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

Mighty Buick
I generally like it anytime I can run a flat top piston and get reasonable compression ratio. I would take that route. I am not a big quench freak because there are other ways to keep things stirred. It takes more parasitic power to squeeze a tight closed chamber head at thetop than an open one and despite all that wold argue that point I have not seen any measurable gains on the engines i am most familiar with (BB Mopars) and i have run both open and closed chambers on the same engine in the same car. I am kind of stubborn that way. If theory doesnt show up on the timeslip i tend to discount it or atleast be very suspicous of it. I would also do the flat top open chamber thing with that engine. If I said otherwise i would have to lie.

 

How about we save that for another TECH thread? I will give my perspective on it maybe next week. OK?
don

 

My .02.. I got this from the grinder I use...
It is important to realize that the engine sees three different compression ratios. One is the static ratio which we are all familiar with: clearance volume + swept volume, divided by the clearance volume. A number like 9:1 is a common static compression ratio.
The second is the effective compression ratio, which the engine sees when the intake valve closes against the valve seat. A number like 7:1 is common. This is determined by the interactions of the static compression ratio, the rod ratio, and cam timing for closing the intake valve. (Wrist-pin offset has an additional but minor effect.)
Third is the dynamic compression ratio which is when the engine is in the peak power range and the volumetric efficiency is above 100% then the cylinder pressure-compression when the intake valve closes is at its highest, example above 8:1.
Building an engine for more performance often means raising the static compression ratio close to
10:1, but keeping the effective compression ratio not much over 7:1. Anything lower gives up power. Anything much higher will not run at low speed with WOT on pump gas without detonating and destroying itself.

 

Alright, this is all understandable yet confusing. So now is dumb question time! I have a standard 350 chev. I believe it to be originaly built in 1970. It was rebuilt before i got it. Obviously has a small cam because it idles smoother than most new pickups. On the bottom end its a dog, like when the base timing is set to say 2 degrees, but i have it at 14, so would i benefit from advancing the cam?


.

 

Many thanks for the write-up and the answer.
And not just cuz I was leaning that way anyway.

I'll be looking forward to an ignition post.
No big rush, don't tie yourself down.

Street mechanical advance curves are something I'd like to discuss.
I'm of the opinion that too many street runners pop in a drag racing curve on an engine that seldom see's the track.

Along with that, it seems that a lot guys are spending their bucks on Official-Racing-Stuff that doesn't necessarily cut it on the street.

Nothing quite like seeing a mildly cammed, vacuum secondary carb'd car kick ass on a big cam double pumper special.

There's no doubt the double pumpers can be made to run, but the great majority I've seen on the street weren't anywhere near optimum.

 

If you could, or draw up "how to make your own" directions, or even post a picture of the one you made, I would be very greatful(I'm one of the afore mentioned visual learners who needs to see a pic of what your describing )

 

AMEN. you wouldnt believe all the horsepower theory I have seen scattered on a dyno room floor .......... or not show up at all!


great post...very well explained.


Fred

 

Can you talk about these ways a little bit?

 

Maybe but I would check some other things first. Usually when you install any kind of a pefformance cam you should recurve the distributor some to match the cam. If you want to know would the engine really benifit do a physical compression tst. (Engine warm, throttle blocked open and five kawump kawump kawump kawumps kawumps. If it is anywhere in the area of 130 to 150 then dont worry abouthe cam but if it is in the 95 to 110 range well then something needs to be done. First check the timing chain for stretch, (wiggle the crank back and forth , you can feel the slack in the chain quite easily) If that is good pull the cover and check the timing marks. if they are ok then you could try moving the cam ahead with an offset keyway (sometimes when doing it it is easier for some reason to think of moving the crank backwards) Otherthan that you would need to raise comppressionratio. But FIRST before you do anything else do a compression test.
Don

 

Of course as you also know learning what doesnt work is just as important as knowing what does. I figure even failed experiments have benifits. They are just not as enjoyable

 

