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How close did you get?
483 HP from 5300-5600
549 ft lbs at 3500
over 520 ft lbs from 2800 to 4400
over 460 hp from 4800 to 5700
We are not even sure we ran it low enough to find the TQ peak
or high enough to find the HP peak
But I'm happy and it should drive Great!
It's not a racer we built it for the torque curve and she delivered!
The guesstimate is the 500 ft lbs may start near 2000 rpm and then it carries all the way to 5000 rpm!
Last edited by leigh1322; Jan 18, 2020 at 10:23 PM.
Nice numbers man!! That should scoot you along pretty well. That's enough for solid mid 11s in a C3. AFR looked a little lean, may had a little more in it too... Did they tune it as well or just make pulls?
I'd say you replicated a LS6 VERY well with your combo....
Ok I appreciate what you guys are saying about being a little lean.
There was a lot of tuning and about 10 dyno pulls over 4 hours, but we still ran out of time to tune it even more. We never even touched the 35 degree timing.
We had a number of carb issues due to the 46 yr old #4803 carb. But we finally got it running right.
The first start up was a disaster so we pulled the carb and tried another one to ensure the engine was running right.
A float had gotten bent and was hanging up, flooding the engine, came out the vent tube like a geyser!, This low rise dual plane can hold 1/4" of gasoline in a puddle inside!! (Ask me how I know.)
A brand new power valve was leaking and made the main jet pig-rich. Who was that that said: we need to double-check every single brand new part these days!!!
Even though the carb had been cleaned it still had a main air bleed plugged somehow, making the main jet very rich. #28 wire drill fixed it.
After that all the major issues were solved, and we had confirmed the engine ran OK.
The main point of this dyno session was to get the old LS6 carb working well.
IIRC factory jets were 70/74. We went up & down a few sizes and wound up with 74/72.
At that point we weren't getting any more power and all the EGTs were very even and within 20 degrees of each other!!. The front bowl and rear bowl fuel flow meters were also within a % of each other.
And the AFR was within a couple of tenths from low to high rpm. From 13.7 to 14.2. Lorenzo was very impressed with how well done the factory carb calibrations were, and how little the AFR varied everywhere .He said it was one of the best running carbs he had ever seen and certainly took the least amount of work to get it there, so I am glad I got an LS6 carb. He pointed out how small and smooth the afr changes were as the carb transfered from idlle-transfer slots, to main jet circuit, to power valve. He has made other carbs run that well, but it takes a lot of dyno time and a lot of tuning with AFRs, air bleeds, pvcrs, etc.etc.
BTW the idle vacuum was 16.9" at 850 idle, zero at WOT, and it did climb slightly to 1.7" vac at 6000. (Lorenzo lost that coffee bet with Jeff). Did I mention it was really interesting having two very experienced engine builders present, with different opinions and build philosophies?
So after a little research this morning I found that a better AFR would have been 12.7-13.5 at WOT. Sound about right?
Found another chart that says that much difference may add 2.5% power. That would add 15hp and put me at an even 500hp.
That synopsis sound about right to you?
I do intend to do some more AFR testing on the road once it's running, so I'll fine-tune it then. I'll have a O2 sensor bung installed after the headers, and use a remote device in the car, I can borrow Lorenzo's.
What would you suggest to start with? Up 2 jet sizes all around?
However it plays out, a 3000rpm wide 500ft lb torque curve is gonna be fun!
Last edited by leigh1322; Jan 19, 2020 at 01:33 PM.
I have a Demon 850 carb on a 496. I have an LM-2 hooked up and it's running close to what your AFR numbers are. It was suggested by someone that knows a heck of a lot more than I do to add some bigger jets in my secondary side-- and have a 4 number split between front and rear. Then it got too cold to mess with. #1 on my "soon as it warms up" to-do list.
I may have my LM2 AF ratio stuff. I have not used it in years. If you are interested I can see if I can dig it all up for you. Your welcome to it if needed.
You do know that the difference between "485 HP" and "500 HP" is way more than just 15 HP.
Absolutely! Bragging rights are worth a LOT!
Now I was building a street motor with stock 50 year old SHP parts.
I found it very interesting to compare the horsepower and torque curves of the three SHP Big Blocks of that era. the 396 L78, the 427 L72, and the 454 LS6. And my S.R. 454.
