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Post by bowtay on May 22, 2019 21:23:32 GMT 1
I'm confused so my compression test is too high at 380 psi if my ration is 15.5:1
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Post by Deleted on May 22, 2019 21:34:46 GMT 1
I'm confused so my compression test is too high at 380 psi if my ration is 15.5:1 How did you test your engine to receive the figure of 380 psi. |
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Post by remmington on May 22, 2019 21:35:03 GMT 1
I'm confused so my compression test is too high at 380 psi if my ration is 15.5:1 380psi / 14.4 = 26.4bar Yes maybe a tad high - but chuck in some more variables. 1. Gauge accuracy 2. Injecting diesel 3. Engine cranking speed - Valve wear 4. Air temp 5. Height above sea level I would not panic too much. If you google this subject - you will gets loads of differing info - there are more people on this planet disagreeing on this subject than just me and Guardian. The ish factor of x20 and x15 - is also very ish - very very ish - it is not an exact science the factor of multiplication.It is more drawn from "have I got enuf compression to ignite the diesel in my engine - for real world on the job field testing with a simple dial gauge than actual engine design" The only industry who really have sussed out compression testing is - compressed air manufacturers - they state compression readings in service documents for air heads - but they are dealing with simpler porting (valve gear). You now can see why car manufacturers don't give out expected compression readings to authorised repairers - with exception to Mazda with rotary engines. |
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Post by bowtay on May 22, 2019 21:52:33 GMT 1
Thank you
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Post by remmington on May 22, 2019 22:01:16 GMT 1
You are welcome...  |
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Post by remmington on May 22, 2019 22:34:09 GMT 1
Hi thanks yes there all the same first crak 170 psi and then crank to highest reading I get 380 on all 4 cylinders So your OK with the compression's then. You need to move on to other areas of testing. Just as a rough guide for future. When the manufacturer only quotes compression ratio data simply take that ratio as in your engine at 15.5:1 and multiply it out as 15.5 x 14.7 = 228 psi. From one posting in this thread from a compression ratio of 15.5:1 you advise the chap the compression should be 682psi - 46bar. (This is bent conrod pressures.) Then from the above quote in another posting - you advise from a compression ratio of 15.5:1 the compression should be 228psi - 16 bar ish. (This is too low to ignite diesel pressure).
Can you not see - why both he and I am confused with your input... |
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Post by Deleted on May 23, 2019 0:09:15 GMT 1
So your OK with the compression's then. You need to move on to other areas of testing. Just as a rough guide for future. When the manufacturer only quotes compression ratio data simply take that ratio as in your engine at 15.5:1 and multiply it out as 15.5 x 14.7 = 228 psi. From one posting in this thread from a compression ratio of 15.5:1 you advise the chap the compression should be 682psi - 46bar. (This is bent conrod pressures.) Then from the above quote in another posting - you advise from a compression ratio of 15.5:1 the compression should be 228psi - 16 bar ish. (This is too low to ignite diesel pressure).
Can you not see - why both he and I am confused with your input...When I get a little free time I'll do a video and show you how the formula works in the real world on my Transit. I'll explain it as I do it and then you can see it in real terms. In years gone by my research has shown that diesel fuel will ignite with a compression pressure of around 200 psi, but that was before CRD. |
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Post by remmington on May 23, 2019 0:44:32 GMT 1
I look forward to it... No I really do... |
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Post by remmington on May 23, 2019 7:50:16 GMT 1
I have PM'd bowtay (that is the sensible chap asking a sensible question).
He has a transit - popping out of the intake manifold - he has removed the head - changed the cams,lapped valves new chain and upper cam carrier. And it is still doing it...
He reckons it has been apart before...
I can fully see why - he is compression testing it again and again... (I can fully understand his frustration!)
Now we have the facts...! Suggestions on a postcard please?
He is worried it has too much compression? (sensible question).
I am yet to establish if he has ever seen the engine run OK? Or it came to him as a "sick dog".
It possible it has the wrong head (valve gear) on it or pistons in it? If it has been apart before - because searching thru Autodata - these have compression ratios of between 15.5:1 to 17.5:1. There must be different parts to get different ratios.
Used transit engines do get thrown together from a collection of used - unmatching parts I am sure.
Valhalla...? Lord of the north - without any debate - what sort of compression readings should he be getting?
Because even with the ruck I had with Guardian last night. Re-reading the posts. They did look a tad high to me....
