Technical MOTOR relationship of cam advance & cylinder pressure and why we raise compression wi

It was suggested this be a tech entry Obviously that was wrong

 

I agree with your theory about the effective compression being lower due to "lost" stroke... but isn't that made up for, more or less, by the inertia of the intake charge? Or, do you think that the very slight vacuum at wide-open throttle really affects compression that much? I can't decide if 1-2 lbs of boost/vacuum really affects effective compression... I'd think not, but maybe I'm wrong.

 

don,

along the same lines, if you had an engine that was pinging on pump gas, could you retard it 4° to lower the compression? what's the effective difference in compression ratio by advancing/retarding the cam?

great post by the way


thanks,
ed

 

Good post.

Shows why installing a smaller cam in a low compression engine is the way to go.

Well done.

 

Back in the 70's ford regularly retarded the cams in their detonation prone motors to lower cylinder pressure at lower RPM's. The 351/400M motors had timing chains that had a factory retard of 5 degrees built in. With a 194 duration at 50 camshaft and 5 degrees of retard on top of under 8:1 compression, its no surprise they were dogs.

And it was all in an efford to make the detonation prone cylinder heads work on crap gas.

 

At idle there is less inertia because of decrease velocity in the port so there is a decrease in dynamic compression ratio. At higher RPM the inertia is high, so there isn't a loss of compression. Also most intake designs are tailored to a specific RPM range, so a high RPM intake/port combination further hurts the low RPM port velocity (ie less inertia).

If you want a long duration (ie late IVC) cam to idle well then you raise the static compression to make up for that loss.

On a side note, a friend once had a small block olds that he threw together on the cheap with a "stupid cam", low lift long duration. He had two different heads on it though. One bank had about a point higher in compression over the other. With open headers you could hear the difference in idle quality from bank to bank.

 

Another side note:

This is one of the reasons why variable valve timing works so well. One way to set it up is so that you have an earlier intake valve close at low rpm, and as the speed raises, close the intake later and later. The result is a lower static compression ratio engine which idles very well and makes lots of top end power.

Also emissions are very good. One of the problems with long duration cams is loping. This is basically when the intake charge actually reverses up the intake runner at low rpm. This causes a lot of obvious flow disturbances and basically the engine "misses". This leads to high HC output and CO output from incomplete combustion. That is no longer a problem with variable valve timing.

 

To an extent you can cheat the gas octane by installing a bigger cam except as you aluded to a high RPM in a real good engine the inertia may make up for the loss of compression anyway and detonation can be a problem. I run my 63 max wedge (over 10 to 1 comp ratio) on 92 to 94 pump gas even though it has a big cam (250 duration at .050 (310 advertised) with .525 lift) but i am very careful if i hammer it not to stay on it too long. Still it goes to 6250 nicely and scares the dickens out of passengers.
The Ford info is correct by the way and if we do a Ford build we get a pre 70s timing gear set for that reason.
To be flat out honest this relationship between compression ratio and cam duration is probably the biggest speed secret there is. Once I figured it out and it took a few years to understand its significance even though the evidence was there all along.(hey, I was too young to think things through then.) Since understanding this in my late 20s I have been able to go faster than many on way less cash than most. Lest you think I mean i think i went faster here is a link to the last customer engine ever to leave my shop before I retired. I spent a lot of time matchng the combo exactly to get the power (ca 500 HP (flywheel, 408hp rear wheel on dyno) on 95 octane http://www.youtube.com/watch?v=hpNy2l1wq6Q&eurl=
This is the most important relationship in the whole engine ball game . The next would be duration vrs Rpm relationship. Understanding these principles will get you off the bigger better louder more expensive bandwagon and into being one of those guys where they stand around in the pits wondering "how in the dickens do they get that car to go like that when it looks almost stock" If you can get your head around this and ignore the baloney that is sometimes out there and really understand this you will go much faster i promise. It is hard though not to run with the herd and i understand that. How do you know what i just said is true when nobody else believes it that you know? Trouble is truth doesnt care who believes it . It is either true or not, period, regardless of opinions. If you try this it works . What more can one say?
Keep in mind that pro stockers run compression ratios of 14 to 15 and more to one(theoretical compression ratios) on restricted fuels . (gotta be the same gas stockers and super stockers run on. no cheating and it is checked) How do they do that? This is a clue. There is calculated compression ratio and then there is effective compression ratio. The engine understands effective compression ratio and doesnt give a hoot about calculated compression ratio.
Don
I am sorry if my explaination is not plain enough. Bare with me i am trying to explain best i can.

