Compression ratio vs engine speed?

So the thread on the AC page over at YTMag is trying to blow up over theoretical CR vs engine RPM.  One guy is saying that in a stock WD45 or D17 that 7.25:1 is it without running special gas or soemthing due to the low rated engine speed.  The 170 is 8:1 with a rated speed of 1650.  Dr Allis and others say that 8:1 is fine.  I know this is a pulling page, but how do CR and engine RPM relate?  Is there theoretical max ratios for given engine speeds?
AaronSEIA

maybe i'm wasteing my time, but its the pulling forum so I'll give a brief explanation of compression ratio related to speed.  Some say compression is king, well they haven't thought it through very far.

Compression is good, the higher the cylinder pressure is, the better the fuel mixture burns.  The higher the pressure is in a cylinder, the faster the flame will travel across the bore.  This is why a higher compression engine would need less ignition timing than a lower compression engine with all other things equill.

But a higher compression ratio has a faster expansion rate, meaning that when the cylinder fires and the gasses expand, the piston moves down the bore the piston is moving at a higher rate, or the bore volume is increasing faster than in a lower compression engine.

So we fire the engine at say 25* BTDC, the mixture starts to burn and it's still getting compressed.  Piston hits TDC and starts down the bore, the high pressure and rapidly burning mixture pushes the piston down.  The effective stroke length of the crankshaft isn't much though for the first few degrees of crank rotation,  At 90* of crank rotation for example a 6" stroke has a 3" leaver, or there's 3" of offset from the center of the crankpin to the center of the crankshaft.   But that mixture is done burning long before 90* ATDC or down in the power stroke.  A high compression ratio releases it's energy sooner and more violently than a lower compression ratio can.  And the leaver or stroke length at TDC is basicly 0.  So this high compression engine releases it's energy early in the power stroke, and it releases it faster and the cylinder volume expands faster as the piston comes down.

The lower compression ratio will "push" with less force but push for more degrees of crankshaft rotation than the higher ratio.  And the cylinder expands at a slower rate.

Now lets look at the fuel burn rate.  It burns faster in a higher pressure than in a lower pressure.  But more energy is released in a higher pressure cylinder.  So we need compression to get the energy released from the fuel.  In a high RPM engine we have less time to burn than in a lower RPM engine.  So a lower RPM engine would need higher compression to get the fuel burned in the same time as a higher RPM engine with all other things being equill,

Now we hear that more compression is better, jack the compression ratio up.  Well higher compression will release the available energy faster and it's great for an engine running at a higher speed.  In the antique tractor engines though we don't have much speed, RPM is low and our milliseconds to burn that mixture are longer.  A lower compression ratio will allow the "push" to continue for more time, but we still need more compression to get the cylinder pressure high enough to release that available energy.

A 14:1 compression ratio may be fine for an engine running 7000 RPM but it will destroy an engine running 1500 RPM.  The only way that some can get away with it in the antique tractor engines is because the induction system is so poor, the cylinders are just starved for air and not nearly as full as in an engine with a good induction system.

So if we built an engine that has to run low speed and starve it for air, we can get by with say a 12;1 compression ratio and run it slow.  But if we can figure out how to feed it efficeintly we can build it with a 10;1 ratio and have a better performing engine that is easier on it's parts, and the 10;1 will lug longer.

I run methanol in an engine, that doesn't quit burning.  It's pushing on that piston a long time and that engine will not go to sleep.  The power stroke is long compared to a faster burning fuel.  It's also very easy on the engine parts.  A stock crankshaft and light alumnium rods hold up fine.  Even the hardware holding the cylinder head on is nothing special.  It's got lower cylinder pressure than a diesel and pushes longer on the crank, that's how we make 3500 horsepower from a 505 cid engine.  The top fuel cars burning nitro can run less boost and less cylinder pressure and make over double the horsepower because nitro just doesn't stop pushing on that piston untill the exhaust valve is pushed into a high pressure cylinder.

yes that is a good explanation low octane gas is designed lower compressoin engines try this take your socks off soak one in regular unleaded and the other in race gas  the one in regular will go up and be gone before the match gets close the one in race gas will take a box of matches to get burned up 

Doc, What pistons give you 8.0 to 1 compression with a 4.5" stroke (226) engine?