Lorenzo has dyno'd all of the above and confirmed my desktop dyno findings. All four are 460-488 HP when bone stock. Recall that they all ran exactly the same cam, heads, carb and manifold.
HP curves
As you can see it was the torque curve we were concentrating on.... Torque curves
Turns out DD is a little optimistic on torque (maybe by 25). His 396 L78 actually made 430 and my 454 SR actually made 549
Last edited by leigh1322; Jan 19, 2020 at 08:50 PM.
Add 2 all the way around.......I think that will get you right there. Your previous post says you ran a 74/72......is that a typo?
Anyway....I was way off.....I thought this would make another 50 but I think there is more in it......did you not try anymore timing or did you not have time?
Well...the chart info is a little conflicting to me but....
Assuming the calibration is correct on the dyno’s air fuel meter, you are a little lean. Normally 12.9-13.0 is the sweet spot for NA engines in real world driving, 13.2-13.3 AFR “may” show a little more power on the dyno.... then usually after that, you lose power. Anything in the 14 afr range is way too lean.
However, looking at the chart, one column says “AFR”, yet those numbers actually appear to be BSFC numbers, and if those calculations are right, then being in then low .500s is actually a bit rich. Judging by the EGTs, it looks lean too me. 1300*+ EGT seems a little hot to me, especially for a mild NA combo.
But you had two pros there doing the tuning and they know their equipment better then we do so.....
The best way to tune it is at the track, reading plugs and jet it for the most MPH....but you may be surprised at what it really wants. Mine runs its best MPH at 11.9 afr so....I just jet it for winter air and leave it like that.
It would have been nice if they had played with the timing as well. 35* is a good average number but....it may have liked 36-37* also....Mine gained 10 hp going from 35 to 37*. You just never know what they want.
For now, leave the timing but I’d definitely go up 2-3 jet sizes in the rear....minimum.
Yeah we definately ran out of time. It was 4:00 on a Saturday and we had been there all day.
Troubleshooting the carb issues ate up more time than the jetting changes. I am glad we were able to find all the hidden problems in that OEM carb and get it working well, but ....All three of us and several of you guys were predicting something closer to 530, so maybe it does have a little more in it.. I guess the real number is somewhere in between.
Lorenzo mentioned the BSFC numbers, and that getting down to .45-.48 is really good and efficient, but that is one I really don't understand well.
He spent a lot of time balancing the front/rear fuel meter flow (not shown) and EGT temps. That's where the odd 74 fr & 72 rr jet split came from. It started with higher jetting and fuel flow on the rear, then square jetting, then this. He said it's what it wanted. I thought it looked odd, but he's the dyno expert. He mentioned he could change it by fooling with the air bleed sizes, but he loved how the AFR curve was so smooth and flat that it was perfect the way it was, as calibrated from GM.
It's running good, so I guess I'll tune it some more whenever I get it on the street. Have to finish the whole rest of the car before that!
I appreciate all of your help, advice and comments. That made this process a lot more fun, knowing I wasn't all alone doing this. "Cause most of the time it just feels like me, an empty house, the woods and an old Corvette. It's terrific to have someone else like-minded to chat with!
My hats off to you guys!
Last edited by leigh1322; Jan 20, 2020 at 09:17 AM.
Compared to the others who have commented here, I am the least knowledgeable... especially when it comes to carb jetting and ignition timing on these motors, as my experience is primarily with late model, EFI stuff. But, I'll throw this out there.
#1... very, very good numbers all the way around and you will enjoy it on the street, with mountains of TQ in the low and mid-range.
#2... if your engine could efficiently pull another 500-600rpm, you would be at 515-520HP easily. But it is going flat at 5300, and even though it holds that power to 5600, it is not going to make more power past that, IMHO, based on the graph. If it was still making power to 6000-6100 rpm (peak at 5800/5900 vs 5300), you'd see an increase in peak HP, for certain. Would it really drive any differently in 95% of conditions on the street? Doubtful.
#3... so I think the combo is working extremely well, and if anything is missing it's just a little "top-end" power above and beyond the rpm where you made the dyno runs.
Last edited by WA 2 FST; Jan 20, 2020 at 11:03 AM.
Those number look great and it should be a fun drive on the streets.
