383 tow motor cam question

[n2omike]

I have used an Isky 264 Mega in 383 trucks with great success. It is 214 at 050, cut on 108 centers. It is very good in the 2500-3500 RPM range.

JOE SHERMAN RACING

Hey thats my favorite cam I still use it in almost every flat tappet truck towing motor I build, Chev, ford or dodge. Sometime I get it ground on a 110 -112 if I want a smoother idle.

Sounds good for power, but how does the fuel mileage compare to something ground on a 110-112º lobe seperation? I might be building a Vortec headed 383 in the near future for towing.... and fuel mileage is definitely a factor.

Thanks!

[PackardV8]

Just to clarify, it would help to note if these truck builds are carb or EFI.

I like Joe's definition of a street motor as one which peaks at or below 6kRPM and needs no maintenance other than oil changes.

FWIW, I consider a real gas-burning truck engine as one which has a flat torque curve from 1k-4.5k. I've built my share of hot rod trucks and the longer cams are fine if run lightly loaded most of the time. However, the longer the cam, the peakier and shorter the torque curve and the lower the fuel economy. In most real trucks with a longer duration cam, the wide-ratio transmissions require them to be revved higher to pull a real load in the next gear. This increases wear and lowers fuel economy. The extreme example of this is today's Class 8 diesels have a 300-500 RPM operating range. To get maximum economy in a gas truck, the same principle applies - keep the RPMs low and the throttle wide open. The shorter the cam, the lower the RPM at which the truck will pull easily at full throttle. It won't be as much fun to drive short-shifted, but it will get a lot better fuel economy.

thnx, jack vines

[n2omike]

Just to clarify, it would help to note if these truck builds are carb or EFI.

My truck is carbed. A new Vortec headed 383 would likely consist of a 750 Edelbrock, Performer RPM intake and headers. It's a half ton 1976 GMC half ton 2wd, and presently has a TH350 trans and 3.08 rear. Tires are 235/75/15, not anything tall. The truck actually seems geared pretty well with the stock 350 w/Q-jet. However, it just gets 12mpg empty... and that's driving it easy. Loaded, and going up and down hills, it drops to around 8-10.

I have an Innovate LM1, and believe I could tune an Edelbrock equipped 383 to get significantly better fuel mileage. I was just wondering about lobe seperation. I would -think- the wider lobe seperation would be more efficient.....

Fuel mileage isn't the -only- issue, but it is significant. I just don't want to use a 108LSA cam if it's going to drop the mileage by 2-3 mpg.

Thanks!

[PackardV8]

Hi, n2omike,

Just for the sake of discussion, what factors would cause a performance-built 750 CFM Edelbrock 383" to get better mileage than a stock Quadrajet 350"? I'll take the bet it won't, assuming an equal condition and tune.

thnx, jack vines

[n2omike]

Hi, n2omike,

Just for the sake of discussion, what factors would cause a performance-built 750 CFM Edelbrock 383" to get better mileage than a stock Quadrajet 350"? I'll take the bet it won't, assuming an equal condition and tune.

thnx, jack vines

The Q-jet is set up just like the factory built it in 1976... so I'd say the tuning could use a lot of help. 12mpg, driving easy, isn't that good... so there's a lot of room for improvement. I'm not expert at tuning the Q-jet, but the Edelbrock is easy. Add in the LM1 to get the the cruise, etc just right...

Thing is, if I build a new engine, a 383 crank is only a few extra bucks, and those extra cubes will come in handy when towing. The Vortec heads are also a no-breainer... especially when feeding the extra cubes. Getting them off a junkyard engine, and getting all the required parts... head bolts, pushrods, rockers, etc, makes them a great deal. No sense in building an old-headed 350.

I'm not comparing both engines, all else being equal. I'm comparing a STOCK 350 Q-jet with factory tuning to a Vortec headed 383 with a LM1 tuned Edelbrock carb. I would think the 383 could be built to run circles around the old engine, and still get better mileage... Just more efficient, overall.

The old engine also has 146k miles, but still runs good, even though it uses a little oil.

Cams? I'm aware the 108 LSA will pump up the midrange, but will it also kill off 2-3 mpg?

Thanks

[PackardV8]

If you can tune an Edelbrock, you can tune a Q-jet. Also, it would be unusual for an Edelbrock 750 to get better fuel economy than a Q-jet in equal fettle. I've talked with a couple of fuel system engineers and the Q-jet was the last clean-sheet carburetor designed in this country. They tell me it is the best all-around economy and performance carburetor ever built.

And yes, a new, completely sorted 383" might get better economy than a worn-out 146kmi stock 350", but that's not a reasonable comparison. Sort of bad idea for me to be in the performance engine business and talking you out of building a more expensive engine. However, if your 350" is a usable core, we can do a solid stock long-block rebuild for less than 1k$ with all new moving parts other than the crank and rods. A budget 383" long block with a performance cam goes for twice that, if you furnish the Vortec heads. When a customer comes to us with the goal of better performance and better economy and low cost, it's the old game of pick any two. Your truck, your money, your decision. If it were my truck, I'd build the 383" also, and maybe even tell the wife it would get better fuel economy. I'd also never let her see me doing the math after a fill-up.

thnx, jack vines

[bigjoe1]

Used vortec heads are VERY RISKY-- they are VERY prone to cracking- my loclal machine shop says they are one of the worse about cracking. I have NEVER used a vortec heads that was not BRAND NEW. SOOO, be very carefull hereThe only place a 108 center cam would be less effecient is when it is ideling... Pulling a load, the 108 will be quite a bit stronger at 25-3500. I always tell my customers to pull hills ect at 3500, even it means 2 nd gear, then engine will not be working as hard as when you try to pull the same hill in high gear.Believe it or not, a 750 double pumper is a VERY GOOG carb on a big tow rig. I have seen it work on many differant combos.

