Jon Ison
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| posted on 31/7/02 at 07:55 PM |
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2p ere too.......we could see power go up n down on dyno by sliding 2 pipes inside each other to get optimum length for "headers", there is some
"black art" therory to this 
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johnston
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| posted on 31/7/02 at 08:34 PM |
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right hold on till i explain this better (if i can)
increasin the velocity has the same effect as less back pressure the exhaust gases move out quicker this helps the high end power but takes it from
the low end ...Good for a race engine or on b roads when right foot is through the floor 
But you lose out on low end torque i.e. you have to rev the nutts of it to get it to go.
makin it a pain in the arse to drive (anyone who has driven a car with a race engine slowly in traffic will agree)
Now when ur exhaust valve opens (which is b4 bdc) u lose pressure on that cylinder which is still going down on its power stroke. so with some bac
pressure it means the gasses although some are escaping the remainder are still pushing down on the piston till it starts to go bac up (on the exhaust
stroke)
this helps when ur going slow with low revs i.e in traffic (anythin other than an empty b road or motorway). and when pulling away from corners in a
low gear
But the bac pressure means it robs yhe power when the piston is going up pushing the gases out at low rpm this doesnt matter as the gasses have time
to get out but at high rpm it slows the piston down on the up stroke robbing power cos it has to force the gasses out.
its the horses for courses you have to compromise you cant have everything scenario
wot u gain down below you lose up top and vice versa
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interestedparty
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| posted on 31/7/02 at 08:48 PM |
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quote:
As I understand it the object of not going too big on bore size is not to create back pressure, but to increase velocity which is required at
the junctions to create the extractor effect.
It's a similar situation with inlet manifolds, but in that case the high velocity is needed for good cylinder filling at low revs, to promote good
torque spread. This is why a 180 manifold (each side of 4B carb feeds 4 cylinders)is better for a V8 at street rpm than a 360 (each side feeds all
cylinders)
John
As some day it may happen that a victim must be found,
I've got a little list-- I've got a little list
Of society offenders who might well be underground,
And who never would be missed-- who never would be missed!
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Alan B
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| posted on 31/7/02 at 08:49 PM |
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Yeah but increasing back pressure i.e. restricting flow increases velocity not the other way round! It's the increased velocity that helps the
scavenging effect at low rpms.
Watch a stream that widens and narrows. To maintain a constant flow rate it has to speed up through the narrow sections and slow down through the wide
sections. It's the fast moving sections that create the venturi that helps "pull" from the other pipes. It's the same way air travels faster over
the top of a wing and creates lift.
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Alan B
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| posted on 31/7/02 at 08:57 PM |
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I knew I'd seen it explained very well:
http://www.ssheaders.com/header.htm
And, yes John you are right the same basic(but not necessarily intuitive ) physics applies just as much for intakes.
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stephen_gusterson
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| posted on 31/7/02 at 10:22 PM |
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watch out....this is turning into a useful tech discussion
my old mazda 323 zxi had summat called variable intake geomery. I guess thats to help keep the air flow in the 'zone'.
When I had my jag V12, there was a company offering to modify the inlet headers. Basically, they opened them up and inserted pipes to make all the
intake tracts the same tuned length. This was claimed to add about 10% power and 13% torque.
They also sold exhaust pipe mods, claimed to give 10% more power. Basically this took out the front silencer boxes, and just left the 'glass pack'
straight thro ones at the rear. In this case less back pressure.
They also once did cams. No one bought em. reason was they gave big torque increase, and more accelleration. And thats a problem - yes - cos the power
output actually fell - it was the extra torque that helped with the shyte 3 speed auto.
Imagine - all that time spent designing a v12 with 300hp then you stick an ancient totally crap gearbox with only 3 gears on it.
