strange lean condition
01-27-2008, 08:58 AM
#137
Nordschleife Master
It has been my experience that the stock fuel pump will deliver adequate fuel at the stock fuel pressure of 55psi or 3.8bar on engines modified well beyond yours, so unless the fuel pump is failing, it should be sufficient. To measure determine if the fuel pressure is falling off, you'll need to do it under full load which should best be done on a dyno.
If you used different valves than the Porsche factory valves, they may have a different displacement in the head. I have seen a differences in compression ratio due to resurfacing, valve jobs, and different valves. However, if someone did the valve job incorrectly, you'd decrease the compression ratio, not increase it because the valves would be sunk into the seat more, not the other way around. A single cut of .010" on the head will reduce the head cc by about 1cc which increases the compression ratio about .2:1, so if the heads were only lightly skimmed .003" or so, that should not alter the compression ratio all that much.
The deck height clearance is important, and if you are running an excessive deck height, then the engine will be prone to preignition because the efficency of the squish is significnatly reduced.
Running an aftermarket camshaft will alter your dynamic compression ratio, so if you are using stock pistons and cylinders with a longer duration camshaft, your dynamic compression ratio will be lower than with the stock camshaft. This is one of the reasons that when you use a larger camshaft, you can also increase the compression ratio.
The fuel pressure regulator is manifold referenced so that the fuel pressure ratio at the injector remains constant. At WOT, the fuel pressure is the highest, under overrun it is at the lowest. At idle, if you disconnect the vacuum line to the fuel pressure regulator, the engine will run richer, and when you get your pressure gauge, you can test this. At idle, with the fuel line disconnected, you should see 55psi or 3.8bar. if you reconnect the line, you will see it drop by the amount of vaccum the engine is drawing. On an engine with stock cams it idles at 55kPa and I'd think yours is in that range, maybe 58 or 60. So, your fuel pressure will drop by 46.3kPa (101.3 - 55kPa), or about 6psi. So, at idle, your gauge should read about 3.4bar or 49psi.
If you used different valves than the Porsche factory valves, they may have a different displacement in the head. I have seen a differences in compression ratio due to resurfacing, valve jobs, and different valves. However, if someone did the valve job incorrectly, you'd decrease the compression ratio, not increase it because the valves would be sunk into the seat more, not the other way around. A single cut of .010" on the head will reduce the head cc by about 1cc which increases the compression ratio about .2:1, so if the heads were only lightly skimmed .003" or so, that should not alter the compression ratio all that much.
The deck height clearance is important, and if you are running an excessive deck height, then the engine will be prone to preignition because the efficency of the squish is significnatly reduced.
Running an aftermarket camshaft will alter your dynamic compression ratio, so if you are using stock pistons and cylinders with a longer duration camshaft, your dynamic compression ratio will be lower than with the stock camshaft. This is one of the reasons that when you use a larger camshaft, you can also increase the compression ratio.
The fuel pressure regulator is manifold referenced so that the fuel pressure ratio at the injector remains constant. At WOT, the fuel pressure is the highest, under overrun it is at the lowest. At idle, if you disconnect the vacuum line to the fuel pressure regulator, the engine will run richer, and when you get your pressure gauge, you can test this. At idle, with the fuel line disconnected, you should see 55psi or 3.8bar. if you reconnect the line, you will see it drop by the amount of vaccum the engine is drawing. On an engine with stock cams it idles at 55kPa and I'd think yours is in that range, maybe 58 or 60. So, your fuel pressure will drop by 46.3kPa (101.3 - 55kPa), or about 6psi. So, at idle, your gauge should read about 3.4bar or 49psi.
01-27-2008, 10:02 AM
#138
Rennlist Member
Thread Starter
Geoffrey
Great post. Thanks for sharing that info. When I get the test kit I'll be able to validate these issues. In my view, the engine will need to be custom tuned on dyno. I've heard from very good porsche mechanic friend with 964. His car would ping too until he tuned it for its unique elements.
I am getting closer, I feel optimistic!
Thanks to all of you for your help.
James
Great post. Thanks for sharing that info. When I get the test kit I'll be able to validate these issues. In my view, the engine will need to be custom tuned on dyno. I've heard from very good porsche mechanic friend with 964. His car would ping too until he tuned it for its unique elements.
