467-Ratman

Ok, I know that most people told me the 850DP was too much carb for my 454. The performance is great, but my mileage has been around 5-7 around town, and thats taking it easy! On the road its about 9mpg, highway speeds. I am running 78 jets up front and 82 in back, power valve is 6.5. Any suggestions on jet size or power valves? At idle the exhaust really spits, sounds wet. I do love to wind it out , but most of my driving is 4500 rpms or less, I know that sounds wimpy but the nearest drag strip is one hour away! How about a Q-Jet or the new Edelbrock performer 800 or 750?

The Money Pit

I run a Holley 3310 vacuum secondary on mine and the stock jetting seems fine.Idles at 650 with quite the fender shake,and pulls to 6500 rpms easily.I was getting 15 mpg till the recent BTO 700r4 swap,and with the crippled trans I still get right at 14 mpg driving easy.The tail pipes look clean,but I haven't pulled the plugs lately to see how it's burning.Got too many other things to do.

I know the big carbs seem macho,but truth is a 750 vac carb is all most anyone needs for the street.Holley knows this and that's why the universal 750 vac carbs are so cheap.They work,and lots of people have already tried them and found they work.

Another tidbit. I replaced my Q-Jet not because it didn't run well,but because I found it starved the 406 at high rpms. The 3310 has no trouble keeping up.




[Modified by The Money Pit, 7:52 PM 8/28/2003]

Brettmc

So what's the problem? Is it the fuel economy? I would think if the 850 is wrong for your motor it wouldn't run right...but it sounds like you're happy with the performance. If it's the fuel economy...well...lookin at your profile, I'm not surprised. Tell ya what, I'll trade ya my stock-headed, small-cammed 468 for yours. Mine gets around 10 in spirited street driving. ;)

Brett :thumbs:

Fevre

Few things to try in order of cost:

1. Play with lower your pump shots and jets sizes.

2. Get an O2 sensor then do #1.

3. Get a dyno tune.

4. Get a tbi system, better than a carb but not as good as mpfi.

5. Get a mpfi system.

ykp53

im running a edelbrock proformer #1910. is basicly a 850cfm q-jet. it has been rejetted to lars specs, the plugs are not fouled it runs good and strong, but not to rich. and my gas mileage isn't much better (8 in town 11 highway). lets say a new carb increases your gas milage from 7mpg to 9mpg for every 100miles thats about $5. so that means to justify a new $400 carb you would have to drive about 8000 miles. so is it worth it?

ORVette

I run a stock jetted 3310 too. My cam is shaky, but I idle around 800 with no problem. I only have about 11 inches of vacuum at idle, so I switched to a 5.5 power valve. Also, I drilled holes in the primary plates and cracked open the secondary just a little to help tune my idle. I noticed a pretty appreciable gain from running a bolt in my secondary linkage to get them to open mechanically. I wasn't getting full opening even with the lightest(white) secondary spring. I average between 8-12mpg depending on how I drive-usually leaning towards the 8.

467-Ratman

One more thing, after I run the car for a couple of days my back bumper and lower valance gets a black coating. I know with the way we set up the motor it really likes the extra gas, and with the MSD system it runs better. Just not sure if I should lean it out some to clean it up? Maybe 75 or 76 jets instead of the 78 size. Next week I am taking a drive to a shop that has a chasis dyno and they said they can check many items, including A\F ratio, should be interesting to see how much HP this Rat really makes!

Brettmc

I'll bet it's a bunch too!! Take some extra jets with you. Also, have you checked the vacuum on your motor? If it's low, the power valve might be opening at the wrong time, like at idle and part throttle. I bought a vacuum/pressure gauge at AUtoZone for like $20. I've used it many times to tune my 850.

Brett :thumbs:

467-Ratman

I only have about 10 to 11 inches of vacuum at idle, seems like the power valve might be the problem.

