Topic: COMPRESSION RATIOS

[paulages]

is there something unique about the design of these or all air-cooled motorcycle engines that allows for higher compression ratios with pump gas? i'm fairly certain that the pistons for my 718cc project will have far to high a ratio right now, meaning that i'll be machining the piston crowns to suit whatever numbers i want. MReick suggested i should go for 10.25/1- 10.5/1, but dan (the machinist) thinks i won't be able to run street gas on anything over 10/1 (i believe the 650 is close to that as is correction: the 650 runs 9:1). though i see plenty of old vincent, guzzi, triumph, etc. parts laying in his shelves waiting to be worked on, he primarily works on car engines.

in the end, he'll do whatever i ask him to do, but i do trust the advice of a guy that's been building and racing since the 50's. insight anyone?

[bwaller]

Paul, although I'm not qualified to answer your question exactly I'll tell you that this 592cc engine I built ended up with a CR of 10.1 to 1. I've been using more premium gas and absolutely no sign of detonation, so I fiqure Mike has it right.....again!

[Pinhead]

The small size of the combustion chamber coupled with the high-RPM nature of these engines allow them to run a little higher compression than comparable car engines. The "big" Hemi had trouble with detonation due to the huge, open nature of the combustion chamber. We're using the same design, but in a MUCH smaller package. Smaller chamber = faster burn = less detonation.

[paulages]

gotta remeasure tomorrow to make sure, but it seems to be sitting at 10.88/1 right now.

[TwoTired]

The cam grind comes into play when calculating effective comp ratios, as compression isn't made while the valves are open.  What part of the comp stroke is made with the valves closed?

Have you CC'd your cylinder head?  Do you have a burette?

[MRieck]

The cam grind comes into play when calculating effective comp ratios, as compression isn't made while the valves are open.  What part of the comp stroke is made with the valves closed?

Have you CC'd your cylinder head?  Do you have a burette?

Yes...overlap effects the ratio.

[paulages]

The cam grind comes into play when calculating effective comp ratios, as compression isn't made while the valves are open.  What part of the comp stroke is made with the valves closed?

Have you CC'd your cylinder head?  Do you have a burette?

i haven't cc'd the cylinder head individually, only the assembled chamber at TDC and BDC. i'm not exactly sure how this would be a useful figure unless the pistons were flat- topped.

a true burette? no. the medical suppliers were all closed today, so i had to go with a 10ml syringe, but it measures with decent accuracy. the only problem is that it takes a little while to fill at BDC with 10ml doses. as i said before the difficult part is in the last 2-3cc's at the top of the combustion chamber.

i have no idea how to compensate for valve overlap when calculating the CR. seems like valve and port design would play a big role in finding that, as a high-velocity intake should still be breathing in as the piston begins rising on the compression stroke. if none of you guys know how do find this variable, i'm sure dan will have some answers.

TT- you always seem to be an idea guy, got any suggestions there?

[TwoTired]

I don't know your cam specs.  But, the CB 550 cam closes the intake valve 35 degrees ABDC.  Therefore, the compression doesn't begin until the crankshaft pushes the piston up in the bore by this amount.  Position the crank at BDC. Note measurments from dome to cylinder deck.  Move the crank to 35 degrees ABDC.  Measure the dome to deck height again.  Your effective compression stroke is the difference. And, knowing the bore, you can now calculate the compression stroke volume.

I assume your domed piston stands proud of the cylinder deck.  Measure the dome peak to cylinder deck height.
You can calculate a cylinder volume using that height and your bore measurements.
Push the piston down the bore so the dome peak is the same as the deck height.
seal the piston to the wall with grease.  And fill the cylinder with fluid until it has reached deck height.  Note the volume it took to fill.  And subtract that from your calculated volume for a cylinder without the dome.  You now have your piston dome volume.

Now, if you cc the head chamber and add area for your head gasket, you will know the volume of the chamber when the piston is at TDC.

With compression stroke volume, and chamber volume at TDC known, you can calculate the static CR.

This will not account for inlet pressure boost from intake runner air velocity inertia, or supercharging/turbo charging inlet air compression.

