Any C2 BB owners have there cars on a dyno? Did mine today.

[Steven S.]

Having a renewed interest in the performance of my car after reading about Duke's seminar (how about doing it again at the 2013 nat??), I dug into the tune step by step as was laid out in his presentation. After I was convinced I had the spark map as good as it was going to get I set up an appointment to put it on a dyno, mainly for the air/fuel, but to see what it was actually making at the wheels. Of course the day I chose happened to be almost 100 degrees and humid here it Pennsylvania (yuck). What I have under the hood is sort of a "defanged" L78 (hydraulic cam), and the end result was 310hp and 365 lb-ft of torque. I was very rich all around and we made a solid 10 hp improvement across the board after jetting down.

Anyone else dyno there C2 (bb or sb)? I'm curious how mine would compare to a stock L78.

Steve

[Steve B.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

If those are rear wheel numbers, I would be pretty happy with those considering the heat.

[Steven S.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Yes sir, those are at the wheels. I was sorta expecting 280-290 so I was happy with the results.

[Steve B.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

I really don't think that a stocker will do much more than that. I seem to remember someone who dynoed their L71 with N11 exhaust and he put down 370 RWHP. I am not sure if the motor was 100% stock though.

[Steven S.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

I searched around here and on CF, found a few posts with dyno results on 427 cars. From the few I could find there were a couple 427/390s that were in the 280-290 range and a couple SHP versions in the 350 range, but like you said who knows how stock they are so it's difficult to compare. No L78s though.

I must say it was "fun" to do. Little nervous in the higher revs but all in all a neat experience. This particular dyno owner was great to work with though which really made it more enjoyable.

[Dennis O.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

They dynoed the L79 in my '67 coupe just after it was rebuilt. It put out 336 HP. This, of course, was on regular dyno, not a chassis dyno. Anybody got an idea of the %age difference between the two?

[Gary C.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Here is my L78 at the fly wheel.





Gary,
Perth Australia

[Duke W.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Those are decent numbers, especially the torque (.92 lb-ft/CID) I suspect the cam is fairly high performance, but probably not as big at the OE cam. A good 327 will make (SAE corrected) about 270-280 lb-ft (.85 lb-ft/CID), and the "327 LT-1" configuration with massaged heads makes about 290 SAE corrected RWHP at 6500. Testing on a hot day can consume 15 lb-ft and at least 10 HP because it's tough to keep the fan clutch from tightening.

My recommendation is to test on cool day when the SAE correction factor is near unity, and it's a lot easier to keep the fan clutch from engaging.

I'd like to know the RPM for peak torque and power? What kind of dyno?

Was "SAE correction" used for the final numbers?

If you got a printout of the torque and power curves, can you post it?

If it was a Dynojet and you were able to get the test files via disk or email, can you send them to me?

What are the final specs for your spark advance map?

It sounds like your session was productive and worthwhile. If you leaned out the primary in addition to the secondary the combination of an optimized spark advance map and leaner cruise mixture should reduce fuel consumption.

What were the original and final jet sizes?

Providing all the details can help others optimize their tune.

Empirical factors I developed for converting SAE corrected RWHP to SAE net and gross at the flywheel for under the car exhaust are in the "Tale of Two Camshafts" article, The Corvette Restorer, Fall 2010.

Duke

[Steven S.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Duke,
It was a Dynojet using the "SAE" correction.

I did ask for the files and downloaded the software per your suggestion, you are welcome to them if you like. The problem is we couldn't get a good tach signal from my car for reasons I don't understand, and so the curves only have data for horsepower, mph and conditions. Only when we were finishing up and I realized the absense of the torque curve output did I question it, and then he set up a pickup from the balancer to make a final pull to chart the torque. So no baseline to compare to. Had I understood more about it I would have asked for him to do that from the beginning.

The cameshaft is a Crane #103072, you can view specs at http://crane.carshopinc.com/product_...d/15858/103072
I would say their description of the cam is accurate, idle is fair.

The peak power was at approx. 4800rpm. The engine is a .03 over 396, it was built to what I was told was a "true 10.5" comp. for what thats worth. Heads cc'd, etc, no port work done. Intake is an Edelbrock Performer RPM dual plane which I am assume is basically an equivalent to the factory manifold. Exhaust is stock manifolds with an Allens "off road" system.