 

Don, let me guess- effective compression ratio and calculated ratio are different due to valve timing and the fact that without a supercharger you are never completely "filling" the cylinder with fuel/air? The more efficient the induction system- including ram effect and intake charge pulse timing- the closer the effective and calculated compression ratios become?
Brad

 

right ON!

 

Killer tech!!!!!

Danny

 

So, is variable valve timing controlled by an advance mechanism like a distributor?

 

I have a few questions-now that you have "outed" one on my favorite speed tricks. (worked on a nitrous pro mod team for a few years-you would be shocked how many people don't get that a motor is a motor, and internally, they all act the same.)

#1- wouldn't a dome piston ADD an additional 1/8th on top of the stroke while traveling up the bore? ( we used dome tops in the D.R.C.E., but our function was primarily to add as much to the top of the piston as possible- lots of nirtous=lots of piston to eat.)

#2- taking into account that the piston is indeed moving back up the bore, but the charge is still moving into the cylinder due to scavenging and flow rate, at what point does the column stabilize? the reason I ask is because the function of ignition advance is twofold- to (of course) ignite the charge when it is as close to peak compression as possible while taking into effect the time it takes for the flame kernel to fully expand, and to avoid the dreaded "ping" which knocks the tops off rod bearings because the piston and rod assy are shocked by the charge explosion. if the ideal timing is say 38 when bench raced, and you make more power at say...36, do you think it is due to having a fully stabilized air charge versus one where the hot spots in the cylinder begin to effect the overall combustibility of the charge?

and yeah, I agree...going fast with stock parts is good clean fun.

 

It's usually controlled by a computer. Some lower tech systems have a mechanical or hydraulic system. A VTEC honda uses oil pressure to actuate a completely different lobe on the camshaft. It has a "change-over" point at somewhere around 4500 rpm. So it has two distinct cam profiles. I think their latest version has three distinct cam profiles. Some other low-tech VVT's use a cam chain tensioner which varies the tension on the chain/belt therefore altering the cam timing (these were the first VVT systems). The highest tech way to do it is an actuator on the valve (no cam) and it is completely computer controlled. Then you have infinitely variable timing.

 

the dome of the piston doesn't change the stroke of the engine only the volume at BDC (by a little) and the volume at TDC (by a lot), hence giving higher static compression ratio.

As for spark advance, this is more a function of the combustion chamber shape than anything else (although it is affected by the charge velocity/distribution.

Basically the peak cylinder pressure is ideally 20-30 degrees after TDC always. As the engine speed increases the flame has less time to propogate and you need more advance to put the peak pressure at the 20-30 ATDC. Advance however is bad. Because the spark is iginited BTDC, cylinder pressure is building while the piston is still coming up the cylinder, this is negative work. So the less advance the less negative work. This is why fast burn combustion chambers (like a chevy vortec) are so good. They increase power and efficiency by decreasing the amount of advance needed to get the peak pressure in the optimal spot (because the flame propogates faster). So if your engine uses a lot of spark lead (like an open-chamber BBC head) it means that the power gains from moving the peak pressure to the optimal spot outweigh the detrimental effect of the earlier spark ignition. Ideally though you would be better off with a faster burning combustion chamber.

Hot spots basically just add to detonation/preignition and it might just mean using a better fuel to compensate. Basically if your engine needs more advance to make peak power but you are limited by preignition/detonation, get a better fuel.

 

I Vote This Best Tech Post Ever

 

btt because this rocks!

 

holy shit man! this wasn't what i was meaning in that thread...way to go the extra mile!!! i don't think i've learned this much about engine functionality in a long time!

seiously fucking awesome, and thanks goes to you other guys for what you've added as well! this is all perfect timing for the engine i'm working on right now too.

 

This is all dynamic compression ratio and cylinder pressure and how the intake valve closing point affects it.

Two things you want to remember though is that a cranking compression ratio of about 175psi or a dynamic compression ratio of about 8:1 is the limit for pump gas and iron heads. This is with a good combustion chamber design....with a bad one like a open chamber big chevy chevy, 165psi or 7.75:1 is about it.

Aluminum heads can tolerate a little more since the aluminum tranfers heat from the combustion chamber better.

I bought a car with an 11:1 motor that had over 200psi cranking pressure because of the short camshaft in it (204/210 duration, 260/268 advertised on a wide 112 lobe sep). I have also built a 12.2:1 motor that had 165 cranking psi that ran fine on pump gas because its 280 degree at 50 camshaft bled off so much cylinder pressure.

