Compression ratio vs engine speed?

wi50 wrote: I've just been to busy to spend much time on here, but Hudsonator is exactly right.  It's easy to get the pressure wave from combustion to outrun the piston.  Or in a sense set off a bomb in the cylinder compared to a push.

I'll make it easy for ya pank. This is what you're hung up on, and you have it backwards when you say that I say the piston can outrun the flame.....which doesn't happen.

What I'm explaining here is on the order of detonation.... you know that phenomenon that destroyed your engine.

Butch watch the YouTube videos blowing the tires off is not a sign of lack of HP or poorly tuned engine.

wi50 wrote: I'll give up 10% on engine size for the ability to run better rod to stroke ratio and fill the cylinders to a higher %.

Rod ratio is the most abused, overlooked, yet critical elements to effectively using the rest of one's numbers in the CID equation.

I also default to lower cubes when either livability or effectiveness start to go away. Big numbers are one thing, making them work effectively is another.

Interesting stuff.

Spent a long evening at cotncrazy's yesterday picking up parts for the working fleet. Talking to him and his Dad made me remember how much I enjoyed pulling, and why.

mgburchard wrote: Oh so its a problem that occurs in.long rod engines that have increased dwell at tdc . I see never had that problem with the big inch engines rods are short. The high rving engine finished better in the 15 mile an hour classes . The big 420 inch won the class. No aluminumed up d17 in those classes but I see why the guy needs help timing it now

I know ac farmer. But don't you want to see what he will make up next? Now it's "aluminumed up" d17 and the guy needs help timing it...... well I said before there was only 1 d17 at the pull....and it certainly wasn't "alumed up". It did however beat on pankeys boys pretty bad, and it's got a bunch of bone stock parts....our troll never even went to the pull, he's just lying. Good for Sunday entertainment, but not much use for any technical help.

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Oh so its a problem that occurs in.long rod engines that have increased dwell at tdc . I see never had that problem with the big inch engines rods are short. The high rving engine finished better in the 15 mile an hour classes . The big 420 inch won the class. No aluminumed up d17 in those classes but I see why the guy needs help timing it now

Edited by mgburchard - 29 Mar 2015 at 6:02pm

Roger with the 420 chic inch WC had the biggest engine and biggest tire and walked away with the wins no flame propagation problems for him

mgburchard wrote: 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.

I see.  Last week my old WC engine was in a D-17 out there you said, you were looking for video of it.  Now there was only one D-17 at the pull and it had a D-17 engine in it, that's the truth.  That guy is running a stock cylinder head, stock manifold and carb on an engine I sold him a few parts for.  I see he managed to put 30 feet on your crew.....that has to bee a big kick in the nuts.  All pankey's knowledge and someone with mostly a stock engine beats up on it by 30 feet.  It's not suprising, but it is funny. 

Today you say you were supposed to help tune it.  So this must mean that you seen it..... are you unable to tell the difference between a D-17 and a WC?  It appears so.  You're a bit on the slow side to catch on here (as always).  How would a fella run an engine like that at low RPM and in a 3 mph paced class?  I'm just curious, as it would be useless.  It would work well however in the 15 mph class.  After you've been collecting pictures of it for 5 years and obsessing over it, you must be bright enough to realize that it would not even light at a low RPM or speed.......

The lies continue, I see you have not learned anything.  But I'll let you in on the truth.  That engine is here in the tractor.

But it's nice to know you drove 9 hrs to watch a pull.  You avoided disapointment by watching and not participating.  That I'll give you credit for.  I hope you got a chance to stop by and check out my cylinder head on display at the mid atlantic super pull. Friends of mine were there with one on display and talking to people. Lawrence and Richard mentioned they were approached by some guys with an AC that were not happy with it's performance.  I think they said something about Ac nightmare or morison or something to that order.  I see the results from the pull confirm a lack of performance.  Lawrence is pretty sharp with engines though, I'm sure he can help them out.  If not I've got cylinder heads, camshafts, etc. on hand and can ship out shortly.


