I have a 1988 LT500 that I have since 1990 and have raised the compression to 180 psi at 4500 ft elevation that I ride at the dunes in Idaho and Utah. I have used 100LL av gas in it for years and it works quite well. The carb is bored out from 39 to 40.5 mm. I am going to Dumont in a couple of weeks (700 ft elevation) and wonder if I can run a mixture of av gas and race gas(110 octane) at that elevation, straight race gas, or do I need to lower the compression ratio before I go. The gas would be the easiest solution. Also, does anyone know what jetting will work for that elevation with the bored carb?

 

The guy on these forums who can probably help ya is Dune Addict.....I would also recommend going macdizzy.com and becoming a registered member becuz DR.Q on that site is the most knowledgable guy on the LT500Rs.

later
Superstar

 

The 'LT500 motor RX' thread on macdizzy is worth alot more than the $10 reg fee.

 

Thanks for the vote of confidence Superstar!

6ppc... I need a little more info if you have it.
I think your heading down the right path. The mighty 500 is very prone to detonation even in stock form, merely raising the compression makes the situation worse.

How did you raise the compression? Just milling the gasket surface? Was there any machining done to the 'squish' band area? (Angled area just inside the dome from the outside)
What size piston are you running? (Overbore size?)

The reason I ask, is the stock squish area is not optimized, but is easily corrected if done properly.
If it is stock, I'd recommend running straight 110 or even 112 race gas at low elevation.
Watch your plug and jetting closely!!! That big drop in elevation will change your jetting significantly....a lean condition promotes detonation also.

Good gas is cheap (Figuratively speaking) insurance... detonation will kill your motor quickly.
Don't skrimp on your fuel, and you shouldn't have any issues...

As others have suggested... MacDizzy has a great forum regarding these machines... there are many shortfalls on these machines... Dr. Q has a really good Rx to fix these. I highly recommend his suggestions.
Unfortunately they charge for membership these days [img]i/expressions/face-icon-small-sad.gif[/img]

Good luck!

 

Dune Addict,

Here are some more details on my LT 500 engine. I raised the compression years ago by milling the head - both the combustion chamber and the squish band. I matched the squish angle to the angle of the piston - 15 degrees I think. The squish band is about 1/2 inch wide but I don't remember how much clearance there is between the piston and squish band - I think it was 0.050 or 0.060. I machined the combustion chamber to stay hemisherical while keeping the squish band around 1/2 inch wide and opened it up to clear the larger diameter piston. The cylinder is 3rd overbore which give 517 cc displacement. The combustion volume (not head volume ) is 39cc. I used to have detonation problems when climibing long steep dunes when the engine was hot but since I changed the pipe from and FMF to a Paul Turner, that problem went away and I have more mid and top end power. Several years ago I had a piston (ART) come apart and mangled the head so I went with Wisco and bought a new head which I remachined the same as the original head. At that time I thought there was something wrong with the original head causing the hot detonation but the sencond head did the same thing. I tried racing fuel but the hot detonation problem was still there. I still have the original head that I could open up to lower the compression ratio, but I would rather just buy 110 fuel for this trip. When I originally raised the compression, I used 100/130 octane avgas but about ten years ago they stopped making it and I could only get 100LL. I remember being worried at the time about the change in octane but it runs fine on the 100LL. If I remember correctly, normal stock compression at this elevation on the 500 was about 125 to 130 psi. I have a friend that had an LT500 with the compression raised to 150 at this elevation and ran 91 octane without any problem. He took it to Dumont and ran 100LL avgas with no problem. That I why I think I can run 110 octane in my LT500 at Dumont but just wanted to know if anyone else has done this. I also had the engine ported by a local engine builder and I really couldn't see any difference in performance so I took it to another builder who did some more work on it and I still couldn't see any difference. The machine runs pretty good - it has to be a very fast banshee to beat it although it was beat last year on Devil's Dune by a couple of bike lengths by an LT250 with a GS1100 engine in it. I have signed up to MacDizzy and will see if they have additional info for me.

Thanks for the input.

 

Good info!

1/2" Squish is pretty dang wide!!! I'll bet that makes some really good torque (Lower RPM motors tend to have wider squish bands, Higher --- Narrower or multi-angle squish bands)

If you signed up for MacDizzy... I don't need to go into great detail, because most of the info you need is right there..

