PARP - How does it happen?

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Andymoor94
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PARP - How does it happen?

Postby Andymoor94 » Fri May 22, 2020 9:26 am

That old Morris bumble, fart, parp, burple, overrun - Whatever you know it as, my 1963 model 1000 did it, but my 1954 803cc doesn't, which saddens me greatly!

What actually causes the fart? I'd like to restore it, indefinitely!
I have an 803cc engine for now, but for those who've seen my other posts will be aware that I have a 948cc going in at some point.

I will be looking to replace my crappy-looking pea-shooter exhaust in the future, so if this contributes at all, then I'd like to know!

Cheers
Image
Eugene - My daily driver
1954 4 Door Saloon with original 803cc Engine.

Murrayminor
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Re: PARP - How does it happen?

Postby Murrayminor » Fri May 22, 2020 9:33 am

The distinctive sound from the original Morris Minor is usually heard on the overrun, I should imagine the small bore exhaust has a lot to do with it, changing to a large bore exhaust will allow the exhaust gases to escape more freely therefore you may lose the sound.
Proud owner of my first Morris Minor

myoldjalopy
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Re: PARP - How does it happen?

Postby myoldjalopy » Fri May 22, 2020 9:47 am

Not sure - I have a wider than usual pipe and its really loud!
Tweaking the throttle settings can make a difference.
Once I fitted a new exhaust and the sound was gone - but came back after a couple of hundred miles......no, I can't explain!

pgp001
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Re: PARP - How does it happen?

Postby pgp001 » Fri May 22, 2020 10:29 am

My Parp disappeared when I rebuilt the SU carb and it now has the later version with the little spring loaded valve in the throttle butterfly plate.
I believe that is the reason on mine, and next time the carb is off, I intend to loctite the little valve closed just to find out.

Phil

Sleeper
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Re: PARP - How does it happen?

Postby Sleeper » Fri May 22, 2020 10:32 am

I have a 2.0 Zetec in mine , 2" pipe to the ( nearly - stainless ) original silencer and tail section.

Lovely fart on the over-run , but tends to loose it when hot...

John ;-)

MattsGPOvan
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Re: PARP - How does it happen?

Postby MattsGPOvan » Fri May 22, 2020 6:23 pm

Wow, have been really getting into this at the moment with David Vizards book.

There is so much more to exhausts than I can list here.

exhaust velocity is a big deal and reverse flow , exhaust lengths, exhaust diameter all play a part.

Design of cylinder head , exhaust manifold etc.

Please remember old leaded fuel around 84/85 octane rating now it’s 91 ... Which in theory should increase BHP ??.

I would say it could....could... be back pressure caused by poor silencer design because there is the first silencer quite close the engine ???.

I laugh at the modern boys with there BIG bore Systems... is it great to have a VERY FreeFlo exhaust system..... But according to David vizard for each exhaust stoke you need the right size diameter and velocity speed to exhaust that Gas...

Having a very large diameter exhaust the gas can reverse more and can contaminate the next combustion cycle ... which will decrease BHP.

Monty-4
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Re: PARP - How does it happen?

Postby Monty-4 » Fri May 22, 2020 6:32 pm

I would love to get the PARP back. My car, now with twin SUs, an uprated cam, an LCB manifold and the Maniflow system is PARPless.

Given I don't spend much time at high RPM so am not realising much benefit from the wider bore I often consider trying too attach a standard exhaust to the LCB manifold.
68' 4-door Saloon, another 'Monty'.

MattsGPOvan
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Re: PARP - How does it happen?

Postby MattsGPOvan » Fri May 22, 2020 7:09 pm

Great stuff, what David says is The importance of getting the right diameter on the 1st metre of exhaust to get the exhaust gas speed up ... then you can increase the diameter size of the exhaust further back.

Also having a step from the cylinder head to the exhaust manifold to stop the reverse flow at low RPM.

He also says that manufacturers make exhaust silencer,s to a cost.... and this creates back pressure.

I love all this technical stuff but I’m really into metal shaping.

Check out old metal shaping on Instagram.👍 and on here.....mattsGPOvan

James k
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Re: PARP - How does it happen?

Postby James k » Sun May 24, 2020 2:18 pm

I would be really interested to know what exhaust you end up going for, if you replace it, and how it is noise-wise. I have a mild steel exhaust from ESM on my Traveller and it is horrendously loud. I enjoy the parp sound and it did make that when new but it disappeared when I added a short chrome tailpipe extension. I did this as it did not clear the bumper and exhaust was getting into the cab through the back doors. The problem with mine is that it has a deafening low to mid pitched drone, like a loud resonant hum at low revs and more akin to a jet engine when driving over 30.

I recently had some chassis work done at the Morris Minor Workshop and they commented that it was unusually loud so I wonder if mine has been badly manufactured.

James

ianmack
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Re: PARP - How does it happen?

Postby ianmack » Sun May 24, 2020 2:35 pm

There can be quite a lot of variation between exhausts as different manufacturers pack their silencers differently. In addition stainless ones tend to be noisier as stainless is more brittle and resonant. As a final variant deposits in the system will muffle noise, for example if the engine runs rich or burns oil.

