[BillyGman]

I don't read a lot of magazine articles. In fact, I don't even have any subscriptions to any car magazines. I believe that a number of articles are merely a disguised means to advertise aftermarket car parts, and nothing more. However I came across what I think is a pretty good tech article in the Chevy Hi-Performance magazine that I located on the INTERNET, which is about using cranking compression pressure readings taken with a compression gage to determine camshaft choices vs. static compression ratio compatibilities for running engines on pump gas.

I'm merely sharing this with those of you who might be contemplating a new engine combo for a street car to be run on pump gas, who might also be interested, and would want to or even need to broaden your understanding on static compression ratio requirements for pump gas, and how they relate to dynamic compression ratio requirements, and camshaft valve timing specs and duration specs. These topics are touched on in the article also. So please don't be completely caught up with the concept of determining pump gas compatibility with a compression gage. Even though the article is about that for the most part, the reason that I'm posting a link to the article is merely because it gets into explanation of how camshaft duration specs and more specifically, valve timing specs can and will make an engine pump gas friendly, or unfriendly to pump gas.

This isn't the end all article, nor the last word concerning pump gas engine requirements by any means. I just believe that it can be of help to those of you who might be new to these concepts, and who might be assisted in your future engine building choices by the knowledge of the things mentioned in this article.

DISCLAIMER:

Keep in mind that the article mostly covers basic concepts, and doesn't get completely into every minute detail nor tricks of the trade of engine building. I'm merely posting this, that it may me be of help to some of you who aren't familiar with the things contained in the article. I believe there are too many threads here in Team Chevelle which start out with somewhat simple and novice questions that require merely basic answers, but then get taken waaaay off topic by some who want to jump in and make claims about what they have accomplished, but who do NOT want to tell the whole story nor share details, nor the tricks of the trade that they have applied to be able to accomplish what they have.They merely wish to debate in order to somehow prove that their theories are correct rather than be of help to the original poster who asked for help. So the purpose of such threads are therefore lost and/or undermined, when they merely were started as a means for one person to ask for some help and advice on some basics. Enjoy.....

http://www.chevyhiperformance.com/techarticles/81679_tuning_engine_compression/index.html

 

[aukai]

Billy have you been accused of something before? The whole intro was a disclaimerl

 

[BillyGman]

Billy have you been accused of something before? The whole intro was a disclaimerl

Who me? No...never.........I was just trying to avoid needless debates about nothing. Somethig that I know would NEVER happen here at Team Chevelle. :noway:

 

[aukai]

Your cool Billy:thumbsup:

 

[Highway Star]

Billy,

I know why you posted this, and thank you for doing it.

Through spending half of the weekend in front of my computer plugging combos into my "Pat Kelley" SCR/DCR calculator, I think I've got a grasp on how all these numbers make up the identity of an engine and its responsiveness.

The dynamic stroke and its relationship to static compression is really what makes an engine a hustler or a slug. Head chamber shape and cylinder dimensions also factor in, but mostly the intake closing event is what determines how much dynamic compression will be made.

Here's what I want to know though. It seems to me that the intake closing angle (ICA) is ground in to the cam. When I plug numbers into my calculator, it figures the ICA off of adv duration and the intake lobe centerline. What happens when you put the cam in a few degrees back or forward? I know it will change the ICA but does it change anything else adversely? I can't see the whole picture yet. Will it make the opening too early or late and negate the point of installing it at a different point?

By the way, here's another good read on SCR/DCR, by a guy that was a TC member until he passed away a little while back. I was talking with Mike Lewis "Wolfplace" and he told me that he contributed Pat's work and the site I am about to link. RIP Pat.

http://www.empirenet.com/pkelley2/DynamicCR.html

 

[BillyGman]

Billy,

I know why you posted this, and thank you for doing it.

Through spending half of the weekend in front of my computer plugging combos into my "Pat Kelley" SCR/DCR calculator, I think I've got a grasp on how all these numbers make up the identity of an engine and its responsiveness.

The dynamic stroke and its relationship to static compression is really what makes an engine a hustler or a slug. Head chamber shape and cylinder dimensions also factor in, but mostly the intake closing event is what determines how much dynamic compression will be made.

Here's what I want to know though. It seems to me that the intake closing angle (ICA) is ground in to the cam. When I plug numbers into my calculator, it figures the ICA off of adv duration and the intake lobe centerline. What happens when you put the cam in a few degrees back or forward? I know it will change the ICA but does it change anything else adversely? I can't see the whole picture yet. Will it make the opening too early or late and negate the point of installing it at a different point?

