Nascar engine specs vs. others

[Milwaukee Midget]

When my head was flowed (at 10" H2O), the flow at .400" lift was 127, at .450", 132, and at .500, 135.  Using a simple conversion to 28" H2O will yield 212, 220, and 225.  So I don't actually have 225 at .430" (I misquoted), but it appears that increasing the valve lift is producing diminishing returns.  This is why I haven't tried the new rocker arms yet.  Does this mean that my head is now port restricted?  And how do I know how much my motor will handle?  I could probably work on the head to give higher flows, but might that reduce the velocity of the airflow and possibly reduce atomization of the mixture?  As Bo (Wobbly) said above, I don't want to do anything that is irreversible, as I might already be at Nirvana 

It always will - it's not a linear deal.  Below is from my build diary from a few months back - the flow bench results of my Midget head -



Lift   cfm
.100   44.0
.200   87.6
.300   111.7
.4500   124.1
.500   125.6

Mel C&S tested it all the way to .600 (127.9 cfm), but it’s pretty clear that the party is winding down at ~.450, which I believe is well beyond the demands of the cylinder at that point.

Keep in mind that the flow rate during the intake stroke varies. 

If we assume a static situation where no ramming effect is in play, peak flow requirement will be at 90 degrees after TDC because that is the point that the cylinder is drawing air in at its fastest rate (cfm).  Of course, it's not as simple as that, but it plays into your observation about diminishing returns. 

Let's look at the small end of the cam, because peak lift and flow is only half of the equation. 

We'll use my flow numbers, because they are a more complete set.

Let's say that at .100 lift, the cylinder demand at that point of the intake stroke is 50 cfm.  At that point, we're port restricted at the valve opening - 44 cfm.  If we are able to increase that lift to .120 - a 20% increase in lift brought from your new rocker ratio - looking at an extrapolated graph of flow, we're seeing about 62 cfm available flow, which exceeds the need - at that point.  So you've effectively started filling your cylinder more efficiently earlier in the stroke.

We both run normally aspirated - we need all we can get, wherever we can get it.

[Koncretekid]

MM,
What you (we) are saying is that we should graph our piston position relative to our measured flow at progressive crank positions.  I will need to degree my cam lift at various crank positions to do this (although I may be able to get this info from Megacycle.)  Then what you're saying is that as long as the flow is greater than that required by piston position, we should be good.  The other problem is that we really need to know the velocity of the piston at each crank position, because the velocity, and hence the air requirement is constantly changing. The maximum velocity of the piston actually takes place near 75 degrees ATDC.  Gets complicated.

Even if I determine that I would like, say, a steeper ramp on the cam to open it faster, I don't have much choice, if any, in cams that I can buy.  Megacycle is not very receptive to making custom cams (I'm told, as I've have never actually been able to talk to the man in the driver's seat, always too busy!) Know where I can get a custom cam ground?

Tom

[Milwaukee Midget]

Hey, Tom -

Darn tootin’ it gets complicated!

There are issues of ram charging and extraction, swirl, squish that also play into this, and I’m not comfortable speaking to those, particularly in an engine I’m not familiar with.

I guess the general point I’m trying to make is that by using the larger ratio rockers, the biggest gains in airflow don’t come at wide open valve lift, but at partial lift.  And that is still usable.

Let’s go back to your numbers, 212 @ .400, 220 @ .450,  225 @ .500.  You mentioned diminishing returns, and yes, if you could push this valve out to .600, it’s not likely that you’d break 235 cfm.  You’re close to maximum.  If you think of it in terms of an ascending curve on a graph, you’re at the top on the right hand side, and flattening out.

What would be good to know is what the flow numbers are at .100, .150, .200, .250, .300, chart them on a graph, and then extrapolate what the flow figures would be if the valve opening were to be increased by 20% at each point – essentially shifting the graph. 

The piston will be in the same relative position to the cam timing, but the valve will be open 20% more.  At .120 versus .100, you’ll see a lot of potential for better flow, at .180 versus .150, the effect will be a little less dramatic, but still to the plus, etc.
 
So during the entire intake stroke, overall flow potential is higher, yet the valve still opens and closes at the same points on the intake and compression stroke.

The greatest potential to achieve better flow is actually between the point that the cam starts to open the valve until you meet that point of diminishing returns toward a wide open valve.

If it doesn’t cause the valves to get too comfy with the pistons, or the springs to buckle, I’d try the larger ratio rockers.  You’ve already got ‘em! 

[Koncretekid]

Chris,
I'll drink to that 

I have some ideas to try, but really need that dyno time.  Too bad it'll have to wait till springtime. 

In the meantime, I realize I should have asked the same questions above of the guys running Dodge Hemis, as they are probably even more closely related to the BSA than a wedge head Nascar motor.

My conclusions are that I have a motor that has the right architecture and doesn't know it isn't a Nascar motor, so there should be more to gain.  Easy things are dyno time with different intake and exhaust lengths and diameters; difficult thing is optimizing internal head flow and cam characteristics.

