  But, you can’t know everything when you need too now can you, at least not me, but always evaluate what you do so the next time you will do it better.  Anyway, later the same guy wanted a newer setup after exploding his 12 bolt during a street race up by Fogg’s farm. We updated the drivetrain to a narrowed 9 inch Ford axle with at 4:56 gears as he decided to driving his Chrysler Crossfire to work and wanted the ‘55 for mostly Friday night drag racing.  We decided to put in a new set of heads with a 355cc.  

Alan’s Research Notes:  AFR - Airflow Research 3570 - AFR Big Block Chevy 357cc Rectangle Port Magnum Aluminum Cylinder Heads; Big Block Chevy 357cc Rectangle Port 24°
Manufacturer's Part Number:3570, sold as a pair at $3,081.69

Magnum Basic Package Components
100% CNC Ported Combustion Chambers
100% CNC Ported Exhaust Ports
100% CNC Ported Intake Ports
Competition 5-Angle Valve Job
Intake Valve Size - 2.300'' (Valve not included)
Exhaust Valve Size - 1.880'' (Valve not included)
Intake Valve Seats
Exhaust Valve Seats
Bronze Valve Guides

Big Block Chevy 357cc Rectangle Port 24° Magnum Head
Head Torque w/Moly Lube
75 Ft. Lbs.
Rocker Stud Torque
55 Ft. Lbs.
Intake Port Gasket, 1.775'' x 2.500'' with 1/2'' radius
Fel-Pro 375-1275 - DO NOT PORT MATCH Intake Manifold to Fel-Pro Gasket.
Exhaust Port Gasket
AFR 033-6858
Head Gasket Mark IV
4.540'' Bore, Fel Pro 375-1017
AFR 033-6850
Head Gasket Mark V - VI
Up to 4.500'' Bore, Fel Pro 375-1047
Head Gasket Mark V - VI
4.500'' to 4.600'' Bore, Fel Pro375-17049
Head Bolts & Studs
Standard ARP
Head Bolt Washers
Manley Standard
Stud Girdle
AFR 033-6210
Suggested Manifold
Edelbrock 350-2907 & 350-2927 or Brodix 158-HV2000 &158-HV2001
Spark Plug Starting Range
Champion C59C, Autolite 3932
Combustion Chambers
121cc Fully CNC'd
Intake Port Volume
357cc Fully Ported
Exhaust Port Volume
Spring Pocket can be cut to 1.750, no deeper.
Valve Spacing
Rocker Arms
Valve Angle
24° / 4° Intake ; 15° / 4° Exhaust
Angle Mill, 104cc Maximum
.009'' per cc
Flat Mill, As Cast 108cc, CNC 114cc Maximum
.006'' per cc
Flat and Angle mills might require a .120'' thick head gasket.

Alan;  How does compression and cam selection affect the best intake-runner size?

BR;  Well, camshafts and compressing ratios are so important, but camshaft choice plays a slightly larger part in a hot mill.  Let, me explain.  Let say, the ‘55s owner now wanted a strictly dragstrip door-slammer.  The guy wanted to keep running pump gas in a 11:1 somewhat street-friendly car.  He was running a .714-inch-lift solid mechanical roller which, you might know, is fairly aggressive. When push comes to shove, that cam will barely push the valve up to .700-inch lift after valve lash and deflection of valvetrain parts.  You gotta figure, .680 is more realistic.  So, in reality the flow data of the 335cc against the 357cc, you would clearly see that there ain’t much difference between the two until you start lifting the valves past .700. Now that’s not a problem even our race ‘55.  You see, all the larger head would have accomplished would have been a been a reduction in the overall airspeed ‘cause of the larger port and would have not made any additional volume of air.  You would find that, the valve lifted high enough to best use the benefits of the larger head.  I betcha, you would have a loss of low-end torque and have a wash, at best, when it comes to the actual peak power.  I have found that, such an engine would be lazier when driven at part-throttle.  You don’t want that!  No less, the fuel milage would go to shit, That rich bastard who owns the ‘55 would not care, but I would and I stand by my engines.  Maybe worse, the larger head would have wanted to run best at a higher rpm, which is no good for the camshaft we chose for the 11:1 street engine.  I say, take the exact same package and add two to three full points of compression, a cam with another 20 degrees of duration at .050 and a gross valve lift of .800 and that 540ci mill would be a G.. D..., F’en monster at 7,500 rpm’s with those 357 Magnum heads.  Anyway, that is what I think would be the best choice for this car.  

Alan;  When upgrading to aftermarket cylinder heads, how does overall flow affect camshaft selection?

