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Model 20 Compression


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Hi All,

 

Just wondering what people thought was a good cylinder compression reading (PSI) for a Model 20.  I am getting 55 PSI on 1,2 & 3 and 25 on No.4.  Obviously there is a problem with 4.  I did a leak down test and it looks like rings are not seating.

 

Any thoughts appreciated.

 

Cheers, Andrew.

Edited by 1912Minerva (see edit history)
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Atmospheric pressure time compression ratio gives a good estimate of compression readings.  Obviously number 4 needs some attention.  Even readings are more important than how high the number is.

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I wonder how they figure compression ratio. Atmospheric pressure is 15, but if the pressure in the cylinder is 60 that's not really 4 to 1, because the 60 is a gauge pressure not the real pressure. So, with a 60 gauge pressure, its real pressure of 75, which would be a 5 to 1 ratio. If you use gauge pressures, you are comparing 60 to 0 which is impossible. Gauge pressure of the atmosphere is 0.

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Compression ratio is simply swept volume (pi x bore radius squared x stroke), over combustion chamber volume. Compression pressure is another matter. Dr. Spinneyhilll posted the more complex formula some several months ago. It does take into account your observations, but is more involved. Volumetric efficiency (in this case think cam profile/duration rather than gas fluid dynamics), is a factor. Let me see if I can conjure up an appropriate key word to put into the search box in order to bring up the posting. I think I will try "atmospheric". Stand by.........   -    CC 

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Yes, that got it. I was trying to bring the topic back from its last contribution (June 3rd, 2018), with a comment regarding volumetric efficiency. There is a statement in the yellow slide show box claiming compression pressure drops with increasing rpm due to lessening intake charge from reduced pressure (partial vacuum) behind the fast moving piston on intake stroke. This is not categorically true. As we know, volumetric efficiency at speed depends on  intake and exhaust tuning (sonic effects), along with valve timing and scavenging at overlap. It can and does exceed 100% on some normally aspirated engines at higher rpms. Anyway, page 2 near the top as accessed by "atmospheric" in the search box. I kept getting tripped up, so I had to resort to this cumbersome technique. I am generally patient to a fault, but I am somewhat short fused when being second guessed by the high speed moron (my iPad mini with which I have a love/hate relationship), I am getting tired of diddling with my somewhat arthritic thumbs.   A little grouchy, think I'll go do something else for a while. By the way, Morgan, you were a contributor to the referenced posting regarding a 1933 Buick. Compression ratio, compression pressure : two different things.   -   Carl 

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I'm not sure that the  actual figures are  important in a Model 20 . Lets face it  they don't go very fast  and your aren't  going to improve that by increasing the CR by planning the  the head  cause you don't have one . What is significant is  difference between the figures . As in Andrew's case they are an indication of something not  right on the out of whack pot . Sounds like a valve grind  is in order . I always repeat the  test with some thick oil  down the offending pot to  check its not a ring problem . A vacuum gauge is also a useful  diagnostic  tool  but  no facility to attach one on the Hupmobile . I find a  timing light to be quite useful  I use a  standard timing light which I run off  a  12 Volt battery and use Lead one  and  the  markings on the  flywheel and block to see where we are at.   Certainly on my car  if set up by the  book  ie slightly retarded after TDC it performs  and starts much worse than  if  i adjust the  ignition to about 15 to 20 degrees advanced 

 

 

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Well yours may  not  go  very  fast  but  in 1995  on  a  Hupp tour in  Ohio,  I  followed  Ed  Kiley with  his  1909  Model  20   between  45  and  50 Mph.  Now  thats  hauling  A_ _.

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I've had mine up to 40-45 mph   so they can get up there  -but certainly wasn't fun with the steering and lack of shocks   and I could feel I was thrashing it - I found myself asking myself why am I doing this and how would I  feel  when I blew it up - so I backed off. Mostly the max I get up to is about 30 mph and at that speed I figure  that I  might get some longevity out of   my engine.  I drive mine frequently but  only short distances so the  difference in travelling at 30mph and 45mph  is about  2 mins when you get there .  I do hear  of Model 20's  achieving  up to   50 mph ( Edgars , Eds and Davids  off the top of my head ) and while I'm sure they do achieve this speed I don't think they represent the  vast  majority of Model 20's  . The technical adviser for the Model 20  Bill Cuthbert once told me that in his opinion  any  Model 20  that gets to 30 mph is a good one  and  one that gets to 50mph is  on a trailer ! LOL 

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I've had mine over 40, it does get a little busy with the steering being so quick, if you sneezed while doing it you'd be out in the field next to the road.....

