Ham
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| posted on 19/2/07 at 08:37 AM |
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X Flow pinking
Hi all, a bit of a strange one,
After melting a piston on a recent run I repaired the damage by honing the bore and slapping a new Piston in, the head was cleande of debris and seats
were re lapped.
The engine is now running fine except when pulling hard at around 4-5K I can hear pinking. This is a concern as I am sure advanced timing caused the
first melt down (+- 15 deg @ 1000r/min)
The strange bit is at low revs and wide throttle openings there is no pinking, before the melt down I never heard any pinking, I am currently running
at 11 Deg @ 1000r/min and the car definately feels retarded - any ideas???
Cheers
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britishtrident
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| posted on 19/2/07 at 08:44 AM |
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(1) Do you have a working by-pass conection on the cooling circuit ?
(2) Weak mixture ?
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Ham
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| posted on 19/2/07 at 09:07 AM |
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If by bypass connection you mean the outlet from the head to the pump, then yes I do, My mixture seems O.K. maybe slightly rich if anything, as
mentioned why the pinking only at one spot in the rev range?, surely it should pink throughout
Cheers
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macnab
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| posted on 19/2/07 at 09:16 AM |
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I'm wondering what your compression ratio is, have you got it right??
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Ham
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| posted on 19/2/07 at 09:35 AM |
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Hi Macnab, compression is high around 11:1 running on 95 octane with octane booster
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macnab
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| posted on 19/2/07 at 11:11 AM |
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yip thats certainly high, I've heard from the vdub guys that 11:1 is the limit for use on the road and above that needs special fuel. I know its
tempting to go as high as possible but I've shy'd away from that after friends also melted holes in their pistons. I'd lower it or
at least go for the coolest running plugs. Why not super unleaded? its 97 octane...
My cortina use to pink like crazy at the slightest wiff of a hill and that was in the days of real fuel, if I remember correctly that was at 10:1
after if flowed the head. Never melted anything but it sure sounded bad.
Just a thought, you haven't flowed it have you and left any sharp edges?? probably not.
[Edited on 19/2/07 by macnab]
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BenB
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| posted on 19/2/07 at 11:12 AM |
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 that's high comp!!!!
Can't see why it should suddenly start pinking after the rebuild unless you've changed anything... but I'm suprised with that CR on
95 (even with octane boost) that it wasn't pinking like crazy all the time....
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Ham
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| posted on 19/2/07 at 11:48 AM |
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That comp ratio is a theoretical one, I measured and calculated, in practise it may be a touch lower, it is still high.What is strange is that as
mentioned it does not pink at low revs with a wide open throttle but only at one spot on the rev range, the head is flowed, any idea what timing I
should run, I have always believed that the correct setting is to advance untill it pinks and then back it off slightly. thats how I was running
before the piston and plug went on holiday!
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macnab
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| posted on 19/2/07 at 12:34 PM |
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I think (correct me if I’m wrong by the way) but there are two things here not one.
First it your high 11:1 ratio, that is more likely to cause the pre-detonation which burns through the piston and is caused by the mixture getting to
hot and going off before the spark, rather like a diesel engine and slamming into the piston when it’s still coming up.
The other is the timing which is suppose to be set for the octane of the petrol and should be set so that the flame front reaches the piston as it
reaches the top, this flame takes a very short time to travel from the sparkplug to the piston and so the ignition is retarded back to allow this.
The too are very different although can have similar results, so setting your ignition is not the best way of dealing with to high a ratio as even
when the timing is spot on the engine can still get hot enough that it starts detonating.
The reason I believe that going up a hill or accelerating sets off the pinking is that the throttle is wide open and so loads off air is getting in,
hence more air to compress and higher temperatures are reached.
Hopefully some off this will be of help
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MikeRJ
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| posted on 19/2/07 at 03:20 PM |
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Are you using a stock distributor, or have you had the advance curve modified to suit your engine? Sounds like you have too much mechanical advance
(which would be inevitable with a stock distributor) and you are trying to compensate by retarding at idle.
It's usually pretty easy to limit maximum advance by dismantling the distributor and modifying the stop on the centrifugal advance mechanism.
Idealy ditch the distributor entirely and fit a megajolt wich will allow you to make full use of the CR.
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britishtrident
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| posted on 19/2/07 at 05:09 PM |
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11:1 with a cast iron head is very high even with hotter than standard cam.
An alloy head can go one ratio higher or slightly more than a cast iron head but I wouldn't have used more than 9.5 on a cast iron head even
back in the old days of real five star.