Cheers,

[bwaller]

Not to run over paulages thread, but thanks for that TT. I'm sitting here wondering why I couldn't have fiqured that out for myself while I went through this process.

Somebody has to keep us honest, huh?

[paulages]

Not to run over paulages thread, but thanks for that TT. I'm sitting here wondering why I couldn't have fiqured that out for myself while I went through this process.

Somebody has to keep us honest, huh?

thanks, TT. sometime for your thoroughness in response i wonder how thoroughly you read the prior posts.  i have already measured and determined the cylinder volume at TDC and BDC. your suggestion for determining when effective compression begins is good though, and i'll give it a whirl. your last sentence of that is what i think makes it something of a wild card though: there's no way of determining the point at which compression takes over the intake's inertia. theoretically, if the charge is still filling the cylinder as the piston is rising on the intake stroke, this would actually increase the compression except that of course when it does catch up it forces some of the mixture back out the valve. 35 degrees would make for a huge difference in the compression ratio if it was considered all a loss. everywhere i've ever read about determining compression ratios does so while ignoring the valve overlap variable. this of course doesn't mean it doesn't exist..



bwaller- a quick google search will turn up all kinds of pages describing formulas for calculating the displacement based on the methods TT is describing-- measuring all of the components and adding them together...

OR you can simply do what i did, which is to measure the actual volume by filling the cylinder at TDC and BDC. same concept minus the theoretical aspect.

[paulages]

if this isn't the page TT got his info from, it's close enough if anyone casually reading wants to catch up to speed ( do note that though much of the language is very similar to what TT was using it is mostly talking about 2 strokes):
http://72.14.253.104/search?q=cache:w7_5gPZBTrIJ:www.sacoriver.net/~red/uccr.html+measuring+displacement+head+volume+engine&hl=en&ct=clnk&cd=2&gl=us&client=firefox-a

okay, so i talked to dan. there are a couple of wrong concepts going on here.

first of all, he said that valve overlap is never taken into consideration when he is determining the CR. you simply determine volume at BDC and divide it by volume at TDC. i realize that there are different schools of thought with this, but he has good reasoning. while there is no way to determine the efficiency of the intake in this overlap, it does increase with high RPM, which our engines tend to like (well, i do anyway  ). if i ignore the corrected method, then i would come up with an actual CR lower than expected in this scenario. though this is obviously undesirable, it seems safer than calculating based on full valve closure, only to attain an intake efficiency at 10K rpm that pushes my CR far too high. if i started at 35 degrees, for example, my CR drops to 8.4/1. if i base my CR on that 35 degrees and it's wrong, i could end up with a ratio that won't even run on street gas (12.4/1).
   
i imagine when you get into really fine race tuning (like i said he's been building and racing since the 50's) where you want the CR as optimal as possible there must be some fluid dynamics formulas to determine the exact moment (at what RPM?) when the compression stroke catches up with the intake velocity in a particular port design. if someone comes across some let me know (i don't have time to google myself). in the meantime, i guess i'll trust my local expert.

second of all, he corrected my displacement figures as apparently the head volume is not considered at all. he says bore x stroke is it. back to 718cc..    


that said, i am still trusting mike here and going for a CR of 10.5/1. if my trapping efficiency is poor, then i'll still at least have an engine that will run on street gas. i like ideas, though! keep googling!

[Sam Green Racing]

Paul, we run the CR836 on 10.25 to 1 on pump fuel with no problems.

Sam.

[paulages]

Paul, we run the CR836 on 10.25 to 1 on pump fuel with no problems.

Sam.

thanks, sam. good to know.

[TwoTired]

thanks, TT. sometime for your thoroughness in response i wonder how thoroughly you read the prior posts. 

 I read the prior posts in this thread, only.

i have already measured and determined the cylinder volume at TDC and BDC. your suggestion for determining when effective compression begins is good though, and i'll give it a whirl. your last sentence of that is what i think makes it something of a wild card though: there's no way of determining the point at which compression takes over the intake's inertia. theoretically, if the charge is still filling the cylinder as the piston is rising on the intake stroke, this would actually increase the compression except that of course when it does catch up it forces some of the mixture back out the valve. 35 degrees would make for a huge difference in the compression ratio if it was considered all a loss. everywhere i've ever read about determining compression ratios does so while ignoring the valve overlap variable. this of course doesn't mean it doesn't exist.

