Was The XR750 Engine At Its Displacement Limit?

[nicholastanguma]

I know the XR750 engine was based on a standard Sportster Ironhead mill, nevertheless was its own unique powerplant.

Was the 750 engine designed to be at its displacement limit as a 750cc unit, or could it be bored and/or stroked up to larger displacements?

[swampbuggy]

Surely there is " some " room for boring to fit a larger diameter piston, the question is how much is the limit ? Mark H.

[hoodeng]

The XR750 engines displacement was the maximum capacity under the rules of the AMA for flat track racing, not design limiting parameters.

This is still one expensive engine, even earlier examples in sound condition command a lot of money. From what i have seen of the few that have shown up for sale here are butchered beyond belief. Even if you did hot rod one of these you would only be faster than another unmodified one of the same.

Cheers.

[hoodeng]

Although not a XR750 flat track motor, Harley Davidson did build a street version of a twin carburettored engine around 1983/84. It was the XR1000, pretty much a Sportster drive line with a pair of XR750 heads that were prepped by G Branch.
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The 750 had a 3.100" bore and 3.000" stroke, the 1000 3.188" bore and 3.812" stroke. The 1000 was an excellent street performer for its day, Harley did performance cams for them that were manufactured by Erson, also JE high comp pistons were offered which upped the performance considerably, these were Harley Davidson's street performance bike of the day.

Harley Davidson manufactured around a thousand of these bikes with another of similar quantity of stand alone engines, it was one of these engines that i built my own CR version of the bike around.
I tried adding photos but they would not upload in this part of the forum.

Cheers.

[Geoff2]

A little known fact.....

In 1965, a 883 cc Sportster went over 170 mph at Daytona.....& it had one carb & just 4 gears.

[swampbuggy]

How many modifications were done to this 883 to reach that speed ? Mark H.

[Geoff2]

Heads were ported & 'different' cams. The only changes from stock.

[SchmidtMotorWorks]

I found this.

https://magazine.cycleworld.com/article ... per-sports

[nicholastanguma]

I found this.

https://magazine.cycleworld.com/article ... per-sports

Great find, thanks for posting!

[rebelyell]

I found this.

https://magazine.cycleworld.com/article ... per-sports

Thank you, sir

[Ks Fats]

Not all that familiar with Harley cam timing numbers but found the pb specs very interesting; coupling those with the carb used probably led to the rideability issues below 4,000. Smoking the tire down the straightaway should have given even more "pucker" factor to the ride; great article.....thanks

[SchmidtMotorWorks]

The thing that surprised me the most was the speed accomplished with the low power level.

[Truckedup]

The thing that surprised me the most was the speed accomplished with the low power level.

I do land speed racing with bikes..Number of gear box speeds is not a factor..A naked frame bike can reach 160 mph with about 85 hp on a long track..A fully faired bike like the Harley on a long straight can certainly do 170..Triumph 750 triple factory faired race bikes could do about 160 mph with a rated 80 hp. Back then a 82 HP Sportster was quite the tuning trick. Harley had an excellent racing program, they were getting 50 hp from 45 cube side valve racers ...Do not confuse a well tuned Harley with a loud open pipe stock engine.

[PackardV8]

The thing that surprised me the most was the speed accomplished with the low power level.

The unsurprising result of making power in air-cooled iron heads and barrels was it was only good for a couple of laps at those speeds before melting down. That's the reason the V.1 XR750 H-Ds are so rare today.

The other thing to which our friend truckedup can attest, the H-D racers were little bikes with minimal frontal area. The H-D Racing Department spent fifty years honing a stone ax to a keen edge.

[hoodeng]

This was a version of a XR1000 that i first built in the eighties, it got an update in the late nineties to what you see here. This one started life as one of the spare engines that Harley built to augment the 1000 or so complete street bikes.
This had the E cams, JE pistons as well as Leinweber valve gear, Mikuni TM carburetors, Supertrapps, Andrews CR trans, etc, It was my main road bike on and off over those years. I retired it in 2000 or so. Still trying to buy it back off the guy that has it.

For Harley Davidson to get 85hp out of an 900 iron head still surprises me, i have built a number of iron head XL's using all sorts of combination of engine parts, the later 1000 heads are much better to work with than the 900's but even those all have a shallow port floor and no short turn to speak of. I would hazard a guess here and reckon Harley cast some quite different heads for this project, and lead everyone involved with these things on a wild goose chase to replicate what the factory had done with a stock casting.

There was a company that manufactured alloy heads for the iron motor in the early eighties that were called 'lightening' these things would transform the stock motor.

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