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Mercedes heat shields? by belchfire on 05 Feb, 2011 21:16
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So, I'm working on my injectors for the 100th time and buying heat shields by the gross. I noticed that some of them have been leaking like they never sealed. I believe that I solved that problem by lapping the nozzle face so it was dead nuts smooth. Always looking to improve things, I heard that the mercedes nozzles were the trick set-up. I bought a set from Prothe & installed them. It pulls strong from about 75 mph on up but seems "softer" down below. The stock nozzles seem to have more snap & pull harder. Anyway, I was checking out some mercedes sites and saw that their shields had a bigger hole in them. I have a 300TD lying around the shop (doesn't everyone?) and pulled an injector from it. Sure looks the same to me. Matched the two shields and they look the same as well except for the hole. Do they know something we don't?
Also, just for fun, I pop tested the mercedes and it cracked at 1900 psi. The VW goes at 2245. Both are turbos
Has anyone done a swap?
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#1 by knowtwodrugs on 07 Feb, 2011 03:42
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I haven't ever seen anything on here about this. If you try it out please post results!
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#2 by Mark(The Miser)UK on 07 Feb, 2011 06:58
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This copper shield may look like the Merc ones. This is actually off some kind of UK Ford.
I suspect that for any particular engine, they will run hotter. This may be required if it is a prechamber rather than a swirl chamber. Does not at least one type of Merc engine rely on hitting an incandescent gumball in the middle of the chamber. Maybe for the Merc chamber the injector would otherwise run cooler than the swirl chamber...maybe
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#3 by mystery3 on 07 Feb, 2011 17:36
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How many hp/L or ft-lbs/L does a 300td produce?
What other work have you done to your truck? Why do you think having a bigger hole in a heatshield would be beneficial? Wouldn't it be less effective at it's intended purpose-to shield the injector/nozzle from heat? -
#4 by belchfire on 09 Feb, 2011 10:16
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I don't believe that the hole diameter would have any effect whatsoever as long as the spray pattern was not affected. since it apparently is not with the smaller hole, then there should be no change with the larger hole. It's not like a depth change either, as the outer ring is the same thickness and is the ultimate travel stop. What I was getting at is the quality of seal on the injector face I have had a few seal failures even though they were properly torqued. That seemed to disappear after I lapped the nozzle faces so the slightest amount of irregularity seems to be enough to cause failure. I was wondering if the larger diameter hole somehow provided a better seal and if so, why not use them instead? I don't have a picture to post but they look even larger than the "ford" ones shown.
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#5 by Mark(The Miser)UK on 09 Feb, 2011 15:30
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I don't believe that the hole diameter would have any effect whatsoever as long as the spray pattern was not affected. since it apparently is not with the smaller hole, then there should be no change with the larger hole. It's not like a depth change either, as the outer ring is the same thickness and is the ultimate travel stop. What I was getting at is the quality of seal on the injector face I have had a few seal failures even though they were properly torqued. That seemed to disappear after I lapped the nozzle faces so the slightest amount of irregularity seems to be enough to cause failure. I was wondering if the larger diameter hole somehow provided a better seal and if so, why not use them instead? I don't have a picture to post but they look even larger than the "ford" ones shown.
Clearly you seem to have had success with lapping the face of the nozzle, giving improved sealing.
However I find this astounding that the finely machined nozzle needs to be refaced, when it is the crudely made mild steel shield that is built to distort to the shape of the nozzle, and literally smear.
If you look at my previous thread on this that Vincent locked [or did he
]
I claimed that sealing of the heatshield improves with reuse [when used in the same hole,]
I am now on the eighth reuse, and over 30000 miles, due to some recent dithering/experimentation in order to try and improve mileage.
It could be that your lapping did nothing but your reuse of the shield did, or, asuming that not all shields failed, your choice of a replacement was 'lucky' if that was what you did.
Back to your question of sealing with a bigger hole, in the case of a flat washer, the smaller the surface area the better the seal, a bit like an inlet/exhaust valve seals better with a fine contact line [so does the injector nozzle EDIT pintle needle come to think about it].
However in contrast; due to the shield not being flat, and the seal being created by inner edge contact only [and outer edge contact which is met wth greater contact with the head flesh]; so, EDIT the nozzle seals to the inner edge of the shield and the shield seals to the head on it's bevelled outer edge there is an insulating air gap inbetween these two anular contacts which I believe aids temperature control of injector.
The larger Ford and even larger Merc hole, I still believe puts injector into the correct operating temperature for that particular engine design....
Noone had a guess at which shields were single use and which were 7+ did they Are you feeling lucky... punk?