 

I think it's a combination of condition and which rear end one has in a 20, either 3.50 or 4.00.  My best friend up here used to own a worn out Model 20 (which subsequently was restored to the nth degree and at one point sold for 80K or so).  We were driving mine, long after he'd sold his, we came to a hill and he said good luck getting up that one.  My car just loped along, slowing some but easily climbing the hill mentioned. I will admit I had a good head start.  His comment was similar to "it's a good one".

 

If Ed Kiley's car is the one I'm thinking of, I've seen it on a tour, and it is HIGHLY modified, from light weight flywheel to different transmission, and so forth.  Not that there's anything wrong with that, but it's not a true representation of Model 20 performance.

 

The sweet spot on my car seems to be around 35 mph, and out in the open as you sit in the car, that's plenty fast!  I have a windshield, but not on the car, and of course mine is a 1910 with no doors.  I once drove the car to work and my then secretary walked out, looked at the Hupp, and said "David, it's a shame you can't afford a whole car....."

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No,  Eds  was  not  modified,  you  are  thinking  of  one  of the  Hupp  club  members  that  put  a different  engine  and  a 3  speed  transmission in  his  Model  20.

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23 minutes ago, Hupp36 said:

No,  Eds  was  not  modified,  you  are  thinking  of  one  of the  Hupp  club  members  that  put  a different  engine  and  a 3  speed  transmission in  his  Model  20.

Ok thanks for correction.   A stock Model 20 would be scary at 50, I'd think.  

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I agree David rear end ratio is crucial . I'm not sure  what ratio I have in mine . I suspect it is 1: 3.63  as  I believe that  my diff internals  are Model T and that is the standard  T rear end .

What I need to do is  jack up the rear end and in top  gear  turn the engine over  once  and  count the rear  wheel revolutions .  I know that the  Model T had an optional  1:3.10 ring gear which increased  speed  by about 7 mph Not sure a good idea if you want to climb any hills . 

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We did not  travel those  speeds  very  long. The  road  in Ohio we  were  on  was  very straight and  flat  with  no  turns. Ed  had  driven his  Model  20  many  miles and  was   a very  experienced driver.

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I know I've been over 40 in my Hupp, my wife was following behind and told me speed.  It definitely has to be a flat road, wind at your back doesn't hurt either!

 

I'm running a stock ring and pinion, 3.50 ratio.  As to the Model T innards, at one point my spider gears chewed themselves up, and I was devastated since had no idea how I was going to find another set.  I started looking at early Model T spider gears and noticed how close they were in size. Ordered a set, and I don't remember specifics, but one minor modification and they slipped right in and worked perfectly.

 

The only other thing scary about that speed in a 20 is the stopping, not so much the brakes, but if one hits the brakes and forgets to hit the clutch.  At that speed, the momentum of that big flywheel on the front of the crankshaft can do really bad things.

 

 

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Interestingly I took my car for  a run this afternoon  despite it being  in the middle of winter here  . In light of this  thread I opened it up and  30 mph was about it . Pondering on this I took  particular note of  where the  throttle arm on the Breeze was at full throttle  and it appears that  the throttle plate is only about  2/3 open at full throttle . Further inspection revealed that there is  about 4 .5 cm  of movement of the  throttle lever on the carb from closed to  fully open  throttle  plate in the X plane  . However   fully depressed  gas pedal only moves the  gas pedal carb linkage   open 3cm  in the X plane  ie with the present set up  I'm never going to get fully open throttle plate in the carb.  The solution is to make a shorter lever  between the throttle plate and the  gas pedal linkage  Which I will get onto on the weekend .  The 40-45 mph I have had previously was with the Holly  NH set up -I wonder if the getting the throttle plate right will make a difference -I suspect so !

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