[wobblywalrus]

Tom, that piston tells a big story.  Good benchmarks to compare to are Harleys and Buells.  Some of your solutions to getting more power might be similar.  For example, dual plugs are used on some racing harleys to make sure all of the mixture on both sides of the piston top are ignited and burned.  Don't hesitate to do this.  There a is a lot of info out there and many of the country's best tuners work on these bikes.

[Koncretekid]

Tom, that piston tells a big story.  Good benchmarks to compare to are Harleys and Buells.  Some of your solutions to getting more power might be similar.  For example, dual plugs are used on some racing harleys to make sure all of the mixture on both sides of the piston top are ignited and burned.  Don't hesitate to do this.  There a is a lot of info out there and many of the country's best tuners work on these bikes.

Bo,
Harleys and Buells do not have hemi head combustion chambers, to my knowledge, and are not really designed to run much over 6000 rpms.  Dan Dunn at NRHS performance is my local Harley expert, and I'm in frequent contact with him.  His wife, Lucille, holds the current record in 500 M-PG and MPS-PG on a Buell Blast at about 115 mph and 118 mph, respectively.

As for the twin plug, I am already using twin plug ignition.  That said, I haven't really played around with timing on the dyno to see what can be gained.  That will be part of the next program.

Tom

[jacksoni]

You might look at this Speedtalk thread and follow the links to Stan Weiss's website and software. Lot easier than the calculations you are planning to do!

http://speedtalk.com/forum/viewtopic.php?f=15&t=33017

[wobblywalrus]

Tom, I was thinking about my old shovelhead.  The pistons looked similar.

[panic]

All H-D OHV motors beginning in 1926 are hemi, until the bathtub Evo (1983 big twin, 1986 Sportster).

The spark lead for a dual-plug H-D engine is generally around -5° from stock.

[panic]

WebCam does custom work, they've been doing side-valve stuff for us for 30 years.

http://www.webcamshafts.com/

What exhaust percentage does the head flow, meaning will you need a dual-pattern cam?

[Koncretekid]

All H-D OHV motors beginning in 1926 are hemi, until the bathtub Evo (1983 big twin, 1986 Sportster).

The spark lead for a dual-plug H-D engine is generally around -5° from stock.

Panic,
Does -5* from stock mean retarded 5* as I think it would?

As for exhaust flow, that hasn't been flowed, yet.  I have a simple flow bench I made that uses a flowmeter within a 3" PVC pipe hooked to a shop vac and I will be doing some research this winter, using a stock head for comparison.  I will check out Web Cams.

You might look at this Speedtalk thread and follow the links to Stan Weiss's website and software. Lot easier than the calculations you are planning to do!

http://speedtalk.com/forum/viewtopic.php?f=15&t=33017

I will be checking this out shortly.  Interesting that last post -- suggests that the software may have a flaw.  When I've graphed piston position/ piston speed, hence flow required vs. valve lift/flow bench results, I'll have a pile of data that I probably won't be able to interpret, but it should make an interesting bench racing topic.

Thanks
Tom

[panic]

Typically, a good hint is "does the hottest factory cam have split duration", and if yes, which way?
General rule: if the exhaust flow is below 75% of intake, the exhaust needs more duration (not lift). If the overlap period is enough, the XO point is bumped back to add the degrees. If it wants more, the extra can be split (same XCL) or put any way you want.
if the intake isn't 125 to 133% of exhaust, stretch the intake - but this is more delicate because both ends have consequences: overlap vs. DCR.
Typically, the head porter's input is really helpful here.
Exhaust: open? megaphone?

[Rex Schimmer]

Tom,
I am re-opening this post based upon some reading I have been doing in a book by Kevin Cameron, "Classic Motorcycle Race Engines". One of the engines he analysis is the BSA Goldstar. He states that in 1956 the factory was making 42 hp at 7000 for a BMEP of 156. If you could get your engine to have this BMEP at your 7800 rpm you would be seeing additional 9 horse power. Cameron is not a "friend" of hemispherical combustion chambers, which, after reading most of his book, are not the optimum shape for maximum power. You should get the book and read, lots of stuff on early single cylinders all very interesting.

Rex

[gearheadeh]

Tom,
 Cameron is not a "friend" of hemispherical combustion chambers, which, after reading most of his book, are not the optimum shape for maximum power.

Rex

Yes but it would appear that "Nitro" is a really good friend of the HEMI!

[Koncretekid]

Rex,
I ordered my copy of Cameron's book the day you posted a reference about it.  But how much do we trust what the factory said they got out of the old motors?  I'm guessing that it was not rwhp for example.  On my last dyno run in Longmont, CO, I got 44 hp (rwhp) at about 7700 rpm with maximum torque of 31 ft-lbs at about 6350 RPM.  I don't know what bmep that is, but I'm sure someone can tell me.

 I also know that the FIM record for a 500cc production bike is 108 mph on a BSA Goldstar (T.Meadows, who also holds the PPV 500 AMA/BUB record at 104mph).  I made 109 mph on my B50 roadracer which was orignally built by the late Ted Hubbard, a well known BSA race motor preparer,  which dyno'ed at about 33 rwhp. I would have thought that a 42 hp Goldstar would have gone faster than 108.
Tom