496 Stroker Tuning Questions

[Warrenavenue]

Hey guys, feel free to redirect me to a better place to post this but im looking for some tuning advice with a fresh 496 build in a truck. I figure you B Body guys are a wealth of info on the subject of tuning. Build specs are down at the bottom.

I have about 450 miles on this build and broke it in on a chassis dyno at a friend's shop a few weeks ago. When I broke it in I had the initial timing set around 12* and overall was around 33*. After seating the rings we did a couple full pulls and made 423hp at 4980RPM and 465tq at 4040RPM at the tires on 91oct with a very linear torque curve and a clean and consistently increasing HP curve. Im assuming somewhere around 18% parasitic loss since this is in a 4x4 with 35" tires so not too disappointed in the numbers. I was running an MSD Atomic EFI that did well to fire up and quickly learn the engine but I maxed out the capabilities of the Injectors at 5200 RPM and noticed over the next week that it smelled fat at idle and would diesel when I shut it off. I went around and around trying to track down issues with MSD Tech but ended up finding out that the Atomic can't handle a 107* lobe separation and decided to change it out for a 750 double pumper last week.

Now with the 750 I set the floats, pulled the 73 main jets and put in 72s since im at 2000ft, evenly adjusted all 4 idle screws to a 12.9-13.2 AFR at idle, and an idle speed of around 700 RPM. All 4 screws are a tad less than 3/4 of a turn out from seated. At first I was running 12* initial timing and an idle around 800 but found that it still smelled really fat at idle and wouldn't tolerate anything leaner than 13.5 AFR. It would also want to run on when I shut it down which is really frustrating since its almost winter and ambient temps are down in the 50s so summer is gonna be a real joy! So I bumped initial timing to 16* which has slightly helped with the dieseling (slightly) and it doesn't smell quite as fat but now im concerned that I might be flirting with detonation, especially since this is in a truck with a stick.

Are any of these numbers looking crazy to you guys? Should I be chasing a different idle or bringing initial timing up even more? Im assuming an overall timing in the neighborhood of 36* is healthy for a build like this but I can use all the tuning input I can get from you seasoned tuners out there. Am I running the right plug gap? Are my plugs too hot for the application?

Thank you ahead of time for any insights with this, my head and my wall are getting to know each other way too well.

383 Block
4.310 Bore
4.25 Stroke
11.3cc dish Ross forged pistons
84cc Edelbrock Performer RPM Heads
Comp Ratio is right around 10.39
Long tube Hedman headers
Edelbrock Performer RPM Intake
Holley 750 Double Pumper w/Elec Choke
MSD 6AL
MSD Pro Billet Distributor w/Vac Advance
NGK R5671A-7 gapped at .034"

Manual Transmission
NP435
4.10 Axle Gears

Cam- LS 107*
Dur. @.050 I-229* E-242*
Lift- .345 x .345

 

[Kern Dog]

From what I see....
I would not run a double pumper on a street car. Vacuum secondaries are a much better fit.
California gas...Ethanol "up to 10%" means that you will need a lower AFR number to run right. My 440/493 in my Charger runs strongest in the 13.0-13.5 range at idle and cruise, mid to high 11 range at WOT.
Any higher/leaner and it loses power. The Ethanol gas needs more fuel to do the same job as 100% gasoline.
The cam isn't huge but the lobe separation angle is narrow. You may need 18-20 degrees initial but no more than 35-36 total. The distributor may be adjustable to get there.
Often times, dieseling and run-on is due to the throttle being partially open at idle. If the timing is brought up at idle, RPMs may rise enough to back off the idle set screw. I'd still prefer to see an 850 Vacuum secondary rather than a 750 double pumper.

 

[beanhead]

The 750 Holley is WAY small for a 496 but you already have it, so....
Get the initial to 20 and yes you're right, the total timing will then be too high so a curve adjustment will be needed. How much total is right for your combo is undefinable over the internet but I'd shoot for 34 to start.
Cam is on the small side for the cubes also, which can work okay but increases your cranking psi....do a compression test.
Don't sweat the idle smell unless you know the difference between rich and lean 'stink'...they both stink, and the 107 LSA isn't helping that. That screws with the a/f reading at idle as well.... so try getting the initial timing up, make certain the transfer slots/butterflies are set correctly and get it idling on the lean side of best vacuum reading with the mixture screws. You may need to get into tuning the IFR if it stays too fat at idle.
MSD generally likes more gap, start with at least .040. NGK 7 heat range is likely a step too cold already.
Remember....our gas is a completely different animal. It takes longer to color plugs and takes more initial timing...and all the stuff Kern Dog said about gauge readings and ethanol.
Happy hunting!

