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Racing Cylinder Head Selection Guide

💥 head GUIDE 💥

Honda GX, Predator, Briggs

SPEED PARTS SPEED KITS ENGINES HEAD VALVE CAM
Parts | Stock | Hotrod

Stock

Racing

Predator

Billet

Briggs

Gaskets

SMALL BLOCK

Available heads for small engines:

Spring Selection:

Genuine Honda cores flow better, are less porous, and are stronger than aftermarket heads. Single springs are recommended for 306 and smaller cams. Dual springs are recommended for 307 and larger cams.

GX200 heads will fit GX160s 

BIG BLOCK

Available heads for big engines:

The Small Port Torquer uses small port castings to increase velocity and maximize low-end power at some loss of high-end RPM. It is the best choice for most applications.

We also offer a Large Port, High RPM head, which uses large port castings to increase high RPM power at some loss of low-end torque and reduced throttle response. They can provide increased cooling for endurance applications.

390 heads will fit 340s, and 270 heads will fit 240s.

Spring Selection:


INFO

  • Fully ported and bead blasted
  • Milled to increase compression

Why aren't they polished? Bead blasting provides a boundary layer that flows better than polishing! 

They include:

  • Lightweight stainless valves
  • Aluminum retainers and split keepers
  • 3-angle valve job
  • Lash caps
  • Choice of single or dual springs
  • Optional titanium retainers

BUILDER TIPS:

  • Honda 18cc is the best choice for quality
  • 14cc has an easy compression increase, but does not flow well due to valve Shrouding
  • Dual Springs are needed for 7500+ RPM 
  • The 25/28 combination adds more flow (7500+ RPM only)
  • 27/24 is the best for large torque  
  • Predator heads have larger dowel holes. Any head can be drilled to accept
  • Be sure to always use lash caps

Notes on 14cc versus 18cc Heads
For flow improvement: as you mill the head, it removes part of the chamber and actually un-shrouds the valves. A 18cc head milled .080" has much better flow potential than an unmilled head. We say potential because edge treatment can affect the flow (i.e. round off the edges). We have a digital superflow bench and have flowed hundreds of heads in all kinds of different configurations with every type of porting one can imagine! We have backed up a lot of this with dyno testing (which does not always agree with the flow bench). In the end, unless major work is done to the chamber, the 18cc heads will always outflow the 14cc! The only advantage to a 14cc head is that the compression can be increased to around 10.3:1 by just bolting one on. You need to mill an 18cc head by .050" to achieve that. However, a .050" 18cc head can make more power than an un-milled 14cc head.

Notes on Hemi Heads

  • Hemis come with oversized lightweight stainless valves (28mm intake, 25mm exhaust: +.070" longer with single springs, +.230" with dual springs).
  • Hemi spring height is .100 shorter than clones, so the coil bind may be an issue with single springs (duals are not an issue with the +.230").
  • They require 5.34 to 5.54 for the +.070" valves, depending on rockers, and 5.11 to 5.16 for the +.230" valves.
  • Hemi heads are prone to piston-to-valve clearance. They must be checked carefully!
  • We do not recommend 1.3 ratio rocker arms for these heads due to valve-to-piston clearance issues.
  • The +.230" valves may create improver valve geometry when using 1.2 or 1.3 ratio rocker arms.
  • .24 and 1.2 ratio rockers are the maximum recommended valve lift for +.070" valves.

Porting Comments

The key to making power with a Honda head is mild porting, smoothing out the short side radius, blending the bowl, and shortening the valve guides by .100". Together, these will yield about 60% better flow than stock. Trying to remove any material (i.e. making the port larger) tends to hurt performance, and there is really not enough aluminum to do so. In fact, raising the floor of the port improves flow quite a bit. Honda ports – especially the exhaust – are intentionally oversized to promote cooling. The ports should also be left rough since this creates a boundary layer that improves flow. Sandblasting the ports after porting/smoothing helps considerably. This type of surface will flow much better than a polished surface; plus, it helps fuel atomization. The exhaust is not so critical since carbon buildup creates a rough surface on its own.

Valve Size Comments
A good three-angle valve job will allow up to 15% improvement in flow on a GX160/GX200 head. Unless running huge Mikunis or Tillys and turning 7000+ RPM, the stock valve sizes work very well and are capable of making up to 16+ HP. Stainless valves do flow a bit better than the stock valves. For high RPM operation (with big carbs and cams), the 27mm intake valve will help and will allow 18hp.

A 28.5 intake will only show improvement above 7,500 RPM.

WARNING: 32/28 valves are subject to seat failure! They should only be used in methanol applications. And, to be honest, the 28/25s make as much, if not more, power (the velocity is much better)!

Lift versus Flow
It is important to mention that actual flow through the head is also a function of valve lift. Needless to say, the more lift, the more flow. Note: higher lifts can decrease velocity, which in turn, decreases torque. For the most part, GX200s show improvement as lift is increased. However, on the flow bench, a GX160 race head (big valve, ported, milled) does not show any additional flow after about .350 lift. Dyno testing combos above .350 lift (e.g. .400 lift) has shown only minimal high RPM improvement, usually at some losses at the low end. Also, the Honda valve train does not do well with a lot of lift. There are a lot of geometry issues that one must address. 1.3 rockers should be avoided on high lift as they really play havoc on the geometry. We have seen many failures from using 1.3 rockers with big cams. You can get them to work, but custom valves, spring pockets, and other head work is needed.

General Rule:
  • 1.3 with no greater than 274 lift cams
  • 1.2 with at least 280 lift cams
Notes on Milling
You will need to mill the head to get to your desired compression ratio. It all depends on the piston, deck height, and gasket.
  • 10:1 pump gas
  • 12:1 50/50 mix of race and pump or race gas
  • 13:1 methanol
Don't go beyond these, and single cylinders don't like a lot of compression as they don't have an opposing piston!
If unsure, mill to .050".

BIG BLOCK INFO

Like our small block heads, the big block heads are fully ported, bead blasted, and can be milled to increase compression. Again, we don't polish these since bead blasting provides a boundary layer that flows better than polishing.

Big block heads include:

  • Lightweight stainless valves
  • Aluminum retainers and split keepers
  • 3-angle valve job
  • Lash caps
  • Choice of single or dual springs