 

This is a great post, can someone elaborate on valve overlap, and some of the other events which play a role in combustion?

Thanks again all who have contributed to this post.

h-bob

 

Finally, Dynamic Compression explained in a way that I can understand it In college, I went to a High Performance Engines seminar. I learned a ton(info overload even. I'm not a note taker, and I took 8 pages of notes). About half way into the Cam section, I was completely lost. It makes much more sense now...

 

Valve overlap is the period that both the intake and exhaust valves are open at the same time. There is alot that goes into designing and choosing the right cam. Lobe centers are one way to change overlap. That is done when the cam is ground. I don't have much exp. with naturally asp. engines but in my T/AD, advancing the cam always made the car run quicker. Sometimes as much as 2 tenths.

 

variable cam timing. Years ago there was a device called varicam which used a big spring in a two piece cam gear and relied on the increasing load as speed increased to give more or less variable cam timing. Actaully thinking about it I have three units downstairs a neighbour gave me and would take a photo of them of anyone would like to see it. personally i never used one myself but i did work on a VW dunebuggy build where it was used. To be honest back then in our hormone driven youth we didnt have the gray matter working well enough to understand the benifits it could offer. We just figured it was trick and shiny and must be good. I am cured of that completley (maybe almost too much) but I constantly fought this even with some of my best racers i did engines for. We would be beating everyone and they would go and buy a rinkydinky thousand Dollar carb or something to put on my engine to replace the one I made for them because someone told them over a beer that was trick and I would get a phone call at the from the track about how the car slowed down (This has actually happened as described 4 times) Only one guy ever told me up front he had replaced the carb , The others i dscovered when i got to see the car next time. In all four cases reinstalling what we made for it despite the low cost restored the performance ,( Smallest loss/ gain was 3 tenths) Three of the carbs had purple parts so you can figure that out from that. Anyway i got sidetracked making the point about how we all gravitate towards the tricky stuff. Anyway, Jags (I was a Jag mech in my mispent youth)had vernier cam gears and the cam timing could be altered individually in small increments on either the intake or exhaust. That is where variable cam timing really make a wooping difference . When you can change the intake cam timing or exhaust timing independant of the other cam. My brother had a W8 VW for awhile and it had variable cam timing but in typical german fashion it was a bit ovecomplicated (dont get mad, ich bin auch deutsch) and he sold it after a couple of years before it gave trouble. It worked great. Someday and not too far in the distance i believe we will have no camshafts but will have solenoid controlled valves with as our one friend already mentioned Computer control and then super chips will really be SUPER! Talk about variable cam timing! The posibilities are staggering to say the least.
It is my belief as far as ignition timing goes that ignition timing is a function of cylinder head design more than anything else. Most of you are chevy guys and i have done a few of those but my most experience is with BBMopars . We run normally 32 degrees but always run a loop to see what each indiviual engine likes. They are not all the same even though you think (or hope)they are. My Old Reliable motor would take 32 and could handle as much as 36 but it didnt make one bit of difference in perfomance. I have done other motors that liked 34 and one we are working with now (a 426 made from a 400 (3.53 stroke) likes even more and gives perfomance to back that up. To be honest i have spent a bit of time playing with ignition especially advance curve requirements for modified engines and such but have not gone into it in the detail i have in this area. One thing I know is find out what the engine wants give it to it. After that dont monkey with it. Exploring the cam vrs compression relationships and the rpm duration link and such i thinkwill keep me busy till they are throwing dirt on my face. Others who are more gifted in the ignition thing can explore that. Hopefully they will share what they know as some even here already graciously have. I believe that especially with engines that knowledge is POWER and i mean Horse power.
Don

 

#1 Domed piston
Wouldnt a domed piston --
Yes to a point but it still wouldnt be quite the same as having the stroke so as you aluded to in # 3 you might have to add more than just the actual stroke difference. Cranking compresion is an excellant reference point. You can get a real sense of what is going on in an engine by the cranking compression.
Years ago when i was just beginnig to understand this i had a home built 69 dart with a 12.5 to 1 426 hemi. I had two cams , A crane which had been recommended to me by a shop who did good work but didnt really understand engine function and a cam i had picked for myself a Sig Erson 999xx (Which those turkeys discontinued)
The crane was 310 duration or so and the Erson was 318 duratrion (both advertised figures. I sometimes used the crane when we sorted out a new combo like carbs or something to eliminate variables. (i still keep milder cams for that even now , cams i know work and cause no trouble . It keeps the number of variables down when sorting through a new deal. ) anyway with the crane crankin compression was 225 psi. With the Erson it was 185 psi. The crane was all done by 6600 and the Erson "bless its pea picken heart" would pulll to 7400 and would got to the sky if you let it.