Fevre

Stock pv's are usually 6.5 for Holleys. I am using staggered pv on my 750 dp so it gets a little juice under light acc and a second shot when the vac really drops. I am running lean on my primaries so I am running a pv that is rated 2 lbs below my vac at idle. I too use a vac gauge along with a O2 sensor and it has helped a ton in making it easier to tune. Now if I can get the valve stem seals to stop leaking. :mad

467-Ratman

Next step after the chasis dyno work is off to Ubly drag strip, I am hoping to be a solid member of the 13 sec club!

fireball

I have a 3310 Holley vacuum secondary with stock jetting. I ran on the dyno at Carlisle and was AMAZED that I was running WAY RICH. I've got 72's up front, 76's in the rear, 6.5" power valve, with the lightest spring. My A/F ratio was around 11 from 3k rpm and slowly getting richer at higher RPM. My setup is a 468ci, rect heads, and high torque cam. Doesn't seem right that a big block would need SMALLER than stock jets, but the dyno doesn't lie (unless I have a problem somewhere).


[Modified by fireball, 12:00 PM 8/29/2003]

467-Ratman

Sounds like in addition to the new power valve I will be trying a set of 76 jets as well...... instead of my 78's

gkull

The carb size or whether it's a double pumper really doesn't change the mileage.

Gas mileage is a factor of engine efficiency + gear ratio + aerodynamics & mechanical drag.

I don’t know what you really have. In one of my old motors I got over 21 mpg average on a 5000-mile trip. My last hot rodded motor with a 825 CFM Race Demon got almost 17.4 on a nearly 3000 mile road trip driving 85-120 mph most of the time. My new setup with a bigger roller cam got about 17.4 average on my last 1000-mile drive over the weekend. That’s with a 750 Speed Demon and driving excessive most of the time.

I’ve been driving to and from work and screwing off this week and it’s near empty on my 25-gallon tank and the trip set OD is 287 miles on it.

High over lap cams are a big No No. That’s why I now have less mileage than before.

Jetting the primaries leaner will gain some if you’re way rich on your cruise A/F ratio.

I used to have a street rod with the monster tunnel ram and twin 660 cfm Center squirter Holley four barrels. If it got 8 MPG it was a good dayJ

467-Ratman

Is it the lift or the duration of the cam that makes the overlap? My Crane powermax has 236\246 duration at .050 and 553\571 lift.

Fevre

The duration factors into it but it is how long the two lobes are open at the same time. Just think of what a cam looks like then imagine taking one of the lobes of a cyl and rotating it away from the other, that is decreasing overlap, now think of moving it closer, that is increasing overlap. More overlap makes for choppy idle and high end power while less over lap makes for smoother idle but less top end power.

zwede

You probably have the same problem I did (back when I had a carb). The low hood clearance made me use a drop air cleaner and the lid was too close to the choke horn and acted as a restriction. Basically I was always running around with the choke on... Only way to solve it (besides getting a L88 hood) is to cut the choke horn.

zwede

Actually, it does. Holley DP's are meant primarily for the track. Because of this, the part throttle mixture is much richer than their VS counterparts. You can't lean the main jets as that would make it run lean at WOT.

Techie explanation: AFR on a holley is determined by the main jet size at cruise. During high load/low vacuum the power valve opens which adds extra fuel through the PVCR (power valve circuit restriction). On a DP the PVCR is much smaller than on a VS, so to get the same AFR at WOT, you need larger jets which in turn will richen the cruise AFR.

Fevre

Can't you run richer jets on the sec side to balance it out? If not then what affect does changing jets on the sec side do if it won't make it richer?

norvalwilhelm

I always run 850 double pumpers on my motors and find the carb after reworking to be good on gas. My blown 460 has twin 850's and pulls 15 mpg anytime.
I run the carbs very lean. My oxygen sensor reads off the scale at 15-1 and I have run it this lean for years. The plugs are white.
To jetting doesn't lean the carb out around town. 1st the idle circuit needs to be leaned out and that involves drilling the throttle plates, all 4 with about .100 hole and holley recommends this. Also a .015 wire should be put in the idle circuit passages of all 4 idleing jets. By closing the throttles with the holes drilled the transfer slots are covered leaning out that area. I still jet 80 square and it runs lean.
At speeds the jets start to take over but not at low speeds.
Lean it out down low and the top end will be fine.