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Hi all, I'm having a few problems with an engine I've put together and am fast running out of ideas/patience/mojo/will to live, so hoped someone else could help! So. Engine. Mercedes 190e 2.5 16v. 4 cylinder twin cam. From the factory it puts out 204bhp. When I got my engine, it had low compression (8 bar), due to damaged valve seals. Had the head rebuilt with new guides, etc. At the same time, fitted twin weber 48 dcoe carbs and megajolt. Took it to the rolling road for setting up and it only put out 170bhp - less than standard! I was gutted and confused. Rolling road guy suggested a few things to check so off I went. Turned out I had low compression again! Leakdown test suggested bottom end. Compression check after a few drops of oil down the bore suggested this was also the case. 8 bar to start and 11 bar after I wet the bores. Put this down to too much idling on the driveway whilst trying to tune the carbs/ignition myself. So. Last few months I put together a fresh bottom end. Got a lowish mileage bottom end. Stripped it and re-honed the block. Lightened balanced rods, pistons, crank (also knife edged), flywheel/clutch. Put it all back together with new seals, bearings, gaskets, etc. Put carbs and igition back on 'untouched' - same set up from the first rolling road session. Got it runnning a few weeks ago and have put around 700 miles on it. Seems to drive great, feels lively. All good. Went back to the same rolling road as before, and now it only has 155bhp!!!!! WTF? Came away assuming I'd screwed something up in the rebuild. Just checked compession - 12bar, spot on. Cam timing spot on. Only changes are a sealed airbox. The 170bhp session was open carbs. I now have a sealed airbox running to a front mounted airbox. 3" intake pipes, 4.0 litre capacity. Would this effect power that much? Only other thing is the exhaust system has taken a few good smacks recently (a new one is next on the list) and is a bit restricted in a couple of places along its length - again, could this have such an impact on output? Any help or suggestions appreciated. Many thanks. Pic of the engine in question as thanks:
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Adey
Posted a lot
Posts: 1,404
Club RR Member Number: 171
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did it come with carbs from factory or injection? also how mappable is the ignition on it?
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Sort the exhaust. (I'm assuming its dented)? Maybe a larger airbox on the carbs?
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get injection back on it, I bet it'll make more power!
how badly dented is the exhaust?
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1986 Panda 4x4. 1990 Metro Sport. 1999 Ford Escort estate.
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v8ian
Posted a lot
Posts: 3,824
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Cams badly timed or even fitted in the wrong sides, lash adjustmens incorrect assuming the cams are mechanical??
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Atmo V8 Power . No slicks , No gas + No bits missing . Doing it in style. Austin A35van, very different------- but still doing it in style, going to be a funmoble
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ChasR
RR Helper
motivation
Posts: 10,256
Club RR Member Number: 170
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The exhaust would not help matters, but that is quite a fall in power! Injection may help get the power back, but even on DCOEs I would expect more power!
I would be tempted to recheck the cam timing. Has it been checked just by him, or by someone else?
What did the bottom end come off (Are all 2.5 Merc 4 pots the same, or were there variations over the years)? I have heard of owners (Cosworth 24V guys) who used BOB cams on BOA chains and sprockets (since the latter are duplex and not simplex) with the results in power output being dire).
However, that engine setup does look lovely, with the manifold details etc. What is the spec on the engine?
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Was the orignal merc BHP figure given at the crank or rear wheels? Obviously on a rolling road its going to be at the wheels......
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if you replaced the rings, perhaps they are in the wrong position and thus causing a loss of compression. This happened on one cyl of a mini engine i did a while back, only way to resolve was a strip and reposition rings properly. did the trick for me :-)
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'88 Cadillac Brougham hearse (white) '91 Carlton GSi 24v '72 Dodge dual cab pick up '99 Mercedes S55 AMG
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stealthstylz
Club Retro Rides Member
Posts: 14,926
Club RR Member Number: 174
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Did you check the rear tyre pressures before the 2nd rollingn road sesh?
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There are so many variables on a dyno... even the same one Two clicks of a mouse will give you 30% more or less, and don't think that because the guy "does a lot of dynos" means he actually knows what he is doing
Fact If you have the same air volume with more compression without detonation and a similar AF ratio and equivalent revs you have more power. In fact if any 3 of the 4 are the same and the 4th is better - you have more power EFI vs Carbs mean nothing, power is air and fuel at optimum timing (timing is only an issue if you cant get what you want), although that said I personally think EFI will be better as it can meet the fueling requirements over a larger range.
