Rage Racing Inc.

Rage Racing Inc. Rage Racing is a Harley-Davidson specialty shop that provides repair, maintenance, and high performa

Part 5 of 6M-8 Undersquare engine design.I left off talking about Oversquare (big piston, short stroke) vs. Undersquare ...
07/18/2026

Part 5 of 6
M-8 Undersquare engine design.

I left off talking about Oversquare (big piston, short stroke) vs. Undersquare (small piston, long stroke) engines. In specifics as how it related to the Twin cam engine. Other previous Harleys were pretty much the same setup. Today I will go into some detail on the newer M-8 Harley engine.

As a quick recap, a smaller piston has less room for valves to move air in and out of an engine. A big piston has more room for larger valves so more air can be moved easier. A shorter stroke crank has less distance to travel per revolution so piston speed is slower than a long stroke engine which has to cover more distance therefore has higher piston speed at the same RPM. Big pistons have more surface area for the combustion pressure to act on so provides more force, but a longer stroke has more leverage over the center of rotation. There are a bunch of other factors that go into this but these are the general principals.

In the Twin cam's situation, the cylinder head was developed for a 3 3/4" bore size. So the valve spacing and layout are for that bore size. Even if you get big bore pistons the original valve layout was for a 3 3/4" bore. In the example of a 103" twin cam that has a 3 7/8" bore and 4.375" stroke the intake valve is only 1.805" diameter from the factory. The biggest I have gone up is to a 1.900" valve in a stock head. Aftermarket heads can go a little bigger. Lets take the stock 1.805" intake valve. That is about the size of a 305" or 265" Chevy small block valve. The problem is that tiny valve is trying to fill a cylinder the same size as a Big Block Chevy. See the problem from a high horsepower per cubic inch standpoint? (Harleys have their strengths, I'll get to that soon)

Now to the M-8. The M-8 is still an Undersquare engine. The stroke length is longer than the bore diameter for the piston. If you divide the bore size by the stroke length you get the bore to stroke ratio. A number bigger than 1 is Oversquare. A number smaller than 1 is Undersquare. 1:1 means it is a "Square" engine. The 103 Twin cam in the above example has a bore to stroke ratio of .885. The 114 M-8 is slightly better at .905. Not too shabby, but still restrictive to air flow.

Harley engineers developed a really good cylinder head with 4 valves I will go into detail about next time. 2 Intake valves. 2 Exhaust valves. This greatly helps the lack of flow situation. But was it enough? The M-8 is obviously getting a bunch more power and easier than the twin cams for sure. The 4 valves are doing their job. They could be doing better.

Lets look at a few generic numbers again. The 114 M-8 is the equivalent of a 456" Big Block cylinder for cylinder. Most guys these days are upgrading to 128" kits with a 4.25" bore to go with the 4.5" stroke. That puts the bore to stroke ratio at .944. Getting closer to that 1:1 mark. Pretty good! Unfortunately, the cylinder head was designed for a 3.937" bore size of the original 107" M-8. Same valves. Same valve layout. You can increase the valve size by a couple millimeters which helps but 4 valve heads as good as they are do have restrictions too. The valves of a good 2 valve head are more efficient than the valves of a 4 valve head. Why? The 4 valve heads are competing against each other for air. Where I might see a 90% efficient 2 valve head I will see a 80% efficient 4 valve head. The larger you make the valves the closer they get to each other and the closer they get to the cylinder or chamber wall where they start fighting for space to breathe. Don't get me wrong. 4 valve heads will (generally) way outflow 2 valve heads especially at low lifts. The flow numbers stock out of the M-8 is impressive. Modified guys are getting crazy floe numbers out of these heads. At least by Harley standards. That's the key.