I am getting closer, I feel optimistic!
Thanks to all of you for your help.
James
01-27-2008, 10:24 AM
#139
Nordschleife Master
In my view, the engine will need to be custom tuned on dyno.
01-27-2008, 02:35 PM
#141
Nordschleife Master
You can take any engine to the dyno and test it. If you are mapping the engine, then you will want the engine in the condition it will be used in. For instance, it doesn't make sense to map an engine needing a rebuild because once rebuilt, the mapping will be wrong.
01-27-2008, 09:47 PM
#143
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An encyclopedia of knowledge. If not for our PM's I'd think his forum presence computer generated. We're fortunate. The racers, enthusiasts, and technical support make this the best forum on Rennlist.
01-29-2008, 02:56 AM
#144
Rennlist Member
Thread Starter
Geoffrey
Thanks for everything, I aways appreciate your posts. The car ran pretty darn well on the 200 mile ride to St Louis. She is a bit rich down low and as we know leaner in the mid to upper range. So once I finish the final testing to eliminate the other variables I will plan to put the car on the dyno and tune it right. The piggyback will be a huge asset in this respect.
James
Thanks for everything, I aways appreciate your posts. The car ran pretty darn well on the 200 mile ride to St Louis. She is a bit rich down low and as we know leaner in the mid to upper range. So once I finish the final testing to eliminate the other variables I will plan to put the car on the dyno and tune it right. The piggyback will be a huge asset in this respect.
James
01-30-2008, 03:10 AM
#145
Instructor
James, out of curiosity, do you use or plan to use an LM-1 to measure the A/F ratio. That would be my suggestion. Probably a worthwhile investment.
Cheers (from Korea)
Ingo
Cheers (from Korea)
Ingo
09-10-2008, 12:04 AM
#150
Rennlist Member
Thread Starter
UPDATE
I still have some pinging issues but now think it is pre-detonation caused by plugs that are too hot for my engine.
Last week I ran the car on the dyno at Automotion in Louisville KY. We quickly determined that under load when on came the car pings. We used the Vitesse kit I have to adjust fuel and timing to get them optimal and the condition improved but still happened. This led us to pull the plugs. On the 'arm' (part that goes over the tip of the plug) was light brown all the way down to the head of the plug.
The tuning expert working with me thinks the plugs are too hot. I was running top of the line denso plugs. So we've ordered a set of a step cooler than stock plug and are re-running Friday at the dyno.
We arrived at the conclusion that due to a rebuild that included cams, head work, port and valve work and cyl head update, we probably have an engine with higher than stock compression.
If Geoffrey or any others want to chime in I'd welcome opinions. We did a number of runs, stopping when the pre-detonation happened. The last run up to 5100 RPM the car made just shy of 220 to the ground.
So we think once we get the pre-detonation resolved and can do a full run to red line we'll find a strong engine. leak down was hardly on the gauge (1-2 percent) so we feel the engine is very strong. All external ignition components have been replaced. The only thing I've not replaced is the distributer (yes the belt is good) so if it is not the plugs I am yet again, lost :-(
I still have some pinging issues but now think it is pre-detonation caused by plugs that are too hot for my engine.
Last week I ran the car on the dyno at Automotion in Louisville KY. We quickly determined that under load when on came the car pings. We used the Vitesse kit I have to adjust fuel and timing to get them optimal and the condition improved but still happened. This led us to pull the plugs. On the 'arm' (part that goes over the tip of the plug) was light brown all the way down to the head of the plug.
The tuning expert working with me thinks the plugs are too hot. I was running top of the line denso plugs. So we've ordered a set of a step cooler than stock plug and are re-running Friday at the dyno.
We arrived at the conclusion that due to a rebuild that included cams, head work, port and valve work and cyl head update, we probably have an engine with higher than stock compression.
If Geoffrey or any others want to chime in I'd welcome opinions. We did a number of runs, stopping when the pre-detonation happened. The last run up to 5100 RPM the car made just shy of 220 to the ground.
So we think once we get the pre-detonation resolved and can do a full run to red line we'll find a strong engine. leak down was hardly on the gauge (1-2 percent) so we feel the engine is very strong. All external ignition components have been replaced. The only thing I've not replaced is the distributer (yes the belt is good) so if it is not the plugs I am yet again, lost :-(