I’m building my 496 for mid range tq and hp as well because mine will probably never see track days unless I drive it up to Vintage Day at Sonoma just for fun.
Sounds like they tried to balance it all out pretty well. The jetting tricks don’t surprise me. When we had mine on the dyno the last time, Mark was actually stagger jetting it up on the left side... meaning the left front and rear circuits were 2 jet sizes larger then the right side. It did pick up power. Maybe it’s a dual plane intake trick or something.
I think the dyno guys did ok for you, I just wouldn’t have left it that lean personally if souly relying on the o2 sensor. As I mentioned, he was monitoring other things as well...if the BSFCs are in the .450-.480 range, that’s pretty good. I do think it maybe had 10-15 hp more with a little more jet and possible 1-2* timing more but... not always the case. As Way 2 Fast mentioned, it peaked pretty early so, it wasn’t gonna make THAT much more power regardless... I think the power numbers are pretty close to what it should be though.
When we did our 65’ L78 396, we also ran out of time on the dyno, chased issues with our TI ignition system and misfires etc... luckily the #4777 (650 DP) carb (that we thought was a #4779 750 DP 🙄 metered fuel very well and needed no jet changes. I too was also hoping for 500 hp through stock exhaust manifolds but came up WAY short....at 441hp/435tq @ 6300. I figure with headers and a 750 carb it would likely hit 470hp or so but.... a little less then hoped for. It is what it is tho. In the car, it drives great and runs pretty decent. I think it’s a solid 12.50-12.60 car in good air. Maybe better if you really ran it hard.
Put it in the car and enjoy it man, it’s a nice engine. Should be enough for 11s and burn tires for days.
I was too optimistic as well, but still good numbers. It would be interesting to see what a different intake would do. The thing has some serious grunt and should be really fun for pedal mashing, thanks for posting all the details.
As others have said the AFR is a little lean for WOT acceleration, I'd be considering Power Valve tweaking as well as Mains and the AFR Meters make it a breeze compared to track runs and plug reading (I am at the point of more runs less wrenching = happy driver). The beauty of AFR data logging is the part throttle acceleration data that can really let you dial it all in (near EFI ratio's).
Sounds like they tried to balance it all out pretty well. The jetting tricks don’t surprise me. When we had mine on the dyno the last time, Mark was actually stagger jetting it up on the left side... meaning the left front and rear circuits were 2 jet sizes larger then the right side. It did pick up power. Maybe it’s a dual plane intake trick or something.
I think the dyno guys did ok for you, I just wouldn’t have left it that lean personally if souly relying on the o2 sensor. As I mentioned, he was monitoring other things as well...if the BSFCs are in the .450-.480 range, that’s pretty good. I do think it maybe had 10-15 hp more with a little more jet and possible 1-2* timing more but... not always the case. As Way 2 Fast mentioned, it peaked pretty early so, it wasn’t gonna make THAT much more power regardless... I think the power numbers are pretty close to what it should be though.
When we did our 65’ L78 396, we also ran out of time on the dyno, chased issues with our TI ignition system and misfires etc... luckily the #4777 (650 DP) carb (that we thought was a #4779 750 DP 🙄 metered fuel very well and needed no jet changes. I too was also hoping for 500 hp through stock exhaust manifolds but came up WAY short....at 441hp/435tq @ 6300. I figure with headers and a 750 carb it would likely hit 470hp or so but.... a little less then hoped for. It is what it is tho. In the car, it drives great and runs pretty decent. I think it’s a solid 12.50-12.60 car in good air. Maybe better if you really ran it hard.
Put it in the car and enjoy it man, it’s a nice engine. Should be enough for 11s and burn tires for days.
Interesting post regarding the stagger jetting, I am seeing a back to bank afr difference on my tri-power that somewhat mirrors my dyno pulls. I will be investigating this spring after I get some bigger fuel line installed.
metered fuel very well and needed no jet changes. I too was also hoping for 500 hp through stock exhaust manifolds but came up WAY short....at 441hp/435tq @ 6300. I figure with headers and a 750 carb it would likely hit 470hp or so but.... a little less then hoped for. It is what it is tho. In the car, it drives great and runs pretty decent. I think it’s a solid 12.50-12.60 car in good air. Maybe better if you really ran it hard.
