Milwaukee Midget

[wobblywalrus]

Sometimes it is a good idea to choose a cam that has reasonable specifications, then tune the intake system to work with it, then tune the exhaust, and last to optimize fuel composition and spark timing.  At that time, after everything else is optimized, a person can make a judgement about how to alter the cam timing.  Experience has shown that all sorts of problems a person would associate with cam performance disappear when everything around the cam is adjusted to work with it.

[jacksoni]

Thanks Chris. I think I get all that and my understanding and what I was trying to say agrees pretty much with everything you said with perhaps the exception of the last sentence. There are huge differences of opinion about the importance of rod ratio as opposed to the actual stroke, piston speeds and acceleration and position in the bore, which is very small. The engineers and physicists calculate and show only a few thou difference in position or piston speeds at some point and suggest that rod ratio is of (not none but) considerably less importance than other engine parameters. Other engine builders and racers and cam grinders suggest it is considerably more important. Never the twain shall meet ( republicans vs democrats, Christians vs non Christians etc, etc). truth may be somewhere between but is a hard to prove deal as every change effects every other system and optimizing one, changes another. Like the EFI vs Carb argument and which makes more power. I have a rod ratio similar to yours and thus have tried to follow such arguments. Personally, and readily admit may be wrong, I don't think in the scenario I was trying to present/ask about, that rod ration makes any difference. Piston location in the bore is microscopically different with small vs large rod ratios. But what is happening in the intake port (read the whole induction system) and exhaust at overlap and then after the exhaust valve closes and there clearly is further cylinder filling (allowing greater than 100% VE- at least in some engines) determines that important greater than 100% VE resulting in the greater dynamic CR that FordBoy so eloquently described some time back. What I was trying to say was closing the intake valve to take advantage of that is where it's at, Just right, not too soon, not too late. I do understand the rocking the torque curve to the right for top end. and yes, messing with cam timing often does that alone, not magically pick up the whole curve.

[Milwaukee Midget]

Wobbly, yes - our cam selection is what we're working with and around.  It's maximized for this application - lift, duration, overlap.  Our experiments last year led us to the intake length and size we ran, and other than a rejet, I'm convinced that side of the equation is optimized.

Harold, we were a bit stumped last year when Pipemax predictions didn't match up with what we were seeing on the dyno.  I've got the header I ran last year and a somewhat larger one to test, and we'll be bringing along exhaust pipe to determine a good length.  Might be a bit more to be had in that department.  My LCA is 108 - a function of valve clearance parameters.

As far as rocking the cam, again, I'm not expecting huge gains.  But if a couple of pulls bring up no extra horsepower, but a better looking curve, I'm going to take it.  

It's all nickel and dime at this point - cleaning the coins out from between the seat cushions.

[Harold Bettes]

Hey Chris and Mark -

Thought I was making a smirking funny with my A, B, C comments, but the joke didn't work..... The only comment of merit was about the exhaust and that comes from testing and dyno results not just program calcs.  Ricochet Hp is not the best approach.

Obviously a tighter lobe center gets things closer and as Mark has stated before, when things are tight you have to work with what you have. Afterall, a cam can't make airflow.

OK, now I will go back to my cave and hibernate some more as spring is not here yet.

Regards,
HB2

[Milwaukee Midget]

Harold, you're always welcome here.  We're getting slowly smart on this lump, but it has thrown us a few curve balls - the exhaust in particular.

After last year, compared to WOS in 2010 and Maxton in 2011, I've made up my mind that I simply will not EVER take an untested motor to the salt again.  It's so much easier dialing it in on an engine stand in a dyno bay than bending over a knee-high fender in 95 degree heat with your neighbor's blown BBC making it impossible to think.

OK, now I will go back to my cave and hibernate some more as spring is not here yet.

Regards,
HB2

When did you move to Wisconsin? 