Back to numbers. Lets look at surface area of valves. The stock M-8 Intake valve is 1.571". Combined, both valves make that 3.87" square inches of total valve area. That equates to a single 2.225" Intake valve out of a 2 valve head in area. Considering the fact that the bore size (3.937") the M-8 head was developed for is smaller than a 350 Chevy (4.00") that is pretty damned impressive. The biggest stock 350 Chevy valve was 2.02" and even aftermarket heads have a limit of 2.08" without changing valve placement. So talking flow for a 350 Chevy Harley has hit it out of the park! In fact, on my flow bench the M-8 head flows stock as good as a set of aftermarket stock valve angle racing heads for a small block chevy! Awesome!

BUT we are not talking small block chevy displacement are we? The 114 engine has the same cylinders as a 456 Big BLock. The 128" big bore kit everyone uses has the equivalent sized cylinders as a 512" Big Block. These guys with giant 143" M-8 engines have the same size cylinders as a 572" Big Block! Thats huge! And still breathing through a head developed for a 3.937" bore.

Flow numbers. This is what I've found on my flow bench. Others will get different numbers but I'm just talking in general here so bear with me. There are a couple ways to test a cylinder head's potential. One is using the flow at peak valve lift. Another is flow at a lift equal to 1/4 the diameter of the valve so (.25xD). For an M-8 from stock to even larger valves that comes out to .400" lift as a test point (I test all the lift points, these are the important ones)
Max lift = .550"25D lift = .400"

Stock M-8 @ .550" = 324 CFM
Stock M-8 @ .400" = 315 CFM

Gen 2 S/E @ .550 = 342 CFM
Gen 2 S/E @ .400 = 328 CFM

Gen 1 Ward+1mm @ .550 = 354 CFM
Gen 1 Ward +1mm @ .400 = 333 CFM

Those are good numbers! No doubt! Even for a Big Block Chevy that is good. Lets look at the Big Block 2 valve head.

Bone Stock #088 1971 casting25D = .550" = 293 CFM
Max lift = .800" = 320 CFM

Hand Ported #088 1971 casting25D = .550" = 325 CFMMax lift = .800" = 375 CFM

So the stock M-8 head flows a little better than a stock square port BBC head. Thats awesome! Still barely did that with a 4 valve design competing against a 1970's casting 2 valve, but we will call it a win! Port that old 1970's casting and even with 2 valves the modified Ward M-8 and Screaming Eagle Gen 2 heads get beaten pretty badly.

Lets look at aftermarket BBC heads. After all who has a 572 Big Block with stock cast iron heads right?

Brodix BB-3 380cc IN runner Aluminum Heads CNC ported25D = .550 = 380 CFM
Max lift = .800 = 443 CFM

OK. I'm sorry. I know that was unfair of me. Apples to Oranges right? Race engine Big Block Chevy that has had 60 years of development compared to a Harley. I was just having some fun with that last one.

By the way. Exhaust flow! I never mentioned that!
Thats because the exhaust flow on the M-8 is garbage for a 4 valve head. Slightly worked over twin cam single exhaust valves out flow the M-8 head. The M-8 exhaust does freaking awesome up to .200 to .250 lift then it craps the bed and falls on its face. The reason is packaging. The exhaust port must make such a sharp turn the flow of on exhaust valve interferes with the other valve and its just a mess.

So, conclusion?
The shared Intake that splits the slow lazy charge at a terrible sharp angle and feeds two cylinders directly front to rear combined with a mediocre flowing cylinder head because of a small bore size in relation to the total displacement with a horrible exhaust port despite having 2 exhaust valves is why Harleys usually hit a wall at around 6,000 RPM. They can't spin enough RPM to make the really big power number. Its also not supposed to. They can make good torque and decent horsepower because of huge displacement but not good horsepower per cubic inch at all considering all the good things it has going for it. It can be fixed. They can run high RPM. But that would take a lot. This is why horsepower per cubic inch for the M-8 even modified to the hilt is around 1.35-1.40 to 1. (My 1987 1200 Sportster is 1.35 to 1 with modified 883 heads and turns 7500rpm) Where that's pretty decent street car territory, anything that's going to hit a racetrack is usually more. I've built 2 valve racing engines naturally aspirated with a single carburetor and a flat tappet cam in the mid 1990's that had 1.95 horsepower per inch. Stock 4 valve V-rod engines are about 1.7
The M-8 does make crazy power. It just doesn't have good horsepower per cubic inch numbers.

