Mild (less than ideal) 466 chassis dyno results, guesses for conversion to flywheel horspower

After having wanted to take my 69 Mustang to a chassis dyno for years now, I finally got it tested and tuned today.  Of course this leads to the inevitable follow up question of I wonder how much horsepower I am losing through the drive line and what would it's flywheel number be so I wanted to poll the knowledgeable folks on here to see what the opinions are.  The basics is of my engine (originally built/put together way back in the early 90's and not finished until a few years ago) are listed below as well as the driveline info.  You'll see two separate graphs because the shop I went to used a Mustang type dyno that applies a load versus the more conventional Dynojet inertia only type dyno which will yield numbers approximately 12% higher which is the second graph.

466 cid, 11.4:1 compression, forged pistons, stock crank and rods
Dove heads ported and polished including CJ gasket match (which is basically wasted on the intake side using a Torker II intake manifold), but retaining the small valves ***
Old single pattern Comp Cam 294S solid flat tappet, 0.605 lift, 248 duration at 0.050***
Holly 850 DP (box stock)
Torker II intake
MSD distributor and 6AL box
FPP 1 7/8" shorty headers ***
2" exhaust with H pipe and FlowMaster mufflers ***

The items followed by *** above denote the known weak points or bottlenecks to HP potential to the best of my understanding.

Drive line is a C6 (heard these consume up to as much as 60 HP??) w/11" 2500 stall converter, Ford 9" rear end with 3:89 gears.  
Also the car has stock Ford power steering.

Thanks in advance for any opinions.  I'm thinking that 30% drive line loss is not out of the question.  The dyno operator said maybe even 30 to 35%!

Edit: I can't seem to upload the graphs which are 1 Mb jpg for some reason.  
Dynojet number: 401 HP @ 5836 rpm;
Mainline dyno: 352 HP @ 5836, 343 ft/lbs @ 5017 rpm

What about the 1/8 or 1/4 mile dyno what do they say?

Didn't get that information.  Just HP ant torque curve vs RPM and KM/hr. Trying to convert the files so I can upload, but this site say max file size 0 Mb? I'm thinking some sort of glitch or something.

I mean the dragstip

For a somewhat close comparison:

468 cid, 10:1 compression, hyper pistons, stock crank with H-beam rods
429 PI heads, hardly touched
Same cam as you have - Comp Cam 294S solid flat tappet, 0.605 lift, 248 duration at 0.050***
Holley 750 DP with the choke horn milled off
Offy port-o-sonic single plane intake
Duraspark distributor and 6AL box
Hooker 2" LT headers, 3.5 collectors

I had the above combo on the engine dyno, it made 475HP, 502ftlbs on 91 octane pump gas. I ran this at the track in my 87, it would run 10.60's in good air, 10.80/90's the heat.

I don't think the C6 will absorb anything close to 60hp. jmo

IMO,Cam,exhaust size and mufflers are the biggest power killers in that combo.

Mark Miller wrote:IMO,Cam,exhaust size and mufflers are the biggest power killers in that combo.

Yea, I agree with that for sure and to an earlier comment, I have never been able to get it out to a drag strip for any test and tune (been working out lots of little bugs here and there for some time) so no idea on what it runs unfortunately. As far as guessing on flywheel horsepower when somebody asks in the future, I guess I'll play it conservative and use 20% loss which would be 501 hp. That's respectable enough I suppose and don't want to accidentally exaggerate.

BigRigTech wrote:For a somewhat close comparison:

468 cid, 10:1 compression, hyper pistons, stock crank with H-beam rods
429 PI heads, hardly touched
Same cam as you have - Comp Cam 294S solid flat tappet, 0.605 lift, 248 duration at 0.050***
Holley 750 DP with the choke horn milled off
Offy port-o-sonic single plane intake
Duraspark distributor and 6AL box
Hooker 2" LT headers, 3.5 collectors

I had the above combo on the engine dyno, it made 475HP, 502ftlbs on 91 octane pump gas. I ran this at the track in my 87, it would run 10.60's in good air, 10.80/90's the heat.

Awesome! That is a pretty close comparison and very encouraging. I assume that was running slicks or drag radials right?

Your power is about where I thought it might be . It is not uncommon to see around 200 HP loss through the drivetrain and tyres . A chassis dyno really doesn't give a 100% accurate power figure , it IMO is a tuning tool .

The areas you have marked could improve . A 2" exhaust definitely is choking it .

As a comparison though was quite a few years ago , I dyno'd a 560 HP 535 ftlb Motorsport Crate engine . In the car with a 3500 converter and 3.5 gears showed 361 RWHP

I have seen several other similar cases with about same losses

all my passes are made on 28 x 10.5 ET Drags.

2 inch exhaust and flow master is killing it , also 2 inch long tube headers would help.

That single pattern CAM is smoking gun. All you have to do is change that and it will respond in leaps and bounds.

I appreciate all the advice and as money allows I definitely want address the noted weak points/bottlenecks, but overall, considering the obvious weak points I'm pretty happy with 401 RWHP which conservatively translates to at least 500 flywheel HP. Also this engine was put together back in the early 90's on a moderate budget so I think it is a good example of BBF cheap power crammed into a small engine bay. Besides, I can't make it hook up with street tires as is and more power would be a waste without suspension upgrades and drag radials