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Old 06-28-2009, 01:23 AM   #1
clarkakirby
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Default Need Advice on L76 Engine Dyno Run

Guys, I took my car to Corvettes of Dallas Saturday afternoon for a dyno run. I have a NCRS Top Flight 1965 L76 (327/365) A/C coupe. This engine is the solid lifter, Duntov cam, big Holley 327, with 11:1 pistons.

I knew going in that the '65 factory engine figures were "SAE gross". This engine's factory rating was 365 hp. The air temperature during the dyno run was 103. I asked the technician to "only" take it to 5800 rpm, not the redline at 6500. The printout he gave me shows rear wheel max power: 203 @ 5200 rpm and max torque: 247 @ 3800 rpm. The technician told me that the carb was running way too lean and was not developing max power due to the lean condition and the high air temperature. He recommended that I change the carb jets as soon as possible.

Can you tell me why the torque value is higher than the hp value? I have never seen this before. Should I be disappointed in my engine's performance? Any carb jet recommendations? -Clark
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Old 06-28-2009, 04:15 AM   #2
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Horse power is only a mathematical function of torque and RPM. The dyno does not actually measure horse power. I suspect the fact that you limited the pull to the specified RPM caused the curves to never make the classic crossover that occurs at higher RPM.

If it was leaning out that early, I would definitely look into rejetting. The air temperatures were killing you as well.

Last edited by Vogie; 06-28-2009 at 04:17 AM.
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Old 06-28-2009, 04:22 AM   #3
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What was the TQ and HP reading at 5250?......... they should be the same.

A well known company IMO would (should) have had a AFR reading from your exhaust that would have been the only acceptable basis for the diagnosis of running lean.

A car so lean that it is loosing that much performance can end up with some expensive repairs.

A hot car on a dyno on a hot day and just the lack of good air will starve it but then my guess would lean towards that tending to enhance a rich condition.......... what type of fuel are you running?


Also you are denying the motor the RPM that it was designed to run at........... but if it is in that bad of tune maybe saving it

Good tuning is an absolute necessity but without knowing the bottom line on the quality of things like your air filter, plugs, wires and points, fuel pump/filter, just from reading your post I am hesitant to automatically agree with the operators conclusion.

Doug
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Old 06-28-2009, 09:43 AM   #4
Dicecal
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http://www.lbfun.com/warehouse/tech_...rb%20Paper.pdf
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Old 06-28-2009, 09:57 AM   #5
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with a heavy breather like the 327-365.. yet limiting it with exhaust manifolds and the L76 intake, does in fact make perfect sense that your torque reading might be higher especially if you didn't run it up to the 6500 that the Cam is capable of, not to mention that your AFR is probably way off.

i agree that if your 2818 holley has factory jets... the car is WAY WAY WAY to lean and will not come anywhere near peak HP. yet your torque might not be that far off.

with the carb dialed in, and timing corrected, you should see somewhere between 285-300 HP at the Rear Wheels with a properly built blueprinted L76...

assuming you didn't drop the compression.

Alot of guys simply rebuild the Holley to the factory specs and leave it at that... indeed the car will run fine, and most are happy with it, however there is alot more power under the hood if you fool with the holley and get it where it needs to be.

Good Luck

A
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Old 06-28-2009, 01:12 PM   #6
ghostrider20
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5800 RPM !!! Take it to 7000 RPM. Can't tell from the pic, but are you running side pipes?

That 30-30 is not a torque stick and you need open flowing exaust and RPM to make HP with that cam.

Mark
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Old 06-29-2009, 11:51 AM   #7
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Doug (Shurshot) asked: "What was the TQ and HP reading at 5250?......... they should be the same."

I looked on the printout and yes, the torque and Hp curves meet at 5200 rpm. I suppose this is normal for a stock L76 engine. The value where the two lines meet is about 203 hp, the peak hp reading. Interestingly enough, the hp is almost flat line from 5000 -5600 rpm. At 5600 rpm, the power starts to drop gradually.

