I also found that Desktop Dyno was a total wash compared to my dyno charts.

Using the valve timing, matched cylinder head flow files and all engine dimensions
down to a science, the HP/TQ figures were more than 100 units off (considering
driveline losses)
and the peak RPM's were nowhere near the real world figures.

Just wanted to share the info for those that are using DD2K to setup race
cars, or get 'ball park' values for benchmarking.

Don't rely on that program.

 

There are better programs available, but GIGO always applies.

 

Agreed.

For < $25.00 and ~ half a megabyte of executable "math", you can't
expect much.

 

Then I'd try to set my car up like the "test" car. I've seen what was in the car and it had no tricks, just done right, including dyno tuning, select weight reduction, but still in a rollbar equipped T/A. Pretty impressive. And no, I don't own an LS1.

 

Well normally you can run a more aggressive lobe on the exhaust due to it's lower mass.... but normally you run the same valve springs since you by them in sets of 16 but it's not required.

Lobe aggressiveness is not always easy to spot, there is a relation between durations at different lifts and max lift... You can easily have a low lift aggressive lobe that's hard to control than a high lift lobe that controls the valve better.

Bret

 

Conversely, aggressive lobes on the exhaust have proven to be unnecessary.

 

Depends on your application Mike.... I can find a few people to debate that point. A fast opening exhaust valve is actually a good thing IMHO.

Bret

EDIT:http://www.thunderracing.com/custome...cid=36&x=8&y=8

The cam went fast in a race car... good **** to see!

"4.56 gears with spool"
"Home Town: Baton Rouge, LA"

Those two things help! A guy with a posi, 4.10's and the best DA of the year might get to 1500 ft ain't gonna beat that.

 

Right, but because the exhaust valve lifts more, it will have more open pressure with the same installed height and the same springs. My thinking is that perhaps less seat pressure is necessary on the exhaust valve because it is lighter. If that is correct, it would make sense to set up the springs for the same open pressure, and then the exhaust valves would have less seat pressure. Of course, if the combination isn't really wild, the pressure difference might be small enough that shimming the exhaust and intake springs differently wouldn't make a measurable difference in longevity of the cam or rpm ability.
For example, with a 400 lb/in street-strip spring and a 0.020" lift difference, the pressure difference would only be 8 psi. I wouldn't be at all surprised to see that much variation in a set of springs. After the springs break in, there could be even more variation in pressures, so I guess this idea is pretty much just a whisper on a scream. If I needed every hundredth I could get just to be competitive, then I'd think about it more.

 

I would rather have more than less pressure if it's that small, just do to spring pressure bleeding off with spring life.

Bret

 

gm knows how to make a cam that will put out and stay legal emission wise , but to match the bottom end to the power that could be made would cost too much ...my thoughts on why the numbers don't seem right, or am i way off base here

 

Bret- thank you for that post. I actually could follow most of that. And a lot of it lines up with my (limited) experience as well.

That's really getting to the heart of what I'm talking about. Remember I build STREET motors with mostly off-the-shelf parts. High peaks and soggy torque curves don't get it done for me. I like to get slammed in the seat and held there through at least a 3000 RPM powerband (preferrably 3500 or as far as I can stretch the envelope) with a little something left beyond peak in case I need to hold a gear a little longer for some reason. Remember I'm talking about a drivetrain that usually has 3 forward gears and a pretty big RPM drop between shifts (TH-350) and rear gearing/torque converter is not usually optimized for dragstrip use, either.

For the sake of this discussion let's just say that I'm working with "known quantities" as far as intake (single plane Vic Jr, dual plane Performer RPM), decent off-the-shelf heads (AFR 195 or simialr), longtube headers and good flowing exhaust, but with mufflers, and the combo is properly matched as far as cam and compression. That's pretty common for me. No "weird" quirks like odd intakes, bizarre compression ratios or corked-up exhausts. Vanilla.

Up to recently I had one basic "formula" for getting results that often exceeded expectations- a single pattern cam that is probably too small but with good flowing heads, intake and exhaust. I would use the good flow into and out of the engine to "stretch" the usable RPM range of a given cam usually well beyond what many people would think is possible. 220* cams shifted at 6300 or more for best ET and with street manners that just couldn't be beat. That kinda stuff.

Now what I'm trying to add to my bag of tricks is some knowlege about how I can use duration splits to fatten things up even further- maybe EVERYWHERE in the RPM range, if that's even possible.

When I run my simulations (admittedly cheesy DD2000 simulations) It seems I can use a dual pattern cam with at least 10* more exhaust duration to beat my current single pattern combos pretty much everywhere. Instead of a 230* single pattern cam I could plug in an otherwise identical dual pattern cam with 225/235* durations and beat the single pattern across the board- particularly on the high and low ends of the RPM range. Less intake duration helps low end torque, more exhaust duration "stretches" the top end. EXACTLY what helps most on the street.

Does this seem "real world" to you? I'm a big fan of single pattern cams in my motors and have been for a long time but the promise of a dual pattern cam that can give me improvements just about everywhere in the RPM range is just too tempting to ignore.

 

Well if you have exhausts that aren't plugged up I think you are on the right track already, no need to go to a huge split duration cam. I personally like the small LSA small duration cams myself to get the job done on a street motor. I would bump up the duration a little more on both sides and see what that does first... remember more exhaust duration means a higher LSA to get the same overlap which I don't think is a good thing.

I like the good heads, small cam, basic intake idea. Works for me.

Bret

 

Yeah - I always try to error on the side of a "smaller" cam.