Edited by wi50 - 29 Mar 2015 at 4:10pm

Lots of good information Wi50

And the pankey sags continues. Don't worry folks when he's done licking his wounds he'll be back with more make believe and science fiction.

Anyways hudsonator. I'll add a little compression to the big engines to make up for their inability to fill the cylinder if they are limited by rules on certain components. But a big empty cylinder coupled with a faster expanding cylinder only result in an engine with a narrow power band. Generally they run the same gear ratio, and of course a better managed engine will maintain more rpm when both are already starved for air. 9 times out of 10 you are better off with a little smaller and better managed engine......and the results pankey is referring to show it. I'll give up 10% on engine size for the ability to run better rod to stroke ratio and fill the cylinders to a higher %.

Edited by wi50 - 29 Mar 2015 at 8:56am

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

Edited by mgburchard - 28 Mar 2015 at 6:46pm

I've just been to busy to spend much time on here, but Hudsonator is exactly right.  It's easy to get the pressure wave from combustion to outrun the piston.  Or in a sense set off a bomb in the cylinder compared to a push.  Next is the expansion ratio, the higher compression ratio has a higher expansion ratio so it dumps it's pressure faster, that is good....but not in the case of a low rpm engine or one that needs to make big torque numbers at low speed.  In this case we want to push on the crankshaft a bit longer, the longer we push, the more effective stroke length gets used.  Or if I dumb it down for a certain crowd, "the leaver gets longer".

We have a certain amount of time to push on the piston, in a 4" stroke engine that makes great power at just say 5000 RPM we have a mean effective piston speed of say 3333 feet per minute.  We have a verry brief amount of combustion time pushing on the piston.  The "bomb" can explode and be contained while being effective.  Lets apply that same time to a 6" stroke engine, like we may build for the tractor pulling engine.  We would have to run that engine 3333 RPM in order to get the same time / pressure applied, except for the connecting rod to stroke ratio plays a little into this, as the poorer the ratio, the quicker the piston will acclerate.  Again it will have negative effects and positive effects.

The people who will sit back and preach that more compression is better are not the people out there doing well and winning.  They are the people who are still trying to figure things out, some of them do and some of them never will.  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.

dont do it boys, i checked his profile he is sittin watching this topic waiting for someone to take the bait.

Dr a runs them at 13 /1 read his post on y t

Yep got to have good parts

I always looked at the situation from a feet per minute (fpm) or feet per second (fps) in relation to the burn rate and expansion front.

I didn't want the speed of the piston to be "outrunning" the pressure wave of the burn. Its entirely possible to "outrun" your pressure wave near the bottom of the stroke and create a "negative" pressure that offsets some of the positive pressure of the burn. Slight? Yes. But any decrease in the force applied to the "leverage" Marty outlined, is a loss - plain and simple. However, your fpm has to begin to exceed the rate of burn/expansion to see this loss.

Its also possible to "under-run" your pressure wave - which is what Marty explained.

Longer stroke engines have a faster fpm than a shorter stroke engine at the same rpm.

WD 4" stroke vs WD45 4.5" stroke is classic.

FPM = 2(stroke in feet x rpm)

4" stroke @ 1650 rpm = 1100 fpm
4.5" stroke @ 1650 = 1237.5 fpm

Now, lets compare a 6" @ 1650 = 1650 fpm

I'm not in any way arguing with Marty, just enhancing his contribution. This is why excessive high compression and efficient induction in a low speed engine kills it - the low piston speed and inadequate cylinder volume per force basicly creates a pipe bomb in the cylinder. When the force exceeds the ability of the rod, rod journal, and/or piston to contain it - breakage occurs.

I've seen a bigger problem in antique tractor engines from the "pipe bomb" effect than I have a loss of power due to compression. In high rpm engines, I've seen the opposite or "outrunning" effect put a cap on Hp because the piston speed is moving faster than the pressure wave.