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mis matched cylinder size engine
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BFB
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PostPosted: Sun Aug 01, 2021 4:36 pm    Post subject: mis matched cylinder size engine Reply with quote

anyone actually built an engine with mismatched sized cylinders? like 2 94mm and 2 85.5's..
ive read its possible as long as opposing cylinders are the same but cant say ive heard anyone actually did it
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CR MAD MAX BUGGY
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PostPosted: Sun Aug 01, 2021 7:09 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

Why in hell would somebody do that?
What is the benefit?

Sounds extremely stupid to me…. Just match the cylinders.
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PostPosted: Sun Aug 01, 2021 7:39 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

I have built one before - I was curious. I intended to run it in a lawn tractor.

Just think of it as two - 2 cylinder engines. Very Happy

I balanced the parts as per usual process. Motor runs fine. Try it man.
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PostPosted: Sun Aug 01, 2021 7:48 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

I have wondered if it would help smooth out the poor idle imbalance problem on single port engines. Not sure if the larger bore would go on the leading or lagging cylinder...
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PostPosted: Mon Aug 02, 2021 4:53 am    Post subject: Re: mis matched cylinder size engine Reply with quote

I have a goofy spare parts motor.
At one time it had 3 x 90mm NPR's and 1 x 90.5 Cima.

It worked....
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PostPosted: Mon Aug 02, 2021 5:57 am    Post subject: Re: mis matched cylinder size engine Reply with quote

I want a weird cam, and head modded, so it's two 77mm cylinders being supercharged by two 94mm pistons.
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UK Luke 72
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PostPosted: Mon Aug 02, 2021 6:04 am    Post subject: Re: mis matched cylinder size engine Reply with quote

FreeBug wrote:
I want a weird cam, and head modded, so it's two 77mm cylinders being supercharged by two 94mm pistons.


VW tried that, had heat issues they couldn't fix.
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PostPosted: Mon Aug 02, 2021 1:03 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

UK Luke 72 wrote:
FreeBug wrote:
I want a weird cam, and head modded, so it's two 77mm cylinders being supercharged by two 94mm pistons.


VW tried that, had heat issues they couldn't fix.


Are you talking about the different prototypes they worked on before settling on a flat-4? Please....tell us more!
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PostPosted: Mon Aug 02, 2021 1:08 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

And just because the hundreds of engineers at VW couldn't solve the heat issues, doesn't mean that I'm not going to get better results with just a ball-peen hammer and an adjustable wrench, in my basement garage...I think we all think that way sometimes...no? Laughing
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PostPosted: Mon Aug 02, 2021 2:10 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

Ive seen world reckord holding class drag v8's with different size cylinder and different strokes.... and headers made so you cant pull the plugs on the big ci cylinders.
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UK Luke 72
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PostPosted: Mon Aug 02, 2021 2:14 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

FreeBug wrote:
UK Luke 72 wrote:
FreeBug wrote:
I want a weird cam, and head modded, so it's two 77mm cylinders being supercharged by two 94mm pistons.


VW tried that, had heat issues they couldn't fix.


Are you talking about the different prototypes they worked on before settling on a flat-4? Please....tell us more!


I would love to be able to... no idea where I read about it but yes, I do believe you are correct about it being one of the prototypes.
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PostPosted: Mon Aug 02, 2021 3:52 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

UK Luke 72 wrote:
I would love to be able to... no idea where I read about it but yes, I do believe you are correct about it being one of the prototypes.


I think they were experimenting with 2-strokes, too. Maybe a piston-supercharged 2-stroke? I think NSU had made something along those lines: 2 smaller combustion cylinders, being supercharged by one big piston. There's probably about 10 reasons no one is doing that today.

I did open an engine where there was a dish on some pistons, others not. So, different C/R. I did not see the engine run, but it had. Vaguely on topic.
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BFB
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PostPosted: Mon Aug 02, 2021 9:28 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

i think i read something about using one cylinder to " supercharge " another and it didn't work because the supercharging cylinder couldn't move enough cfm to feed the other cylinder efficiently.
think of it like a shops reciprocating air compressor, high psi but not really alot of cfm. where as a shop with a screw compressor can put out alot more cfm.
at least thats how i remember it being explained..
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Build it your way not the way someone else does it. Their way might not work for you." - clonebug
An interesting thing happens in forums where everyone starts parroting the same thing and "common knowledge" takes over.
“ The monkey see monkey do mentality seems to run deep in VW people. "Gene Berg said it was so 30 years ago so thats the way it is" “ - bdkw1
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PostPosted: Mon Aug 02, 2021 10:08 pm    Post subject: Re: mis matched cylinder size engine Reply with quote

In the 70's in the Bronx my dad and brother and I built station cars, bugs that only were needed to get someone to a train or bus station or commuter parking lot.