King Kenny
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Re: PARP - How does it happen?

Postby King Kenny » Sun May 24, 2020 2:53 pm

I used to test how well my Traveller was tuned by driving down the Bradford on Avon hill. If it Parped loudly all the way down to the town bridge the engine was in fine fettle, but if there was no Parp I knew it was time for a service.

pgp001
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Re: PARP - How does it happen?

Postby pgp001 » Sun May 24, 2020 3:04 pm

King Kenny wrote:
Sun May 24, 2020 2:53 pm
I used to test how well my Traveller was tuned by driving down the Bradford on Avon hill. If it Parped loudly all the way down to the town bridge the engine was in fine fettle, but if there was no Parp I knew it was time for a service.
I am curious to know what part of the service it was that restored the Parp when needed ?
Carb settings or new air filter maybe ?

Phil

burnham28
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Re: PARP - How does it happen?

Postby burnham28 » Mon May 25, 2020 10:38 am

My Panda has a really loud parp. I think the silencer must be made of recycled baked bean cans
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myoldjalopy
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Re: PARP - How does it happen?

Postby myoldjalopy » Mon May 25, 2020 10:45 am

Not much good for sneaking up on crooks then - they'll hear you coming from miles away! :lol:
Lovely looking car though.

MattsGPOvan
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Re: PARP - How does it happen?

Postby MattsGPOvan » Mon May 25, 2020 11:28 am

James k wrote:
Sun May 24, 2020 2:18 pm
I would be really interested to know what exhaust you end up going for, if you replace it, and how it is noise-wise. I have a mild steel exhaust from ESM on my Traveller and it is horrendously loud. I enjoy the parp sound and it did make that when new but it disappeared when I added a short chrome tailpipe extension. I did this as it did not clear the bumper and exhaust was getting into the cab through the back doors. The problem with mine is that it has a deafening low to mid pitched drone, like a loud resonant hum at low revs and more akin to a jet engine when driving over 30.

I recently had some chassis work done at the Morris Minor Workshop and they commented that it was unusually loud so I wonder if mine has been badly manufactured.

James
Love this James , about the Chrome tailpipe scenario..... is it because now the exhaust has a radius and that the noise disappears ...... We need to experiment 😉😉👍

Most exhaust silences are poorly made in far away countries.

StillGotMy1stCar
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Re: PARP - How does it happen?

Postby StillGotMy1stCar » Mon May 25, 2020 11:49 am

pgp001 wrote:
Sun May 24, 2020 3:04 pm
King Kenny wrote:
Sun May 24, 2020 2:53 pm
I used to test how well my Traveller was tuned by driving down the Bradford on Avon hill. If it Parped loudly all the way down to the town bridge the engine was in fine fettle, but if there was no Parp I knew it was time for a service.
I am curious to know what part of the service it was that restored the Parp when needed ?
Carb settings or new air filter maybe ?

Phil
Or perhaps the points being replaced restoring the ignition timing.
The points gap affects the ignition timing, smaller gap due to nylon heal wear retards the ignition.
Just a thought!
Regards John

MattsGPOvan
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Re: PARP - How does it happen?

Postby MattsGPOvan » Mon May 25, 2020 12:41 pm

We need a scientific sound test to hear where the sound is strongest.....

Is it near the engine? Design of the Cylinder head with Siamese exhaust port..

Is it near the Siilencer box ?.


Is it at the sharp tip of a small bore exhaust ?

Is it when the engine is cold or hot ?.


As the Morris Minor owners club we need to find out...... it’s the most distinctive exhaust note out there.😉👍

We’re all bored at moment we can do this even from home.

James k
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Re: PARP - How does it happen?

Postby James k » Mon May 25, 2020 6:40 pm

I imagine that the 'parp' is caused by a standing wave created in the exhaust system, effectively behaving as a brass instrument. Adding the chrome tailpiece moves the node about 6" further along and therefore 'tunes out' that resonant frequency.

King Kenny
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Re: PARP - How does it happen?

Postby King Kenny » Mon May 25, 2020 9:36 pm

pgp001 wrote:
Sun May 24, 2020 3:04 pm
King Kenny wrote:
Sun May 24, 2020 2:53 pm
I used to test how well my Traveller was tuned by driving down the Bradford on Avon hill. If it Parped loudly all the way down to the town bridge the engine was in fine fettle, but if there was no Parp I knew it was time for a service.
I am curious to know what part of the service it was that restored the Parp when needed ?
Carb settings or new air filter maybe ?

Phil
Usually just a general tune up, carb and timing. But to be fair this was often overlooked until the performance had dropped dramatically.

Monty-4
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Re: PARP - How does it happen?

Postby Monty-4 » Wed May 27, 2020 6:51 pm

This might be lockdown madness talking but - could one replace a larger bore performance exhaust with two parallel peashooters? A split at the bulkhead and install one with the silencer towards the rear instead of the front so they don't rattle against eachother.

Double PARP?
68' 4-door Saloon, another 'Monty'.


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