The poor chap is days and money into this - he is fence sitting from compression problems to injector problems.
What we have here is a sensible frustrated chap asking a sensible question.
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Post by bowtay on May 23, 2019 8:12:27 GMT 1
The van starts fine drives fine but has a slight miss fire
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Post by valhalla on May 23, 2019 12:46:35 GMT 1
Valhalla...? Lord of the north - without any debate - what sort of compression readings should he be getting?Because even with the ruck I had with Guardian last night. Re-reading the posts. They did look a tad high to me.... Unthrottled and just at a cranking speed of 200rpm, I would expect to see compressions of 24BAR on this engine. This is for a new engine at standard compression, and is the peak of each stroke.
You have raised an important point, which is that there are errors left, right, and centre in the way that this is measured. That is why, if I really want to dig down on something like this, I will adapt the diesel-compression kit into my Picoscope WPS500 pressure transducer, and look at the compressions in real-time against crank-angle (or as close to the crank angle that is possible, given the changing engine speed throughout each rotation). Even then..... I have dead-volume error in my pipework, etc. etc. so I don't get too excited about anything unless it is really, really out-of-bounds.
There is nothing too suspect about 390psi (circa 26BAR), as this could be a new engine, or very well overhauled and with top-end-tolerances for compression ratio, or with a decent and powerful starter motor/battery/leads (unlikely on any Transit I have seen recently, but I hear that they do exist, somewhere.......). It could suggest an exhaust valve mis-timing or possibly a blockage at the exhaust (OP has removed this for the popping noise, but is it still off for the cranking compressions?) or VNT turbine stage back-pressure with "closed" vanes - Ford have used this as a cold-crank strategy on other calibrations, and BMW do the same thing. So basically, not really the focus of attention here. Something to come back towards if all else fails.
Engine design has come on a bit since I did a bit of dabbling with head-gasket sealing faces - around the early 1990's - but we used to get very worried if peak cylinder pressure (that is the maximum pressure just after ignition of the fuel, not the averaged Indicated Mean Effective Pressure, or IMEP, derived from the dynamometer brake on the flywheel) went much over 37BAR. Our aim was to limit it all to around 33BAR on some engines, which made things a bit awkward to say the least. Engine design has come on a long way since then, and the head designs and cylinder head bolt designs have allowed peak cylinder pressures to increase a little since then. What has not improved (much) is head gasket design. so that is why you tend to see an increase in the number of head gasket failures since the 1990's, as the advantages afforded by new materials and casting methods in the cylinder block have been "leveraged" (I believe that is the modern terminology for "sticking your neck out"). The very best designs of engine could just about cope with 46BAR at full-load, but as you say, the con-rods would be at their limit, and we would have to be considering racing design.
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Post by remmington on May 23, 2019 12:50:55 GMT 1
Thank you Valhalla - lord of the north.
You are so more articulate than I am.
I am just a fat dumb mechanic....
Thank you again for accurate response to the simple question.
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Post by givusaclue on May 23, 2019 13:05:08 GMT 1
i don't want to scroll back through it all  last time i had one popping back one exhaust cam lobe was significantly less lobe like than the others & when the intake valve opened it vented the compression out through the intake manifold or wrong exhaust cam if timing is correct, can the lobes move on the shaft like the psa 1.6 tdci? can the sprocket move on the cam? had that as well assuming they're sweated on, but can't remember |
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Post by bowtay on May 23, 2019 13:36:05 GMT 1
I have changed cams to known good ones
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Post by remmington on May 23, 2019 16:34:20 GMT 1
i don't want to scroll back through it all last time i had one popping back one exhaust cam lobe was significantly less lobe like than the others & when the intake valve opened it vented the compression out through the intake manifold or wrong exhaust cam if timing is correct, can the lobes move on the shaft like the psa 1.6 tdci? can the sprocket move on the cam? had that as well assuming they're sweated on, but can't remember We had a Skoda Superb engine (CBAA) do this to us - hollow cams - slightly twisted - owner would not stump up the £175 ea from TPS. Fitted "known good ones" it was doing it on differing cylinder. I took pulse readings from exhaust tailpipe/against crank sensor - with homemade pressure transducer and a scope. It was only because I used duel channel scope and overlaid the waveforms - did I note I moved the problem about (to another cylinder). Lots of camshafts - have pressed on lobes now - they are on sort of "splines".... |
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