Compressoin ratios

How did they change compression ratios on the little engines, like the BE/CE/D10/12,138,149, and the D14/15s?

I know when they increased the bore the compression goes up, but there were different ratios with the same bore. Did they have different pistons? Heads?

For instance, how did they get 6.25 CR on the CA engine, when the CE (same bore 125 CI) had 5.75?

Thanks, Bill b

wbecker wrote: How did they change compression ratios on the little engines, like the BE/CE/D10/12,138,149, and the D14/15s? I know when they increased the bore the compression goes up, but there were different ratios with the same bore. Did they have different pistons? Heads? For instance, how did they get 6.25 CR on the CA engine, when the CE (same bore 125 CI) had 5.75? Thanks, Bill b

Larger pistons could very easily have a lower com ratio. The higher ratio comes from more of the piston above the wrist pin or less chamber in the head(milling the head off some). Most likely the original engines just had a different piston with the wrist pin farther away from the top.

Since the bore squares in the cubic inch displacement formula . In all cases bigger bores pull more air . more air compressed to the same height naturally means more compression. This is also why when evaluating performance cylinder head airflow advertised numbers one should take note of the flow bench bore size that advertised numbers were taken from.  A 260 cfm head @28 inches  at .600 lift on a 4 inch bore fixture is a better performer than a 260 cfm head @28 inches at .600 lift on a 4.25 inch bore fixture.

DaveKamp wrote: Since he didn't ask anything regarding airflow ratings, advertised numbers, flowbench bore sizes inches of draught or valves... Compression ratio is a very simple thing:  Initial volume (piston down, including volume of the chamber) and final volume (piston up, including volume of chamber). The difference between engines and CR was typically the result of either a piston design (Compression Height - wrist pin location) and the shape of the top of the piston (dish, dome, or flat), and the amount of volume found in the cylinder head... and a small amount granted by presence of the head gasket.  Another possible variation is the length of connecting rod, but it wasn't typical for Allis to change conrod lengths to alter compression ratios. Typically, compression-ratio variations WITHIN a model existed for fuel-tolerance issues.  Low-grade fuels (down to 82) and multifuel operation warranted lower CR,  while gaseous fuels (Propane) would work better at higher CR levels.  As a result, the compression ratio wasn't an 'option', but rather, would be varied to suit the fuel-choice options... and frequently, the fuel-choice options were a regional-market thing.

Seams like he stated in his post increasing bore size raised compression. Dont recall him asking about fuel though either..... Seams we all have our tangents dont it that we run off on . How does rod length increase compression ? Cam advancing or retarding will affect compression altitude etc.  

Edited by mlpankey - 26 Apr 2012 at 2:37pm

Different piston. The C's CA's and late B's all used the CE engine.

Edited by CTuckerNWIL - 27 Apr 2012 at 8:42pm

Yes, piston design.  Shallower 'dish' for dished pistons, taller dome for domed pistons, and 'compression height' (distance from piston top to wrist pin center) on all.

There was no appreciable cylinder head 'chamber volume' change, because Allis's valve-in-head design centered around a volumeless chamber... flat.  The 'hemispherical' chamber design was the result of dished pistons.

In answer to Panky's question:  Extending the connecting rod locates the piston's minimum and maximum travel farther UP the bore, which makes the 'final' volume smaller by the equivalent displacement of the extension distance.  Why he asked, is beyond me- it's not proprietary knowledge.


Edited by DaveKamp - 27 Apr 2012 at 9:45pm

HEY Rod B,Quit trying to start trouble on this site,go somewhere else!!

I think I'm going to look for taller boots or should I say ones that are longer at the top [over flow area ] in relation to the pivotal area [ ankle  ] or find a taller chair to reside in. Jeeze, all the guy wanted to know is what parts they used to change the compression.

OK I got tired of the debate and pulled out Dealers Repair Parts Catalog D-26 dated April 1955 and it shows same head and connecting rods. Some differences in crank, but no doubt not different in stroke since that would affect displacement.

Only variation significant to CR were Pistons and Head Gaskets - yes Head Gaskets. On early engines - mostly the 116 CI BE, but also used for the earliest 125 CI variant in the R tractor - they used 2 head gaskets to lower CR for Low Octane (Distillate) fuel.

And the differences in the pistons - as pointed out by another poster would be height from piston pin to top of piston and possibly piston top geometry.

Pistons for early CE Low Octane were 4.7:1 Cast Iron - these would be CE engines with K suffix

Pistons for early CE Low Octane High Altitude were 5.2:1 with both CI and AL variants
Pistons

Pistons for early CE gasoline engines in B & C tractors up to early 1950 were 5.7:1 Cast Iron - presumably these would be the G suffix engines

Pistons for CE gasoline engines in mid 1950 up to s/n CE153961/CR47264 were 5.7:1 Aluminum - presumably the GA suffix engines in early CA and some B tractors

Pistons for late CE gas engines beginning mid-1950 at CE153962/CR47265 were 6.2:1 Aluminum - presumably the PA suffix engines in late CA and B tractors

As for the reasons for CR selection, the Distillate required Low Compression as well as Tractors which did not have Electric Start. Remember some of the early B's were designed to operate on 40 Octane fuel. The early Gas BE engines were only 4.9:1 and the Distillate were 4.6:1. As gas became more consistently higher octane, it was possible to incrementally increase CR and benefit from the increased performance.

Best, Ed
  

Camshafts have NOTHING to do with Compression Ratios

And in looking back to the original question, I realize I failed to address the differences in Compression Ratio between the 125 CI CE engine and the larger displacement engines up to the D15.

I am NOT going to pull the research on the D10, D12, D14, D15 - but any time you change DISPLACEMENT everything starts from scratch when you change either Bore or Stroke - the 2 ways to increase displacement. Over time, as gas improved CR continued to increase with each new design.

But as far as I know there are only 4 ways to affect CR without changing displacement - the 2 noted before 1. spacing the head up with a double gasket or 2. changing piston geometry - or 3. changing connecting rod lengths or 4. changing volume in head. I don't remember anyone doing it with rods though that is possible, but others including MM frequently changed head volumes over time to effect CR changes.

Look back to original question - it was about standard CR as defined by AC and everyone else at the time including SAE - he asked about CR of 6.25:1 vs 5.75:1

As I said in my first post on this subject I'm tired of this BS - sorry I took the time to respond.

Best to all the sane members of this list. Ed

If you have a wd 45 and a fresh226 175 engine. And you cboose to install the 175 pistons in the wd 45 you will have identical static cr ratios now between the two but the wd 45 will not have as high a cylinder pressure reading as the 175 untill thw wd 45 cam comes out and a 175 cam goes in the 45 motor as well and there fore wouldnt run as well on a flywheel dyno

Edited by mlpankey - 28 Apr 2012 at 5:06pm

Didn't see that the opriginal poster asked for compression psi numbers.  He asked about ratios.  So pankey is answering the wrong question in trying to stat an argument.  As usual, he's arguing with himself.

Edited by JimD - 28 Apr 2012 at 6:32pm

You keep making my point.