To answer another postees question, yes, the dyno tech had a "sniffer" in the right side exhaust, but not in the left side exhaust. So, he was only "reading" the right side engine cylinders. The printout shows a flat line at about value 4 (which means nothing to me). He told me that it means it is way too lean. He recommended that I change the jets asap. He did not want to run it again due to the 103 air temp and due to the fact that the engine was running too lean. -Clark
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Old 06-29-2009, 11:59 AM   #8
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To answer other questions:
I bought the car in Nov. (last year) with advertised 46,000 miles on the odometer. It is possible that the engine has never been rebuilt. I just do not know. The previous four owners were businesses and only owned it for a few months each and then sold it for a profit.

I was running Exxon/Mobil premium, rated at 93 octane. The car's ignition system was changed to a Pertronix unit and coil a few months ago. At the same time, the plugs were changed. Only about 1,000 miles since then. The spark plug wires were changed the week before the dyno run with new Lectric Limited date-coded original-style repros. The air filter and fuel filter were changed about 6 weeks ago. The car has "factory" side exhaust. The carb was vatted and rebuilt in January because the floats were sticking, the idle screws were inoperative, and the car was running too rich at that time. -Clark
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Old 06-29-2009, 12:03 PM   #9
Donny Brass
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Clark, the eninge is a system. Folks seem to think throwing parts at them is the way to fix them, and I disagree.

have someone look at both the distributor and the carb for proper operation..

you hear all the time that guys find 50 horses on the dyno, as if the dyno is some magical device, but really all it does is let you know where to look for problems

I had Lars rebuild my distributor and the darn thing was retarded, the car was much happier after the rebuild. Same for the carb, a local guy found all kinds of issues, and not it's just fine.

after those issues were fixed, I had the motor on an engine dyno and could not improve on the sets ups provided by knowledgeable people
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Old 06-29-2009, 12:21 PM   #10
Shurshot
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Quote:
Originally Posted by clarkakirby View Post
Doug (Shurshot) asked: "What was the TQ and HP reading at 5250?......... they should be the same."

I looked on the printout and yes, the torque and Hp curves meet at 5200 rpm. I suppose this is normal for a stock L76 engine. The value where the two lines meet is about 203 hp, the peak hp reading. Interestingly enough, the hp is almost flat line from 5000 -5600 rpm. At 5600 rpm, the power starts to drop gradually.

To answer another postees question, yes, the dyno tech had a "sniffer" in the right side exhaust, but not in the left side exhaust. So, he was only "reading" the right side engine cylinders. The printout shows a flat line at about value 4 (which means nothing to me). He told me that it means it is way too lean. He recommended that I change the jets asap. He did not want to run it again due to the 103 air temp and due to the fact that the engine was running too lean. -Clark
Reading (sniffing) the right side for AFR is good...... or at least it is the better side for BB's because that is the leanest side. Nice that the man did not recommend doing another pull because something is definitely amiss with your readout.

IMO peak HP being in by 5000 is to soon and so is the fall of at 5600. For sure not enough fuel can cause that scenario to happen but so will other things such as to tight a valve lash...... something that I am suspecting.

There are other reasons than jet size for not enough fuel than just to small of jets although it is mentioned above that your motor did need more jet than the factory provided. If your fuel is getting hot and starting to boil before entering the metering system of your carburetor that can cause a lean condition so severe that a hole can burn through a piston in just one high rpm blast.

BTW was your AC off for the test? Just free wheeling the AC belt draw should IMO be without any consequence but if the compressor was running there would be a significant loss of HP.

True that your motor would benefit from some mild head work that would not show on the outside however the rarity of the combination of options that your car has would make me think twice before doing that...... and because to appreciate head work you really need to switch to using headers.

However your car should be producing quite a bit more power than it presently is. Although it was a hot day and maybe not enough airflow to carry the fuel to the cylinders I definitely suspect a tuning problem with your motor.

