Last year I had the opportunity to attend a dyno session with a NCRS chapter. Free breakfast & lunch and 4 pulls

My best pull was 194 rwhp or (x 15%) about 223hp fw . The car is a 78 L82, 4spd, 3:70 rear, and 99,000 miles on the original engine. All smog stuff gone with open air element. The car was rated in 78 as 220 hp. The car does have a different cam (of which I know nothing about) and Edlebrock intake, original carb. Restricted exhaust (2-1-2)but no cat.

However during the pulls, after taking off the air cleaner, the air valve was not opening even at 5000rpm 104mph. Came home read about the adjustment and made the change. Worked perfect, as you could tell on the highway. No bog and a steady stream of power. No "seat of the pants" feeling.

Now to today. Changes made were the correction of the spring tension on the air valve and true dual exhaust with Magnaflow muffers, no headers. Guy did explain exhaust helps but headers would be
better
Here are the new numbers:

246rwhp & 271 torque -flywheel hp (246x15%) 283
245rwhp & 269 torque
248rwhp & 272 torque
242rwhp & 271 torque
@110mph

Now my questions.

Relative humidity was high 44%. According to one online calculator I should deduct 4% for the air. True? should it be more.

If I deduct 4% my best would be 238 rwhp. Can the opening of the 2 barrels have such an effect on hp gain (238-194= 44hp)?

How much would the exhaust contribute if anything?

thanks.
click

 

Opening up the exhaust makes a big difference.
http://www.corvettefaq.com/c3/ganeyexhaust.htm

 

Didn't the dyno operator account for the conditions of the day and give you corrected readings?

 

"Didn't the dyno operator account for the conditions of the day and give you corrected readings? "

He talked about the correction and the air and the effects it has. I was curious if my changes resulted in a little more hp. Suprised it was this much. I did learn a little something about relative humidity today

 

Most dyno results are corrected to "standard conditions" so that results are comparable to other tests done by that dyno under different environmental conditions. So, I don't think you should have to make that correction.

P.S. The Q-jet on your L82 would never open the secondary air valve fully, as the capacity of that carb (with fully opened air valve) is 750 cfm...the L82's volumetric capacity (even with a better cam) is probably less than 600 cfm. The Q-jet is 'self-regulating' [force of airflow over the secondary air valve opposing the return spring] for whatever engine size it is placed on.

 

The dyno corrects for pressure, temp & humidity. On the screen you can switch the graph between "raw" and corrected power. There can be quite a difference between the two.

 

Well Bill your results confirm what I have been saying for years about the changes that I made to my 78 L-82 4 Speed-the OEM exhaust is a killer on our motors!

I agree with 7T1vette that the Q-jet has much more capacity than the stock engine requires and his comments makes sense which is one of the reasons that I run a 650 CFM Holley 4175 and have since 1985. I also think that a correction factor of 15% to go from RWHP to Net HP on these cars is way too conservative-it is probably closer to 18-20% since the drivetrain loss is much greater on these older cars which puts you right about 300 Net HP-12-15% correction factor is probably more for newer cars with much less drivetrain losses-more efficient drivetrains.

The L-82 is a terrific engine and is VERY similiar to the 72 LT-1/73-74 L-82's the major differences the exhaust (true duals) and the change from gross to Net HP ratings in 1972. The earlier 350/370 Gross HP LT-1's had higher compression 10/11:1 which is a big difference.

It is well documented that the OEM 2-1-2 OEM cat exhaust on the 75-82 vettes is total crap and VERY restrictive. It is also well documented that changing the exhaust alone (no headers) could add as much as 30-40 HP on these cars. Heads, Cam, Intake, and Carb can add 100 Net HP to an internally stock L-82 (see below).

http://www.superchevy.com/technical/...ver/page2.html

I have a bone stock L-82 (65,000 miles) except no pollution control at all, 2.5 true duals/Monza turbo mufflers with McJacks shorty headers, Holley 650 CFM 4175, and comp cams roller rockers and the car has Sooo much more power than when stock-it is that different! Next up is the replacement of the timing chain with a roller timing chain and I will advance the cam 4 degrees since these cars all had the cam retarded from the factory due to emissions.

Great numbers!

 

I always assumed my car was putting out about 300hp now. It has the original L-82 w no smog, headers into true dual exhaust, bigger cam, and a few minor other goodies. this pretty much confirms what I assumed. I'd be curious to one day put mine on a dyno, or even better, take down the 1/4 mile.

 

This was a long time ago, but the operator of a Clayton chassis dyno told me the torque readings also need to be corrected for the rear axle ratio. And I would assume for a transmission with a less than direct drive high gear; eg, overdrive. But I cannot remember which axle ratio gave the most accurate dyno readings.

 

Must have been back in the vacuum tube days.

Modern dynos have a plug wire clamp so it knows the engine rpm. It then calculates the overall ratio between road speed and engine rpm.

You will, however, get lower readings if you dyno in a gear other than direct drive as the transmission has more losses when power goes via the cluster gears.

 

Actually, I think vacuum tubes were still being used in 1970 when I rolled my rat-motored '57 Nomad onto the dyno. I had 4.11's and the operator said there was some kind of fudge-factor needed to correct for it.

No wire, no cables, no digital readouts. Just a max torque reading left on the dial from the tell-tale needle. The dial face was huge like on a grain store's feed scale. Horsepower was factored from the torque reading.

All very crude by today's standards. But that's all we had back then.