Only REAL way to test power is a stop watch, 400 mt, total weight and a terminal speed, can't argue with those facts!
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Contrary to popular opinion, I do have mechanical sympathy, I always feel sorry for the cars I drive.
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Thanks for all the sugestions.
I've had a fiddle and can confirm: Cylinder compression spot on (around 190psi both hot and cold, throttle wide open). Cam timing spot on and cams in right sides! Tried running it without the airbox in case that was a restriction, made very little difference - a lot noisier!
Havent checked valve gaps as yet but they were spot on 700 miles ago.....usually need checking every 10,000 miles or so, so should be fine still.
Exhaust is pretty poor to be honest! Still a 3 box system (think 2 would be better) and the rear section looks like it was shaped over someones leg! Am on the hunt for a replacement as we speak.
Orignal 204bhp is at the crank. I would have thought 210bhp upwards would be reasonable to expect with the mods I have done. The factory ignition is untunable and the factory mechanical fuel injection leaves a bit to be desired! Can't think if the power the rolling road gives is crank or wheels but assuming its the same both times I visited, I'm still 15bhp down on last time with a stronger engine?.....
Didnt check tyre pressures before, but just did - o/s/r 30psi, n/s/r 24psi - oops! I guess that wouldnt make too much difference?
Guys said the fueling was spot on aside from a little rich at idle. Gave the idle mixture screws a half turn in today.
Plan is to fit new exhaust and go back again to see what it does. Then fit larger chokes and re-jet to suit to see if that does anything....
Current spec:
2.5 16v Block checked and honed Lightened/balanced rods, pistons, crank (also knife edged), flywheel/clutch. New piston rings, seals, bearings, gaskets, etc. Twin 48 DCOE Weber. 36mm chokes. 130mains. 160airs. Megajolt ignition ECU - fully mappable, runs with Ford EDIS system.
I want to swap out the 36mm chokes for a set of 42mm ones I have. The weber tuning manual states a choke size of 45mm for a 2.5 engine making max power at 7000rpm. I can't find any 45mm chokes so my 42mm ones will have to do - hope it will keep some of the 'driveability'. We'll see!
Thanks again all.
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Last Edit: Aug 4, 2013 16:54:03 GMT by ukmercman
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omega
Posted a lot
Posts: 1,060
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low tyre pressure,worn exhaust not giving the car a fighting chance really
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I will bet that if you back to the rolling road you will get a different figure again. The video I posted clearly shows that rolling road figures are pretty much meaningless unless conditions are EXACTLY the same between runs. If its running well, I would suggest you don't worry about it.
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1986 Panda 4x4. 1990 Metro Sport. 1999 Ford Escort estate.
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If it feels like its driving well leave it. You'd be able to tell of you 20bhp down easily. If you took it there 3 times in 1 week you'd get 3 different read outs.
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If you can't beat em, knock em off
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maybe so, but not 30 bhp different.
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^^^ watch the video. They "gained" near enough 100 bhp just by moving the dyno temperature probe around.
Dyno figures are useless unless conditions are EXACTLY the same between runs....its all too easy for a slight detail....ambient temperature, tyre pressures, different operator, etc etc to be slightly different which can have a huge impact on the figures. I have the print-outs to prove it myself....my old V6 calibra gained 30-odd bhp between runs at the same rolling road with no changes to the car at all.
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Last Edit: Aug 5, 2013 5:48:49 GMT by dave21478
1986 Panda 4x4. 1990 Metro Sport. 1999 Ford Escort estate.
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Darkspeed
Club Retro Rides Member
Posts: 4,845
Club RR Member Number: 39
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36mm Venturi in the 48's will be giving good torque and likely making the car feel quicker up to 5000 but robbing power at the top end - I suspect that is part of the problem. As you say Weber guideline is a 45-47mm venturi. As 42mm will be the biggest recommended venturi for a 48 whilst maintaining good atomisation and all-round drivability that's where I would start. Would also look at seeing if the exhaust back pressure can be measured. There is a rule of thumb with Webers and Dellorto's that the venturi should always be 6-7mm smaller than the body.
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stealthstylz
Club Retro Rides Member
Posts: 14,926
Club RR Member Number: 174
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Have you asked any other owners for their rolling road results?
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Why do you care what numbers the dyno tells you? If it drives well and goes fast then enjoy!
For the record, your method is nowhere near scientific enough, as others have said. The factory power figure is quoted at the flywheel in a lab, the dyno will be measured at the wheels. The figures are not comparable. What you should be more worried about is getting the setup that you have tuned right.
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