On the spark advance map, I installed Moroso #72310 advance curve kit with one light and one medium spring. I tried two light springs but had detonation and so backed it off. It worked nicely with the combination of the light and medium, I need to go back and verify where it was coming in at (I'll edit post). B28 VAC and using a pertronix ignitor unit. 37 degrees total timing.

What I'm uneasy about is what the jetting ended up at. The carburetor is an original 3247 for a '66 L72, it had 76 secondary and 72 primary jets. After the 1st pull he told me it was so rich on top that we were approaching cylinder washout! We ended up with a 71 secondary and 69 primary jets, which to me just doesn't sound right but we were in the 12.9-13.1:1 range on the A/F with that combo. It concerns me that there is something wrong with the carburetor to have such small jets to get the A/F right. I am running "pure gas" from a station that is still selling it. I had taken this car on a 800 mile round trip prior to the tune and had an average 16mpg fuel economy, which I thought was very good. At one point during the trip I had a friend following behind who later told me he was choking on fumes, which then alerted me that it was most likely "way rich". I'm very interested in seeing what the fuel economy would be now.

I do plan on a return trip this fall on a nice cool day, this time I'll be sure to get all the data on file to analyze.

Duke, if you or anyone else would like a copy of the data files, just PM me the e-mail address you would like them sent to and I'll send you them ASAP.

Steve

[Steven S.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Gary,
Very neat! Still pulling at 6k! It would be very interesting to see what that car does at the wheels. Thanks for sharing!

Here is my L78 at the fly wheel.





Gary,
Perth Australia

[Duke W.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Thanks for the info. The cam (226/226/106/108) has nearly the duration of the OE SHP cam (231/231/108/114) but is phased earlier and has more overlap, which kills low end torque and yields a rougher idle.

I have PM blocked and apparently you have email blocked, so please send the files to dukewilliams at netzero dot net

Let me know your rear axle ratio, tire size, and whether all the pulls were in top gear. I can then convert to RPM.

The dyno reads horsepower directly and the operator normally places a magnetic pickup on a plug wire, which gives an RPM signal, which is used to compute equivalent engine torque. At least you got A/F data, which solved a problem.

I don't have any insight into the jetting situation, but as long as it doesn't go leaner than 13.5:1 at peak revs, it's okay and it meets that criterion.

As long as you don't have any lean surge at cruise it's okay, and you could even try to go leaner on the primary jets.

Duke

[Steven S.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Files on the way!

[Bill M.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

They dynoed the L79 in my '67 coupe just after it was rebuilt. It put out 336 HP. This, of course, was on regular dyno, not a chassis dyno. Anybody got an idea of the %age difference between the two?

The August '12 Car and Driver, p.47, has it for a ZL1 Camaro (13%) and GT500 Mustang (15%); both manual transmissions.

[Duke W.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Assuming a lab dyno test uses STP correction, the OE manifolds and no mufflerts, you first you have to convert gross to net. My empirical factor based on my test data base is 0.89, assuming a small block with 2.5" under the car exhaust and the fan clutch is kept cool enough to not significantly tighten during the test. Make it about .88 if power steering is installed on the car.

Then you multiply net times drivetrain efficiency, which I estimate at 0.85 for vintage Corvettes with manual transmissions assuming the pulls are in top gear (1:1). Make it about 0.82 if pulls are done in an intermediate gear where torque is transmitted through the countershaft. This should yield a reasonable estimate of SAE corrrected RWHP.

If headers were used during the lab dyno test reduce peak torque by about another 8 percent and top end power by 3 percent with SHP cams. With base small block cams decrease peak torque by about 4 percent, and top end power is about the same. Headers have minimal effect with low overlap cams at high revs and may even cost a little top end power relative to the OE manifolds.

This was all in the article I referenced.

Chassis dyno tests are usually cheaper and yield more useful data than lab dyno tests. If a shop needs to do a lab dyno test to verify that the engine runs okay - find another shop that has the confidence to put the engine together right the first time, and spend the money on a chassis dyno test session to optimize the tune.

Duke

[Steven S.]

Re: Any C2 BB owners have there cars on a dyno? Did mine today.

Lets try this again...
Centrifugal advance is 30 degrees all in at 2800 rpm. That is with one light and one medium spring. Two light springs resulted in detonation. I do have what I would call some "light transient" detonation while road testing on a HOT day.

Before reading about Duke's presentation I had the wrong VAC installed in conjuction with being connected to ported vacuum, what a difference it made in idle quality when correcting those two problems!

Steve