Hemi chamber. Mods to increase thermal efficiency?

[Pinhead]

I've got a 1979 Honda CB650 motorcycle that I would like to rebuild to increase my engine's efficiency. It's got a classic Hemi-shaped chamber (a nice big dome), with 2 valves per cylinder. The ports look decent to my eyes, but I have virtually no hands-on experience with porting. I know the downfalls of the hemi chamber and the slow-burn characteristics of the head. I don't know how to correct these downfalls, however. And here's my question. What can I do to this chamber to be able to run higher compression and get a faster burn? I won't be able to replace pistons, as there are no aftermarket options for this engine. I may be able to deck the cylinders, shave the head, and do some mild porting if I can see what the ports should look like. However, being a poor college student limits the amount of money that I can spend on the project.

Engine: Air-cooled, 8-valve, SOHC inline-4
Bore x Stroke: 59.80 x 55.80 mm (2.354 x 2.197 in.)
Capacity: 627cc (38.2cu-in)
CR:9.0 : 1
Carburetion: 4 Keihin PD50A or PD50B Mechanical
Ignition: CDI (Capacitor Discharge Ignition)

I'm going to try to incorporate the carburettor mods that David mentioned here. It has 4 Kei Hin carbs.

I will also incorporate PowreLynz and Surface Turbulence Valves to help with fuel vaporization. I might try Metric Mechanic's Grooves to further increase chamber turbulence, but I worry about the depth, width, length, and all things that are unknown about this particular mod.

I may be able to incorporate Singh Grooves if the piston sticks out of the cylinder far enough to create some squish around the perimeter of the chamber. This area will be very small, however, so gains will probably be minimal. But adding a lot of small changes can add up to one big change.

[automotivebreath]

Can you post a pictue of the piston and chamber?

[Pinhead]

Oops, forgot to in my original post. Here we go.

[automotivebreath]

Pinhead,

I would seriously consider creating as much squish action as possible using the
sides of the piston and chamber. Adjust clearances in this area carefully where
the piston comes about 0.050" from the head in this area at assembly. If you
try to get tighter than this piston rock may cause contact.

[Pinhead]

What do you think about the Metric Mechanic grooves? I'm pretty sure ST Valves and PowreLynz will help quite a bit. If I can get the piston within .050" do you think Singh Grooves would work on the perimeter of the chamber?

[big block fiero]

Lately the dynamic duo have me rethinking everything I know back to old school engine building techniques, But one thing Darin morgan said that I believe completely that applies to all engines is never run A tulip type intake valve because the 12 degree has better sheering effect. Hemi's usually run a tulip type valve so this would be a start. Just immagine the one engine that you would think would run a tulip valve, A 10, 000 rpm pro stock engine in warren johnsons car but yet darin runs a standard 12 degree (valve stem to seat) angle. Darin would tell you to forget about any grooves as long as you have good vaporization as seen on the wet flow. I would tell you to give it a shot but do just one thing at a time, test it at each phaze, Then report back to us. But I'm sure that would be impracticle.

matt.

[Stan Weiss]

First looking at pictures it is sometimes hard to see the whole picture. But this looks like what a will call a flat chamber and does not have the depth that the 426 did. This is something that was done in the D-5 heads. It reduces surface area and needs less of a piston dome. With the chamber and piston shown what is the CR. Have you looked into thermal coatings for the head and piston? Also as automotivebreath said on the sides of the piston what is the piston to head clearance? This can to used as a squish area.

[Pinhead]

The stock head and pistons produce a 9:1 compression ratio. This is a 626cc motorcycle engine, and therefore has a very small chamber, in the area of 17cc's. Redline is around 9700 rpm.

From Honda CB650 Forums:

Originally Posted by pae

Combustion chambers are ~ 60mm diameter by 17mm deep, and pretty well spherical in shape. At a rough calculation that makes them about 20cc each, but I'll do that accurately later with a pippette.

The old head gasket measures 1.25mm.

I'll check the deck height for your (top of barrel to top of piston at TDC) so you know how much you might have to play with. The pistons are of course crowned so that'll reduce the volume, I'll check on that too.

An original brochure lists compression ratio (UK) at 9.0:1. So with bore/stroke of 59.8 x 55.8mm that gives 156.7cc per pot (627cc overall). If compression is 9:1 then the total combustion volume should be 17.4cc. With a head gasket of 1.25mm thick that's 3.5cc, leaving 13.9cc in the deck and combustion chamber combined.

