VE and ACFM seems low on Boss 347 build

[c9zx]

This has been nagging at me for almost two years. I'd like some input from the members here. Details; 1969 Boss 302 stroked to 347, 10:1, SFT cam 296/304 Adv., 244/252 @ .050, net lift .612/.609, L/C 108, in on 104, Mild port work, Edelbrock E-Boss dual plane intake, Holley HP 750, 2" Dyno headers. Dyno results; Standard Correction, 526.6 HP @6900, 434.4 TQ @5100, peak VE @ 6800=92.5%, peak ACFM @=642 cfm.  Fuel (lb/hr) @ HP peak =215.1 At 1.5 HP/CID the VE and CFM seem low to me, but I may be very wrong. The runner volume on the Edelbrock are a LOT Smaller than stock, a good thing I think. Any input or opinions will be appreciated. Thanks, Chuck

[blykins]

Dynos are dynos.  Without a baseline of other engines, it's hard to really point a finger at anything.  That's why it's so good to dyno a plethora of engines on the same dyno to get a feel of what's good or bad.  Contrary to popular belief, numbers vary and they can vary big time. 

One of the guys that dynos where I dyno put a circle track engine on the pump, then took it up to Bischoff's and it made 60 more hp there. 

[c9zx]

Brent, I can believe the 60 HP difference. There are a lot of variables, atmospheric conditions, elevation, a differences in what "correction" SAE formula is being used. What kind of VE numbers do you see on builds similar to mine? Any insight is appreciated. Chuck

[Rory428]

I just looked at the dyno sheet for my own 347, close in power, but different setup. Mine is a "big bore" 347, it has a Dart block at 4.125", with a 3.25" stroke, 10.6 compression with flat top pistons and 58cc heads. (AFR 185 Renegade). It has a hyd.roller cam (.571"I, .587"E, 231/239 dur@ .050", 112 lobe sep.). It has a Parker Funnel Web single plane intake, and was dynoed with a 4779 Holley 750 DP. Best numbers were 528HP @6800 RPM, peak torque was 451@5200. My VE and ACFM numbers were much higher, 106-114% VE from 4400-6800 RPM, ACFM peaked at 719 cfm at 6800. Over the past 2 weeks, I put 26 1/4 mile passes on the engine in my 2850 pound 4 speed Fairmont, at 2 different dragstrips. Both weekends were in the 80s, corrected altitude up to 2600 feet. The first weekend it ran a best of 10.018@132.48 MPH , using the old Holley 780 vacuum carb from my FE, last weekends best was 10.033 @ 131.84 MPH, using a 750 DP. My engine made the best power on the dyno with 32 degrees of timing, so far I have tried 32 and 35 at the track with no noticeable difference. On the dyno, the AF ratios from 5200-6800 RPM were between 11.56 to 12.75, but were richer at lower revs.  No idea how your big valve Boss 302 heads and dual plane intake stack up by comparison, but hopefully this gives you some input.

[jayb]

So, I couldn't resist digging into the dyno math on this    On my Superflow 901 dyno, Volumetric Efficiency (VE) is calculated as follows:

200 X (1728 X cfm) / (cubic inch displacement X RPM)

So the tricky part here is that cfm has to be calculated with the cfm numbers shown by the dyno's air turbine(s), and then adjusted via the weather conditions to normalize the airflow to standard conditions.  This requires the barometric pressure at the dyno, the vapor pressure at the dyno, and also the air temperature that the engine is ingesting.  Most dynos will have that information printed in the results; did you get the full dyno sheets from your day there?

Here's an example from my dyno mule, that ran recently with my cylinder heads.  The engine is 510.7 cubic inches, and we'll calculate the VE at 7000 RPM.  The total cfm from the two airbells at 7000 RPM was 1028.9 cfm.  Barometric pressure was 29.09, vapor pressure was 0.42, and air temperature going into the engine was 73 degrees.  To correct the cfm reading to standard conditions, here is the formula:

Corrected Airflow = Measured  Airflow / ((Baro prs - Vap prs)/29.92 X 520 / (460 + Air temp))

This works out to 1119.9 cfm.  So, plugging that back into the VE formula, VE for this engine at 7000 RPM was 106.4%.

The source of error in this calculation is always the measured airflow.  The turbines that measure airflow into the engine need to be precisely calibrated, and if they are off, this will dramatically alter the results.  On my dyno, the air turbines are in the ceiling, and they connect to the carb inlet with a flexible foil tube.  That tube can get holes in it during normal handling, and this will make the measured airflow read low, because some air will get in through the holes, thereby bypassing the air turbine.

One other comment; your parameter "ACFM" may be AVERAGE airflow, across the entire dyno pull.  At lower engines speeds, VE will be lower, so using the average VE number and looking at the higher engine speeds would be misleading.  But I think it is equally likely that the air turbine at the dyno is out of calibration, or that there was a leak of air going around the air turbine, leading to a lower than expected cfm reading.

And I hate to say it, but it is also quite possible that you ran on a "Happy" dyno, and the HP and torque numbers are optimistic.  Hopefully not...

[c9zx]

Rory428 and jayB, Thanks for taking the time to respond. Rory,I wish I had a 4.125 bore block. The 1969 Boss 302 heads had 2.23 intakes and 1.71 exhausts. No hollow stem 2.23 valves so 2.25 hollow stem Ferreas were used. The heads flow 310 on intake at 205 exhaust at .600 lift. based rapid drop off at 7000 RPM, I thing the springs lost control of the intake valve.
jayB, I think you nailed it. The "hat" on the carb was a very loose fit. The sheets show no atmospheric information at all and I didn't ask about calibration accuracy. The dyno is a "Land&Sea". The dyno has been accurate in the past based on 1/4 mile MPH, weight, and estimated drivetrain loss.

Thanks Again,
Chuck