OK
One way I like was developed by Jag and is using a slight offset in the port in relation to the cylinder bore centre to induce a swirl which helps distrubute the mixture in a even and nicely stratified manner. The 426 hemi also uses this and the Chevy vortec as well as the late 80s 318 swirl port head. One thing i discovered working on my most popular engine (BB MOPAR)was we tended to always see the port and valve pocket in a vertical maner. The valve cover side was the roof of the port and the deck side the floor however on further investigation I have decided it aint so. There is a very definate bias built into these ports and valve pockets so if we considered the sides of the intake valve pockets that are close together to be the floors of those ports and the long side or side away from the floor side to be the roof then we port in a very different manner. The mixture comes down the port mostly on the wide or outside side and is in a gentle circular pattern. It flows around the open intake valve like this and draws whatever else it can from the floor or short side. I believe (and i might be just dreaming but) in my heart which i have learned to trust that it continues this swirl right into the combustion chamber keeping the charge well distrubuted in an orderly and repeatable manner. Biggest enemy of port flow and combustion chamber flow is turulance. (Airplanes dont like it much either) When a port is turbulent you can hear it on the flow bench. It almost farts for the lack of a better description. When you induce an consistant swirl into it tends to stablize. (best way to understand turbulance in a valve pocket and port is to watch various tea pots pour. You will never see a better visual example) A better word for turbulance to me would be confusion because that is what it is.The air is runnnng into its self. If we get an orderly flow with a consistant swirl going on the port flow goes way up and the mixture enters the cylinder in a definate and orderly fashion. It would appear when we work at this with our BB Mopars we get real strong numbers with good torque and a broad horsepower range. James Bond liked his drink shaken not stirred but motors would much rather have it stirred, not shaken. To me a carefully induced swirl is stirred and a tight quench is shaken. Maybe i am nuts.(Well actually i probably am) but when i do this i get real good perfomance on reworked stock parts. The link i gave earlier is one such engine i built for use as a practice car and driver training car for the Lemans classic race in France. It was built under FAST rules so i had only 95 octane (9.6 to 1 compression ) reworkd iron heads which i spent weeks getting the swirl the way i wanted it, constantly checking as i ground with a quick blast of air up the port and watching the grinding dust blow) Because of the compression ratio i was limited in cam choice and i set it in 3 degrees advanced. They tested it in june on a chassis dyno and at 5550 rpm it was producing 408 hp at the rear wheels.(approx 480 to 510 flywheel hp depending on driveline efficency -80 to 85 %) To hit that that early on an engine of only 400 cu inches told me that some of my work was paying off. That is a lot of moderate rpm Hp for such a short stroke engine (3.38") and it only varied about 1.5 hp all the way to 6240 (where he had set his redline). The dyno error would be more than 1.5 hp. It is because of results like this i believe what i think about this is correct. It shows up on the timeslip or dyno sheet. i think a lot of that power came from the way the heads were ported to induce a swirl. Where else did it come from.? Not the compresion ratio, not the cubes, not the cam because it is an hydraulic streetable grind running on stock stamped steel rockers. This is why i think it works . i cant afford the equipment to do extensive testing but i am liking how things work since i started thinking this way. That my story and im sticken to for now at least. I hope somewhere in here is what you wanted to know.
Don

 

Speaking of swirl, would something like an earily Hemi, which has the valve smack dab in the middle of the chamber and is prone to tumble, benifit from having some swirl induced? How much swirl is to much? hos wan you induse it? Poducts like "The Tornado" and "Turbulator" induse swirl very early on in the intake(at or above the carb/throttle body) and have been shown to decrease performance. Why don't they work like swirl induced further down the intake track does?

 

I dont know if you could get much swirl in an early hemi. The last set of those heads i worked on was almost 20 years ago so i am having trouble remembering what they actually looked like in detail. Swirl in intake. i think you just want a gentle swirl not a tornado. You could try with as set of heads that didnt matter to port the intake port slighly off centre but you know they are decent heads and were way ahead of their time. Doing too much to them would be like touching up the Mona Lisa. They seem to me from memory to be like you said very straight in type of port entry.. It is possible they might not lend themselves to this method very well. They are good flow-ers anyway where as the ones i was describing were sufficient for their intended application but we wanted more than that.

 

Swirl is a compromise. Basically it takes energy to induce a swirl in the flow. So you are paying for this energy during the intake stroke in a higher pressure difference to fill the cylinder. The benefit is a more evenly distributed charge which will more effectively combust. Is there such a thing as too much swirl, yes it completely depends on what you are doing. A 18K rpm F1 engine does not have any swirl. The combustion process happens in such a short time at that engine speed that any benefit from the swirl is negated. However in a relatively stock engine AKA under 6K rpm, added swirl benefits outweigh the loss. Especially because at these lower port velocities the energy needed to induce swirl is simply not that much.