1st went to 65
2nd went to 100
3rd supposedly 150.
a lot of scope for downshifting as you can imagine! It would change out of 1st on a normal throttle at 6mph or so!!!
atb
steve
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johnston
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| posted on 31/7/02 at 10:46 PM |
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yeah produtction cars can have too much but too litle can be a bad thing 2 especially under normal drivin conditions which is wot i was orginally
tryin to say
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Alan B
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| posted on 31/7/02 at 11:04 PM |
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quote:
watch out....this is turning into a useful tech discussion
Sorry Steve, I'll try and lower the tone in future
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theconrodkid
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| posted on 1/8/02 at 06:09 AM |
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This is getting too much,ill fit a diesel 
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stephen_gusterson
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| posted on 1/8/02 at 07:49 AM |
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quote:
This is getting too much,ill fit a diesel
Ah, well there is another conversation!
there was a dicussion on TOL a while back over fitting a mondeo diesel to a locost. Its been done and apparently the torque and resulting
accelleration is actually really good!
atb
steve
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johnston
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| posted on 1/8/02 at 05:05 PM |
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wot sorta ratin would you need on ur front springs though a diesal is a heavy beast!!!!!!!!
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theconrodkid
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| posted on 1/8/02 at 05:26 PM |
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Ive changed the carb jets,ill see what happens now,the mondeo td is reasonably quick but do drink a bit when given wellie and dont like running hot.
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theconrodkid
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| posted on 14/8/02 at 07:08 PM |
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Right problem solved and you were all wrong.
Q- If i drive to work everyday,the journey is 6 miles each way in my tin top,how many miles using the same route is it in my 7?
A- 3 miles,speedo is right milometer is under reading by 50% 
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johnston
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| posted on 14/8/02 at 07:42 PM |
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no we were all right u're question was wrong u said u had a fuel consumption problem but u dont
heard of a girl once took her car to a garage numerous times claimin it had an oil leak of course the mechanics found nothing till 1 day she mentioned
it had ruined a new pair of shoes............
CONFUSED???????????????
it was leakin at the pipe goin to the oil pressure guage??
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stephen_gusterson
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| posted on 14/8/02 at 09:20 PM |
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quote:
wot sorta ratin would you need on ur front springs though a diesal is a heavy beast!!!!!!!!
well, my v6 is pretty heavy - and i have 350lb springs on the front and they are suffering a bit. however my shocks are angled at about 45 degrees and
that wont help. gonna give em a less shallow angle.
atb
steve
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stephen_gusterson
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| posted on 14/8/02 at 09:23 PM |
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quote:
no we were all right u're question was wrong u said u had a fuel consumption problem but u dont
heard of a girl once took her car to a garage numerous times claimin it had an oil leak of course the mechanics found nothing till 1 day she mentioned
it had ruined a new pair of shoes............
CONFUSED???????????????
it was leakin at the pipe goin to the oil pressure guage??
My mates alfa 156 had an oil leak once in the passenger footwell.
yeah.
we put it down to the misses shopping leaking mazola on the carpet, but we couldnt get her to admit it!
Mate of mine also had a lady come in to garage saying car was making a knocking noise. the engine was FULL of oil. she had been taking off the oil
filler, couldnt see oil, and so slapped a bottle full in each time she checked it when filling up.
atb
steve
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theconrodkid
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| posted on 14/8/02 at 11:09 PM |
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is this a have a go at women day?if so ill be back when i have lotsa time sorry dear
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johnston
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| posted on 15/8/02 at 08:22 PM |
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the vw transporter i mentioned in another topic (the one with a tractor attached to the grill) never really went too good when i mentioned it i was
told it should go well it was only rebuilt with bout 20-30thou mile i of cousre asked why had it been rebuilt
the reply "the driver b4 u was very kken on topping up the oil" which is gd most van drivers never bother checkin the's till they seize EXCEPT
He never dipped it eventually it blew every seal in the block requiring a full rebuild wit feck all miles on the clock..
one of the service manager's favorite fuk ups
customer brings in car complaing of it leaking water from door seal boot sunroof etc etc
mechanic gets told......
that cars got a water leak..
result 2 hours wasted trying to figure out where the engine is supposed to be leakin from
this has happened
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