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Old 02-26-2010, 08:40 PM   #1
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Default HD Roadkill turbo project

My dad has a 1997 HD road king, and lets just say up here at 7400ft elevation it lacks power. We were thinking of going with a cam etc, but for the price we could fab up a turbo kit for it. From my research thus far, I think we will either go with a GT12 or a GT15 series turbo.

GT12 TurboByGarrett.com - Catalog
GT15 TurboByGarrett.com - Catalog

As far as turbo selection, which one do you guys think is more fitting to the 1340cc road king motor? Realistically were only boosting the motor to get back to atmospheric pressure, and maybe a few kpa above that. Nothing extreme here.. which has me thinking going with the smaller GT12.

I kind of doubt there will be any noticeable spool difference between the two, but I know nothing about these tiny turbos.

As far as the manifold, we were thinking about taking his current aftermarket header and welding a collector where the two pipes come into one. Or something budget along those lines, hopefully we will be able to brace it well and have it last.

If this doesnt work, we will build a scratch manifold from thicker tubing.

I have no idea how to tune a boosted carb motor, but apparently you can tune for boost with a draw through carb that injects more fuel as the air is blown through it.

This is still in the research stage, but what are your guys thoughts on the idea? Logical? Or not at all?
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Old 02-26-2010, 09:20 PM   #2
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We have quite a few customers who have put turbos and MS ECUs on Harleys. Most convert over to EFI and take control of the ignition too, which is what I'd recommend, but I've also seen a couple of guys just add injectors for supplemental fuel while making boost. You can do that with the staged injection function.

I'd not want to get involved in a draw through or blow through set up with a turbo and carbs. They just don't run well.
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Old 02-26-2010, 09:45 PM   #3
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I'd not want to get involved in a draw through or blow through set up with a turbo and carbs. They just don't run well.
While I've never turbocharged a motorcycle, I do have some exposure to carbureted turbo setups by way of the air-cooled VW community.

The earliest such setups were of a draw-through design, typically using a large two-barrel (or small, progressive 4 barrel) carburetor of Holley / Rochester design. These tended to work rather poorly when not in boost, and rather particularly poorly at idle, as gasoline which has only just achieved an atomized state does not, it would seem, enjoy being drawn around the complex and rather tight curvatures of a compressor wheel and housing. And, of course, forget about running a bypass valve. Additionally, turbochargers rather dislike having vacuum pulled across them (such as by a closed throttle plate) to the extent that when exposed to such conditions, the oil seals within them tend to adopt a rather casual attitude towards their job.

A rather ingenious blow-through design was then undertaken, wherein the entire carburetor was enclosed in a sealed box, and the box itself simply pressurized by the compressor. Of course, it was necessary to add a carefully calibrated boost-referenced fuel pressure regulator, to prevent all of the fuel from being blown backwards out of the carburetor when in boost. This was, to say the very least, cumbersome. Not to mention that a concept which we take for granted (fuel enrichment under boost) is simply a foreign concept to a carburetor of this design.

Later efforts refined this concept employing discrete-barrel Webers, eliminating the box and carefully sealing the carburetor against the outside world, manipulating the pressure applied to the emulsion tubes in complex ways to achieve boost-referenced enrichment, and produced a system that did in fact work, but at the cost of being indescribably finicky and bothersome to tune.

At about this time, the concept of electronic fuel injection was discovered and, despite the exceedingly high cost attached to aftermarket systems of the day, was universally accepted as the Right Thing™, whereupon all attempts at carbureted turbocharging were unceremoniously binned.

I can think of few bikes better suited than the Road King, with its vast panoply of unoccupied space and its robust electrical system, for an EFI conversion.
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Old 02-26-2010, 10:58 PM   #4
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Quote:
Originally Posted by Fireindc View Post
From my research thus far, I think we will either go with a GT12 or a GT15 series turbo.

GT12 TurboByGarrett.com - Catalog
GT15 TurboByGarrett.com - Catalog

As far as turbo selection, which one do you guys think is more fitting to the 1340cc road king motor?
I wonder if anyone here has any experience with the 1544? Oh wait...

https://www.miataturbo.net/forum/t14691/

two 1544's spooled instantly with 0.9 liter feeding each, and that's reving to 7200rpm. I hope that leaky pig of a harley revs higher than that.

My dad had the 1548 on his 1.3l suzuki, that also spooled instantly. I have two 1548's now, and their laggy with only 0.9 each. Go with the 1544.

MarkP ended up using a chinese ebay 1544 for like $200 on his SV1000 I think?
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Old 02-26-2010, 11:35 PM   #5
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i'm going to say don't bother, my parents have harleys and in my opinion they are awful and would probably only be worse with more power.
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Old 02-27-2010, 02:11 AM   #6
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Yeah sorry to say this, but if that Harley can't put out enough power with a 1340 than you better get rid of it. Turbos are for performance applications. This is why my friend Wayne has a turbo on his yamaha r1. It even has ebc. Perhaps some head work and a good cam might do the trick? Check out I think it might be megaflo.com? I know there is a company that does really good head work for specifically harleys. You also may simply need a good tune. I had a ducati monster 750 a few years back, it was a dog. I jetted it, cut the airbox and the mufflers, and the difference was like night and day.
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Old 02-27-2010, 03:20 AM   #7
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Turbos are for performance applications.
I suspect that quite a lot of OTR truck drivers (and ship captains) might beg to differ.

(Well, depending on what you mean by "performance.")

The OP is on target here, I'm just not sure that retaining the 19'th century fuel management system is a good idea.
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Old 02-27-2010, 07:43 AM   #8
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Thank you guys for the wealth of info.

I will start looking into what a EFI conversion will require, thanks again..
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