 

[Warrenavenue]

From what I see....
I would not run a double pumper on a street car. Vacuum secondaries are a much better fit.
California gas...Ethanol "up to 10%" means that you will need a lower AFR number to run right. My 440/493 in my Charger runs strongest in the 13.0-13.5 range at idle and cruise, mid to high 11 range at WOT.
Any higher/leaner and it loses power. The Ethanol gas needs more fuel to do the same job as 100% gasoline.
The cam isn't huge but the lobe separation angle is narrow. You may need 18-20 degrees initial but no more than 35-36 total. The distributor may be adjustable to get there.
Often times, dieseling and run-on is due to the throttle being partially open at idle. If the timing is brought up at idle, RPMs may rise enough to back off the idle set screw. I'd still prefer to see an 850 Vacuum secondary rather than a 750 double pumper.

Thank you for the replies, I’ll mess with these tomorrow and see what I can get with it. Even with a manual trans you’d prefer a Vac Secondary? I did have a Vac Secondary on this truck before and man, it was a bear to tune the transition to secondaries in with the gearing in this transmission. Is it worth nicely asking summit for an exchange?

 

[Warrenavenue]

S

From what I see....
I would not run a double pumper on a street car. Vacuum secondaries are a much better fit.
California gas...Ethanol "up to 10%" means that you will need a lower AFR number to run right. My 440/493 in my Charger runs strongest in the 13.0-13.5 range at idle and cruise, mid to high 11 range at WOT.
Any higher/leaner and it loses power. The Ethanol gas needs more fuel to do the same job as 100% gasoline.
The cam isn't huge but the lobe separation angle is narrow. You may need 18-20 degrees initial but no more than 35-36 total. The distributor may be adjustable to get there.
Often times, dieseling and run-on is due to the throttle being partially open at idle. If the timing is brought up at idle, RPMs may rise enough to back off the idle set screw. I'd still prefer to see an 850 Vacuum secondary rather than a 750 double pumper.

Speaking of vacuum, I’m lucky to see 11in at idle warm, is that par for the course with a cam like this?

 

[Dan64]

Thank you for the replies, I’ll mess with these tomorrow and see what I can get with it. Even with a manual trans you’d prefer a Vac Secondary? I did have a Vac Secondary on this truck before and man, it was a bear to tune the transition to secondaries in with the gearing in this transmission. Is it worth nicely asking summit for an exchange?

Yes, it's worth asking.I bought a 850 Brawler for my 493 from them, dyno'd the engine and had the carb for 7 months. I didn't like the carb and the engine builder told me to send it back. I figured that i just eat that money but surprisingly they gave me 100 percent refund AND sent me a shipping label. I was so shocked that i actually asked the women on the phone why they were taking it back and she responded "Covid". I love Summit,always great to deal with.

 

[beanhead]

Speaking of vacuum, I’m lucky to see 11in at idle warm, is that par for the course with a cam like this?

Normally I would say it's a little low, I have a solid cammed 505 with [email protected] and I get 9-10" at idle.... BUT I'm running a 112 LSA. So with your 107 that could very well be close to all you're going to get. Bring your initial timing up some more, and get the carb set right and then whatever you get, you get. You can stray lightly from the .020 T-slot exposure to get your idle speed where you want, but don't get crazy.
As far as running a double pumper on the street...
It's all personal preference and how much you like to play with carbs. I have a 68 Plymouth (not light) with an automatic and it doesn't suck extra fuel, bog, burp, fart or anyting else that people say they experience with a DP carb and auto street car. Cruises around just fine. There's a little extra effort involved to get them right, but when you get there it's worth it in my opinion...

 

[Warrenavenue]

Normally I would say it's a little low, I have a solid cammed 505 with [email protected] and I get 9-10" at idle.... BUT I'm running a 112 LSA. So with your 107 that could very well be close to all you're going to get. Bring your initial timing up some more, and get the carb set right and then whatever you get, you get. You can stray lightly from the .020 T-slot exposure to get your idle speed where you want, but don't get crazy.
As far as running a double pumper on the street...
It's all personal preference and how much you like to play with carbs. I have a 68 Plymouth (not light) with an automatic and it doesn't suck extra fuel, bog, burp, fart or anyting else that people say they experience with a DP carb and auto street car. Cruises around just fine. There's a little extra effort involved to get them right, but when you get there it's worth it in my opinion...

Been a couple busy days so I haven't had time to try these tweaks but today I finally got some time to play around with things. Thanks for the input, I ended up getting some great results.