Next I will talk all about the fantastic features and amazing engineering the M-8 has going for it. The truth is it doesn't need high horsepower per cubic inch. It does something very special and has one piece of amazing engineering that few people even realize. I am excited to talk about the wonderful M-8 engine platform despite always putting it down in my posts. The advancement in power levels and technology are leaps and bounds beyond the Twin Cam and the engineers should be applauded for their efforts. Next post will be a look at how good M-8 owners have it.

More Undersquare vs OversquarePart 4 of 5 (or 6)Last time I got into the general benefits and downfalls of both Undersqu...
07/17/2026

More Undersquare vs Oversquare
Part 4 of 5 (or 6)

Last time I got into the general benefits and downfalls of both Undersquare engines (meaning the crankshaft stroke is longer than the piston bore size) and Oversquare engines (meaning the crankshaft stroke is shorter than the piston bore size). Now I’m going into a few specifics with Harley engines and why this is one of the two major reasons Harley’s arent regarded as high horsepower per cubic inch engines. They can make big power but require big displacement to do it.

Airflow. Horsepower comes from how fast can you create BTU’s of energy and convert it to work. So that depends on how fast you can burn the fuel. You need oxygen to burn the fuel. Naturally aspirated that oxygen comes from air so how much air can you pump through the engine?

The amount of air you can get into an engine is based on the entire combination of parts but the most important factor is the intake valve(s) in the cylinder head.
Oversquare with big piston and short stroke can fit bigger valves for better breathing.
Undersquare with smaller piston and longer stroke has less room for valves so they are smaller.
Let’s start with 2 valve Harleys. The twin cam 103 has 3.875“ bore and 4.375” stroke. If it was an 8 cylinder engine it would be 412”. Big block size. The intake valve is 1.805 inches. About the size of a 305 Chevy. About the biggest valve you can put in a stock head is 1.90”. The 110 CVO with a 117” big bore kit has the same displacement as a 468” Big Block. The largest valve I put in one of those heads was 2.12 inches. A small block Chevy sized valve feeding a 468 Big block. Not good.

The M-8 is much better with its 4 valves and way better breathing but it should be even better than it is. The M-8 also increased displacement. Bore size is 4.075” and stroke is 4.50”. With a 128” big bore kit that is like a 512” v-8. I’ll go into the valve sizes and head flow for that combination tomorrow! Stay tuned!

PART 3 of 5 (or 6)Oversquare vs Undersquare If you missed parts 1 and 2 go back to read those and be sure to catch Part ...
07/16/2026

PART 3 of 5 (or 6)
Oversquare vs Undersquare

If you missed parts 1 and 2 go back to read those and be sure to catch Part 4 tomorrow.

There are 3 different ways to make an engine. Oversquare, Square, and Undersquare.

Oversquare = Bore larger than Stroke

Square = Bore same as Stoke

Undersquare = Bore smaller than Stroke

Most engines in the past were all Undersquare Using large stroke crankshafts and small pistons on the bore. Today you still see that setup in things like tractors and trucks. An Undersquare engine is usually a little more thermally efficient as it loses less heat from combustion and provided lots of torque with the long crankshaft throw providing leverage over the center of rotation. This is good for low engine speeds. At high engine speeds the piston can outrun the pressure front of combustion and piston speeds become too high also causing high stress on parts and breakage.

An Oversquare engine has a smaller stroke than the size of the bore. This Setup has several advantages. Slower piston speed so more rpm is available and less stress on parts. The larger bore allows larger valves and much improved breathing. The bigger piston allows more force to be applied to the crankshaft for the same combustion pressure. Downsides are usually less torque at very low speeds and the flame front must travel farther across the combustion chamber which means more advanced ignition timing. nearly all modern cars, motorcycles, and race engines use an Oversquare engine.

