Desktop Dyno versus real world...

[JayBee]

I don't have the means right now to have the engine dynoed, so I am wondering if it sounds like I'm making the power I should with the following engine specs. It's a Chevy 396 configured exactly like the numbers shown at the left in the picture below. Only real difference is I have pretty decent flowing logs versus the small tube header open exhaust setting I used on DD.
http://webpages.charter.net/jlbarnes/boat/xm278dyno.jpg
The pump is a Berk JC with a Berk A impeller. My usual boating conditions are mid 80's and high (70% - 90%) humidity, about 1700ft elevation. The pump is tight - rebuilt with HTP wear ring about 100gallons of gas ago... Right now I see 4300 - 4400rpms max, and I have confirmed this with a digital diagnostic tach against the one on the dash. Since this is a heavy V-hull boat I am seeing about 48 - 50mph GPS.
I had been trying to decide if cutting the impeller to a B would gain me anything. It would take me a bit further up the HP curve, but on the other hand I'm already starting down the torque curve... So, I've pretty much decided against that. I have been considering advancing the cam a bit (it's straight up right now) to see if that helps get me a little closer to the HP peak. I've played with less aggressive cams in DD, but didn't see much improvement in mid 4000 rpm range... They dropped the peak HP back a few rpm's, but also dropped the hp at 4400, too.
I have searched through a ton of old posts, and while I haven't found any 396's with max rpms versus impeller size listed, I have seen enough mild BBC's listed spinning Berk A's in the mid 4000 rpms range to figure mine doesn't have any major tuning issues... What are some of your opinions on what I could do to dial this thing in for the best power? Not expecting any huge gains in speed, but any small gain would be nice. I might have to borrow a set of old OT headers and slap them on to see what it pulls then. I have an enclosed engine bay, and definitely no budget for jacketted headers right now but it would be nice to see what potential they had...
Thanks,
Jason

[flat broke]

Jason,
It looks like your following pretty close to the curve posted in DTD. Bear in mind that DTD doesn't account for your environmental conditions, and your logs are deffinitely not flowing as well as a set of small tube headers. If you have the means to advance the cam (remember to recheck your valve to piston clearance just to be safe), and don't mind the work, you could try advancing it 2-3 degrees to see what it does. Typically it will just shift the curve around and not really affect the gross number too much.
Actually I believe you can run an iteration in DTD with the cam advanced. Down in the cam profile section, there is an area where you can advance/retard the cam timing. Doing so should move the curve in close approximation to what you would see in the boat. I would also use the HP manifold and muffler selection for the exhaust as it probably better emulates a very restrictive log setup.
All in all I'd say you're setup is running pretty close to what should be expected and I defintely wouldn't cut the impeller down to a B as I don't feel there would be an adequate tradeoff.
If you're looking for more speed, concentrate on getting the boat out of the water more, and that should allow you to run faster.
Good luck,
Chris

[malcolm]

Those numbers are pretty high I'd guess. You need to get some airflow files for your heads and plug them in to get a little more realistic numbers. That, and set the exhaust for manifolds and mufflers, those logs are terrible HP stealers.

[JayBee]

Thanks for the input. So, the logs are really that restrictive? I've noticed some logs look worse than others, and at least mine have the bigger ports and swept back flow design. I do see that the risers are horribly restrictive, though. I just figured that DDyno was referring to automotive exhausts, and I would think auto manifolds, mufflers, and 10 feet of exhaust pipe to be way more restrictive than the logs...
How are the high performance aftermarket Mercruiser style manifolds? The ones that go up from the heads and then have an elbo at the top for a riser. I see those on ebay quite often a could budget them much easier than a set of jacketted through transom headers...

[P-Money]

Not to totally hi-jack the thread, but I was looking at some EMI Thunders for a blown, intercooled 496. Anyone have expereince with them?

[malcolm]

I have heard the center rise are better than logs, but don't have any experience with them. I know going to headers from logs gave me 300 RPM and 2+ MPH.

[JayBee]

I guess it would be a good experiment to get a set of old rusty headers and see if they raise the max rpm's. My engine cover is easy enough to remove, so it wouldn't be much work...

[malcolm]

That's exactly what I did. Rust and all. My engine cover and jump seats are still out under the deck. :wink:

[LVjetboy]

JayBee I wouldn't be surprised if density altitude cuts your power by 30-40 hp. DTD uses 68 F/ 0% / 0 ft and 29.92 inHg. Also, as others mentioned, logs/w risers, water injection and flappers may be as restrictive as a muffler and stock exhaust. Those two alone could easily account for the difference you see.
For example, I ran DTD with your engine and stock exhaust and got the following...
http://members.cox.net/lvjetboy/JayBeeChevy.jpg
Notice at 4400 rpm your predicted power is now 310 instead of 375 hp...a loss of 65 hp from restriction alone. Don't know how DTD's stock + muffler exhaust model compares to your logs, but a 40-50 hp loss over open headers wouldn't surprise me. Then you factor in density altitude. With the high humidity, temperature and 1700 feet you're running nearly 5,000 feet DA. Here's an online calculation...
http://members.cox.net/lvjetboy/JayBeeDA.jpg
I used 94 F instead of your 85 F because this calculator's based on a standard of 77 F for “rated” power instead of DTD's 68 F. So I added a 9 degree delta, a bit of a wag but better than nothing. I also used a standard 29.92 as your altimeter setting because you didn't record one. That density altitude cuts your DTD rated power by 12% (88% of rated) or at 4400 rpm with restrictive exhaust you now have 275 hp (0.88 x 310) instead of 310. 35 hp less. Does that make sense when you look at Berkeley pump curves (JPC)? Sure, at 4400 rpm the curves say a Berkeley A absorbs 270 hp...see above inset. Those types of adjustment are why some (not saying you) are confused when they think their engine’s rated peak power is a lot higher than what they spin at the lake. 430 hp sounds a lot better than 270 hp right? But 270 it is, and that where it counts.
As for cutting to a B? I agree with Chris. Once DTD’s adjusted for DA you're about 30 hp off peak. Even if you cut a perfect match to peak, and you ran a smaller 18-19’ jet, you'd only see 2-3 mph. With a larger jet maybe one mph or so? On the lake you may not notice that small of a change. And you may loose a bit of hole-shot. Also, if you do uncork the exhaust, you’ll likely move your match point bit closer to peak.
jer

[flat broke]

Jer,
Since you have an iteration built to his specs in DTD and I'm lazy, does advancing the timing 2-3 degrees put him right on top of the peak?
Chris