Doc a is right and Allis Chalmers is right the higher the compression the lower the rpm peek torque will be even with the largest factory cam as allis did with the 175 cam. Higher torque and hp comes from compression wither it's from piston blower or turbo top fuel runs 65 psi of boost per professor Pankey

Edited by mgburchard - 10 Mar 2015 at 9:20am

Yep got to have good parts

All this time fooling with four cylinder engines I thought one piston was being pushed by the pressure wave of the explosion during the power stroke for the other pistons on intake compression and exhaust stroke to reach 1650 fpm. Also a six inch strokes pistons speed is identical to the rpm

Mitch is the MAN

You are learning blue when someones wrong and doc a and Mitch are right

wi50 wrote: ... If I have an engine that breaths well and build it to 10:1 compression ratio, I will have more cylinder pressure upon firing than a poor breathing 14:1 engine will.  The 10:1 has a slower expansion ratio and applies pressure to the crank longer than the 14:1 can.  If I were to build it at 8:1, it may be to weak, simply not enough compression to do an effective job, apply some boost though and see what happens. The same people that claim compression is king, are not getting their induction system right.  They are compensating for lack of air with excessive compression to try and get enough cylinder pressure to do the job, but the higher expansion rate and the inability of the piston to outrun the pressure wave just make for a bad combination.  The engines beat themselves up, don't last and really don't make any impressive power, the power band is also very narrow in comparison to one that is designed and built right.

Have you worked out a general "rule of thumb" in regards to expansion rate and piston speed?

I'm one of those who has used increases in compression to compensate for an induction system that just could not be modified to approach anything efficient without "forcing" the issue - which wasn't an option.

When there is nothing else left to do, it works - but has a practical limit. It does narrow everything: timing curve, fuel curve, and resulting power band.

mgburchard wrote: Be good if you had proof to back it up . The facts are at the mid Atlantic pull in pa last week .High compression engines won most the classes even in three mile an hour with a little high compression case build. To end this discussion Taylor and Taylor published a internal combustion book . That says as engine rpms increase flame propagation increase proportionaly Said this is why a spark advance can be all in at a low rpm and still be correct for 7000 rpm

That would depend on your definition of "high compression". But speaking of the pull results, be sure to congratulate yourself and crew on last and near last place finishes. When I checked the results it looks like the guys I work with put 30-70 feet on team spankey. I always thought the objective was to pull the furthest.

Your guy wi50 came over wanting help tuning his engine . i believe it was the engine out of your WC. Your guy cotton crazy wants a larger engine after performance at tunica . The high winding old WC engine 2100 rpms stayed in mile an hour classes didn't 15 mile an hour . Roger Ellwood's 420 inch WC runs good doesn't it.

Edited by mgburchard - 29 Mar 2015 at 9:25am

AC200Puller wrote: Lots of good information Wi50

yep roger elwwood shown everyone come with big cubic inches on the biggest tire and win . give up twenty cubic inches and one tire size and loose.

Edited by mgburchard - 29 Mar 2015 at 12:05pm

Everyone seemed happy with Pankeys stuff . The guy with WiFi stuff wanted some tips on timing his .Lawrence would have traveled further than us . Heres a question what mountain would the tractor have to roll down in road gear for the piston to out run the flame propogetion. I thought a pulling engine pulled on a flat track dragging the sled behind it and its flame propagation drove the piston down turning the crank to what ever rpm it can reach due to combustion .isn't that how internal combustion engines works. So what external force let's the piston out run the flame and wouldn't that external force be what's doing the work so the external force would be defined as the motor.Maybe in Wisconsin the motorized sled pushes the tractor making the piston over run its flame propagation. Lol

Edited by mgburchard - 29 Mar 2015 at 4:27pm

mgburchard wrote: Everyone seemed happy with Pankeys stuff . The guy with WiFi stuff wanted some tips on timing his .Lawrence would have traveled further than us . Heres a question what mountain would the tractor have to roll down in road gear for the piston to out run the flame propogetion. I thought a pulling engine pulled on a flat track dragging the sled behind it and its flame propagation drove the piston down turning the crank to what ever rpm it can reach due to combustion .isn't that how internal combustion engines works. So what external force let's the piston out run the flame and wouldn't that external force be what's doing the work so the external force would be defined as the motor.Maybe in Wisconsin the motorized sled pushes the tractor making the piston over run its flame propagation. Lol

You need some geography lessons too?  You have it backwards.  More lies.

I wouldn't be happy with last place and next to last place finishes, but if you say they're happy, then I suppose they're happy.  Happy it didn't blow up, but that's about it.

Anyways go back and read what Hudsonator and I are trying to explain, you seam to have that backwards also.  I'll dumb it down to a level I hope you can comprehend. Problems occur when the piston doesn't get away from the "bomb"