[Milwaukee Midget]

The engineers and physicists calculate and show only a few thou difference in position or piston speeds at some point and suggest that rod ratio is of (not none but) considerably less importance than other engine parameters.

Jack, I highlighted that in red for a reason.  Even a few thousandths difference in the closing event of this combination - keep in mind, we're talking less than 250 cc per cylinder - can make a big difference in the amount of dynamic compression ratio we'll be able to achieve.

I can loose a 1/4 point of static CR by simply using a .006 thicker head gasket.  It's not quite the issue on larger capacity engines.

[fordboy628]

Conversation overheard @ Pro-Motor Racing Engines circa 1981:

Fordboy:  "I'm struggling to reduce the chamber volume by 1 or 2 cc's to raise the compression ratio on these 1000cc Cosworths."

Big Bob:  (Noted AHRA Pro-Stock driver & engine builder of 600 cubic inch Big Block Chevies)   "Who gives a f*** about 1 or 2 cc's?"

Frame of reference can really affect your attitude . . . . . . . . . .   like, by a factor of 10 . . . . . . . .    Just a thought.
 
Fordboy

[jacksoni]

The engineers and physicists calculate and show only a few thou difference in position or piston speeds at some point and suggest that rod ratio is of (not none but) considerably less importance than other engine parameters.

Jack, I highlighted that in red for a reason.  Even a few thousandths difference in the closing event of this combination - keep in mind, we're talking less than 250 cc per cylinder - can make a big difference in the amount of dynamic compression ratio we'll be able to achieve.

I can loose a 1/4 point of static CR by simply using a .006 thicker head gasket.  It's not quite the issue on larger capacity engines.

\I agree 100%. I was talking about piston position with respect to rod ratio, not the valve closing point. Anyway, looking at my post last night, which was done late and after a few  might have sounded argumentative. Was not at all meant to be and if came off that way I apologize. I am just trying to learn here as well. As I have mentioned before, I am in similar boat as can't decrease chamber size, domes on my  pistons just make sharp edges sticking in places they shouldn't be. I have the thinnest gasket I can get away with, with maybe .033 P-head clearance, minimal P-V clearance etc and had a custom gasket made to fit bore and notches in top of cylinder which are there to clear the chamber to minimize any dead space. Will continue to follow and look forward to results on the dyno.

[Milwaukee Midget]

No problems, Jack -

As long as we keep my momma out of it . . .

[krusty]

     "Conversation overheard @ Pro-Motor Racing Engines circa 1981"

     Peter Guild's ProMotor in the old Shadow CanAm building in EGV? Perhaps that's why your name is familiar to me, Mark.

     vic

[fordboy628]

     "Conversation overheard @ Pro-Motor Racing Engines circa 1981"

     Peter Guild's ProMotor in the old Shadow CanAm building in EGV? Perhaps that's why your name is familiar to me, Mark.

     vic

The very same.   Peter rented the building from Don Nichols if I'm remembering correctly.   All the old UOP Shadow Can-Am cars were upstairs in the storage area.     Would like to have one of those now . . . . . . . 

I used his Heenan & Froude "meat scale" water brake to test the F/Fords & Cosworths I was building at the time.    Peter was very generous to me to allow the use of his facility.    Although having Peter be married to the sister of my main client at the time sure didn't hurt . . . . . .      And I was occasionally crewing for Raghead #2 on his Trans-Am effort.    Pro-Motor built all his engines and he was competitive at the races he attended.    Just not enough dough for a full season effort.    BTW, Peter was another of the guys who was generous to me when I was at the beginning of my "career" . . . . . . .

Incidentally, that's how I got the "Fordboy" moniker.    Pro Motor was basically a Chevy shop, and "Big Bob" and the other guys who worked for Peter were 'somewhat' offended that I was allowed to drag in a bunch of 4 cyl Ford stuff.     You know how racers are, everybody else's "stuff" is crap, until they kick your a** on the track . . . . . .  or set a record/win a championship, etc.    They were impressed that the stuff I was building didn't leak or blow-up.   And it turned out that they made competitive bhp.    As a "pro" you are doing your job when you can accomplish that.

