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Post by Six Stroke on Feb 28, 2006 20:12:48 GMT -5
Hello all, my name is Adam. I'm in the final stages of the engine rebuild/parts hunt of my '78 SR500. It's been three years now since I picked it up with a blown motor. Long story short, the exhaust valve stuck and destroyed all kinds of stuff. My question concerns aftermarket piston choices. I plan on running a Supertrapp pipe, the mild WB cam, hi volume oil line kit, and probably an aftermarket Mikuni carb. I can't decide whether I want to go with a 10:1 Wiseco with a small overbore, or a 9:1 Wiseco big bore piston. Just looking for some opinions from experienced SR builders, because I want good power, but I don't want the thing to hatch on me. Rebuilds arent cheap!  |
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Post by G Man on Mar 2, 2006 0:04:52 GMT -5
Hi Adam: Welcome to the forum. I don't have too much to offer on the rebuild, mine is still stock internally. There are some very knowledgeable engine people here. I am sure someone will chime in with some opinions. Looking forward to updates on your progress.
G Man
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Post by StewRoss on Mar 2, 2006 1:38:32 GMT -5
Hi Adam,
I have used two different Wiseco pistons. The 88.5mm is a piston that I have used 3 of. All have been OK. It makes a reasonable difference to power and flexibility. It works better with higher octane fuel as well, prevents detonation.
I destroyed one of these, but that was down to the last person who owned it leaving the bolt that holds the sprocket to the cam loose....it interacted with the exhaust valve...not good...headless valve and a corresponding large hole in the top of the piston...
The other piston is an 89mm one. It is a much better design and is a lighter, slipper style one. I haven't done much with this one yet, but others that have speak highly of it. Both types are 10:1 or so.
Alays ensure that you check the squish and piston to valve clearance. It can be rather messy if you dont.
I have heard people indicating that there is not much left of the cylinder liner with the 90mm pistons and bigger; however I've never used one, or done the measurements to ascertain if the problem really exists...
Hope this assists in some way.
SR
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Post by skyebike on Mar 2, 2006 19:37:43 GMT -5
Hi Adam, I've just fitted a wisco 87.5mm 12:1 cr piston to the '78 SR I'm putting together. I didn't choose this piston, the local Yamaha dealer said standard 3rd oversize pistons were no longer available so they supplied this piston!! It was cheaper than the price they quoted for an original. Engine started first kick yeehaaa. Quick blat up the road revealed the most savage pinging I've heard, checked the timing, slightly advanced, fixed that, still pings (pre ignition). My question is...Why? I had no air filter fitted, could this contribute? Petrol was regular unleaded, will premium high octane make that much difference? Anyone out there got any ideas? Skye |
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Post by manxman on Mar 2, 2006 20:50:36 GMT -5
Skye,
with 12:1 cr you'll ABSOLUTLY need high octane premium fuel.
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Post by yamatopdog on Mar 2, 2006 20:56:12 GMT -5
skye,run the highest octane you find.i'm new to this motor but not new to engines and that a very high cr for an air cooled street bike.and please put a filter on it.bob
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Post by skyebike on Mar 2, 2006 23:49:31 GMT -5
Thanks all, I'll get onto it!!
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Post by wotavidone on Mar 3, 2006 7:16:59 GMT -5
Dood,
You are seriously going to struggle with 12:1 CR.
I've been doing a bit of research on octane ratings of fuel. I predict you will need 100 octane or better. You may remember the old Ford V eights of the early seventies (really showing my age now) were a bit prone to pinging. My brother in law was able to run 12:1 pistons in his by using avgas. Hard to get now. From the information I have been gathering, most of the commercially available octane boosters only give boosts of less than one unit. eg you might be able to boost 95 octane permium to 95.4 or so.
I am thinking of trying 5 or 10 percent methylated spirits in the fuel. Don't spill it on the paint though. Anymore is alleged to wreck the rubber bits, and since it doesn't have the same energy density as petrol, anymore would probably require bigger jets.
Its all part of the fun. Pity it consumes so much time that would be better spent riding.
Mick
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Post by skyebike on Mar 3, 2006 16:28:58 GMT -5
 Oops! What have I done?? Sounds like the piston I've been got is going to be more trouble than it's worth! Waste of money perhaps, wonder if the shop will take back a slightly used wisco? After all I only wanted a standard piston in the first place, my plan was to restore this bike to original, sell it on and start work on the ultimate cafe racer with the other pile of bits. Might have to keep this motor for the cafe? Don't want to sell the bike with a list of instructions on how to mix the correct grade of fuel Its' been bucketing down rain for the last week, so I haven't even had a chance to try some premium fuel. Thanks for the feedback, guess I've got some tinkering to do. Skye
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Post by Six Stroke on Mar 4, 2006 10:03:00 GMT -5
Wow, 12:1 is up there for sure.
Stew - was that 89mm slug newer than the others? An updated design, perhaps?
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Post by StewRoss on Mar 5, 2006 3:41:22 GMT -5
Yes, I got it from Carl at Cafe Racer in Bathurst. Unfortunately Carl's business has now closed.
He had sent off (or phoned thru...can't remember) to Wiseco with a few suggested changes. They apparently work well...squish changes etc..a much nicer looking piston.
The old type Wiseco's tend to cause pinging with the standard head dimensions. I get around this on my black cafe racer by using AVGAS (100/130). Instant torque and the engine pulls from nowhere after this. Not a totally useful fix for full time riding however...
Perhaps 98 octane UL would work as well....not really tried this on that bike. I certainly use it on the other SR's...
Once I build up the new engine...I've been very slack...too many projects at once...I'll post the results. That engine is going to be different to all the others, hope it works as well as it should!
12:1 is fairly high I'd suggest...you'd need to carefully modify the head (...and piston dome...) to get an efficient squish shape etc. I feel...I'd take it to someone who is an expert in that area...
SR
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Post by Six Stroke on Mar 5, 2006 11:00:24 GMT -5
I think what I'll do is go with an oversize 10:1 piston. It sounds like the most reasonable route.
Thanks for the help!
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Post by StewRoss on Mar 6, 2006 1:36:01 GMT -5
Perhaps try for the 89mm and see how it goes...
SR
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Post by theswede on Mar 6, 2006 3:07:35 GMT -5
I'm also rebuilding an sr500 engine, and considering wiseco's 90mm 11:1 piston. Since I plan to use the standard cam I feel this might be a bit much too? Can I run it on 98 octane without pinging?
Skyebike, I think you will get an awsome engine with the 12:1 piston and a wilder cam!
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Post by skyebike on Mar 6, 2006 19:21:24 GMT -5
;DWell, 89 octane fuel & an air filter seem to have stopped the pinging!! Just a quick squirt up the road (bike still not registered), there seems to be a fair bit of grunt available, but haven't ridden one since I sold my last one ten years ago so time may have altered my perspective.
As I plan to sell this bike to concentrate on my cafe racer project, I'll let the next owner deal with the cam.
Now I have to finish the paint work, rebuild the front brake m'cylinder, & it should be complete, I'll post some pics when finished.
Thanks to all for their feedback, a great forum!!
Skye
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Oops!