[Edited on 19/2/07 by britishtrident]
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Syd Bridge
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| posted on 19/2/07 at 06:53 PM |
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Geez there's some tosh sprouted on this forum! 
No more than 11:1 compression on iron heads? Where did that load of bovine fertiliser appear from. I know, 'you read it on the internet, so it
must be true'.
How many Pintos run on oval tracks here with cr's well above 12:1, every weekend in summer?
Tell the yanks who run high 13's and low 14's that their heads are the wrong material for the cr's. What a load of rubbish!  We
ran 14:1 + on the sprintcar engines in Aus 30 years ago and had no problems, with iron heads!!!!
The 'pinking' problem appears to be mixture related, as it is at a specific point in the rev range, but not either side. Points very
specifically to lean mixture, and would also explain burnt pistons.
A good rolling road session is cheaper than a new engine.
Cheers,
Syd.
[Edited on 19/2/07 by Syd Bridge]
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MikeRJ
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| posted on 19/2/07 at 08:22 PM |
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quote:
Originally posted by Syd Bridge
How many Pintos run on oval tracks here with cr's well above 12:1, every weekend in summer?
Then again the Pinto actually has a reasonably well shaped combustion chamber, not something the xflow can be accused of, and I suspect they are
running fairly wild cams. The OP doesn't mention what other mods the engine has.
quote:
The 'pinking' problem appears to be mixture related, as it is at a specific point in the rev range, but not either side. Points very
specifically to lean mixture, and would also explain burnt pistons.
Possibly, but not necessarily. Depending on the cam fitted 4000-5000 could be around peak torque so just as possible for it to be ignition related
IMO.
OTOH perhaps the octane booster isn't particularly good, or not enough has been used. If the cam is fairly mild then 11:1 CR coupled with
crappy combustion chambers is very likely too much for 95 RON.
quote:
A good rolling road session is cheaper than a new engine.
Agreed.
[Edited on 19/2/07 by MikeRJ]
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paulf
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| posted on 19/2/07 at 09:04 PM |
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I am running a 1600 crossflow fitted with 1300 pistons and understand it to be 11/1 compression ratio also.
I found the standard distributor had to be retarded at low revs to prevent pinking at the top end as already stated the total advance is to high with
an uprated cam etc.
I switched to megajolt and now run 36 degrees maximum advance on full load and 45 degrees on overun and light throttle, with no pinking or signs of
over advance.I usually run super unleaded plus valve seat additive as i have soft seats in the head.
It would be well worth checking the total advance and mixtures etc, if on webers could it be one barrel has a mixture problem due to blocked jets or
an airleak.
Paul.
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barraw
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| posted on 19/2/07 at 11:57 PM |
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Could be weak valve springs, or valve springs not matched to your cam. Check your rocker/tappet gaps.
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NOTE:This user is registered as a LocostBuilders trader and may offer commercial services to other users
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Ham
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| posted on 20/2/07 at 06:06 AM |
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Thanks for the opinions, the engine spec is as follows:1660,1300 pistons,block decked,head flowed,internals lightened and balanced,cam - 275 deg,carb
38 DGAS, ign - bosch electronic.
I tend to agree with MikeRJ in that the spark could be over advancing too early hence the pinking at a single point, this would also explain the
pinking dissapearing above this point I will have a look at the mech advance mechanism, Weak mixture would cause problems throughout the rev range,
and at the mo the car pulls well and runs cool, not signs of weak mixture.
Cheers
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Syd Bridge
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| posted on 20/2/07 at 09:47 AM |
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quote:
Originally posted by Ham
.........I tend to agree with MikeRJ in that the spark could be over advancing too early hence the pinking at a single point, this would also explain
the pinking dissapearing above this point I will have a look at the mech advance mechanism, Weak mixture would cause problems throughout the rev
range, and at the mo the car pulls well and runs cool, not signs of weak mixture.
Cheers
If it was overadvancing early, then the pinking would be more evident at lower revs and under load at low revs. Maybe that's as it is.
But, running a Weber, they can give exactly what you are describing for various reasons, if wrongly jetted. Been there, chased the problem, got the
solution. Unfortunately, it only took the rolling road gent 10 mins to spot a problem we'd been chasing for three months!!! Change of a few carb
innards and probs were gone, and a much more driveable car to boot.
Get it to a rolling road, it will be money well spent.
Take your air cleaner/filter assembly off, and give the car a run, and see if it still has the problem. 
Cheers,
Syd.
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