JOE SHERMAN RACING

[CamKing]

Cams? I'm aware the 108 LSA will pump up the midrange, but will it also kill off 2-3 mpg?

If you want to waste more fuel through the engine during overlap, run the 108.
If you want better milage, run a 112.
The 108 will only make more power between peak tq and peak hp.
the 112 will make more power on both sides of that.
When cruising down the road, towing a trailer, you'll be running way below peak tq, and the engine will be much more efficient with the wider lobe center.
My cams have held a few milage records, and I've done enough cam development for the D.O.E. to have a pretty good handle on this.

You need a small cam on a wide LSA. Focus on making the engine efficient below 3,000rpm. If the power noses over above 5,000, so what?
A 383 will still be making good power at 5,000.

[n2omike]

If you can tune an Edelbrock, you can tune a Q-jet. Also, it would be unusual for an Edelbrock 750 to get better fuel economy than a Q-jet in equal fettle. I've talked with a couple of fuel system engineers and the Q-jet was the last clean-sheet carburetor designed in this country. They tell me it is the best all-around economy and performance carburetor ever built.

And yes, a new, completely sorted 383" might get better economy than a worn-out 146kmi stock 350", but that's not a reasonable comparison.

I've never messed with a Q-jet. Don't really intend to. The Edelbrock is simple. Plus, they sell Calibration Kits with different metering rods and jets, that make them easy to dial-in... especially with an LM-1.

I just recently purchased the truck, and was looking at fixing it up a bit and using it mostly for occasional towing duty. I figured building a 383 for it would be fun, and could be made fairly efficient and powerful with some more modern parts and good tuning.

I'll probably go with modern lobes measuring around 210-215 @ 0.050" with a 112º lobe seperation. I figured the Performer RPM was a much more modern design than the regular Performer, and with the extra cubes, it should be about right. Edelbrock carbs seem to be a little over-rated as far as CFM is concerned when compared to Holley style carbs, so the 750 seemed about right. -Maybe- the 600 would be enough for towing, and -might- be a tad more efficient, but the 750 should be more fun when it doesn't have a trailer behind it. LOL

Thanks for the advice, CamKing. Do you sell many cams to people on here? You sure give out a lot of free advice. Hopefully you are getting rewarded a little for it.

The build is off in the future a bit, as the current engine runs just fine. I'll be slowly gathering parts for the next few years before it's time to put it together.

Thanks for the advice!

[bigjoe1]

Today I spent lots of time on the dyno, trying to make an Edelbrock carb run as good as a Demon. After I got it jetted as good as I could. I put on my favorite 750 Demon.. The Demon was 15-15 better on torque, and 1- to 19 better on HP.. It is true, This was a 500 Hp 406 Chevy street engine, But I was real dissapointed that the Edelbrock could NOT COME any closer thamn that.. My brother has a 406 SB in a big motor home. He has tried the Q jet, and a 600 Ededbrock to try to get better milege, but the 600 Edel was way too small. The 750 Edel was a tiny bit better than the Q jet on gas milege. But NO better on horsepower. Again, I am surprised that the Edel 800 is way down on power to the Demon.. Several times I have been told the could be made to run just as good--- NOT TRUE

JOE SHERMAN RACING

[fredo11]

Just a question....if the 750 dp carb worked well then would a 750 vac secondary work even better.Properly tuned it should help increase fuel economy while not giving up anything in the power department.

[bigjoe1]

I drove this truck many times, and the manual secondarys are the secret.. You can open the secondaries just a little, because you can feel the linkage with your foot.. It would scream up a big hill with a car on the trailor, and you never had to push the carb wide open.. I have told several other people about this, and when they tried it, they could not believe how good it works. Most people would never even thing to try it, but it really works good

JOE SHERMAN RACING

[digger5]

Thanks for all the input.

CamKing, I did have one follow on question. Wouldn't additional duration on the exhaust be helpful? The cam you recommended is very close in duration on intake/exhaust and has less lift on the exhaust. The vortec head does not flow very well on the exhaust side. The original GM cam for the 350 is 192/197 and 413/421 on a 114 LSA. The new HT383 crate engines have a 196/206 and 431/451 on a 109. The lift numbers or the HT383 numbers may be due to the lift limitations on the stock vortec head. Below are the factory flow numbers.
This engine will use the factory injection system (CFSI) and hydraulic roller camshaft.
Advertised Airflow Measured at 28 inches of H20
Lift Intake Exhaust
0.100 66 57
0.200 129 101
0.300 186 136
0.400 219 149
0.450 225 150
0.500 216 155

[CamKing]

Thanks for all the input.

CamKing, I did have one follow on question. Wouldn't additional duration on the exhaust be helpful?

For making max power, a longer exhaust would be helpful, but at the cost of fuel milage.
A longer exhaust duration will make the engine more efficient at higher RPM's, but less efficient at the lower RPM's you'll be cruising at.
You never want more lift on the exhaust then on the intake(unless you're running NO2).

On the steady-state engines that never run beyond peak Torque, we actually run much less duration on the exhaust side.

[gnicholson]

i had a 383 truck motor used for pulling with a mild hyd flat tappett cam on 112. later in order to build more torque i had a cam ground on 106 with less duration. the overlap was close to the same as the original cam and a single pattern. adv duration was around 256 if i remember right. anyway, the truck made way more torque than the 1st combo. it was amazing the difference it made. milage was about the same after sorting out the q jet and timing issues