Like I recently said to someone else here if there is only one thing you ever learn how to do your car by yourself it needs to be how to tune it........ no one will care as much as you to get it done right.

Doug
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Old 06-29-2009, 01:26 PM   #11
ghostrider20
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Check your timing, you can search the archives for "Timing". Get the timing curve set up for the 365 SHP. I run #66 jets on the front and #78 in the rear on my 365 SHP. Runs great.

You are going to loose some HP with the side pipes.
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Old 06-29-2009, 01:41 PM   #12
midyear1
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Quote:
Originally Posted by clarkakirby View Post
Doug (Shurshot) asked: "What was the TQ and HP reading at 5250?......... they should be the same."

I looked on the printout and yes, the torque and Hp curves meet at 5200 rpm. I suppose this is normal for a stock L76 engine. The value where the two lines meet is about 203 hp, the peak hp reading. Interestingly enough, the hp is almost flat line from 5000 -5600 rpm. At 5600 rpm, the power starts to drop gradually.

To answer another postees question, yes, the dyno tech had a "sniffer" in the right side exhaust, but not in the left side exhaust. So, he was only "reading" the right side engine cylinders. The printout shows a flat line at about value 4 (which means nothing to me). He told me that it means it is way too lean. He recommended that I change the jets asap. He did not want to run it again due to the 103 air temp and due to the fact that the engine was running too lean. -Clark

HP and torque readings are identical and "normal" for EVERY engine, since HP is calculated as a result of torque:

HP = Torque x RPM / 5252

The disturbing part is your statement that HP is essentially a flat line from 5000 to 5600, and then starts falling off graduallly, rather than continuing to rise. This indicates to me that your engine is definitely starving for fuel, since at the very least your HP should keep rising at a fairly steady and linear rate at least to the 5252 RPM level, and that motor shouldn't stop building HP until it reaches it's advertised peak of 365 bhp @ 6200 RPM, torque advertised peak of 350 lb-ft @ 3400.

As your printout indicates a torque peak at 3800, this may indicate that your cam's valve timing is retarded, either from being deliberately installed that way to favor upper range power, or as the result of a worn timing chain. It's also possible that a different cam was installed at some previous point in the engine's history. I don't think you can assume that it is what it's purported to be, without actually checking lobe lifts and degreeing the cam to determine I & E valve timing and intake centerline, and comparing your results to advertised cam specs.

I would also suggest that you use a piston stop method to verify that your indicated balancer TDC is in fact true TDC, since you will never be able to accurately time the engine if that indicator mark isn't correct.

After those checks, I agree with the previous posts suggesting that you verify the timing curve of your ignition, not only for rpm curve, but stability of the curve as well, since any "wobble" at the measurement points along your plotted graph may indicate worn advance weights or pins, sticky weights, misindexed distributor gear to rotor, worn distributor or cam gear, incorrect distributor end play, worn distributor bushings, or worn and sloppy timing chain issues.

I don't know if you're in a position to borrow a known good distributor and carb from a friend, but that would probably be the quickest way to isolate the problem during another dyno session, since time definitely = $ on the dyno. Change the dizzy at home, get to the dyno, see what that does. Get the readings, they'll either be higher or the same. That will let you know if you have timing issues. Then swap out the carb on the dyno. Bear in mind that you should check the previous parameters suggested for this to be as beneficial as possible.

Finally, your stock exhaust with factory sidepipes are most likely causing a pretty good amount of intake charge dilution, since burned exhaust gases don't tend to produce a whole lot of power the second time they're burned.

Your dyno operator should be able to readily bolt a temporary free-flowing exhaust system to your manifold flanges to determine how much loss is occurring as a result of tiny sidepipes. The only cure for those losses is, as well-discussed in numerous posts on this forum, a better-flowing ( and louder ) set of sidepipes.

David
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Old 06-29-2009, 01:41 PM
 
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