Sheesh, now I'm feelin' my age . . . . .     Guess I'll have a suds, after all it's 17:14 on a Friday, Holiday weekend at that!   
 
Fordboy

[Milwaukee Midget]

Jacksoni is in a similar boat, so maybe this might be of help.  It's had me tearing my hair out all day, so I devised an experiment.



Amongst the huge inventory of junk in my garage/basement/attic/office, I managed to locate a stock 1275 piston and 5 ¾ inch rod.  Seeing as the engine is half way apart anyway, I replaced the front piston assembly with the stocker, and left the other three in place.  And as 1 and 4 travel in tandem, a single cam reference will produce the height example I’m looking for.

I referenced the piston height to 90 degrees ABDC on the crank wheel, and installed blocks on the piston faces to gather height sufficient to measure piston travel with dial indicators.  At 90 degrees, any difference in rod angle with respect to piston travel is 0.

I then turned the crank to 102 ABDC – the point where the cam absolutely closes the #1 intake valve.  Then I measured the distance the 2 pistons traveled from 90 degrees to 102 degrees, and added 1/2 the stroke length, which would be the same for either rod at 90 degrees ABDC.

Corrections in next post.

Here are the results.

Stock piston travel w/5 ¾” rod 2.35:1 R/S ratio – 1.489”  
Current piston w/6” rod           2.45:1 R/S ratio – 1.484”            

So at the point that the valve is completely seated, the longer rod provides .005” additional potential dynamic compression compared to the shorter rod.

The difference calculates to .16cc.

Now NONE of this takes into consideration the dynamics of piston speed, ramcharging effect, relative long duration at the extremes of the stroke with the bigger R/S design, or any other dynamic consideration that a running engine brings to the table.

But I think – and I don’t know for sure – but I think the additional potential volume of compressible A/F mixture at the time of absolute valve closure is likely positive in this application.

Discussion is open!    

[Milwaukee Midget]

Subaru - I just realized I didn't take into account valve lash.  

Corrected to 67 degrees ABDC - allowing for .020 clearance with a 1.6 rocker ratio.

Stock piston travel w/5 ¾” rod 2.35:1 R/S ratio – .636  
Current piston w/6” rod           2.45:1 R/S ratio – .632

So at the point that the valve is completely seated, the longer rod provides .003” additional potential dynamic compression compared to the shorter rod.

About .09 cc difference.

I was sitting there drinking a beer when I thinks, "How is it the valve isn't closed until well after 90 degrees ABDC?"

That's then I had my Homer Simpson moment - "Doah!"  

[Peter Jack]

Beer and quiet times definitely assist the thinking process. 

Pete

[jacksoni]

MM- I have seen your experiment referred to in other forums as well as calculations that produce similar results and the results commented upon. I think you may admit that .09cc may not be worth stressing over particularly if the change is either expensive (new custom rods/pistons for instance) or produces some other issue, theoretical (side loading the piston increasing wear or friction) or real (clearance with the block for instance). People argue endlessly about performance differences- how fast short vs long accelerates out of a corner on a circle track car for instance-but actually measuring a hp difference is harder even with the best dynos. I know really comparing just the ratio change is very hard as-fordboy has pointed out I think- you change one thing, something else needs to be adjusted for optimization. Like I said before, it is not that rod ratio has no effect but that it is down the list of things on which your time and money might be better spent and then if you say I have done everything I know how and am still trying to find that last half HP (you there yet? ) and my bank account is feeling overfull so I am just going to buy a new set of custom rods and pistons to try, have at it. Although they have other design considerations, that they likely feel more important, it is my understanding that Formula 1 engines have rod ratios similar to yours. They don't seem to be going to great lengths to get them in a "better" range.  All this JMHO as usual so take with your grain of salt.  Hmmm, salt and beer. I wonder......