If you want to get to 10:1 then you'll need to take about 1.73cc out, to reduce it to 15.7cc, or about half a head gasket thickness (~0.6mm).

Originally Posted by pae

Hmmmm, did a 'fairly accurate' measure of the combustion chamber volumes at the weekend and they are about 26-27cc which is a bit more than I'd expected. Having looked at the pistons though they are quite crowned so I have to assume for the moment there's about 12cc of crown on them to bring the squish volume back down around 14cc (plus gasket at 3.5cc).

Originally Posted by cb650

I took off about .o25" (inch) on a couple of mine with no probs. Some day going to make the kid play with one and do some clay to find out the valve clearance.



Terry

[Stan Weiss]

What we would do on the 426 is blue the piston or head and see where they hit. Then remove a small amount of metal from the piston and do it all over again until most of the piston hit. Once this was done we mill the heads to set the piston to head cleanance.

[seattle smitty]

Clay the squish areas and see if they all conform and have about the same measured squish. If it varies, do what Mr. Weiss says, even them up. Then mill the cylinder deck surface as needed to get to .035-.040" squish. When you are claying, find out also how close the valves get to the piston; it's possible that after you tighten up the squish, the valves may get close enough to the pistons that you need to deepen the reliefs.

Groove the heads if you want, but you might alternatively try die-grinding fire-slots in the piston crowns, since pistons are cheaper to replace than heads if you don't like the results. Fire-slots were the earlier (30-40 years ago) incarnation of Singh's grooves, were tried by many people, and were eventually abandoned as ineffective. I don't think this entirely discredits either technique, but I believe the effects will prove to be useful only in very specific combinations of combustion chamber and piston shapes and conformity.

Chamfer or round off sharp edges on the piston crowns and then mirror-polish them.

Check the new volume at TDC, and calculate your new compression ratio, which should be a bit higher after tightening the squish, even though a groove or fire-slot will have some small extra volume. Maybe you'll be where you want to be. If the compression is now too high, take material out of the squishband where it shrouds the valves. But DO NOT install a thicker head gasket or otherwise increase the squish dimension. A tight squish (quench), just short of letting the piston contact the head at peak rpm, is your main anti-detonation protection here.

Some of us think that heat transfer from the heads and cylinders can be improved a little with coatings. The rest of this post is about putting a heat-dissipating (we hope) coating on your air-cooled engine, the motivation being to help the engine accept higher compression without detonation. Blast the exterior of these parts with salt, glass-bead, walnut shells, or some not-too-aggressive medium. Scrub the parts clean (I use TSP and hot water, an SOS-pad or Scotch-Brite, and rifle brushes). Dry the parts and quickly rub some WD-40 into the cylinder liners, but don't get anything oily on the aluminum.

Some guys spray stove-black on the cooling fins, but I like a product from KG Industries (formerly Kal-Gard, which was a major advertiser in Cycle magazine during the heyday of 2-stroke roadracing in the '70s) called Gun-Kote. Order the flat black variety, about $35 with shipping. Gun-Kote has the additional advantage of being a good barrier against corrosion. If you want technical or applications advice, phone KG, ask for Chris (who will be running the company in a few years), and tell him Smitty sent you. KG also sells a heat-cured moly-coating called Gear-Kote (marketed by Kal-Gard also as Piston-Kote) which I use on piston skirts, carb slide, rotary-valve cases, etc..

I use an old record player at 33 1/3 rpm for spraying round parts, and an old portable oven for heat-curing. Pre-heat the oven to 300F. Shake the Gun-Kote extremely well, and I personally always warm up the can in hot water first (this also works well for spray-paint cans). Right before you are going to applay the coating, do a last treatment of the aluminum parts by acid-etching them with mag-wheel cleaner for a minute or less. Wash off the acid, then heat the parts in the oven or with a torch until all moisture is boiled off or evaporated off the parts. Now let the parts cool . . . but not all the way; you want them between 100-125F when you spray the Gun-Kote. Load your detailing gun or air-brush with the Gun-Kote, and spray the warm parts enough to cover the metal but not a heavy build. When you have all the parts coated, let them air-dry for 20 minutes or so, then put them in your 300F oven. Heat-cure for 60 to 90 minutes.

These are the tricks I'd use if I wanted to build a little extra compression into that engine. Obviously, close monitoring of fuel mixture and spark timing are always primary, along with using the appropriate heat-range sparkplug.

(How ya doin', Pinhead!!)