BR;  That is a very important thing to consider, as the engine’s flow characteristic and valve timing are the two main things to get right for a fast car.  The two must be matched to make that power we all look for to have the fastest car.  Most people think that better-flowing head actually require less valve timing or shorter duration cams for a given rpm than a lower flow heads, but they’d be wrong.  That’s, ‘cause the better-flowing head can fill the cylinder faster, that is given the same signal from the piston than the less flowing heads.  I learned, that heads with high-flow ports are typically used only in high-rpm setups, they are typically used with relatively long duration cams.  Now, if you need to run the same rpm with a head that flows less, even more duration will be require.  So, on your street car with good heads, surprisingly little duration is needed to make the best horse power at say, 5,600 to 6,500-rpm.  A 350 mill with  250-plus-cfm head should make peak power in that range with a cam as small as 224- to 236-degrees at .050-inch to make great power that high.  One of the biggest myths about cam selection is that people think that big heads need a big cam.

Alan;  With high-flow aftermarket heads, how important is the intake versus exhaust flow ratio?

BR;  In the same way the exhaust-to-intake flow percentage does play a role in the camshaft choice.  To make this percentage right at a given lift, simply divide the exhaust flow by the intake flow and multiply by 100.  For example, a 250-cfm intake port and a 180-cfm exhaust port give a 72-percent flow number (180/250x100=.72)  As a engine builder, I shoot for about 72- to 75-percent flow for peak power.  A cylinder head in this range usually needs about six degrees more exhaust duration than intake duration to perform best between peak torque and peak power.  You see, the exhaust flow gets to 80 percent of the intake, the best cam selection should have about the same exhaust and intake duration  Likewise, heads that flowing the high 60s need more than 10 degrees extra exhaust-to-intake duration. As always, there is always an exception.  That’s when the combination needs to spend a substantial amount of time above peak power.  In that case the pumping losses happen while trying to push out the exhaust can hurt your horsepower output.  When that happens, as I have done for stock car racers’ engines,  I usually widen the separation and increase the exhaust duration that opens the exhaust valve earlier to allow more exhaust gas to escape near bottom dead center as the piston has to push it out.  This happens a lot in your larger-ci motors, due to high piston speeds and lower peak-power rpms.

Alan;   Since most aftermarket heads can handle much more valve lift, how does that affect the choice of the net lobe lift?

BR;  Many of your trick heads don’t flow much more than the older heads I used back in the day. They range in the .100-inch to .300-inch valve-lift range.  Mostly, your trick heads more than likely take off above .300-inch valve lift, The older heads were made to around about .4-inch valve lift, so most of the work was done over time to improve low-lift flow.  Many new street valve-springs can handle lift above .600-inch, and some of the best endurance race springs will run over .800-inch valve lift for more than 500 miles of hard use.  Many a good drag racer engine builder is almost getting 1 ¼- lift, some even more!  With the additional lift available, two-valve cylinder heads can make great flow improvements at these higher lifts.  However, both the cam and the head must be made to work together in the higher range.  

Alan;  How do you make a selection differ with forced-induction applications running high-flow heads?

BR;  Using a turbo or a blower, can change the cam choice ‘cause of the forced pressure differences across chamber.  With a blower I have to consider that too much overlap forcing the intake charge through the chamber and out the exhaust.  So, I typically widen the lobe separation.  You could also go with an earlier exhaust opening to run at a higher rpm.  On turbos you must pay close attention to the boost versus the backpressure to figger’ out overlap.  On older systems, the backpressure was very high and overlap would result in a bad reversion problems.  In later systems, pressures are much more constant across the chamber and cams are closer to those run in non aspirated setups, except, and there is always an exception.  Heads act much better with the dense charge, so duration can be much smaller for the same rpm.  The ricer’ guys with the four-valve motors typically need less overall lift due to the simaller valve diameters, greater curtain areas and increased lift-to-diameter ratios.

Alan;  There are so many variables to think of when picking out a cam.  Does it not make sense for the engine builder to just call COMP Cams for advice before buying a new cam?

BR;  Of course you could.  When you do, make sure you go over the details of the build before buying a cam.  Let me see,  I believe I have Comp Cams’ help line number here somewhere, here it is, 1-800-999-0853  I have found they are very good at helping you select a camshaft that best fits what you want the mill in you car to do for you.

BR;  Let me ask you this?  Are you following any of this at all?

Alan;  It’s a lot to think of, but if I ever get build my dream engine I will know more than I did yesterday.

A Great Tri5 Chat Board


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