By the way, here's another good read on SCR/DCR, by a guy that was a TC member until he passed away a little while back. I was talking with Mike Lewis "Wolfplace" and he told me that he contributed Pat's work and the site I am about to link. RIP Pat.

http://www.empirenet.com/pkelley2/DynamicCR.html

Guys like Mike Lewis would be able to answer this question that you have with more specificity than I can, since I don't have a dyno that I can test cams on in various engines. All I can do is like most everyone else does. try different cams and see what differences are made judging by the butt-o-meter from behind the wheel of a car during a spirited street drive, or even an occasional dragstrip run. But exactly where in the powerband any particular camshaft is making it's best power and exactly how much power is best determined with a dyno.

I had a 350 pump gas engine bored .030" over (355 cid to be exact) in a street car that I tried 3 different camshafts in. It had 3.73 gears, and a Muncie 4 speed trans and it was interesting and a little bit of a lerasning experience for me to see how each camshaft effected the powerband of the engine, and how the car responded on the street to each of the three camshafts. They ranged from a [email protected] dur to a [email protected] dur. and two of the cams were ground on a 110 LSA, wgile one of them was on a 108 LSA. With all three cams I used the sdame 10.2:1 static compression ratio, and with one of the cams, I also tried a second compression ratio of 11.0:1. It was a little bit like a mechanical lab rat on wheels. or in this case, I guess i should say a lab "mouse". :yes:

To answer your question, I don't think that the valve intake closing being varied by merely 2 or 4 degrees one way or the other is going to make a major difference. If you were to vary it 8 or 10 degrees, yes but I don't think that you're going to retard nor advance the camshaft that much during installation. There wouldn't be any reason for doing that. As far as the valve timing events taken at .050 vs. being taken at advertised duration, that DOES make a considerable difference. What I mean is, if you want to compare camshafts, it's difficult to do so using the valve timing events if they're displayed @.050 for one cam, and displayed at the advertised duration for another cam. I hope that I've answered your question well enough.

 

[70GS455]

Also keep in mind that most of these calculators assume that the camshaft lobe profile is symmetric. A non-symmetric profile will have it be adjusted to account for the valve events occurring at some other point. If not, this could lead to a few degrees error in IVC numbers.

 

[Guest]

We actually need a DCR program that will take into account advertised duration from a Cam Doctor at ONE specific lift off the base circle for ALL cams, and then durations at .050", .100", .150" and .200" lifts on each side of the lobe.

This will be more accurate, plus, it should take into account non-symetrically designed lobes.

But it still will be an approximation.

pdq67

Best Car Insurance | Auto Protection Today | FREE Trade-In Quote

 

[oldtimr]

Billy,



Here's what I want to know though. It seems to me that the intake closing angle (ICA) is ground in to the cam. When I plug numbers into my calculator,

it figures the ICA off of adv duration and the intake lobe centerline.

What happens when you put the cam in a few degrees back or forward? I know it will change the ICA but does it change anything else adversely? I can't see the whole picture yet. Will it make the opening too early or late and negate the point of installing it at a different point?

Pat Kelleys does not use ICL to calculate DCR. You can entirely skip Pat's middle tab of 3 and get your DCR.
Neither does KB's, it uses at .050 plus 15°.


You just tell the calculator the new ICA. EG, if the cam card says the ICA is 63° and you intend to advance it 4° .....

 

[oldtimr]

I agree with Paul in spirit but in practice all you need do is mount an indicator on your intake and as soon as it stops moving (gets on the seat)is when in degrees your particular cam is allowing compression to begin. That's all dcr is.

It doesn't take time into consideration (rpm), and as the engine spins faster less airflow can move through the same opening so compression almost begins to start sooner than at lower speeds.

 

[MadmanMark]

Here's what I want to know though. It seems to me that the intake closing angle (ICA) is ground in to the cam. When I plug numbers into my calculator, it figures the ICA off of adv duration and the intake lobe centerline. What happens when you put the cam in a few degrees back or forward? I know it will change the ICA but does it change anything else adversely? I can't see the whole picture yet. Will it make the opening too early or late and negate the point of installing it at a different point?

Duration and installed ceterline are all you need to calculate ICA. The relationship of intake events to exhaust events is ground into a cam (LSA), but the ICA will change depending on how the cam is installed (advanced/retarded). The basic jist of DCR is you don't really start the compression stroke until the intake closes, which happens while the piston is going up. The distance the piston travels, is treated as a lien on your stroke when calculating DCR.

Thanks for the article reference. It's pretty interesting. I sort of disagree with the conclusions of shooting for 170-185 and over 200 is trouble. There are lots of tricks to make 200-210 work on the street and there are strong motors out there cranking 160 and less. If there's one thing I've learned about car stuff, is nothing is simple (especially the stuff that looks like it should be).

 

[MadmanMark]

I do know 220+ is trouble. My first shortblock build was a zero-decked flattop 383 with a wimpy hyd flat tappet. I miscalculated compression and didn't realize my 64cc heads weren't going to cut it until the car refused to turn off. I should've had a clue during the test ride when the gas pedal sounded like it was connected to a paint can shaker. The tester was reading 220-230, so I know that much isn't good for a small cammed iron headed street small black.