Swirl is basically added by the shape of the port/intake runner. By putting a turn ("radius") in the port you induce a swirl. It's that simple. This doesn't mean that you should go out and start grinding and welding on your heads. Port shapes are fairly complex and while you may make a port shape which will induce swirl you could also really hinder the flow of the port with "boundary layer seperation" and other fluids characteristics, if you really don't know what you are doing. Best bet is to simply buy some heads that have swirl ports.

 

I agree with that 100% except you when said that dirty word BUY. :>) Tis true you get nothing for nothing, everything is a trade off in some fashion and all applications have their limits . Always with ports we are fighting the time element.
Thanks for the post. Good info

 

Maybe I missed it, but are we talking about swirl only once the mixture is IN the cylinder, or also in the port LEADING TO the cylinder?

 

my brain hurts...

 

With a wedge port you cant just offset the port a bit because the combustion chamber is not symetrical so you have to get everyone dancing the right way before they get there. A hemi is different. if you dump in off centre the mixture will tend to swirl as it enters the chamber and it doesnt matter if there is port swirl so long as the port is not turbulent. . Anyway swirl is just one way to deal with it . Basically you dont want dead areas or non active area in the chamber . There are a lot of differing ideas and methods seem to come and go then re appear a few years later. That is what so interesting about motors . There is so much yet to be done.. How did we get here from Cam advance discusions? anyway I thinkwe have about beat this to death for now. Don

 

This post reminded me of something I read a few years ago. Luckily it is still on the internet: http://mototuneusa.com/think_fast_intake_porting.htm There is lots of good information on this website. This article also talks about the same things, only he goes about it through decreasing the size of the intake ports. There are four parts to this article. The rest are here (#16,17, 18, & 19): http://mototuneusa.com/thanx.htm . Also a really good one on breaking in a motor.

Didn't Smokey Yunick build his Smoketron to study how air moves through a running engine?

 

Smokey Y was an inspiration to me as I was learning. David Vizard is also someone i admire as well as John Lingenfelter. All three were able to discipline themselves away from the trick of the month club and really delve imto what is happpening inside. Another real good book that has been printed many times and revised (and one I certainy appreciate) is Philip H Smiths book Design and Tuning of Competition Engines. Not many pictures but a wealth of good sound information. Of course nothing beat trying it in the flesh but time and money put some restaints on us all there.

 

Thanks for the compliment. I only said "buy" simply because most people that don't really understand this stuff still go and try to port heads. I don't mean the guy who does a little clean-up with a grinder, I mean the guy who hogs the shit out of the port without any real idea what he's doing. I've seen quite a few good heads ruined by bad porting.

 

Ok You asked for it . git yer sissors out. See the main bearing , make three and put one behind he crank wheel, and one behind each end of the con rod.
this will take a few posts so hang on. device may be made and used but not sold. A real smart fella would make a HD one from wood but stiff paper works for a quick look.
numbering the back board around the crank wheel in degrees wil help even more. use a high school geometry set protractor to get the degree layout.

 

here we are at TDC (top dead centre (top of the stroke)

and here we are at BDC (bottom dead centre or bottom of the stroke)

 

And here we are at intake valve closing of 88 degrees after bottom dead centre, (268 degrees crank position) taken from an actual cam i have run (Sig Erson 999xx)
Look at all the stroke we have left to make compression in. Not much compared to what we use to calculate it. To get decent cylinder pressure we have to put a dome on the piston . If we advance the cam 4 dergrees the intake will close at 84 degrees ABDC (264 degrees crank position)which will give us a bit more distance to make cylinder pressure in but in this case not enough without increased piston dome to get a decent running engine.
Please realize this is not a dramatization. These are actual cam specs and not that unusual for a bracket race type engine.
I hope this helps
Don

 

thought I would bring this bad boy back up...

I spent 3 hours degreeing in a cam on my "new" motor with a good friend yesterday.

it's a solid roller from Crane. It's a custom grind, and I had questions, and arrived at the numbers, and ended it all with "and what is the advance of the cam?" straight up...according to them was 5 degrees.advanced.

when it was all said and done, the centerline of the cam (which was supposed to be 110) turned out to be more like 105, and thier 5 degrees of advance turned out to be more like 10.

the after effect would have been a motor that made big power...right up to about 2800 and "flattened out"meaning it would not have kept puling.

point being- ALWAYS degree in a cam- no matter how "pro built" it is supposed to be!

 

AMEN!

 

How would the height of the dome affect the stroke?The stroke is the distance the whole piston travels up the cylinder,so unless the domes "grow"they won't change the stroke.