Just finished off my 2L 8v conversion using an Audi ABK block and a ported PB head with 272 newman cam. The trouble is it just dosnt run right and after a compression test its showing 195psi on each piston. Now I know the dangers of such high compression and I know I need to lower it. Way I see it is I've got three options: 1: head off, piston out and have 0.5mm (ish) taken off 2: head off and cut out material from the combustion chamber (not easy) 3: head off and a thicker gasket to lower compression like a turbo head spacer. 3 seems like the best option too me seeing as with the other two I'd have to buy new gasket and bolts anyway. So does anyone know where I can get a compression lowering gasket from? I don't currently have the internet on my PC, only on my phone, so can't really follow links unfortunately. All help would be greatly appreciated!
compression test and compression ratio are two totally different things. how can you figure that 195psi is too high a compression? mine is 260psi and is running fine. What fuel are u running and what management? Surely something else is your problem and not the compression ratio of your engine
Mate that doesn't sound too high at all IMHO. I have just built up a 1900 bottom end with a big valve head and 11.25:1 CR and my cranking compression was 220psi, the head and pistons were cc'd so this is a true calculated CR not theoretical. I must add that this is with a 300deg cam, if I were running the cam you are then the cranking compression would be up nearer the 260psi that Brutal has got....but I would not advocate running it that hish on a cam of your duration as the dynamic CR would be over the optimal for that duration of cam IMO.
Compression sounds fine to me. More than likely the lumpy cam (same as my old one) and the fueling not being set up correctly for the engine.
I'm using standard digifant management and 95 petrol as its the only one available within 15 miles of my house. I've got a vernier pulley on my cam and its set to 4 degrees advance for the cam so would that be a contributing factor? Also I noticed plugs 1, 2 and 3 are the showing good colouring and 4 is a little lighter than the others. Could that be showing a possible cause?
Have you checked the cranking compression on all 4 cylinders? My advice would be to fire a PM to Mr Hillclimber regarding a suggested cam timing figure for your spec of engine. I am running a Kent cam and their spec called for an advance siumilar to that you have used but Jason proposed something very different and the engine is running a lot better than it ever did at the Kent timing figure
Maybe the cause is your cam. The digi management might not be happy with it. Why did you set the vernier on 4 deg advance? That's too much of advance. Is difficult to say about the spark plugs without any pictures..
Cranking compression on all 4 cylinders is 195psi and I'm afraid I can't put up pics of my plugs due to my phone being the only source of internet I have at the moment. The cam is advance 4 degrees on the advice of the place I went to to have the engine set up, who are also the people who said that the compression was too high causing detonation in the engine,and who gave me the three options. They also advised me there's no point in setting up the engine on their rollers as its not running right.
One other thing mentioned by the rolling road people yestrday was stacking two head gaskets to increase CC. This sounds nothing but dodgy to me, what are the real results of doing such a thing?
With all due respect to the garage you have been talking to they are talking a load of cr@p IMHO. 195psi cranking compression is NOT too high, if anything it is a little low, 200psi would be optimum if head and cam is matched....cranking compression is in effect dynamic CR and is driven by your static CR and the duration and overlap of the cam you are running. In order for a specific engine/cam/head combination to work most effectively the dynamic CR is almost identical irrespective of the combination but the static CRs would be widely different. E.g. For a fast road engine combo like you have with not to lairy a cam I would expect a 10.5 static CR to be ball park, with mine that is running a 300deg cam I have a static CR of 11.25:1 and with the race engine I am building with a 320deg cam I will have a static CR of 12-12.5:1, but on all these engines I would expect the cranking compression to be in the 200-220 psi range due to differing duration and overlap of the cams. Hope this helps?
I see. Any suggestions then on what could be a cause of my running problems? It misses under load below 2k and then pulls to about 4k nicely where it seems to hit the rev limiter. I've tried another ECU and TCi, no change, its got new plugs, leads, arm, cap, dizzy, coil and blue temp sensor so I would doubt its any of them. I have a gunson exhaust analyzer but we all know they can't be trusted really.
I've just had a play with the cam timing and set it to 4 degrees retard and its much much better. My timing marks are all spot on so clearly advancing the cam timing wasn't the way to solve my problem.
It certainly shouldn't be hitting a rev limiter at 4k.....my gut is telling me it's cam timing that's an issue here, but you will also need to get your ECU tweaked for that cam.
Like I say I've put it to 4 degrees retard and its a lot better. Still not quite revving as high as it used too but its close at least. It feels a lot better to drive and it smoother too.
Mate 4 degrees retarded cam timing should not be right This leads me to ask how you are setting the cam timing? Are you using the valve lift at TDC or opening and closing timing of the valve? Either way can you confirm how you are finding TDC of the crank which you need to know for either method and how you are measuring the valve lift/opening?
I'm using the TDC mark on the flywheel for the crank, and for the cam I'm just undoing the vernier and moving the cam to where it feels right when driving. I'm not an expert at this sort of thing and most of the technical jargen is lost on me.
