Exhaust back pressure?

[Keith Andrews]

Has anyone played with reducing exhaust back pressure?
IE using a slighty larger pipe..
And what effect it has.
I havnt done any calculations, but say , at 2500rpm it seems to me pushing around 1.5 cubic meter per min thru that little pipe could very well be accessive.
I have had a leak between the collectors and muffler and the pressure at the leak is very high...

[PAUL BEAUMONT]

Keith, this is again one of the things that has interested me for years. In the latter years of life with my father's CC it used to regularly burn thru the exhaust close to the cylinders - result was always poor performance. My own CB is currently fitted with a straight thru silencer muffler, cause I destroyed the last one and needed something quick. Result sporty noise but what seems to be livelier performance. I cannot easily reconsile the two unless an exhaust leak close to the cylinders allows cold air to be drawn in.
I will watch the discussion with interest.

[Keith Andrews]

regularly burn thru the exhaust close to the cylinders....currently fitted with a straight thru silencer muffler.....Result sporty noise but what seems to be livelier performance...I cannot easily reconsile the two unless an exhaust leak close to the cylinders allows cold air to be drawn in

This is all therory...
If there is not enough back pressure on older engines, this can cause scavanging of the cylinders, espec with a Bradrord as the cam opens and closes @ TDC and BDC.]
This results in lean running and hotter exhaust temps....hence excessive corrosion at the top of the manifold.
With foot off the accererator, these temps will raise up even further and more raw fuel in the exhaust system, that can cause 'poping' and back fire like a small bore gun shot.
A problem that started not long ago (I posted request for carb jets sizes not long ago) At the same timewas contiually welding up splits in the old muffler..
A few days ago I was supplied from JCC store ( http://www.jowettnz.sytes.net/BuySell.html ) a test baffled muffler, much larger dia than the std muffler adapted to fit the Bradford.
I believe this may have a very slightly more resistance than the old muffler....the reault was the idle mixture had to be screwed in to just under bottom end of the tollerance, the idle is way more stable, and far less use of the choke used..time and amount...and a far more stable idle speed, only just ticking over....and VERY quiet, keeping that suttle comic book "putt" putt" sound I like.
There is still a very occassional small pop with the throttle off...on very rare occassions...keep in mind this engine is very worn and are very suprised it is still running, so the likely cause could be there...
it is not due to inlet manifold leaks or worn carb butterflys, latter is fully rebuilt with new bushes.
Worn butterfly bushes would be the most common cause for leaning out, popping and manifold corrossion.
I did have a popping problem which was solved with the rebuilt carb about 6 months ago....before the muffler started falling apart...

I digest, but maybe interesting...the muffler started to fall apart after a 100 mile open road race with our Club president on the club econonmy run...which I won and he came 2nd
Bradfords are fun to race
1/one cant exceed the speed limit
2/It is the next best thing to NASCAR racing...like NASCAR racing where all the cars are close to the same, catching up (even a few 100 yd) and passing has to be done over very long distances...in slow motion.
Passing is also like tactical pit stops, these have to be timed and chosen carefully at pionts where one pulls over to let faster traffic thru....
3/One can race, and still have heaps of time to watch the country side go past and very low stress.
I have raced Formula Ford, off road (not 4x4), and 1/4 mile...for pure fun the Bradford tops the lot.

[george garside]

some more theoretical thoughts on the matter. I understand that the reason for the 2 into 1 exhaust system on the twins is because it provides an extractor effect which I understand to be the reason why the jupiter front cross pipe joins behind the nearside manifold, thus providing an extractor effect which the javelin doesnt have with the cross pipe going into the front of the nearside manifold.

I have also been given to understand that for good low speed torque a relativaley large ;& unrestricted inlet system together with a relatively narrow exhaust system is best, hence the reason for the IOE valve layout as used on Rover 90etc and post war 6 cyl RR & Bentley, both of which had extremely good low speeed torque. The IOE set up allows maximum size of inlet valve as on OHV set up the max diameter of inlet valve cannot exeed less than half diameter of cylinder (unless of course a tiny exhaust valve was used!). According to Horacve Grimley the CD was 'tuned for torque' and presumably this is why they used the IOE valve layout.

I am not at all ssure about this but was it when the vintage twins changed from twin exhausts & rear cross box to the 2 into 1 system that the power was increased to 17bhp - if so to what extent, if any, did the change in exhaust sytem help.


On the other hand the Weezel had twin exhausts and presumably knocked out more than the standard 17bhp, but then twin carbs & ??? higher compression may have helped - does anybody have power & torque figures for either the CD or the Weezel. Yet another conundrum on the inlet outlet thing is that, as far as i know, the Jason with twin carbs & the 1937 on tens with single carb produced the same power output - have never seen a torque figure for the ten.

finally ,and probablyof no particular relevence, if my understanding that the CD produced 34 bhp is correct and that it was 1005cc capacity its specific power output per litre was the same as the Javelin .

[Keith Andrews]

I understand that the reason for the 2 into 1 exhaust system on the twins is because it provides an extractor effect which I understand to be the reason why the jupiter front cross pipe joins behind the nearside manifold, thus providing an extractor effect

That would fit in with the use of headers on SBC. The length primaries determine the RPM range. The shorter the primaries the higher the rpm range. The principle works on speed of the pulse (wave) in the exhaust which constant regardless of rpm or speed of the gas.
A 32" primary works above the 4500 rpm range on the SBC.
There has been many a SBC with holes in pistons and burnt valves due to using incorrect headers causing scavenging...The tuner reads (more common in pre unleaded dayts) the black exhaust as running rich and leans out the carb jets...when what is happening is that the concentated pool of fuel that sits over the closed inlet valve is dragged right across the chamber and out into the exhaust...(note on these engines the cam duration is such that the inlet and exhaust are both open at the same time, which is not the case on the Bradford)
With this pool of fuel missing from the compustion it fires lean. and shows rich exhaust...hence the importance of reading plugs not exhaust with headers.

[Keith Clements]

Must have been those baked beans.....you can also use the can as a muffler