Okay so yesterday I did a compression test on an 86 I just bought a few weeks ago. I Just held the throttle wide open and kicked it 8 or 10 times until it stopped coming up and it went just above 225 psi. Now I expected it to be a bit high because it is supposed to be a"30" over. and have a cr head basket but 225 seems really high. Did I do it wrong? I have been running it on 91 because that's what I was told he had been using. It runs great but I don't want to burn it down.
Compression test
Bowtie316
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Okay so yesterday I did a compression test on an 86 I just bought a few weeks ago. I Just held the throttle wide open and kicked it 8 or 10 times until it stopped coming up and it went just above 225 psi. Now I expected it to be a bit high because it is supposed to be a"30" over. and have a cr head basket but 225 seems really high. Did I do it wrong? I have been running it on 91 because that's what I was told he had been using. It runs great but I don't want to burn it down.
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Compression gauges can be off and lot's of times they are. If you can double check with a good known gauge...first. Likely your true compression is closer to 175-185 lbs than 225 lbs.
Stock 86 OEM head? How thick is the Head gasket? what does the piston wear or run-in time look like? Plug Chops? Engine temp?
If you truely are pumping out 225 lbs, you need to run 100% Race Fuel...if you really want to know; perform a full UCCR, only waty to really know what you have.
Carlos
Stock 86 OEM head? How thick is the Head gasket? what does the piston wear or run-in time look like? Plug Chops? Engine temp?
If you truely are pumping out 225 lbs, you need to run 100% Race Fuel...if you really want to know; perform a full UCCR, only waty to really know what you have.
Carlos
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dont run your motor on 91 octane with 225psi cause you will destroy your motor, for 225psi you should be using 110/112 race fuel. if you want to run 91 octane you will have to drop the compression down to 165/175psi.
if you run 91 octane with 225psi your motor will detonate and run very HOT.
carlos and neill will beable to advice you better than me.
kevin
if you run 91 octane with 225psi your motor will detonate and run very HOT.
carlos and neill will beable to advice you better than me.
kevin
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It kicks hard is why I checked it in the first place. Its a snap-on gauge but I suppose it could be off. My buddy borrowed it right after and got 185 on his stock bike. It has what looks to be the temp gauge sold by bdt on the hose coming out of the head and the highest I have seen it was 190 after idling on a long slow trail. It runs 170 drag racing pass after pass. I don't know the thickness of the gasket but it is visually much thinner than the one on my buddies 86.
I have no idea on piston run in, I know it lasted through a trip to the dunes and me beating on it for 3-4 hours. Haven't done a new plug chop but it looks good after a long run on an old plug. Probably 3-4 mm fuel ring at the base of the insulator.
I guess I'm down to do the uccr.
I have no idea on piston run in, I know it lasted through a trip to the dunes and me beating on it for 3-4 hours. Haven't done a new plug chop but it looks good after a long run on an old plug. Probably 3-4 mm fuel ring at the base of the insulator.
I guess I'm down to do the uccr.
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Thin CR250R Head gaskets are .0098" or .25MM...if the head/Dome and cylinder have been Decked, than the 225 lbs is very possible. One quick fix is remove the head & thin gasket, and install a new 3 piece metal TRX250R OEM Head gasket (1.6MM thick .063")...much thicker and will reduce compression below 200 lbs and in a safer area for 91 Oct.
Again; only real way to determine what Octane level you need for your cureent or future set-up is to perform a full UCCR., and not by compression alone.
Carlos
Again; only real way to determine what Octane level you need for your cureent or future set-up is to perform a full UCCR., and not by compression alone.
Carlos
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I used a mac tools set and a Matco compression tester set they were 40 pounds different Both new then I switched around the gauges and they were still not comparable to each other. 1 read 145 and the other read 180ish on my pump gas 330. I can believe theres that much differance. You would think a gauge is a gauge (name brand atleast).
For the record, you did do it right. Throttle WOT, kick until the gauge stops moving anymore. That's the way to do it. Since you used a Snap-On gauge, the fastest way to find out if that gauge is correct is to tell your local Snap-On truck guy to test it - he'll be able to say whether it's right or not, and fix it if it's not right. That service that they have to provide is why the tools from them cost what they do. Make the guy check it, and fix it if it's off.