First, I decided to hold on to the 750DP, im not running this thing at WOT down a strip, this motor is seeing a lot more street use and will see some elevation changes so I think im safer with the 750 in this truck. But I did bring my initial up to 16* and overall is around 35* and all in around 3100 with a black bushing and a light silver spring/heavy silver spring combo. It loves it! power is effortless. I gapped plugs to .048" and have been messing with jetting a little today. running a 70 main jet and at a light cruise im running at 13.3-13.6 AFR. I originally put a 4.5in power valve in because idle vac was 8-9 but that is way too low. I put the 6.5in back in today and will be swapping that for a 7.5in. Runs really good though!

But I do have one thing going on that I could use some input on. This truck is a stick and I live in the mountains so I use my gears a lot. Today I noticed that it likes to go fat when the throttle is light or when im off the throttle decelerating. It'll go down to 11.7-12.2AFR until I throttle up to accelerate or until vacuum drops close to idle and then it levels back out to 13.1-13.4AFR. Anybody have any idea why? Where is it getting fuel when the vacuum is higher (15-23in)

 

[beanhead]

As you discovered power valve should be sized according to your cruise vacuum, not that 'half the idle' BS. You can have the power valve open at idle, and it won't cause your idle to go rich.. You're on the right track going up to the 7.5. The whole idea of the power valve is to be able to run the leanest primary jet you can without stumbles or adverse effects, and then have the extra fuel only when you need it (at throttle time!)
I would step down the primary jet again and see if it cleans up those cruise numbers, because it's easy. If not, time to get inside and tweak the idle/transition circuit. When you say 'fat at light throttle' that's an indicator of too much fuel there. That can get tedious, lots of trial-and-error, but it's so worth it to get the tune spot-on. And FWIW that's usually the area of the carb that needs the most work. Very common. Plenty of help with that here and FABO if you need it.
To answer your last part, don't worry about what the gauge does during decel. As the throttle eases closed vacuum jumps around and pulls extra fuel from the T-slots, as well as what was flowing through the boosters ...but it's temporary. Once you get the cruise area leaned a bit it may help with that too. Gotta tune from the bottom up, and since you posted some different vacuum numbers, is it safe to assume you hooked the gauge where you could read it and went driving? If so....YAY for you! That's a big help when dialing in a carb but many folks don't think about it.

 

[Warrenavenue]

As you discovered power valve should be sized according to your cruise vacuum, not that 'half the idle' BS. You can have the power valve open at idle, and it won't cause your idle to go rich.. You're on the right track going up to the 7.5. The whole idea of the power valve is to be able to run the leanest primary jet you can without stumbles or adverse effects, and then have the extra fuel only when you need it (at throttle time!)
I would step down the primary jet again and see if it cleans up those cruise numbers. If not, time to get inside and tweak the idle/transition circuit. Plenty of help with that here and FABO if you need it. Adjust your WOT with the secondary jet and PV channel restrictors (if equipped).
To answer your last part, don't worry about what the gauge does during decel. As the throttle eases closed vacuum jumps around and pulls extra fuel from the T-slots, as well as what was flowing through the boosters ...but it's temporary. Once you get the cruise area leaned a bit it may help with that too. Gotta tune from the bottom up, and since you posted some different vacuum numbers, is it safe to assume you hooked the gauge where you could read it and went driving? If so....YAY for you! That's a big help when dialing in a carb but many folks don't think about it.

Yeah, the 4.5 was terrible! After I took it for the first drive and it fell on its face hard and then lurched out of the hole when the PV finally came in I hooked my vac gauge up and ran it around to the dash so I could see what the heck was going on. Made it pretty clear that my vacuum coming out of cruise is somewhere in the neighborhood of 7-8in, not 4! I might even drop an 8.5 in but we will see what the 7.5 does. I don't have billet metering blocks on this...yet...so im working within the parameters of the preset power circuit. Secondary power valve is blocked off but the secondaries are coming in smooth from what I can tell. Ill update after getting the primary PV right.

Thanks again

 

[beanhead]

Yeah, the 4.5 was terrible! After I took it for the first drive and it fell on its face hard and then lurched out of the hole when the PV finally came in I hooked my vac gauge up and ran it around to the dash so I could see what the heck was going on. Made it pretty clear that my vacuum coming out of cruise is somewhere in the neighborhood of 7-8in, not 4! I might even drop an 8.5 in but we will see what the 7.5 does. I don't have billet metering blocks on this...yet...so im working within the parameters of the preset power circuit. Secondary power valve is blocked off but the secondaries are coming in smooth from what I can tell. Ill update after getting the primary PV right.

