RV/Race Trailer Tow vehicle camshaft change?

[Ron C.]

The "Improve mileage on 3500 tow truck" has inspired me to work on mine.

The vehicle is a Chevy truck chassis RV that also tows my 30' enclosed race trailer. The motor is a 1999 454 Vortec engine without computer controlls or fuel injection. Edelbrock Quadrajet on a Weiand manifold and GM HEI ignition with limited advance for detonation control.
I already have headers on it.

When I installed the engine I changed the cam to a Comp Cams XR264HR which is 212/218 @ .050 on a 110 LSA.

I should never of installed that much camshaft because I tow 98% of the time in the 2400-2600 RPM range.

The stock cam that I didn't use was 204/209 @ .050 and on a 118 LSA.

I'm looking at ether installing the stock cam or possibly the Comp Cams XR252HR which is 200/206 @ .050 on a 110 LSA.

What are some thought's, this engine works in the 2400-2600 RPM range going down the road.

I'm also going to install a 02 gauge to work on the AF a little.

Blessings.................Ron Clevenger.

[new engine builder]

The "Improve mileage on 3500 tow truck" has inspired me to work on mine.

The vehicle is a Chevy truck chassis RV that also tows my 30' enclosed race trailer. The motor is a 1999 454 Vortec engine without computer controlls or fuel injection. Edelbrock Quadrajet on a Weiand manifold and GM HEI ignition with limited advance for detonation control.
I already have headers on it.

When I installed the engine I changed the cam to a Comp Cams XR264HR which is 212/218 @ .050 on a 110 LSA.

I should never of installed that much camshaft because I tow 98% of the time in the 2400-2600 RPM range.

The stock cam that I didn't use was 204/209 @ .050 and on a 118 LSA.

I'm looking at ether installing the stock cam or possibly the Comp Cams XR252HR which is 200/206 @ .050 on a 110 LSA.

What are some thought's, this engine works in the 2400-2600 RPM range going down the road.

I'm also going to install a 02 gauge to work on the AF a little.

Blessings.................Ron Clevenger.

What are your thoughts on doing a gear change instead of a cam change?

[Old School]

I would think something on the 200-204 intake , 206-212 exhaust on a 112-114 may be the best compromise.

[bigjoe1]

The most powerfull 454 truck cams I have ever seen were at 214 at 050- They do work better if your RPM is higher,like 2800 to 3200, I have tried everthing from 108 to 114 centers, but that duration was allways very good-If you have low compression, the 108 is better, but if you have more duration, 112 or even 114 will work-- We allways had 3.73 or 4.11 rear end gears in the 454 dullies I remember. Single pattern cams seem to make more power for a tow truck rig

JOE SHERMAN RACING

[beth]

I did a lot of testing on my P-30 454 GM chassis RV after installing a miles per gallon computer in the late 80's. Gearing changes and installing an overdrive with the turbo 400 transmission showed that the lower the cruise rpm the higher the mileage even with a loaded trailer as long as it was on level ground. I tried all different speeds... 70.. 60.. 50.. 40.. even down to 30mph with and without overdrive the mileage was always better in overdrive. Even with a head wind or very slight grade when more throttle was required the computer showed less fuel was used in overdrive. With a slight tail wind or slight down grade the higher gearing used as much as 50% less fuel. This was with the stock cam, peanut heads and Edelbrock intake replacing the terrible stock intake.

The quadrajet carburetor is excellent on passenger cars. The small primarys give good mileage and the secondarys remain closed until WOT and it makes good power. These is a problem with this carburetor when the GVW is high. The primary side does not flow enough for the power needed for even a modest grade especially at altitude. Under these conditions the secondary air valve opens part way and very poorly atomised fuel is dumped into the secondary. This kills mileage and sometimes will even produce black smoke. If the seconday air valve adjustment is loose this is even worse. Tightening the air valve forces you to shift down way too soon when you run out of torque on just the primarys because the secondaries will not open until you do. Shifting down soon gets the air speed up and the secondarys open but you are shifting down and going wide open on very slight grades.

You have to be very careful but it is very interesting to have someone drive an rv on various grades and watch the operation of the quadrajet.

I built a spreadbore Holley double pumper QJ replacement. It used a cam for the secondary opening and i modified the cam so the primaries were 90% open before the secondaries suddenly opened. The fast action of the secondaries caused a stiffer feeling in the accelerator pedal. You could push it down fully opening the primaries with out moving the secondaries. The larger and much better flowing primaries of the holley gave enough power to pull moderate grades. A slightly harder push opened the secondaries when full power was needed. I even disconnected the secondary accelerator pump as it was not needed with such late opening. This carburetor gave complete control and felt like EFI while using less gas then the QJ. I don't think they make those carbs any more

[bigjoe1]

I had a similiar experiance using a 750 double pumper-- You could open the secondaries just a little when you came to a hill without going to full throttle--- It worked very good on a very heavy load ( 18000 to 20000 ) truck and trailier weight- I also found that if there was a cast iron manifold that was pretty good, it will always get better gas milege than an aluminum one


JOE SHERMAN RACING ENGINES

[Ron C.]

Actually I have one of those Holley Qjet replacements out in the shop someplace. I've never used it. I'll have to dig it out and take a look at it.

I have 4.10 gears in the rear with a 400 turbo/allison converter and behind that I run a brownie three speed that has about 17-18% over direct.

I've run it in direct and over drive and it really does not seem to change the mileage much. I do like the less engine noise in overdrive. And since it's quieter I usually run a little faster too, which may be why the mileage is about the same.