A Square engine splits the difference and has an equal bore to stroke. Therefore it shares both advantages and disadvantages.

The 45 degree Harley is an Undersquare engine in nearly all its forms. This is reason #2 why Harley’s (even the m-8) make poor high horsepower per cubic inch engines. With a small bore size for the displacement you can only fit so Much valve into the combustion chamber. That restriction is on top of the already restrictive intake system.

Part 4 coming tomorrow

Part 2 of a 4 part series of posts(Part 4 I actually explain how good the M-8 engine is). [see, you just have to to wait...
07/15/2026

Part 2 of a 4 part series of posts
(Part 4 I actually explain how good the M-8 engine is).
[see, you just have to to wait for it]

Ram tuning Pt.2

Imagine this. You have a column of air or air/fuel mixture (which weighs more) traveling at around 300 ft/s. This column of air in our imagination is around 13 inches long and has an area about the same size as the 2 inch intake valve. Maybe some of you can calculate how many joules of energy that is.

The piston has completed the intake stroke and is at the bottom of the cylinder. It dwells at the bottom as the crankshaft and rod move to start pushing that piston back up on the compression stroke.
The intake valve stays open
The piston is building up speed upward to compress the air/fuel mixture
The intake valve stays open
As the piston is making the area above it to the chamber smaller and smaller air/fuel is still rushing in.
All the energy of that column of air/fuel does not want to stop so it continues to pack the cylinder as the piston is a good way into the compression stroke. Just as the column of air starts to lose the battle against the riding cylinder pressure the intake valve closes trapping more air/fuel than can normally be consumed.
Ideally!
Does this happen only at high RPM on a race engine?
No.
It happens on every engine in some way.
Look at the picture of the dodge cross ram. This setup is on a 413 wedge engine and it boosts power from 3500-4500rpm. Benefits start at 2500rpm on this long ram style intake. Far from race car stuff.

The Harley shared intake design i talked about yesterday DOES UTILIZE RAM TUNING. It just doesn’t do it very well or very effectively as compared to other engine designs out there. (I’ll go into why Harley does it this way in the 4th post in the series)

The big shared plenum is needed to provide the volume for both cylinders. Then it splits off in a “T” (or “Y”) to feed front and rear cylinders. The air does not like that. That reduces efficiency of the intake system.

There are 2 major reasons Harley Davidson 45 degree v-twins cannot make good horsepower per cubic inch displacement.I ha...
07/14/2026

There are 2 major reasons Harley Davidson 45 degree v-twins cannot make good horsepower per cubic inch displacement.

I have been talking a lot about horsepower per cubic inch. I have also said that the traditional 45 degree v-twin in all its platforms (Evo, M-8, Shovelhead, and so on) are about the worst. There are 2 major reasons for that and it’s not the 45 degree cylinder design. Here I will point out 1 of those reasons.

Look at picture 1 of the 64mm M-8 intake manifold. Big volume plenum with relatively slow moving air that slams into a wall and has to make a 85 degree turn to go into the head. All traditional Harleys have this same setup. Worse yet for the air path for the M-8 is the intake starts low then goes up then back down to the valves while making that 85 degree turn. Meanwhile on the next cycle the air needs to switch directions 180 degrees to fill the other cylinder.

Look at picture 2 of the Gen 2 M-8 head. Why is the intake oval instead of round like before. With an oval port Harley tried somewhat successfully to give that tortuous air path a little better angle of attack making that complex turn. The Gen 2 valves don’t flow any more. The increase is the better path of flow.

Now look at picture 3 of a single plane v-8 car intake. Shared large plenum with open intake runners heading directly to the port in the head

Picture 4 of a tunnel ram improved on the single plane design. Longer runners of equal volume directly aimed at the back of the valve

So is it just the air path? Only partially. Even more important is RAM TUNING! Air is very heavy. Yep. Air when moving fast has a LOT of momentum. How fast? How about 1/3 the speed of sound. Or more! To use that free energy you size the intake correctly and set cam timing so as the piston is riding on the compression stroke air is still rushing into the cylinder. You can achieve more than 100% volumetric efficiency this way. A 350 Chevy could actually burn 400 cubic inches of air. More later…..