Thanks again

So the old trick with non-adjustable orfice carbs was to use a small piece of wire in the desired size, bent in an 'L' shape, and then inserted into (whichever)hole you want to make smaller, and it's sandwiched in between the gasket surfaces and can't go anywhere.....sounds kooky but it works. People did (still do?) it all the time...just a thought! Best part of that method is, it's quickly reversible.

 

[andyf]

Too bad about the MSD. Had you bought a Holley Sniper you would be finished with the tune and enjoying the big block. The MSD was a little limited in terms of what it could do.

 

[Warrenavenue]

Too bad about the MSD. Had you bought a Holley Sniper you would be finished with the tune and enjoying the big block. The MSD was a little limited in terms of what it could do.

Yeah it’s a bummer but I ended up getting it sold. But just like anything else, when I bought it there was so much positive and negative info out there on every system that was available that I ended up goin with the MSD because it seemed to have the best overall publicity at the time. Right after I bought it guys were telling me “you should have done the fitech” then a year went by and “the sniper is the way to go now, fitech is trash!” Makes it tough to pick the setup that is actually the best when what’s “best” changes with every shop you talk to.

 

[Green72]

Just out of curiosity, how did you have the MSD set up? Did you have an in tank pump or a PWM type pump? What fuel pressure where you running. I'm putting together a similar motor and also have an MSD Atomic system that I would like to use.

 

[Warrenavenue]

Just out of curiosity, how did you have the MSD set up? Did you have an in tank pump or a PWM type pump? What fuel pressure where you running. I'm putting together a similar motor and also have an MSD Atomic system that I would like to use.

I had it set up with an in tank pump from a 91 w250 gas pickup. That pump supplies plenty of fuel at 75+psi so I ran it to a bypass regulator originally set to 45psi and returned it back to the tank. Interesting thing about the Atomic is that it needs to have the regulator mounted after the Throttle Body so send full pressure fuel to the inlet fitting and then go out to a regulator and then return back the tank there. At first I used it with a single plane Weiand mid-rise intake but later found out from MSDs tech department that the Atomic, unlike other systems, likes a dual plane so I swapped the Weiand for an Edelbrock Performer RPM. It turned out in the end that my LSA was one degree too narrow for the most dramatic fuel map that the Atomic supports, but I really do think it would an excellent setup on a slightly milder build. Hopefully you don’t break over 600hp because the injectors just can’t handle any more than that.

 

[hunt2elk]

As you discovered power valve should be sized according to your cruise vacuum, not that 'half the idle' BS. You can have the power valve open at idle, and it won't cause your idle to go rich.. You're on the right track going up to the 7.5.

If I could give you 2 agrees's, I would. Can't believe that so many people don't understand how the power valve/idle circuit works, and will argue till they are blue that a guy can not have a higher number pv than your idle vac.

 

[Warrenavenue]

If I could give you 2 agrees's, I would. Can't believe that so many people don't understand how the power valve/idle circuit works, and will argue till they are blue that a guy can not have a higher number pv than your idle vac.

Looks like I’ll be bumping the power valve up even more, 8.5 still leaves a dead spot so I’m gonna go with a 10.5 and see how that works. I’ve also got a dead spot off idle that is killing me! I’ve messed with the accel pump linkage, I’ve messed with cams, I’ve bumped the nozzle size up and added a hollow screw. I have a 37 nozzle in it now and still have a dead spot right when I blip the throttle. It goes dead lean and then takes off. I’m gonna try a 42 nozzle later.

 

[Hemirunner]

That cam is on the small side for a 496. What’s the cranking compression?

 

[Warrenavenue]

That cam is on the small side for a 496. What’s the cranking compression?

Tim at Hughes designed the build for me, and it’s in a heavy truck so it’s really not bad. I haven’t done a comp test yet.

 

[Warrenavenue]

If I could give you 2 agrees's, I would. Can't believe that so many people don't understand how the power valve/idle circuit works, and will argue till they are blue that a guy can not have a higher number pv than your idle vac.

Well, I dove into the primary accel pump circuit today and found a huge part of the off idle stumble issue, and it may help explain some of the decel richness. It seemed that no matter what size accelerator discharge nozzle I used the dead spot wouldn’t go away. I pulled the carb off, flipped it over and dropped the discharge nozzle needle weight out and behind it came the brass seat of the needle valve. I looked in the orifice and saw that there was already a seat in there so I pulled the secondary side off to compare and found that there was no seat at all under the needle on that side. I put the extra seat in the secondary side under the needle valve and used a nozzle screw to press the seat into the body. Then reassembled and dropped the discharge nozzle back to a 31, dead spot is almost gone! This was a brand new Holley!! What they heck Holley?