cams vs. compression

James2 · Post by **James2** » Tue Jun 05, 2001 9:08 pm

John, would you agree with my idea that the carb/ head/ exhaust combo has the greastest effect on VE?

I feel the cam only determines what RPM peak VE occurs at.

So that the carb/head/exhaust combo will decide what HP you will make, and that the cam and engine size will decide at what RPM it happens at?

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Tue Jun 05, 2001 9:18 pm

Dan,

I think this quote fits...

"Frankly, my dear, I don't give a DAMN" That is if you believe me or not...( and no Dan, you are not my Dear)

Last time I checked, I have nothing to prove, so I'm just here to cause hate and discontent..

Who cares, why don't you try some things and see..

I gotta run, slop the hogs real quick..and try to skin that coon, that we got last night, I think I'm gonna make a nice pair of welding gloves out of this one....

Post by **Tom Notch** » Tue Jun 05, 2001 9:22 pm

I still don't understand how dynamic compression could rise on a atmos. engine at high rpm. Pumping loses would make sure that you never get more air into the cylinder as the rpm goes up as at a low rpm level. I don't see how compression affects dynamic comp. Guess I have a rock in my family tree.

Am I looking at the forest instead of the trees? Or vice versa?

------------------
Tom Notch
Tom's Old VW Home

JohnConnolly · Post by **JohnConnolly** » Tue Jun 05, 2001 9:36 pm

>>>Dynamic compression is close to static compression at lower RPMs. At high RPMs the dynamic compression gets LOWER because the port and intake system can't flow enough air to FILL the cylinder in the amount of time the valve is open.<<<

"phew! thank you, now that i'm sober

i can see where i got off into left field."

"john, your example above would be dependent on cam design; you have described a lower-rpm jake cam, one with short duration, and it doesn't appear that you are going to see it any other way."

it's dependent upon cam duration and RPM.

"i'm oversimplifying, but by increasing the duration, the torque band can be moved up the rpm scale radically, so that the engine can breath at higher rpm's.."

yes

". h.p. is torque x rpm... "

divided by 5250.

"so then we raise the static c.r. to make up for the loss of dynamic compression at low rpm's, caused by the long duration... i'm with you there."

cool

"yes, i know that this is how it's done in the v-8 world, as darren pointed out on the clf, the v-8 cam mfg.'s will many times recommend a static c.r. to be run with their cams... apparently there are no vw cam manufacturers who do that."

any why not? THAT is what matters! The VW World is unto itself, and could learn a few things by opening up concepts from other engines. There are a LOT of smart guys out there!

"i think we are in some agreement up to this point; where it goes south for me is in calculating the dynamic compression at high rpm's with a long duration cam."

I'm not saying to calculate it, it's a bitch. What I'm saying is that you can run more static compression with more cam, and the result is the SAME dynamic compression at lower RPMs, and it stays there for longer because as the cam comes on, the port's breathing capability drops off (starts to), which results in a relatively flat "curve".

"about the only thing relative to actual dynamic c.r. figures that i know of is that some v-8 people claim that 200 psi is the limit for pump gas, without detonation."

that prolly' is about right, mebee' a little high for aircooled.

"my claim was that the dynamic c.r. with a radical cam could be unacceptable for pump gas"

but it doesn't matter Dan! For example, you stated earlier that we can get away with more compression in Utah because we live at elevation. Well, the gas companies already took care of that because the best gas we have is 89 octane! Regular is 83.
So, the game here is the same as at sea level.

"jake gave us one example(still don't know if i believe it!)

but it's really a question of how far out in left field i am with long duration cams and dynamic c.r...

i'd sure hate to be proven wrong!

"

Next engine you build, follow my rules and see how it works out. If it doesn't work as I say, I'll buy the bearings and gaskets to put it back the way YOU WANT IT. I can only say to try what I have been pushing for some time (Mark H runs 10:1 in the SSB with an 87' and 80K miles or so, right?). A more drivable combo is 296 duration (adv) and 9:1 with 1.25 rockers, pulls right off idle with big heads and engine.

John

bad62bug · Post by **bad62bug** » Tue Jun 05, 2001 9:38 pm

this is getting crazy someone has to stop this insanity

JohnConnolly · Post by **JohnConnolly** » Tue Jun 05, 2001 9:42 pm

You are right Tom. Compresion has nothing to do with VE from the outside, but it DOES.

You aren't wrong.
Compression doesn't change VE much.