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i used to run a calibration lab and calibrate guages every day some in one year move 10-15lbs we had to check them against a known PSI and Known working guage every 3 months
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Okay uccr is at 14.99 with the .0098 head gasket. If someone wants to check my numbers are 66.75 bore 72 stroke and cc'd 17.5 at tdc, add .5 for the spark plug gives me 18.
Looks like I should be running above 100 octane minimum.
By my crude estimate a stock gasket would put me close to 13.75 and hopefully it would be 91 octane friendly.
Looks like I should be running above 100 octane minimum.
By my crude estimate a stock gasket would put me close to 13.75 and hopefully it would be 91 octane friendly.
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Actually at 14.99/1...you need a Motor Octane (MO) of 107-108, or a race Fuel like VP C12 or Sunoco Supreme (15/1) don't go by RON (Research Octane Number) as it can be misleading.
At 13.75/1 you still need a race Fuel like VP 110 or Sunoco Standard (14/1)...you will need to be below 12/1 to safely run 91 Octane.
Carlos
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322 Posts
Well I wonder why it has lasted this long then? It really shouldn't be far from stock if I change the gasket back should it? It looks like the only things changed are the bore by .75mm, the wiseco piston and the thin head gasket. The bore makes a small difference, head gasket is a big change but what about the piston? How much does it increase compression over stock piston?
Thanks for all the help by the way.
Thanks for all the help by the way.
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13,288 Posts
Possibly the cylinder was machined to increase compression.
At TDC, does the piston (outer edges) sit flush or below the top deck?
You normally don't get 225lbs going from the stock thicker gasket to the thin .0098" OEM gasket. Stock OEM is 165-175lbs...
Have you checked your compression gauge?
Carlos
At TDC, does the piston (outer edges) sit flush or below the top deck?
You normally don't get 225lbs going from the stock thicker gasket to the thin .0098" OEM gasket. Stock OEM is 165-175lbs...
Have you checked your compression gauge?
Carlos
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The piston sits down in the bore a bit, I would say close to .060" but that is just a guess.
I agree that 225 seems high, but it would probably correlate with the 14.99 UCCR wouldn't it?
I have not had the compression gauge checked, I really don't have a way to hook up with snap on anymore. Actually, I could probably check it against the gauge on my compressor since it has the quick coupling on it.
My plan is to put the OEM gasket on it and re-check UCCR, but honestly I'll probably just run with it. It's too much of a hastle to try to get race gas around here.
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Don't get too hung up on gauge readings a buddy's old KTM 300 kicked about 240 psi and ran fine on pump gas; some YZ250s too search the net a bit. More important is volume above TDC, head design, squish thickness, timing and jetting. I would expect a healthy unported cylinder to kick over 200 psi with a thin head gasket. My ATC does and runs great on 100 pump.
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14/1 or 15/1 does correlate to high compression like +225lbs. One reason I asked if you checked the gauge, was not only to determine if it was reading high, it could also be reading low...
One point to consider is even if an engine can and does run on Pump gas with high compression like 225lbs or 240lbs, it does not mean it is not detonating; it is. Many levels of detonation, most are never noticed at the top end level, however; they do directly affect performance and more important bottom end life. Beat on the crank and bearings long enough and you will shorten it's life span.
You ever notice when you tear an engine apart that has been properly set-up, with the correct UCCR, running the correct Octane level, how everything looks like new? Bearings feel new, carnkshaft, rod needle bearings feel New...I have seen 25 year old 250R's running Pump gas on 11/1 CR verses 250R's running Pump gas for 10 years on 13/1 or 14/1...no comparison on component wear or life span; 11/1 or 12/1 looks like new.
Carlos
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I'll check the compression gauge tonight against a couple other gauges that I have, hopefully we can put that one to bed.
I rechecked the math on the gasket change, and I was off by a bit. If I figure volume by using the bore diameter (66.75) and the added thickness of the oem gasket (1.25mm) I get 4.37cc that I would add to my compressed volume. When I redo the UCCR math I come up with 12.26:1. That sounds better doesn't it? Fairly safe with 91 octane?
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12.25:1 is a little edgy for 91 keep good quality fuel in that baby.
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