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oldtuckunder

End of season compression test

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Hi Folks

Well this years competition season is over, so thought I'd run a couple of checks on the engine to see if I need a plan of work for the winter. Have also sent an oil sample off to Millers Oils to compare with one from end of last season.

Stripped engine down at end of last season, Shells all looked good, but changed them anyway because it could, and also popped in a new set of rings as the engine was stripped.

I ran a full compression test when the engine was warm after a couple of hundred miles, when I thought everything should have bedded in and got the following results.

Cyl      1          2          3          4          5          6

Dry    130    130    130    130    130     135

Wet   150    150    150    150    150     160

Engine has now done around 1500 miles total,  approx 50 competition ones at high rpm's and just got the following with the engine Hot as opposed to probably Warm on the first set

Cyl      1          2          3          4          5          6

Dry    115    115    115    115    115     120

Wet   135    135    135    135    135     140

Why #6 is always higher than the rest I have no idea, the chambers were all accurately measured, the rods are identical its just a mystery to me.

The engine was built with a CR of 9.25:1 so the original readings of around 130 dry are in the right ball park.

So question is does a drop to 115 dry now but with a similar +20 wet indicate bore/ring wear I should be concerned about, or can I for once not pull the engine.

Alan

 

Edited by oldtuckunder
hit send too early!

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A better indication of wear would be a leakdown test where you pressurise the bore via a sparkplug insert and measure the amount of air escaping.  Anything more that a 1-1.5% leakdown is no longer a racing engine.  In addition the noise of the escaping air will alert you to which area is weak causing the loss.  I.E.  hiss in tailpipe suggests exhaust seats.  BTW, were your compression tests done with throttle open or closed?  Numbers seem low?

 

Edited by GT6Steve

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Following good advice  from Steve, leak down tester kit ordered at £20, so when I'm back from holiday in 10 days should be able to find out more "Information is power"

Alan

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Odd...... I'd have expected them to have got higher, not lower.......

And still beautifully even.  And still the 20 psi uplift on wet vs dry.   I wonder whether something other than engine condition is causing your offset.

Throttle wide both times?  Engine warm both times?  Something else?

If it's not an offset (and I think the sheer eveness of everything says it is) but condition related, then the constant 20psi uplift makes me think it's valves not rings/bores.

Nick

 

Edit: and I agree with everything Steve says though I'm seriously doubtful that any engine I built ever achieved  better than 1 - 1.5% leak-down.......

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I think Steve put his finger on it, I didn't have the throttles open on the latest test and it was Hot, fairly certain I did have them open on the first set  (cause I do really know they must be) but from my note engine was Warm and maybe not as Hot as this time, will retest again tomorrow. 

Alan

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Ah! Leakdown test.  Good idea.

Hmmmmmmm.  Lots of kits about, some very similar (standard red box?) some very pricey, some ridiculously cheap. (£60-12!)  OTC's kit at £20 is middle range, but with respect still sounds cheap to me.

I've bought tools before that were too cheap (pipe expanders made of chocolate 'steel' that caught fire when I tried to weld-repair them!).   There's no leakdown kit on eBay that could be said to from a "reputable manufacturer".   Compare the list of compression test kits -  Sykes-Pick, Sealey, Laser - at least I know those names, and some of those kits can cost hundreds.  What would you buy?

John

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Same kit seems to sell under a number of names (including Sealy) for a wide range of prices. £ 20 - £ 30 looks to be enough to buy one under a "lesser known" name.  My problem is that I don't have a compressor.......

Nick

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Looked at a few "how to build your own" Youtube videos, and to be honest when you look at all the simple individual components that go into making one, there isn't much to go wrong, just worked on the theory that if the individual bits would cost £15 a finished kit for just over 20 delivered was a good price (especially as if I bought all the bits they are going to end up having been made in China anyway!).

Not even sure you need a compressor, I think you might get away with a bicycle pump, racing bike pressures are 80-120psi and you don't need much volume to fill a cylinder at TDC.

When I've done the tests if you want to borrow and try your welcome, as its just going to be another of those ubiquitous red boxes that sits on the shelf 364 or more days a year, as its cheaper many times to buy the tool (if only used once) than it is to do a job badly. 

Alan 

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Agree with OTC, Nick, uyou could pressurise it with a foot pump, or one of those stirrup pumps that racing cylists use.

Might need an adaptor, but most bike tyres today use Schrader valves like car tyres.

Gosh, OTC!   Did you 'cost' the parts in an exercise to discover  what a 'fair' price might be?  That's a most professional method!

Thanks, both,

John

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Well John I have to admit no, but on one of the "build your own" videos, one of the guys said the bits cost him $15-$20 and as bits like that always seem way cheaper in the States (not to say readily available from numerous hardware chains) I thought I was fairly safe using approx the same figure. I actually expect if I did buy the bits individually over here it would be more than the cost of the kit. All I did do was pick a UK supplier rather than a hong kong one, as I thought if its rubbish the extra couple of £'s means I can at least return it. To be honest I normally find the Red Box tools OK for the odd usage, but you wouldn't like to use them as a professional tool in use week in week out.

 

Have redone compression tests, engine warm, and throttles open, and almost identical figures to start of season.

Actually the reading Dry of 128 is actually with the needle rather close to 130, but as there is no calibration between 125 and 130 and the needle takes up a fair amount of the gap, I may just have put down 130 at the beginning of the year, likewise 135 instead of 132.

Likewise wet readings are all a tad over 145 apart from #6 which had reached 150

 

Cyl      1          2          3          4          5          6

Dry    128    128    128    128    128     132

Wet   145    145    145    145    145     150

So maybe just a tiny tad down on start of year figures (if I wasn't just rounding earlier in the year)

Will post leak down test results in a week or so.

Alan

 

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Here's a couple at random, but you'll get dozens, everyone seems to want to re-invent the same wheel :-) More I think about it £20 odd for the kit is a no brainer!, but I'll let you know if it works!

 

 

 

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