I was figuring the cam in it, the main torque area RPM wise with the 212 @ .050 (264 advertised) might be above the 2400 to 2600 tow RPM.

Why do you think the stock cam is spread to 118 LSA? Maybe smoother idling or increased vacuum for the stock fuel injection?

The rig weights about 20,000 lbs. going down the road.

Joe, I'm not after more power, it's got plenty of power. I'd even give up some for more mileage.

Blessings........Ron.

[Keith Morganstein]

Run some simulations in EA or similar program. Exeriment with cam specs and see where the the "fat" part of the torque curve is in your desired cruise RPM. I find BBC's like low teens on .050" intake duration with 112*-114* LSA for towing and economy. Lunati had some EFI grinds that worked well for me in the past.

In general lower cruise RPM and vehicle speed will result in better fuel economy.

[gnicholson]

i built a 454 for a 3500 that was pulling a 40 ft car trailer. the 1st cam i used about 268 adv on 112 .the engine was about 9 to 1. the power in the rpm range needed to tow was disapointing and it also pinged pretty bad at part throttle even after changing the timing curve. i ended up putting a custom grind that was 260 adv on 106 in the truck and couldnt believe the diiference. it felt like it had 100 ft lbs more torque and you couldnt make it ping. you would think the added cyl pressure from tightening the lobe center would have made the detonation worse but it sure wasnt the case here. mpg was much better towing as well

[raynorshine]

i had a 86 chev with 350 sbc, did some towing with it years ago. i used a rv cam that was 216/216, .440" on 112. i thought it worked very good. can't see how a bbc couldn't handle that just my experience

[MadBill]

For those who might have missed it due to youth or inattention, about 30 years ago Bruce Crower produced a "fuel economy" engine kit that consisted of high compression pistons (>13:1) and a cam with conventional RV valve events except for a very late IVC (sort of a poor man's Atkinson Cycle* concept) to reduce pumping losses, as more throttle opening was required to produce road load power due to the smaller trapped volume of mixture. The reduced cylinder pressures due to the late IVC also meant that the high CR did not cause detonation, but still gave a very long and efficient expansion stroke. I think the deal was too much for the average guy to tackle and it was not a commercial success, but once tuning issues were resolved reported gains were as high as 30%. *Google for details.

Of course SpeedTalkers are anything but average, so for those who might want to get a little unconventional, even without a sky-high CR, consider: A 214°/214° cam with a 110° LCA, installed 4°A would have events of IVO 1°, IVC 33°, EVO 41°, EVC -7°.
Adding 20° to the IVC with no other changes would give 1°, 53°/ 41°, -7° or a 234°/214° @ 0.050" cam with a 116° LCA, retarded 2°.

Low speed torque would suffer unfortunately, unless a supercharger, say an Eaton with part throttle bypass to eliminate parasitic drag at part throttle, was used...

[Ron C.]

Sounds interesting Madbill, today's gas especially in Calif is pretty bad.

I 'm really thinking two different directions on this (cam change vs engine change).

One is, the current cam has the main torque curve above my 2400 to 2600 RPM range. The XR264HR is advertised operating range of 1200-5200 rpm. The other two cams I'm looking at trying are XR252HR which has an operating range of 600-4600 rpm. To that tells me it moves torque range down about 600 RPM.
The stock cam puts 4 degrees of duration @ .050 back in, so should be raising it 200 rpm torque operating range or 800-4800 (50 RPM per degree is the rule of thumb).

My real stumper to me is, the big difference in LSA between the two (110 vs 118) and why the factory spreads it so far more than aftermarket cams.....???

The second thing I'm looking at is going back to the pre-vortec engine. I have a 1991 Gen V engine that I could install. It has a little smaller ports and "conventional" combustion chamber. I just need a cam and pistons for it.
I've heard grumbling that the Vortec (heart shaped chamber) does not like a carb and conventional distributor. Don't have any facts for that though.

So, am I out to lunch on this cam thinking and what's your take on the vast difference in LSA between the cams I'm thinking about.

Thanks for helping me think through this.

Blessings..........Ron.

[CamKing]

for that RPM, I'd go with my hydraulic roller part#HR65327-67312-113
208/210@.050"
.556"/.530" Valve Lift
113 LSA

[Lazy JW]

.... I also found that if there was a cast iron manifold that was pretty good, it will always get better gas milege than an aluminum one JOE SHERMAN RACING ENGINES

Would you please explain further?
Joe

[raceman14]

Ron,
Before you go to all that trouble, check and see if there are any low ratio rockers avaailable for the BBC. I had a similar experience with a 400SBC and had a 218-222 Hi-Energy Hydraulic in it and it was balls out as a daily driver but sucked big fuel when towing.

I put the 1.35 break in rockers on the intake, installed a smaller 2101 intake, and went to a 600 vacuum secondary, converted it to rear metering blocks with dual stage power valves front and back, I think they were 12.5/7.5 I think...once I fnally got it running right the old Suburban got stolen and was never recovered. Truck went from 12/13mpg towing to 18 mpg, back and forth from Atlanta to Charlotte. It is a hilly ride but no major mountains.

E or G range tires pumped up to E@100 & G@120psi make a huge difference in rolling resistance, on my GMC 6500 it is about 2mpg towing with E's all the way around @ 100psi 48"GN 2 ARCA cars and pit boxes prolly 20,000# on the trailer easy. I am going to switch to the GY G's when I start to replace tires.

At least 1/2 of what you are dealing with is aero above about 60mph so you can also look at keeping air out from under the truck and then putting a couple wickers on the a-Pillars and matbe the roof to separate the air from dragging all the way down the vehicle.