Ahh freshly machined engine partsSome of you might be wondering why the wait time to get bikes in for service has been e...
07/10/2026

Ahh freshly machined engine parts

Some of you might be wondering why the wait time to get bikes in for service has been extraordinarily long this year. There are several reasons.

It seems every Harley I have worked on this year has had some hidden issue with normal appearing symptoms. Like the picture here. What came in for a simple oil leak and head gasket replacement turned into rebuilding and remachining the entire top end of the engine. Take another example of a quick in and out dyno tune on a 2002 Harley that got jammed up because of a ECM that would not cooperate. Speaking of ECM’s, how about an ECM problem causing intermittent no start when that wasn’t even the reason the bike was in for service. How about changing a tire and the axle is seized in the wheel. Changing fork seals and find the lower slider is unusable. Bolts that strip or break on final torque or just removing them. I could go on and on.
Parts availability. Parts shortages from Covid were bad enough but now I don’t know what the reason is I have to wait for tires, brakes, spark plugs, gaskets, as well as engine components like cams, lifters, pushrods, oil pumps, and so on. This year has been awful for getting parts.
Then there are personal reasons. I have had to take a lot of time off for physical therapy for my old broken mechanic body.
The good news is I see a light at the end of a tunnel. The body is feeling better. Parts are coming back slowly. And once I get these last several headache bikes fixed and on the road I expect the worst will be over and it will be business like usual. At least I hope. I’d like to ride my own bike. I just purchased a new(used) bike after 20+ years. So far this year I have only ridden 50 miles total on my motorcycles. That was not a misprint. 50 miles Or less. Im always working to catch up with these Harley’s.
I just hope that light at the end of the tunnel isn’t a freight train coming my way.

New Bike Day!Well, not today but I very recently did purchase the 2006 VRSCR Street Rod for myself that I talked about t...
07/08/2026

New Bike Day!

Well, not today but I very recently did purchase the 2006 VRSCR Street Rod for myself that I talked about two posts ago.

New to the Rage Racing family is this beauty. My first new (used) motorcycle in 22 years. I’ve wanted one since they came out in 2006. So far I’m having a blast with it even though I’ve only ridden it twice. I don’t need to go into the specs on this bike again. You can find my other post on these bikes.

Today I’m going to talk about getting a new ride, even if it’s used. I’m gonna get nerdy on you here so bear with me. After having my Sportster for 22 (23?) years and changing it up and modifying it more and more every few years I feel like having a 20th level Dungeons and Dragons character that is time to retire and start over at level 1 (some Gen Xers and Millenials will get the reference)

Basically, I’ve done all I can to the old 1987 Sportster. Rode it everywhere. Hot rodded it as far as I can reasonably go. I know every little thing about it. After 20+ years riding the same bike the only next step on its journey for me would be to totally rebuild it as a drag bike or to let it go to someone else can have new adventures with it.

My new Street Rod is 100% stock. It’s like a blank slate. I get to start over again with something new. Level 1. That’s part of the joy of a new (even if just new to you) motorcycle. It feels different. It rides different. It’s a new experience. It may make you want to ride again if you lost the thrill. You may take old familiar roads again with a new perspective. It feels fresh and exciting again. Even being quite a bit slower than my Sportster (yes the Sportster could whoop up on a v-rod) I find the new cadence of the 60 degree twin with a powerband that works from 5000-9000 thrilling. The handling slightly resembling a sport bike.

Sometimes just a new paint job or different exhaust pipe can give a similar “new” feeling to an old bike.

Soon my cherished Sportster will be for sale.

100% Stock 107 M-8 blown head gasket!Ive been throwing a lot of shade on the M-8 over the last few years. Soon I am goin...
07/07/2026

100% Stock 107 M-8 blown head gasket!