BUT, if you run more compression, you get more suck/push thru the ports, SOONER, than if you have lower compression. Think about 7:1 and 10:1. If the piston moves .100", what's the new pressure in the cylinder (valve closed)? Now, open the valve. With the higher compression engine (the chamber is smaller, and/or the piston is closer to the head), you get more suction, which means a greater pressure DIFFERENCE from atmosphere to the cylinder, which means air rushes in faster than the lower compression engine does (less pressure drop). THIS is why low compression engines are more difficult to tune (tune = JET), the signal is weaker, and it's harder to do. The carbs don't respond to LOW compression as good as they do to higher compression. It has NOTHING to do with jetting, I'm just saying a higher compression engine is easier to tell if the jetting/mixture is right or wrong than it is on a lower compression engine. Run 12:1 and you'll KNOW if the jetting is right, because if it's wrong, it'll run horrible! With 6.6:1, it'll run so-so all the time. Problem with most of these engines is the owners think it's running well, but have nothing to compare it to! bump to 8:1 THEN tell me how it runs (assuming adequate cam)

JohnConnolly · Post by **JohnConnolly** » Tue Jun 05, 2001 9:48 pm

I think one thing that's VERY important to note is that cam "size" depends on the engine.

For example, a 296 duration cam is BIG for a 1600 with stock heads, and SMALL for a 2332 with HUGE heads.

Think of the engine as an air pump. Larger engines pump more air, and the ports NEED the extra duration to move that much air. A 296 cam on a 2332 works off idle-6000, on a 1914 it's from 2K-7K, on a 1600 it's 2500-8K.

On a related note, larger ports and vents "cover up" reversion, which is a problem with more duration and overlap. So, many times an engine with drivability problems can be "fixed" by putting BIGGER heads and carbs on it! (be careful here). The added volume in the ports smoothes things out on the low end.

<BLOCKQUOTE>quote:<HR>Originally posted by James2:
John, would you agree with my idea that the carb/ head/ exhaust combo has the greastest effect on VE?

I feel the cam only determines what RPM peak VE occurs at.

So that the carb/head/exhaust combo will decide what HP you will make, and that the cam and engine size will decide at what RPM it happens at?<HR></BLOCKQUOTE>

James2 · Post by **James2** » Tue Jun 05, 2001 9:58 pm

<BLOCKQUOTE>quote:<HR>Originally posted by Tom Notch:
I still don't understand how dynamic compression could rise on a atmos. engine at high rpm. Pumping loses would make sure that you never get more air into the cylinder as the rpm goes up as at a low rpm level. I don't see how compression affects dynamic comp. Guess I have a rock in my family tree.

Am I looking at the forest instead of the trees? Or vice versa?

<HR></BLOCKQUOTE>
two ways this can happen. both depend on carefully tuned Intake/exhaust systems and VE over 100% only happens at the tuned RPM levels.

In the intake you have a column of fast moving air. Remember the stuff about a body in motion wants to stay in motion? Well the column air is moving quickly into the cylinder, it fills the cylinder 100%, BUT it's still being pushed by the fast moving column of air behind it. This "stuffs even more air in the cylinder. it is now over filled, thus over 100% VE.

The other way depends on the exhaust system as John said. The column of air is moving fast out the exhaust pipe, so fast that is now moving out faster then new exhaust is filling it. This happens because the hot exhaust expands in the collector, pulling the next exhaust pulse in behind it. This creates a vacuum in the pipe. During valve over lap, this vacuum is applied to the intake valve, thus is now have LESS then atmospheric pressure in the cylinder, allowing it to be over filled by the incoming intake charge, which is at atmospheric pressure.( which is greater then pressure in the cylinder.

Understand this is NOT normal on an average VW engine. Usually only full race motors can achieve over 100% VE without a turbo, or SC.

danimal · Post by **danimal** » Tue Jun 05, 2001 10:03 pm

>>>Who cares, why don't you try some things and see..<<<<

actually, jake(?), i think we'd all like to hear some more of those engine math figures... it's a hell of a lot smarter than throwing hardware at a problem.

but i'll leave it up to you to correct the obvious disparity between the math you quoted, and your fellow georgian james2: "Dynamic CR can never be greater then static, UNLESS Volumetric Efficiecy (VE) exceeds 100%."

that may be the essence of this discussion.

dan
oceanstreetvideo.com

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Wed Jun 06, 2001 12:09 am

I will not enter into any more diverse conversations here, I'm just here for the hell of it..

I'll put it this way....Our engines are not built for pure racing, but for driveable HP, and to last as long as the car will...And they do exactly that..

My engine failure and return rate is less than 3% overall, and no one EVER complains about not having enough power. My assembly proceures are different than most, and thats how I like it...

No matter WHAT is stated on paper, in theory or guessing, The real test is when it fires up, and more so when it still fires up in 2-3 years and makes good power.

BTW, Helicopters do not fly "on paper" but did that stop Sikorsky??

I just got done, downing another fifth of jack....Now I hear the frogs a'yellin down at the river, anyone for a Pabst Blue Ribbon and some frog legs??