Oh,it is much worse than that.  Valve overlap means that both exhaust and intake are open at the same time.  Do you think part of the exhaust pulse doesn't reflect back into the intake runner and effect it's inertia?  This, of course, is a variable with RPM.

OR you can simply do what i did, which is to measure the actual volume by filling the cylinder at TDC and BDC. same concept minus the theoretical aspect.

In engineering, you prove your concepts with actual measurements.  It's what verifies your understanding of how it works, and lends credence and predictability to your design success.

if this isn't the page TT got his info from, it's close enough if anyone casually reading wants to catch up to speed ( do note that though much of the language is very similar to what TT was using it is mostly talking about 2 strokes):
http://72.14.253.104/search?q=cache:w7_5gPZBTrIJ:www.sacoriver.net/~red/uccr.html+measuring+displacement+head+volume+engine&hl=en&ct=clnk&cd=2&gl=us&client=firefox-a

I didn't get my info from ANY web site.  What I wrote was from reading 30-40 years of engine build books, build articles, and a rudimentary understanding of physics and solid (liquid) modeling (and a bit of sleep deprivation).

okay, so i talked to dan. there are a couple of wrong concepts going on here.

first of all, he said that valve overlap is never taken into consideration when he is determining the CR.

I disagree.  The concept is NOT wrong.  It simply doesn't fit into his methodology.  I don't know him,  But, perhaps his method is based on experience and empirical testing.  This is not a scientific approach, per se, which is what I thought you were seeking.  This is not to say that Dan's approach is ineffective.  But, its based on skill and craft (art) rather than proven science.  All the end result predictability rests in Dan's abilities.   You either hope and trust, or not.  But, if you ask him, I suspect his art was paid for by several broken engines.

in the meantime, i guess i'll trust my local expert.

You might also learn from him, if he'll allow it and is capable of tutelage.  But, you may be learning art rather than science.

second of all, he corrected my displacement figures as apparently the head volume is not considered at all. he says bore x stroke is it. back to 718cc..    

Yes, displacement is always the volume displaced by piston movement alone.  When was that definition in question?


One thing I omitted in my previous write up for static comp ratio, is that the ratio is actually the valves closed piston displacement PLUS the squish area (the chamber volume minus the dome vol) to just the squish area.
This is still the true comp ratio of the engine cylinder.  The actual cylinder pressure peak calculation will depend on the air pressure existent when the intake valve closes.  Now you know why manifold pressure (negative or positive) play a significant role in engine power management.  Were you to model engine characteristics on the computer, these would be basic, essential factors in the computer model.

Best of luck with your engine!

[bryanj]

When I did my marine appenticship we were told to calculate ignoring the valve timing loss I.E. just work out the theroretical volume to volume ratio.
now if Honda do this, which is what i suspect, the numbers they quote will be higher than actual which is why they are OK on pump gas.
you could do a compression test after assembly and if it is in the range quoted by Honda for any of their engines you should be OK on pump gas

[paulages]

I disagree.  The concept is NOT wrong.  It simply doesn't fit into his methodology.  I don't know him,  But, perhaps his method is based on experience and empirical testing.  This is not a scientific approach, per se, which is what I thought you were seeking.  This is not to say that Dan's approach is ineffective.  But, its based on skill and craft (art) rather than proven science.  All the end result predictability rests in Dan's abilities.   You either hope and trust, or not.  But, if you ask him, I suspect his art was paid for by several broken engines.

tt- if you look back, i never asked how to measure compression ratio, you just chimed in with that help. i was simply asking what aspect of an air-cooled small four allows for higher CR's than allowed in a larger car engine. i do in fact trust dan's 50+ years of actually building engines over any amount of reading and theorizing (not sure why you assume he has no science to back his "art" though) . you have a talent with theory, i just have no idea how all of your 30+ years of engine builds and tests have gone in practice, as you only mention theory. i'm assuming you've actually done these things if you're willing to offer advice so confidently, right? after all, testing is a vital part of the scientific method.

