Thread: head upgrade disappointment

Tweet

OK, I'll step up to the plate because the OP PM'd me and asked my advice.

Every motor is different, there are no two alike because of all the variables involved. We can, however, settle in on a few basic facts that apply to the level of performance that we are seeking. For a street or street/strip motor, I like to use a factor of 0.50 to 0.55 to calculate runner size. I realize that this assumes all ports will flow commensurate with their size and we know that's not true, but we have to begin somewhere. 360 X 0.50 = 180, 360 X 0.55 = 198, call it 200. So, if we split the difference, 190 is the largest runner size you should use for a 360 streeter, assuming the runner volume equates to flow, an assumption which we already know is not necessarily true. This also assumes the motor will operate from idle to 6000 rpm's. If the high end were, for instance, 4500 rpm's, then we might want to drop down to a 360 X 0.45 formula and run a 160 runner head. The idea here is to pack the cylinders with as much air/fuel ratio as we can, for the most powerful controlled explosion we can get in the cylinder for the fuel used, without creating excessive cylinder pressure which the fuel will not tolerate. To pack the cylinders, we need to use the correct velocity as well as volume. If a runner is too large, the mixture slows down and may not pack the cylinders properly. If the runner is too small, the velocity will be excellent, but there may not be enough time to fill the cylinder, depending on cam timing and how long the intake valve remains open. It's a balancing act. Those fellows with more experience in building motors will come closer to building the ideal motor the first time because they have experience on their side. Those fellows who have no idea about the characteristics of each individual part used will probably build a motor that will not reach the potential that it could have with slightly different parts.

Choosing heads without having your very own flow bench to experiment with is a crap-shoot at best. You have to go by word on the street because you have no idea how the manufacturer arrived at his published flow numbers. Actually, there is no "truth" that I know of when comparing heads from one source to another if you depend on the manufacturer's information. What size valves, what margin, concaved or no, what seat dimensions, what size barrel was used to flow them, dry or wet, what air temperature, what humidity, what depression was used (we all assume 28" water, but that may not be the case), what valve angle to bore centerline, what angles and how many angles on the valves and seats and what is the width of the cuts, what distance from valve head to deck, these and other questions make every head different. It is said that one of the reasons the L31 Chevy heads flow so well is the valve job from the factory.

Here's my take on headers: I will cut away or completely change ANY part of the car that gets in the way of equal-length, long-tube headers and an "H" or "X" tube. Period.

You mentioned re-jetting the carb. I am in hopes that you don't think you can make the carb flow more with re-jetting it. Carb flow is dependent upon the area of the venturi and cannot be altered without re-sizing the venturi. You must, of course, jet the carb for the correct air/fuel ratio dependent on atmospheric conditions, but the amount of flow of the carb IS what it IS out of the box. In your case, the carb is the last thing I would change. You'll make max power on a 360 with a 750 carb, but driveability may suffer due to the larger primaries. I am still of the opinion that the very best mass-produced OEM carburetor is the Rochester Quadrajet with its tiny little primaries for mileage and gi-normous secondaries for power. I also would retain the vacuum secondaries unless you plan a change to some 5.13 gears or an auto with a high-stall converter. Some of you younger fellows who have grown up on EFI, where more pressure will support more hp, may make the mistake of running too much fuel pressure with a carb. 5 - 5 1/2 lbs is all the needle and seat will control on a modern 4-bbl carb. More pressure than that will blow past the seat and dump raw fuel into the intake manifold. Not good.

Dave Severson mentioned a gear change and I will have to agree with him, even though you are running short tires. We don't know your 5th gear ratio, but we can be pretty sure that 4th is 1:1, so we can swap apples for apples, on paper anyway. With a 3.08 gear, tire radius of 12" and a speed of 58 mph, the motor is revving 2500. With a 3.72 gear, tire radius of 12" and a speed of 58 mph, the motor is revving 3020 and is that much more "up on the cam". I can guarantee you that with the 280H (CompCams 12-212-2) cam, the motor will be happier at 3020 than it will be at 2500. This cam is right on the money for matching up with your static compression ratio, coming in at a dynamic compression ratio of 8.33:1 on the Keith Black calculator, a good balance in my opinion for a street-driver on pump gas.

Best street or street/strip intake manifold operating idle to 6000 RPM? A high-rise, dual-plane model such as the Edelbrock Performer RPM 7000-series or the now-obsolete Weiand #8116 Stealth (I don't like the 8116-S) or Holley 300-36 or the Professional Products Typhoon #52020 (polished), 52021 (satin) or 52022 (real chrome). Here's a comparison of the Typhoon to the Edelbrock RPM......
http://www.professional-products.com...ests/52020.pdf

In my opinion, the only reason to run a single plane intake is to run a much larger cam and higher rpm's. The circle track guys and serious drag racers can use a single plane to good advantage. Running too much intake manifold volume is as bad as running too much intake runner volume. Velocity suffers. Carb spacers are a science unto themselves. You never know if the spacer you are going to try will help or hurt. It's a seat of the pants, time-slip kind of thing. I recommend trying all different styles and types to find out what works best for you. Your combo may work best with no spacer at all or it may work best with a 1" or 2" spacer, but you won't know until you experiment. And again, just like headers, if you don't have enough room to run a 2" spacer, either 4-hole or open and a 14" x 4" air filter, MAKE ROOM. If you don't make it possible for the motor to inhale and exhale properly, you're just wasting your money and your time.

Other thing is, dial in the proper amount of ignition timing to complement the cam timing. At 230 duration with a manual transmission, I might use 16 degrees of spark advance at the crank, with 18 degrees in the distributor weights, for a total initial/centrifugal of 34 degrees, all in by 2800. Again, each motor is different and will respond differently to input, but this is an educated guess. Crane makes an adjustable vacuum advance that you may want to try. Also, try both manifold vacuum and ported vacuum, see which one the motor wants. Of course, you absolutely MUST know where top dead center is if you intend to be correct with the ignition timing, so if you have not nailed it down......YOURSELF......then read and perform the steps outlined here.....
http://www.crankshaftcoalition.com/w...op_dead_center
http://www.summitracing.com/parts/CRN-99600-1/
http://www.summitracing.com/parts/CRN-99619-1/