No need for the for heat, the engine would not have time to heat up. They just wanted something that got them to the station and started in the cold. Straight 30 weight oil did not help.

We never split cases, had no clue, we got the engines from junk yards, and their were bugs as far as you could see.

Mostly bad heads or pistons. We just slap the junk yard head in, so what if the combustion chambers were different sizes or pistons of different sizes???

If they started we did the job. Buy a bug for $50 and sell it for $200. They started and got them to Stations. Never a return. Customer did not care if the idle was bad, as long as it started. Unlike cars made at that time in the U.S. In NY cold and snow.

BTW forget about what the bodies looked like, amazing to see a fender that was not dented. No one cared as long as it started.
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PostPosted: Tue Aug 03, 2021 5:54 am    Post subject: Re: mis matched cylinder size engine Reply with quote

Or a mind bending "split-piston" 2 stroke
...
http://anarchadia.blogspot.com/2008/03/vintage-thing-41-trojan-engine.html
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PostPosted: Tue Aug 03, 2021 5:59 am    Post subject: Re: mis matched cylinder size engine Reply with quote

On the subject of mismatched cylinder sizes it would not run any different than a used engine with uneven compression.

The following however is a totally different subject that has been discussed before.

UK Luke 72 wrote:
FreeBug wrote:
UK Luke 72 wrote:
FreeBug wrote:
I want a weird cam, and head modded, so it's two 77mm cylinders being supercharged by two 94mm pistons.


VW tried that, had heat issues they couldn't fix.


Are you talking about the different prototypes they worked on before settling on a flat-4? Please....tell us more!


I would love to be able to... no idea where I read about it but yes, I do believe you are correct about it being one of the prototypes.

FreeBug wrote:
UK Luke 72 wrote:
I would love to be able to... no idea where I read about it but yes, I do believe you are correct about it being one of the prototypes.


I think they were experimenting with 2-strokes, too. Maybe a piston-supercharged 2-stroke? I think NSU had made something along those lines: 2 smaller combustion cylinders, being supercharged by one big piston. There's probably about 10 reasons no one is doing that today.

I did open an engine where there was a dish on some pistons, others not. So, different C/R. I did not see the engine run, but it had. Vaguely on topic.

richardcraineum wrote:
i think i read something about using one cylinder to " supercharge " another and it didn't work because the supercharging cylinder couldn't move enough cfm to feed the other cylinder efficiently.
think of it like a shops reciprocating air compressor, high psi but not really alot of cfm. where as a shop with a screw compressor can put out alot more cfm.
at least thats how i remember it being explained..

Let me start by posting a picture of an engine that I posted last time this was discussed. It uses the bottom side of the piston and the crankcase to compress the fuel/air charge for the combustion chamber.

Image may have been reduced in size. Click image to view fullscreen.

So what you have here is 2 strokes of the piston of the same displacement compressing a charge for a 4 stroke engine. The charge pressures in the intake do not get that high either, just a few pounds at best but it does make this engine about 15% more powerful than it's N/A competitors.

So yes look at an air compressor and you will see that the charge cylinder is three times the size of the pressure cylinder. That is the balance that has to be struck for there to be any efficiency in this type of process. In a VW air cooled engine even with the biggest bore you can put in the case and heads and 40 HP cylinders you would be nowhere near that balance.

The next step would be to change the charge cylinders to 2 stroke action and leave the power cylinders 4 stroke. Another way to do it would be to split the crankshaft and gear up the charge cylinders up 3/1 (think supercharger here and you have the picture!) Then you would need to cool the intake charge between these stages... Now you should have a workable volume balance but in reality you have an engine with 1/2 or less displacement of a regular VW engine that with this supercharging effect could possible reach the same power levels as a stock untouched engine.

So the whole exercise would in reality be just an academic novelty.
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