but on the subject of theory,
you really only further demonstrated my point by discussing how exhaust valve overlap affects the intake efficiency. there's a good reason for valve overlap, as it actually increases the efficiency of each cycle. in the example of the  intake and compression strokes, the intake duration is hopefully timed to as close to the breaking point of intake velocity/compression as possible, to allow as much fuel/air as possible to enter. if any more mixture enters as the piston begins upward than is ejected as the charge reverses, then the CR could theoretically be even higher. some modern race engines are designed with smaller ports specifically to increase the intake velocity as much as possible and thereby increase the amount of time the mixture can enter before the charge wants to reverse. even in a totally controlled experiment with sensors in the head and cylinders, this point would vary according to RPM. the point is, i'm told these variables are why compression ratios are traditionally measured at TDC and BDC.

you could do a compression test after assembly and if it is in the range quoted by Honda for any of their engines you should be OK on pump gas

thanks, brian. good advice.

[TwoTired]

TT- you always seem to be an idea guy, got any suggestions there?

Seems pretty clear that you and Dan have far surpassed any ideas I might have to offer.  So, perhaps we can agree that if you don't ask for mine, I won't bother you with ideas and information that only serve to annoy you.

Best of luck!   And, I wish you success with your engine.

[paulages]

TT- you always seem to be an idea guy, got any suggestions there?

Seems pretty clear that you and Dan have far surpassed any ideas I might have to offer.  So, perhaps we can agree that if you don't ask for mine, I won't bother you with ideas and information that only serve to annoy you.

Best of luck!   And, I wish you success with your engine.

  ::)come on...

no need to be bitter, lloyd. your ideas don't annoy me, only your subtle condescension. if you don't want to be disagreed with, stick to newbie threads about running pods or electrical problems. they desperately need you, even if they can't sense your need to have someone to talk down to. if you really want to discuss ideas and not be top dog, then take what you dish out. i'll reiterate: i never asked you how to measure compression ratios. i asked a simple question (which you didn't address at all) and you got offended when my expert disagreed with you. again, please confess your wealth of practical knowledge, or is it all books?

honestly, i'm quite open minded. if you simply had any hard data to back what you are saying (like several people on here have and do), i'd be more receptive. instead, you generally seem to spout lots of theory while chastising anyone who strays from the stock path. seriously, have you ever done any serious engine work? you certainly talk like it. i'm asking honestly. ?? theory is quite useful, but you might consider at least being a little more humble when referencing those who have.

dan hall= 50+ years of engine building and racing (and winning)
you= 30 years of books and message boards.

why the attitude??

[Pinhead]

Woah, I didn't read any "talking down" to anybody... Just nice, thorough posts trying to cover all bases. While it may have been considered somewhat off topic of the original question, I see no disrespect meant, and no arrogance inferred. It seems to me that he was answering/continuing the post right before his. Maybe not the original thread topic, but continuing the conversation. He was asked if he had any ideas regarding how to try to figure dynamic compression ratios and he tried to answer it, only to get a "my mechanic is better than yours" response...

[paulages]

Woah, I didn't read any "talking down" to anybody... Just nice, thorough posts trying to cover all bases. While it may have been considered somewhat off topic of the original question, I see no disrespect meant, and no arrogance inferred. It seems to me that he was answering/continuing the post right before his. Maybe not the original thread topic, but continuing the conversation. He was asked if he had any ideas regarding how to try to figure dynamic compression ratios and he tried to answer it, only to get a "my mechanic is better than yours" response...

well sorry if you see it that way, pinhead. i just found it interesting that he backed out of the conversation as soon as i asked him to back his theory up. i'm not saying "my mechanic is better than his" per se, so much as i am saying that my machinist and engine builder has built thousands of engines and he has read a bunch of books. i quick search through tt's history of posts would yield quite a few examples of his condescension towards other people on this board. he's not exactly the friendliest guy in the bunch however helpful he can be sometimes.