4 Valve Hemi Heads for Harley Big Twin Engines
February 26th, 2008 by Paul Crowe - "The Kneeslider"
Revolution Performance Motors has introduced the RPMHemi 4 valve heads for the Evo V-twin and early Twin Cam engines, Twin Cam EFI kits are currently being tested and will be available soon. If someone asks, “That thing got a hemi?” just install these heads and you can say yes. Sweet! The heads bolt directly on to the stock engine, the only changes required are valve clearance pockets must be cut into the pistons and the exhaust must be modified.
RPM has both conversion kits or complete engines in several displacements. There’s also an intake plenum that mounts 2 carburetors, the second CV carb serves as a vacuum secondary for high RPM runs. The heads fit any bore size from 3.5 to 4.25 inches. The hemi combustion chamber and center location of the spark plug as well as less spark advance allow compression to increase from the stock 9:1 to over 11:1. Evo kits start at $2995, Twin Cam kits from $3099.
4 valve heads can dramatically increase engine breathing and with no other modifications, engine horsepower and torque increase by 15 to 20 percent, with gains everywhere in the RPM range but it’s especially dramatic above 4000 RPM. Modified engines will see even bigger gains. Engine height is 1/4 inch lower with the 4 valve heads than with the stock configuration.
These 4 valve hemi heads seem like a pretty nice setup, either alone or as the first step before further modifications, and they would seem to be just right for a big twin powered sport bike or cafe racer. It will be really interesting to see some back to back performance comparisons.
Press release follows:
Ever hear of a V-twin Evo or Twin Cam motor with approximately 85% of available torque coming in at approximately 1500 RPM? We have! At RPMHemi.com we do it all day long with our patented 4 Valve Hemi Bolt on Head Conversion Kits. “This is simply the best performance bolt on for the buck in the V-Twin market!” states Phillip Holmes, VP of Marketing & Sales for RPMHemi.com. You can estimate an approximate 20% power gain. This is a proven technology with both street and racing applications.
How it works: Utilizing our patented 4Valve system in a hemispherical combustion chamber we can increase airflow 1.5 times, increasing the compression ratio while maintaining a 25° spark advance, which gives an equal full burn in the chamber, while preventing pre detonation. All this in a package that is ¼” lower than stock heads. Our headsets and engines are also low in emissions.
RPMHemi.com manufactures their patented 4 Valve Hemi Head Kits and sells complete 97”, 107” and 114” motors with or without our 4Valve Hemi Heads and we have our patented Ram Chamber for single and dual carburetor V-Twin applications.
Currently, we have several OEM motorcycle manufacturers testing our products and all looks good.
Posted in Motorcycle Business, Motorcycle Technology
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20 Responses to “4 Valve Hemi Heads for Harley Big Twin Engines”
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February 26th, 2008 at 1:13 pm
Inpressive performance figures for sure. A bit expensive but a nice set up.
A question here is added stress on cam bearings. The Evo with one set would be most suspect to failure. From personal experience the Twin Cams are subject to the same failures. Harley makes a kit for the TC for cam bearing replacement that includes everything in the camcase. If the total valve spring pressure from 2 valves to 4 valves along with wieght of the valve gear is the same then failures shouldn’t be any worse then with a stock set up.
February 26th, 2008 at 1:16 pm
Harley in the 21st century-heresy I tell you heresy.
Glad to see it happen. With FI it sold be complete rock and rolling package thumping many rice burners.
I’m a gear head from the 60’s and it gals me to see 4 bangers beat a 426 hemi; or any V-8. Even the marque - Moto Guzzi I ride is going back into 4 valves. Harley and Moto Guzzi are the oldest continues production motorcycles and v twin no less. My next bike-yep a Harley with this conversion. Autobahn Cruiser
Great job RPM
John
February 26th, 2008 at 3:55 pm
You can tell by the dyno charts these heads and cams are designed to flow at RPMs that aren’t typical of the average Harley. I bet if they decreased the size of the carbs and valves they could get even more impressive torque figures. As it is now, you can see the affect of valve float due to the push rods coming into affect around 5500 RPM, right when it’s starting to flow really good.
How high can you rev a big twin?
-todd
February 26th, 2008 at 4:31 pm
THIS IS IT. I LOVE IT, BUT DIDN”T BOB METZ AND ARLEN NESS MAKE SOMTHING LIKE THIS. I ALSO BELEVE IT HAD A OVERHEAD CAM KIT INCLUED WITH. GOOD LUCK RPM.
February 26th, 2008 at 6:25 pm
I’ve heard reference to the Arlen Ness heads before, not familiar with them but I do remember the Rivera/Feuling 4V heads back around ‘88 or ‘90 or thereabouts.
February 26th, 2008 at 6:45 pm
So will a pushrod Harley finally be able to keep up with my 28 year old KZ750?
February 26th, 2008 at 7:04 pm
quatoum motorcycle is Melbourne Fl. had those “Hemi” on thier production bikes for years. Problem is that the air box is a joke. LEAK CITY!!! not a bad start to a great design. Riveria had a hand at this too.
Recently sold a setup on Ebay for a few hundred. Like I said…great start to a great idea. FUEL INJECTION is really the way to go with it.
February 26th, 2008 at 9:04 pm
Aren’t stock evo heads hemispherical?
