theconrodkid
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| posted on 29/7/02 at 04:57 PM |
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fuel consumption
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 
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interestedparty
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| posted on 29/7/02 at 05:11 PM |
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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
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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theconrodkid
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| posted on 29/7/02 at 05:49 PM |
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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 
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stephen_gusterson
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| posted on 29/7/02 at 06:35 PM |
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Yor tank has
Pre Menstrual Tension?
ATB
Steve
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chrisg
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| posted on 29/7/02 at 10:29 PM |
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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???
Note to all: I really don't know when to leave well alone. I tried to get clever with the mods, then when they gave me a lifeline to see the
error of my ways, I tried to incite more trouble via u2u. So now I'm banned, never to return again. They should have done it years ago!
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chrisg
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| posted on 29/7/02 at 10:30 PM |
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BTW John,
Have you checked that the brakes aren't binding??
Cheers
Chris
Note to all: I really don't know when to leave well alone. I tried to get clever with the mods, then when they gave me a lifeline to see the
error of my ways, I tried to incite more trouble via u2u. So now I'm banned, never to return again. They should have done it years ago!
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theconrodkid
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| posted on 30/7/02 at 03:59 PM |
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snot binding brakes definatley engine,ive got some more carb jets and gona fit them when i get time 
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David Jenkins
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| posted on 30/7/02 at 06:59 PM |
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It's not a case of an over-weight right foot, is it?
David
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Jasper
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| posted on 30/7/02 at 07:59 PM |
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Ditch it and get a bike engine - you know you want to ...... (yeh, I'm already ducking !!!)
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johnston
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| posted on 30/7/02 at 08:05 PM |
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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....
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Jon Ison
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| posted on 30/7/02 at 08:32 PM |
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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....
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stephen_gusterson
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| posted on 30/7/02 at 10:03 PM |
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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
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theconrodkid
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| posted on 30/7/02 at 10:15 PM |
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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!
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johnston
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| posted on 30/7/02 at 10:24 PM |
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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 ???????
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stephen_gusterson
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| posted on 31/7/02 at 01:57 PM |
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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
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interestedparty
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| posted on 31/7/02 at 02:33 PM |
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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
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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johnston
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| posted on 31/7/02 at 04:30 PM |
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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
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stephen_gusterson
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| posted on 31/7/02 at 06:16 PM |
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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
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johnston
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| posted on 31/7/02 at 06:38 PM |
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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
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theconrodkid
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| posted on 31/7/02 at 06:38 PM |
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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.
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johnston
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| posted on 31/7/02 at 06:42 PM |
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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
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theconrodkid
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| posted on 31/7/02 at 06:51 PM |
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Just had a look at "how things work"ive started on a locost nuclear bomb and will drop it on yorkshire when its finished 
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johnston
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| posted on 31/7/02 at 07:01 PM |
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now that could be a best seller over here in belfast
"how to build a nuclear bomb for £250"
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locodude
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| posted on 31/7/02 at 07:44 PM |
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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
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Alan B
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| posted on 31/7/02 at 07:47 PM |
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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.
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