 

i forgive ya and i'm GLUED to this thread!

 

Man, I want to understand this stuff. I reckon it's the first time I've really been able to read this kind of info. Good post, even though it's pretty much wasted on me.

Pete

 

The really best way to understand this is to build a paper model with a wheel for the crank (when talkng about cams we use Crank degerees except for lobe seperation) A slot with a sliding piston and a paper link for the conrod. use brads. My wife a former kindergarden teacher may be abe to be talked into making a pattern for me and i could post it so it could be printed out. Take the stroke the same as your favourite engine and the rod length the same as well . Mark the wheel off in degress and the side of the paper by the pretend piston in inches of stroke. Then plot various cams intake closing points by putting the brad for the lower end of the conrod and the wheel at that point on the plot, then see how much stroke you have left. Use that figure to calculate your actaul compression ratio using the cyl and head volumes from your engine. Many people are visual learners and seeing it will go way farther than a million words. In fact it came to me when i was laying out snowmobile port timing when i used to port for a local engine shop. I was watching the difference in compession available after modifiying an exhaust port to 88 degree closing height and was thinking about four stroke theory at the time. All of the sudden it hit me that when we close the intake valve late and into the compression stroke we are dealing with the same problem so to speak and must make the lost pressure up at least with increased squeeeze through a domed piston. That was my light bulb day and i still remember it even though it was now over 30 years ago.
Don

 

don,

please DO go into detail about ignition!

i've read a lot about this, but everything seems to simply say, "find out what the engine wants," and then leaves it at that!

so... what does the engine want and how do i figure it out?



thanks,
ed

 

Awesome post

 

Very educational post, much appreciated that you're taking the time to write it out.

I have a general question about compression and combustion chambers.
Pertaining to 455 Buicks fwiw.
(4 5/16 x 3.9 stock bore and stroke.)

Both engines are/will be .030 over resulting in 462"

My present engine is right at 9/1 CR.
Runs forged dished pistons and 73 heads.
Factory CR is rated 8.9/1 and with the overbore, block decking and head milling (not much cut off either one) it calculates out to 9/1.
Timing is 32 degrees BTDC all-in and it's attained at 2800 rpm.
A street engine in a light car - 2400# - fwiw.
The cam is a mild one with 260 and 266 degrees adv duration (int & exh) and .448 lift with 112 degrees lobe center.
Makes for a good crisp quick responding street engine with an Edelbrock Performer intake and 750 cfm Edelbrock carb.
Nice part is, no detonation problems and it runs great on 87 octane.

At one time I ran a bigger cam and dual quads with this engine.
It idled well, responded well and was a pleasure to drive, but I went to the milder cam due to the low compression of the engine.
Seems to have worked out well and granted the testing is subjective.

I'm building another bored 455 engine for a 2200# car.
The dual quad setup (two 500 cfm Carters) will be used along with a bigger cam.
Said cam being 284 and 290 degrees adv duration with .514 and .525 lift and 112 degree lobe centers.
(I do have a cam with almost identical timing and lift that has 118 degree lobe centers, but am leaning toward the 112 degree lobe center cam.

I did some digging on the head casting numbers and found that I have the less than desirable - to the Buick boys anyway - heads that are rated 7.9/1 CR.
The Buick guys call em 'open chamber' heads.

Perhaps more info than needed for a simple question, but there it is anyway.

What I'm thinking of doing is running the open chamber heads with flat-top pistons.
I understand the flat-tops result in 12/1 CR when run with the higher compression heads due to their smaller combustion chambers.

It looks like running the flat-tops with the open chamber heads will result in about a 10/1CR and maybe up to 10.5/1 CR.
I do want a higher CR ratio and from what I understand 10/1 on today's gasoline is a do-able deal.

Except for digging up a set of higher compression heads for cores, costs will be about the same in the end.

Generally speaking what would be the better setup?
The higher comp heads with dished pistons or the low comp heads with flat-tops?

I'm guessing the low comp heads with flat-tops would be the way to go and for sure it would be easier in view of how difficult it can be to find good rebuildable stuff for the big Buicks.

I realize there's not enough information here for a definitive answer, but a general recommendation would work fine for what I'm doing and where I'm at with the engine now.