February 26th, 2008 at 10:11 pm
Guys, I need some explanations, because I’m both not knowledgeable in harleys and not American either … I thought the Hemi name described either a big ass large displacement V8 that made the good ol days on the muscle cars, or the latest version in use on Chrysler/Dodge cars/trucks that uses 4 cylinders in rotation instead of 8 when cruising at stabilized speed … ? What is left on this Harley heads, it’s not a bigger bore stuff or a VTEC technology that commands 2 or 4 valves based on the engine load … ? Have I missed something ?
Now if I had not missed something, Ducati or Honda or Aprila make twins who have a much better power output than this Harley powerplant ?
Thanks for your enlightened comments.
February 26th, 2008 at 11:37 pm
In response to Nicolas:
Hemi refers to the shape of a cylinder head. If it’s hemispherical, it can be referred to as a “hemi”. There have been plenty of hemi headed engines. Chrysler has historically used the term in their advertising and promotion.
The theory behind multi valve engines is that while the valves are smaller, there are more of them which offers more flow, particularly at higher rpms.
February 27th, 2008 at 5:29 am
Not to split hairs, but it denotes a hemispherical combustion chamber (as opposed to a pent roof or other combustion chamber shape), not the whole head.
For the record, my 28 year old, fifty dollar KZ750 is also a hemi.
February 27th, 2008 at 6:31 am
Given we may be headed for $4.00 per gallon, I ask why?
Why not go to a Direct-Injection Cylinder Head, even one that may burn Gaseous (cheaper) Fuels?
With DI you can go higher compression ratios as well.
Go here: http://www.flexdi.com/
February 27th, 2008 at 10:36 am
I’m not seeing any photo of how they set up the rockers. Wonder how they adjust the individual valves. If they’ve got two valves driven by each rocker, how do they allow for individual valve adjustment? Harley’s stock hydraulic lifters won’t account for differing rates of valve recession.
Also I’m pondering the claim of 85 percent of peak torque available at 1500 rpm. If these guys are after diesel-like torque delivery, why do they need four valve heads that typically perform best at high rpm? Sorry, my money goes elsewhere.
February 27th, 2008 at 4:14 pm
Four valves can flow better at all RPMs. The key is in the smaller valves. A small valve and tract will flow much better at low RPMs as velocity is much higher. This helps fill the cylinder faster and keep the fuel in suspension. Efficiency goes up which increases torque. You can tailor the sizes of all the components to flow best at whatever RPM you want. More torque you add higher in the RPM range will give you more HP numbers than adding the same amount of torque at lower RPMs. Most aftermarket companies shoot for adding torque up high because the HP increase makes it look more dramatic. Like I said earlier, they should have reduced the sizes of the valves and carbs to give it more power at the kind of low RPMs that the typical Harley rider runs at.
-todd
February 27th, 2008 at 5:26 pm
Maybe I can help answer a few of the questions and concerns. First of all the Evo head has a quench shelf on half of a bathtub shaped combustion chamber. The description of the 4valve head being a Hemi, is from a centrally located spark plug.So it
is actually a pentroof Hemi 4valve head design. The
rocker arm is designed with one pushrod arm and two actuating arms with adjustable swivel foot adjusters one for each valve at their ends.As far as torque goes, so goes horsepower, one of the major benefits of the 4valve head design is the ability with the proper application of cam timing,
and compression, to have a wider torque band,
because of the inherent design of the shallow combustion chamber, and flattop piston, that can keep the compression high enough at low RPM, even
with a lessor amount of A/F mixture, this also allows a longer closing duration, because the higher velocity slows down the reversion of left over gases after blowdown.
February 27th, 2008 at 10:04 pm
As far as I know a cylinder head is designated a hemi when the valve faces are arranged in an arc over the piston forming a rough hemisphere, regardless of spark plug location. So yes, the evo head is a hemi.
February 28th, 2008 at 12:28 am
It needs to be an arc in more than one plane to truly be called a Hemi otherwise it is actually a pent-roof head (or “semi-hemi”). The valves all need to be pointing to a common imaginary center that would be the center of the “sphere”. If you look closely at the photos you can see that both intake and both exhaust valves are parallel to each other. Take a look at one of Honda’s old RFVC singles; it utilizes sub-rockers to accomodate the non planar valve movement.
“Hemi” is more of a marketing term which brings about images of purple, monster horsepower, Chrysler family cars. Even Chrysler (Dodge, Desoto, not Plymouth) used hemi heads on motors before and after WW2 long before it was ever a marketing term for them. They certainly weren’t the first but they’ve made the most success out of the term.
RPM should be more proud that they have a satisfactorily performing 4-valve head and not resort to marketing hype and misleading half-truths.
-todd
February 28th, 2008 at 10:25 am
Todd, Thank you, your post backed up almost word for word an E-Mail I sent to RPM. including trying
to capitalize on the Hemi. The description that I posted above was their response to that E-Mail.
It was my response to the Evo not being a Hemi,the original heads from the factory came with the D shaped chamber, that was designed for a flat top piston, and the valves were at a 58 degree included angle. I would agree that the Shovelhead model that was produced before the Evo was more of a Hemi design because it needed a domed piston to
generate any compression, and the valves were at a
45 degree included angle. BTW the 4 valve pentroof design above has a 46 degree included angle.J-Man
February 28th, 2008 at 10:30 am
I should have read over what I typed before I posted the Shovelhead has a 90 degree included angle.
March 14th, 2008 at 11:03 am
Arao Engineering also make 4 valve heads but on there everything stock will fit. no special intakeexhaust.
http://www.araoengineering.com/barley.htm