Ive been throwing a lot of shade on the M-8 over the last few years.

Soon I am going to post about all the good things going for the M-8. There are actually a few things that are really cool about it that are absolutely positives. It’s true! I’ll pull up my big boy pants and stop living in the past for a little bit and acknowledge that the M-8 has some very cool and unique engineering to it.

But this is not that post.

Stock 107 (even the stock air cleaner) in an ultra classic ridden as Ultra Classics should be ridden. Easy cruising at comfortable speeds for long distances and short to medium trips.

Rear cylinder sucked IN the head gasket. IN, not blown out! Sucked it it! Never seen that before. My idea is there was a lack of compression force to hold the gasket still. Once just a tiny bit of it crept into the cylinder the piston would grab it at TDC and as the piston rocks as the connecting rod switches from push to pull the piston crown pinched the gasket between itself and the head and slowly over many miles pulled the head gasket inwards.

You can see from the picture the piston was hitting the head gasket pretty good Smashing it against the head. The gasket is hanging into the cylinder 1/8 to 5/16 inch.

The fix? New pistons. Can’t trust that top ring land anymore. Plus machine the heads and cylinders.
Front head took .006 to get flat
Front cylinder took

The 2nd best Harley ever made!( #1 is the Sportster)The 2006 VRSCR Street Rod. I know anything labeled ”the best“ is sub...
07/05/2026

The 2nd best Harley ever made!
( #1 is the Sportster)

The 2006 VRSCR Street Rod.

I know anything labeled ”the best“ is subjective (except the Sportster being #1, that part is non negotiable) but let me put in the case for the Street Rod. The short lived unknown nearly perfect motorcycle.

Only made in ‘06 - ‘07. The Street Rod was based off the V-Rod but came with a slightly different frame, 5 gallon fuel tank, inverted 43mm Showa front forks, mid mount foot controls, less steering head angle at 30 degrees vs 34. Less rake at 32 degrees vs 38. Shorter wheelbase, lighter 10 spoke wheels, 300mm Brembo brakes (3 rotor), 4” taller seat height, flatter handlebar, a 40 degree lean angle before scrapping parts, 180 rear tire, and 5 more horsepower and torque than the standard v-rod.

The V-Rod engine was based off the VR1000 super bike program Harley built in the mid ’90s. It was a road race platform. The Street Rod was the only production bike using that engine platform geared towards good cornering. Not super bike territory but besides Buell products the Street Rod is the highest performing production Harley ever made.
Let’s look at the V-Rod engine. Based heavily off the VR1000 super bike which made 150 horsepower at 996cc. Porshe helped develop it into a streetable reliable production engine. Oil and liquid cooled. 1130cc (1250cc for 08 and up models). 60 degree v-twin. Automotive style plain bearings. Dual overhead cams. 1.575” IN valves. 1.358 EX valves (bigger than M-8 valves!). 11.3 compression pistons. Automotive style connecting rods. Oversquare 3.937” bore x 2.835” stroke. 9200rpm redline. Peak claimed horsepower 120 @ 8250rpm. Peak claimed torque 80@ 7300rpm. Dual 53mm throttle bodies on independent runner intake ports topped off with velocity stacks. In 2006 there were no O2 sensors so can run on leaded race gas if needed but 91 octane pump is all that is required. Horsepower per cubic inch is 1.739. Modified can easily surpass 160 horsepower

Address

38 West Division Street
Coal City, IL
60416

Opening Hours

Monday 10am - 6pm
Tuesday 10am - 6pm
Wednesday 10am - 6pm
Thursday 10am - 6pm
Friday 10am - 6pm
Saturday 10am - 2pm

Telephone

+18153706435

Website

Alerts

Be the first to know and let us send you an email when Rage Racing Inc. posts news and promotions. Your email address will not be used for any other purpose, and you can unsubscribe at any time.

Contact The Business

Send a message to Rage Racing Inc.:

Shortcuts

Share

Category