Pre ignition in inlet manifold problem
Discussion
I have a problem I'm trying to get to the bottom of and just cannot get my head around..
Rover V8 (5.3) running TB's on a 310 duration cam (M256) lift is 13.05mm Inlet 13.56mm Exhaust - so about as wild as you get.
I've set the cam timing fairly mild at 104 degrees rather than wild (106 ~ 108) and after TDC the ex is still open for nearly 70 degrees (rough measurement not dial gauge verified) which I find incredible..
I've double checked and the inlet is fully open at 104 degrees so cam looks to be set correctly.
Now I'm just interested in peoples view if the overlap of exhaust and inlet is the problem with the fuel igniting in the inlet manifolds which can be seen in 5 of the 8 cylinders.
Could it be down to running wasted spark with so much overlap ?
Perhaps coil pack or two being intermittent (breaking down) causing excess fuel in inlet ? Leads are brand new 10mm items and previous magnecor items gave the same problem, coil packs are second hand and may be faulty.
Should add that the usual cause for this is too lean a mixture but I have it running very rich ... (cold engine and me over compensating for this problem ..)..
Any genuine idea's much appreciated.
Rover V8 (5.3) running TB's on a 310 duration cam (M256) lift is 13.05mm Inlet 13.56mm Exhaust - so about as wild as you get.
I've set the cam timing fairly mild at 104 degrees rather than wild (106 ~ 108) and after TDC the ex is still open for nearly 70 degrees (rough measurement not dial gauge verified) which I find incredible..
I've double checked and the inlet is fully open at 104 degrees so cam looks to be set correctly.
Now I'm just interested in peoples view if the overlap of exhaust and inlet is the problem with the fuel igniting in the inlet manifolds which can be seen in 5 of the 8 cylinders.
Could it be down to running wasted spark with so much overlap ?
Perhaps coil pack or two being intermittent (breaking down) causing excess fuel in inlet ? Leads are brand new 10mm items and previous magnecor items gave the same problem, coil packs are second hand and may be faulty.
Should add that the usual cause for this is too lean a mixture but I have it running very rich ... (cold engine and me over compensating for this problem ..)..
Any genuine idea's much appreciated.
Is this happening on load, over-run, steady throttle?
I'd be surprised if you can have excess fuel in the inlet (causing the fault) as there should be fuel in there anyway.
The wasted spark is an interesting thought, it would still only be getting the extra spark BTDC on the exhaust stroke so it would normally just be surrounded by exiting exhaust fumes but on the other hand isn't the point of having the extra overlap to help pull the intake charge in more effectively?
Might be worth checking with the cam manufacturers whether they have tested it with wasted spark.............
I'd be surprised if you can have excess fuel in the inlet (causing the fault) as there should be fuel in there anyway.
The wasted spark is an interesting thought, it would still only be getting the extra spark BTDC on the exhaust stroke so it would normally just be surrounded by exiting exhaust fumes but on the other hand isn't the point of having the extra overlap to help pull the intake charge in more effectively?
Might be worth checking with the cam manufacturers whether they have tested it with wasted spark.............
The solution to your problem is very simple. Take the stupid drag race cam out and put a sensible one in. I've measured all the Crane cams on my cam analysis system and the 256 actually has something like 320 degrees on the inlet and 12 degrees more than that on the exhaust. It won't want to do f
k all even in a well specced engine below 4 or even 5k and it's hardly surprising it's wanting to blow back all the fuel mixture into the inlet at even lower revs.
k all even in a well specced engine below 4 or even 5k and it's hardly surprising it's wanting to blow back all the fuel mixture into the inlet at even lower revs.rev-erend said:
...the issue occurs at low revs - 1500 ~ 2500...
I agree with puma. If this is a car that your going to use regular on the road with very little track use then the cam is FAR too wild, even moreso if it's as far out of spec as the one puma tested. 
With regard to pumas comment about the spec, wouldn't the manufacturer check the cam when it's been ground? where's the Quality Control?
Dave Baker is spot on, the 256 is too much of cam. In the early 90s we worked on a championship dominating and winning TR8 circuit car. This ran a 248 cam and went very well. It was suggested a 256 would be better. On the dyno the 256 made 30 more bhp at high revs but a lot less grunt. On the track it was slower at all circuits except Castlecombe so we went back to the grunty 248 and went faster again.
Peter
Peter
The lobe difference between the 248 and 256 cams, considering they are basically adjacent cams in the Crane catalogue, is one of the most profound differences I've ever measured on my cam equipment. Nominally the 248 only has a few degrees less duration than the 256 and similar lift but they are really chalk and cheese.
In the first 40 cam degrees of lobe opening the 248 inlet cam only lifts the cam follower 26 thou. Barely enough to take up the valve clearance and crack the valve off its seat. In the same period the 256 inlet lobe lifts the follower a massive 103 thou, four times as much. The accelerations, contact forces and potential for wear so introduced are enormous.
The 248 also runs almost the same exhaust profile as inlet whereas the 256 runs about 12 degrees more exhaust duration than inlet. The net effect of all this is the 248 has an effective inlet duration of only about 6 or 7 degrees less than the 256 but the exhaust duration is nearly 20 degrees less. This vastly reduces the cam overlap when both inlet and exhaust valves are open simultaneously and it's this which causes all the 256's bad behaviour at low rpm and consequent shunting of fuel air mixture back up the inlet tract.
Why would Crane do this? Simply because, and regardless of what the catalogue specs say, the 256 is designed for drag use to get the valves as far open, as soon as possible, to maximise airflow at the expense of low rpm torque, driveability and cam life. The extra exhaust duration is so you can fit the largest possible inlet valves and not worry about exhaust valve flow. Everything designed for pure high rpm power in a very narrow powerband and sod everything else including cam life which might be as little as a few passes up the strip.
On the drag strip I guess the 256 might show a marginal improvement in times over a 248 in a well specced engine but in every other use it's a disaster. Don't even get me started on the 266!
In the first 40 cam degrees of lobe opening the 248 inlet cam only lifts the cam follower 26 thou. Barely enough to take up the valve clearance and crack the valve off its seat. In the same period the 256 inlet lobe lifts the follower a massive 103 thou, four times as much. The accelerations, contact forces and potential for wear so introduced are enormous.
The 248 also runs almost the same exhaust profile as inlet whereas the 256 runs about 12 degrees more exhaust duration than inlet. The net effect of all this is the 248 has an effective inlet duration of only about 6 or 7 degrees less than the 256 but the exhaust duration is nearly 20 degrees less. This vastly reduces the cam overlap when both inlet and exhaust valves are open simultaneously and it's this which causes all the 256's bad behaviour at low rpm and consequent shunting of fuel air mixture back up the inlet tract.
Why would Crane do this? Simply because, and regardless of what the catalogue specs say, the 256 is designed for drag use to get the valves as far open, as soon as possible, to maximise airflow at the expense of low rpm torque, driveability and cam life. The extra exhaust duration is so you can fit the largest possible inlet valves and not worry about exhaust valve flow. Everything designed for pure high rpm power in a very narrow powerband and sod everything else including cam life which might be as little as a few passes up the strip.
On the drag strip I guess the 256 might show a marginal improvement in times over a 248 in a well specced engine but in every other use it's a disaster. Don't even get me started on the 266!
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