'94 R100RT engine mods

[fracture]

I have a '94 R100RT with 37,000 miles. I got the bike in '98 when it had 10,000 miles on it. I have made some modest modifications--K & N air filter, Super Trapp 2 into 1 exhaust, ported heads, carburetor re-jetting. The air injection emissions system has been removed. The bike runs better than stock. It is smoother and has better acceleration. Fuel mileage is a bit better than stock.

The bike is in excellent shape, well maintained, so I would like to keep it. The ergos are just right for me, and I like the mechanical simplicity of the airheads.

However, I would like a bit more power. I know that there is nothing I can do to this engine to get it to match the oilhead engine. I have heard that the easiest way to get more power is to install the 9.5:1 pistons. If I do that, will I need to dual plug it to prevent pinging on unleaded regular? If I raise the compression to 9.5:1 without dual plugging, will I have to run unleaded premium? I want to keep this as simple as possible, and I do not want to affect engine durability.

I have already had to replace the pushrod seals due to oil leaks. That was done several years ago. In time, I will have to do it again and that would be a good time to do the compression modification.

If anybody has done this compression increase mod, I would like to hear from you. Comments and suggestions are welcome.

[rinty]

Fracture:

I went through this whole exercise with my '82 RS and my advice is to not go there. If you spend a lot of money doing improvements, it will be lost on the re-sale. Many potential airhead buyers are quite happy with a stock bike.
If you want more power, get an oiler, and you'll get a better chassis in the bargain.

However...you can get the compression up to 8.8:1, simply by shaving .030 from the heads, and this will give you a bit more (I am assuming the compression ratio on yours is 8.2:1). You will want to re-set the carbs after doing this. I wouldn't recommend spending money on flowing and/or porting and polishing the heads. At the end of the day, these engines only make so much power.

I went full hog on my bike, spending about $9,000 on it, and there is now a very happy purchaser who has benefitted from my good deeds.

But have fun anyway, they're great bikes.

[fracture]

Hi, Rinty;

In some respects, you and I think alike. I have been looking for a used oilhead. There have been some pretty good deals here (in Texas) and think I can find a well-maintained used oilhead at a fair price. Yes, the oilhead makes the airhead look like a dinosaur, but I like the airhead.

If I got an oilhead, I would keep both bikes until I was certain that the oilhead is what I want. I have ridden oilheads only on test rides so I have no long term experience on them. I have to ride a bike for awhile to determine if it is a keeper.

I know a guy can spend a lot of money on airhead engine mods and not gain all that much. I have seen some pretty extensive mods on other airheads. The engines ran great for a while and then self-destructed a few thousand miles later. I do not think the 9.5:1 compression would be excessive since that was the compression ratio on earlier engines before emission controls.

I considered getting parts from a wrecked bike to make my mods. That way, I would have my original parts to go back to the way it was. Milling the heads is something I want to avoid because then I cannot go back to the way it was.

The time and money spent would have to be considered. Is all of this worth it for a few more horsepower? Depends on what you want. Logic says that the money is better spent on a more modern bike. Sometimes, I just like working on bikes, sort of a hobby.

Also, the airhead is a know quantity for me. They have their problems but the low tech makes them relatively easy to fix and not too costly to fix. I fear that the oilhead would be high maintenance. There is just more to go wrong.

I am leaning toward getting a used oilhead and seeing what that bike is like. In time, the greater performance of the oilhead may make me forget about the airhead.

Thanks for your reply.

[Motorman]

Since you are undecided about staying with the airhead I think your idea about getting the oilhead is the better one. Ride it a bit then decide if you really want to do the mods. I think you'll be money and time ahead if you just find the oilhead that fits your desired riding style rather than mod the airhead.

[jdiaz]

Also, the airhead is a know quantity for me. They have their problems but the low tech makes them relatively easy to fix and not too costly to fix. I fear that the oilhead would be high maintenance. There is just more to go wrong.

We've progressed backwards thru the BMW world, starting with Ks, then Oilheads, and now back to Airheads. For any given mileage, the Oilhead requires less work than all of them, and is pretty cheap to operate. Even the stickier tires required with the wider Oilhead wheels didn't wear out that quickly.

To compare with the Airhead, the Oilhead has some higher tech systems like EFI and ABS, but generally these don't require any extra work to maintain and the EFI was about as "set and forget" as you could get. I imagine that things could get expensive if they broke, but since we've already paid for a couple $750+ Airhead tranny rebuilds, nothing shocks me anymore. They're all expensive to fix when it comes right down to it. :laugh

[lkchris]

9.5 pistons were used with the larger-valve-diameter heads, which were discontinued after 1984. Also with 40mm carbs vs the 32mm carbs on your 1995.

[rinty]

Fractured:

I made the suggestion for the .030 head mill earlier on the assumption that your bike's engine's compression ratio is 8.2:1, but I did some checking and your motor is probably an 8.4:1 or 8.5:1 (I do not have the spec for a North American '94 airhead). So the result might be piston top / valve clearance problems.
However, you've posted that the route you'd probably take is to install 9.5:1 pistons. It's hard to say whether that would result in pinging; it would probably depend on the altitude above sea level that the bike is being operated. I have had three cars with 9.5:1 motors, and I could run two of them on regular gas at 3,500 feet above sea level (our home city altitude) without pinging. And a friend of mine operates his 9.5 boxer on regular gas in our thin air with no pinging (he can only do it riding solo, however).

If you decide to do the conversion, I would go into it on the assumption that you're going to have to run high octane gas, and that you're going to have to retard the timing. But even if you get pinging after the conversion, twin plugging isn't really that big a pain in the ass, and there are no additional moving parts.
And I agree with you that the airhead engine can easily handle the additional compression.

Rinty

[flash412]

Turbo. Bolt it on, enjoy the power. Take it off when you're ready to sell.

Or NOS. Same deal. Shorter rush. Money on refills.

[pmdave]

As noted by others, an airhead engine has limitations. Lots of people have tried various tactics to extract more HP (Krauser 4-valve heads, higher compression pistons etc.), but there are also lots of people who have experienced holed pistons, broken valves, pulled cylinder studs, eroded exhaust valve seats, etc. etc. I've also watched a guy with a turbo trying for an hour to get it started behind the motel, so he could cruise over to the rally site and show it off.

If you are fanatic about extracting HP from an airhead, and cost is no concern, you can probably find lots of advice. You can also find shysters who will take big bucks for mods that do very little except reduce reliability. But it's as good a way to spend time and money as anything else.

If you're looking for a bike that will carry you over long distances without a lot of hiccups, an oilhead might be the answer.

But if you're seriously infected with the gofast disease, pick up any of the latest Japanese sports bikes.

pmdave