Currently none are leaking, but one injector body is bleeding slightly. Here's the pic quiz once more:
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#6 by belchfire on 10 Feb, 2011 10:47
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I find it astounding that anything has to be lapped as there should be no movement after the initial torque. I could see some smearing form assembly but the marking looks more like erosion related. Nonetheless, it happens and apparently the trick set-up is to get them spiffy smooth so that there is an almost molecular bond between the parts. (see wringing in a machinist book). I have always heard that it was a felony to re-use the seals. I thought that like head bolts, that once used, that was it. I didn't think that there was any spring back and regardless, how would you know? If you ran them & checked again, you would know if they had worked or not but would you re-re-use them again? They're only a buck apiece. There's a point where you can be too cheap. As for annular contact pressure, your first picture shows different contact points so it seems that there is variation between "identical" parts. I agree about the actual psi at the contact point and a smaller hole would have a higher pressure and therefore a better seal. So why does Mercedes use the bigger one. If the V-dub ones are better for the aforementioned reasons, then why not standardize? Now there's a question about heat shielding. (Is it a shield or a seal?) There is an obvious dead air space but how much temperature differential is there? How much change is there between big hole and little hole nozzles? None of the nozzles get high enough to effect any metallurgical changes so is this a detriment? I'd opt for the hotter one myself. Hotter fuel has better atomization and better burning. I run my SVO at 170* and there's no difference in performance but anything below 140* ran like crap.
This may all be invalid if there's an appreciable difference between the two pre-chambers. I don't have that information. Maybe next time I'll just do it and see what happens. -
#7 by Mark(The Miser)UK on 10 Feb, 2011 14:03
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I find it astounding that anything has to be lapped as there should be no movement after the initial torque. I could see some smearing form assembly but the marking looks more like erosion related. Nonetheless, it happens and apparently the trick set-up is to get them spiffy smooth so that there is an almost molecular bond between the parts. (see wringing in a machinist book).
I have always heard that it was a felony to re-use the seals. I thought that like head bolts, that once used, that was it. I didn't think that there was any spring back and regardless, how would you know? If you ran them & checked again, you would know if they had worked or not but would you re-re-use them again? They're only a buck apiece. There's a point where you can be too cheap. As for annular contact pressure, your first picture shows different contact points so it seems that there is variation between "identical" parts. I agree about the actual psi at the contact point and a smaller hole would have a higher pressure and therefore a better seal. So why does Mercedes use the bigger one. If the V-dub ones are better for the aforementioned reasons, then why not standardize? Now there's a question about heat shielding. (Is it a shield or a seal?) There is an obvious dead air space but how much temperature differential is there? How much change is there between big hole and little hole nozzles? None of the nozzles get high enough to effect any metallurgical changes so is this a detriment? I'd opt for the hotter one myself. Hotter fuel has better atomization and better burning. I run my SVO at 170* and there's no difference in performance but anything below 140* ran like crap.
This may all be invalid if there's an appreciable difference between the two pre-chambers. I don't have that information. Maybe next time I'll just do it and see what happens.
They are both a shield and a seal. Clearly all three types seal, but The hole size difference leads to different amounts of heat transfer.
Wringing sounds good, at least at first, but if you look back up at the shield pictures, you will see that there is not a central flat area to be wrung together, but a fine line.
Having a brushed matt finish to the nozzles rather than polished aids smearing and sealing of the shield, onto the harder nozzle. The erosion you see on an injector is that region where the hole is.
[Amazingly, last night I was reading a Bosch manual that claimed that that erosion was from dust ingress past the airfilter
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There is indeed very little spring in the seal, and for each reuse the heatshield has to be reshaped. We are talking about 1mm of plastic movement in each direction, from use to reset.
The virgin shield probably moves 1.5mm on it's first deformation, but a combination of the smearing of the initially pointed edge, and the lack of need to 'unneccessary' resetting; I have found by experimentation only needs to be of the order of 1mm.
I totally agree with the odd buck scenario, but that is truely only a funny side benefit. My main purpose is to create shields that are matched to their hole in the head, and also assuming there is variation in injector nozzles to mate with it's particular injector.
If you look closely at my pictures of the shields, in no way are they identical. Indeed because their surfaces are machined with varying concentric ridges [alas the pics do not show the underneath ridges]; I am 100% sure that they damage the ally head by digging furrows into that delicate ledge in the head. Using the same shield avoids reploughing the surface, and causing erosion. Would you swap shell bearings around, or indeed their carriers?
I have experimented and standardised the method of resetting the shield. I do not use a drill press, I do not use a hammer. I use the 'Mole' grips, and a ball bearing, and thus can reset repeatedly and equally. I do not know how many times I can reuse them, but I may well stop soon. I do know that the Electric car man M Sharkey reused them 10 x. He used a drill press I believe.
If advocates of single use would hold up their hands and admit how many shields fail to work on first use, it would be interesting data. These 4 shields have between them in 8/9 reuses had only 2 or 3 leaks, which I corrected. So that's approximately 1 in 15 -
#8 by belchfire on 10 Feb, 2011 17:31
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The wringing fit was referring to the face of the upper housing and the cup between that and the injector. Unless the seal were to be lapped too that type of fit won't happen. I tried the ball bearing reset on the shields mainly because I hate to throw out something that can be re used. I used a washer on the backside for consistent distortion and then lapped it to (a) determine how much offset I created and (b) to get rid of the raggedy edge for better sealing. But then I remembered, it's just a buck. I'm not convinced that there is that much of a heat issue. The flame front propagates away from the nozzle and the mixture is incredibly rich in the pre-chamber so that most of the violence occurs at the piston, which it should.