JohnConnolly · Post by **JohnConnolly** » Wed Jun 06, 2001 12:27 pm

"First, I meant when I said that the term "dynamic compression ratio sucks" that tacking "compression ratio" onto "dynamic" throws me off because compression ratio was defined to me as max vol. over min vol which wouldn't change except in the case of bleed off. In my mind, whether more or less vapor is being compressed, the compression ratio stays the same because the volume stays the same. "

why don't you come up with a new name that works better, and if it IS, I'll use it. Would you explain how compression ratio accounts for throttle position? It doesn't. Actual "compression" depends on how much air you are letting into the engine. It's true that 7:1 means whatever you let in gets compression to 1/7 it's original volume, but that doesn't tell you much. DYNAMIC depends on a ton of things, and is probably impossible to calculate (no need to either). If it's too high, you'll get detonation, simple.

"You stated this in my response to my question if a high compression engine equipped with a long duration cam running at high rpms would melt due to that added heat. I do understand that engines run more efficiently at higher compression ratios, but I did learn in thermo that there are limits to what cr's can be run, like material composition, coolings systems, etc. "

Yes.

The VW engine has two limitations. Cooling system is limited to 90hp IMO, it's designed for 63hp. You can have 1000 hp, but as long as you exceed 90hp, the engine is going to heat up. As long as you are below 90hp, the engine is going to be ok. THIS is why the T-1 can make more power, and be OK on the street. Nothing wrong with a 230hp blast for a minute or two, then you have to lay off and let it cool down.

The second problem with lots of compression is detonation (if not matched to the cam). If you are too close or over the line, it's time for new pistons and bearings. But, you also seem to miss the concept of long duration and why it can work. dynamic compression is STEADY because as the cam comes on, the port efficiency decreases, so actual cylinder pressures are close to a constant.

" Anyway, I guess my real question here is now what suggestion do you, or anyone for that fact have for a cam/compression ratio for an essentially stock 1.7L type 4 engine? I posted this on the t4 forum, but didn't get any replies"

no more than 7.5:1.

John

Matt Harris · Post by **Matt Harris** » Wed Jun 06, 2001 12:38 pm

OK here's some practical experience with super low compression and my thoughts. Many of you know that I have a 6.6:1 2007cc engine with a 294 degree cam (e-120), 40x37.5 heads, semi hemi cut etc. And from the outset the oil ran a consistent 231 running at 65 mph on a 85 degree day on the 91 freeway in so cal. So I plugged some holes in the tinware and still got 223-225 on a 85 degree day running at 65mph in so cal. Stock geared tranny, stock 165 tires, etc with a stock doghouse cooler and all tinware painted black by me. Needless to say I was pissed about dropping $6500 and going conservative on the c/r and sacrificing power just get an overheating pig! So I had to put an external cooler on it just so I could drive for extended periods and keep things at bay.

And I got the same engine temps whether I was running 37 or 40 degrees of timing so not running enough timing is not an issue. And I didn't assembled it wrong either as Jim Brown had the same exact combo as me to the "T" and had the EXACT same problems with overheating tuning problems. And is a "tinkerer" and did his best with what he had.

And just to elaborate on the hard to tun issue. I bought an O2 senor and spent over $100 (remember that John) in jets and it still didn't run right. It took Art Thraen, a professional carb tuner/rebuilder, to fly out to California and help me while staying at my house on business. Ended up with 135 main, 180 air, 60 idle, 132 idle air corrector, 34 vents and it runs good.

I also had the heads off after only 5 months of running and you should have seen the massive buildup of carbon in the chamber!! Again, another sign of inefficient running, no vacuum at low rpm! And yes the mixture was right as jetted with a $5000 O2 sensor that reads actual numbers not just lights.

So next time the heads are off, I'm going to 8:1 or 8.2:1 c/r period. I got burned running low low low compression and will never be burned again. I talked to Roger Crawford and he said has noticed that engines with loose deck, big chambers, and low c/r overheat. I figure that my engine can't run much hotter with 8:1 c/r and at least I'll get some decent power out of it and have it be more predictable.

BTW John, remember the guide boss post

, the next set of heads I buy will not have guide bosses, just so I can see for myself if what you and others say is true. See I'm not closed to others ideas

Anyways, I just thought everyone would like to here some personal experience from someone with ultra low c/r.

------------------
Matt Harris
78 x 90.5
2007cc
1971 Super

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Wed Jun 06, 2001 1:05 pm

Matt,

Nice to see that someone else here, besides John, and I have been bitten by low CR....I'm sorry to hear it, but it is the only way to prove that low CR is not the answer to overheating..

type11969 · Post by **type11969** » Wed Jun 06, 2001 2:09 pm

>>But, you also seem to miss the concept of long duration and why it can work. dynamic compression is STEADY because as the cam comes on, the port efficiency decreases, so actual cylinder pressures are close to a constant.<<

John, you're right, I didn't think of that. Well I think I might finally have the theory down, hopefully someday I'll have the money and time to put it to practice. Thanks for the help.

-Chris

James2 · Post by **James2** » Wed Jun 06, 2001 2:38 pm

6.6-1 Matt? man, that's low. Guess I was totally wrong about not running enough timing. Goes to show like Jake said, what I read in books and what happens in real life doesn't always agree.

That's why I value the info John and Jake have, they both have TONs of experience.