I can tell you, that the larger overbore sizes (Wiseco exclusively) the top (crown) of the piston needs to be machined at 15 degrees all the way to the outer edge of the pison. If you look at one, they tend to 'flatten' out towards the outer edge..
Your 15 degree squish is pretty good... but you'll need to run some really good fuel. (That's a pretty fast squish velocity if you calculate it)

Your head (dome) needs to be .5mm larger than your bore also... so as your bore increases, so does this dimention.

all of these factors help to reduce detonation... but you really can't skrimp on the fuel. I run nothing less than 110 in mine.... Especially when I'm down to the lower elevations (St. Anthony, etc.)
I feel that 180psi is a great place to be! although higher RPM motors don't necessarily like alot of compression.... these aren't typically a high rpm motor... uness it's ported that way.

Porting: Seems that most porting jobs I've seen (Especially from the 'puppy' shops) are little more than cleaning up the casting flash, and matching the sleeve..
Dr. Q has some 'wicked' specs that significantly change the ball game...

On your Paul turner pipe... Please please please read the Rx for the pipe... The stinger diameter is too small, and needs repaired before you can 'safely' hotrod one of these motors..
(Small stinger diameters tend to hold heat in more... thus making piston crown temps rise... not good) It's a great pipe... much better than the FMF junk.

Oh, one other thing that comes to mind... If you've had some bad detonation problems in the past.. have you ever done a leak-down test? You might be surprised on what you find...
A leaky bottom end = lean = more prone to detonate.... Just a thought!

I need to ask a favor... Since you became a member of MacDizzy... Would/Could you get me an email address for ROG31 ???
I've had him do some work for me.. and I've lost his address... I'd greatly appreciate it.

We need to hook up in St. A sometime this summer!!! I'll be up for a week over Memorial day weekend... Let me know!


Regards,

 

Hi Duneaddict, LT500 at MacDizzy said you were looking for me? What's going on, and how are those hi-speed piston washers working for ya? Mine work great, I tore my engine down to inspect them and they're as good as new. Were you wanting some machine work done or just wanting to get in touch? Well I'm logged on here now so let's see, how's the quadzilla info on this site? It looks pretty interesting. Can you PM on this site? If you need something give me a PM or if they don't mind talking shop then just post it dude!
Rog-31

 

Duneaddict....do you know of a good custom pipe builder?Vito's performance ran a computer print out of a pipe matching the engine work I had done...right now I have a FTZ Drag pipe on it and it work's ok,but from what I read the stinger diameter maybe too small in diameter,holding too much heat,and that sux.

 

hey, all
seeing the post on st. Anthony's reminded me. Anyone going there june 12 area? I am gunna be down there for about 4 days (fri, sat, sun, and mybe mon. would be great for more riding buddies [img]i/expressions/face-icon-small-smile.gif[/img] speeking of compression also, how high can you run the psi on 91 octane gas? high octane is hard to get around here, with the exception of av gas. ive heard that av gas is not good for bike engines because of the frequent rpm changes. (IE, planes are designed to run at a given rpm load). any thoughts on this?any questions, im wondering on the 500 in my info below.
have fun at dumont by the way, i wish i could go. my ultimate dream is to go riding there someday.
aaron

 

Instygatr, 100LL works fine for running 2 stroke quad engines. It is very consistent from one maker, and batch to another. It's highly regulated because it has to be perfect, it's people's lives we're talking about if the engine siezes due to fuel being mixed incorrectly. For that reason it is great in your quad. The gas stations on the other hand cannot be trusted as to what is in their "chemical soup" as people call their gasoline. AVGAS is fine. I used to run mine on AVGAS. As for your quad, has it ever been overbored? If so then I would recommend running AVGAS to prevent detonation. One your displacement is slightly larger so your compression is probably higher two the larger displacement pistons have an area on the piston crown that will cause detonation. So if you don't fix that I'd do everything I could to prevent detonation. Running higher octane does not improve performance in and of itself, on the other hand you might gain some power simply because higher octane fuels are just cleaner, they also help with carbon build up.
Rog