Its standard gt spec,foam air filter and straightish through exhaust
My 1600 x flow driven gently only gets about 20mpg and is hardly a ball of fire,done all the usual things,any bright ideas?
Its standard gt spec,foam air filter and straightish through exhaust
Sounds like something definitely wrong there, driven the way you describe should be more like 40mpg. Presumably all your calculations are correct,
also presumably your sparkplugs don't show your engine running super rich, got to be a petrol leak, failing that, a visit to a rolling road should
sort it out.
John
No leaks(pmt tank)plugs look ok i run a garage and am not a novice at these things but have to accept the fact i dont know everything and cant suss
this one out 
Yor tank has
Pre Menstrual Tension?
ATB
Steve
No, He means his tank was made by Bob Altzheimer/The Cog-Down Kid/Arsechin/Chief sludge-pumper........
THE PLASTIC TANK MAN
The man who many think is me (including Rory)
Cheers
Chris 
PS how many nicknames does one bloke need???
BTW John,
Have you checked that the brakes aren't binding??
Cheers
Chris
snot binding brakes definatley engine,ive got some more carb jets and gona fit them when i get time 
It's not a case of an over-weight right foot, is it?
David
Ditch it and get a bike engine - you know you want to ...... (yeh, I'm already ducking !!!)
may be exhaust is too straight through with out enough back pressure....
got a book on cam timin that explains it but as usual canny find it....
did i not tell you jasper....get you wallet ready, i struggle to get over 20mpg even when the pump packed up, nearer 15/18 mpg, the buggers thirsty,
do i care NO. i lose a fair bit when the flame thrower starts on overrun n upshifts too, looks good though, don,t sound bad either....
When I made up my twin exhausts, there was a lot of stuff on the net that said that exhaust back pressure as an engine aid is a myth.
In fact, long branches on manifolds are to encourage exhaust 'pulses' to co-incide with the next one and act as a 'suction' device.
As far as the fuel consumption goes.....many years ago, my gf, now wife, had a mk1 cortina 1200 2 door. some wally had tried to tune it. he put a 1500
carb on it. it ran ok, without too much smoke - no power gain but BIG fuel consumption. Wrong jetting I guess.....
And I was NOT that wally......
atb
steve
Tried a bit of back pressure,it slowed it down a bit more and caused it to run even hotter than it allready does,more importantly it made it
quiet,boring!
bac pressure aint a myth have u tried runnin a car when exhast falls off at the down pipe?????
had a 309 rally car once failed mot for bein too noiesy (straight through bar a cherry bomb bac box) put a standrerd middle box on thinkin it would
kill power but it actually helped it pull up hills better in higher gear and give it more pull outta the corners.
besides how do u think the performance exhaust works ???????
More importantly, can YOU explain how stopping exhaust gases from getting out fast can help engine power.
Race cars dont have silencers......
As I said in the other post, branch length of exhaust matters....it allows pulses from each cylinder to 'suck' the next one out - this is in effect a
vacuum effect, and not a back pressure effect.
The silencers on my car are like 'cherry bombs' the yanks call em glass packs. They are the back box froma granda! and you can see right thro them -
no back pressure.
Do a search on google for 'muffler' - you wont be long in finding stuff that says back pressure is a myth.
atb
steve
No discussion of exhaust systems is complete without due consideration being paid to the rev range and the cam timing. All the above opinions can be
reasonably true for different engines at different rpms.
By back pressure what is meant is that if a pressure gauge is inserted in the exhaust system at say the downpipe then it will show a higher reading at
any given rpm and load condition than if it is placed further along the system. If an exhaust system flows too freely for a given engine and load
situation then the effect can be similar to a badly timed camshaft. If on the other hand the cam is designed to run an open exhaust then that is what
it will need to produce its best, and probably at much higher rpm than a road engine
John
bac pressuere helps cos if you have none as soon as your exhaust valve opens all ur compression will be lost. (as u no it doesnt just open a bdc) so
the gasses will stop pushing on your pistons more more back pressure the more they keep pushing down BUT!!!!!! the more bac pressure the harder it
is to get them out. its one of them compromise thingys that keeps you from gettin exactly wot you want from a car.
the tuned length bit just creates more bac pressure when you want it.
im tryin to find my book that explains it but i think it might be in the engine management 1 by dave "rave" walker the ccc jorno guy which ive lent to
my mate
right.... so why do perfomance exhaust get described as free flowing, and have a really big bore?
If you look at a F1 exhaust, they are really short. They also have equal length branches. As I understand your explanation, they ought to have a
silencer or some kind of restriction in the pipe, but they dont. I would have thought that if there was any power behind back pressure, they would
have followed your theory.
http://www.howstuffworks.com/muffler4.htm
try this link and see what you think
atb
steve
dont forget f1 cars run at 16000+revs bac pressure helps low end torque i.e. 1000 revs and f1 cars run air valves not a convetional camshaft and
spring mechanism which can let the valves oen a lot later keepin the pressure in the cylinder
trust me on this 1
As was explained before the exhaust,engine and inlet side are all designed to work together,mess one thing up and the lot goes pear shaped and drives
like a nissan micra,F1 engines were designed to be straight through from the start.
oh i and big bore and free flowin exhausts hellp the high end power and let it rev faster but wot u gain u lose down the low end (its that feckin
compromise thing again)
of course this all fecks up when u put a turbo on
Just had a look at "how things work"ive started on a locost nuclear bomb and will drop it on yorkshire when its finished 
now that could be a best seller over here in belfast
"how to build a nuclear bomb for £250"
Conrod
Two up,loaded to the gills 840 miles to Le mans and back, 1700 x-flow, twin 40 DHLA's, 224 kent cam (torquey!) 25mpg ish. Sorry mate!
Chris PTM
Seeing as everyone else is throwing their pet theories
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. Hence an bigger bore system will help top end but hurt low down torque due to lack of extraction caused by
low exhaust velocity.
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
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
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.
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.
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.
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
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
quote:
watch out....this is turning into a useful tech discussion
This is getting too much,ill fit a diesel
quote:
This is getting too much,ill fit a diesel
wot sorta ratin would you need on ur front springs though a diesal is a heavy beast!!!!!!!!
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.
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%
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??
quote:
wot sorta ratin would you need on ur front springs though a diesal is a heavy beast!!!!!!!!
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??
is this a have a go at women day?if so ill be back when i have lotsa time sorry dear
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