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Camshaft Help
I am looking for help in selecting a camshaft for my engine. I’m looking to greatly improve the gas mileage which now is between 6-9 MPG, also looking for better streetability.
The engine is made up of the following: 1969 351W block bored .030 over with TRW forged 2378 +.030 pistons. Ford Motor Sports GT-40 “Turbo-Swirl” Aluminum heads with 1.94 intake valves with intake volume of 160cc and 1.54 exhaust valves with exhaust volume of 53cc. Ford Motor Sports 1.6 Roller Rockers. Cam is Ford Motor Sports M-6250-A351 with these specs: Intake events open 9° BTC, closed 47°ABC, Exhaust events 62° BBC, closed 4° ATC, Duration intake 296 or 236 degrees, exhaust 306 or 246 degrees, Lift lobe .325 I or .336 E, Valve .520 I or .538 E, Lobe center 108°, exhaust 119°. Cam intake and exhaust events are measured at 0.050 tappet lift. Intake Manifold is Edelbrock Performer RPM. Carb is Holley 750 double pumper with mechanical secondaries. Carb is in a fully sealed Paxton enclosure and boosted with a Paxton SN2000 6-8 lb supercharger. Exhaust is custom 1-5/8” headers for Cobra replica. C5 transmission with 2500 stall convertor and manual valve body. Any help will be greatly appreciated. |
Dave,
I ran a paxton on my 91 lx mustang with good results. I'd change your cam out for one with less duration. I ran the E303 cam, has 220 duration@ .050 . You are probably blowing fuel right though the engine into the exhaust.I would us the comp cams 268H, 218 @ .050. Also to pick up cruising fuel mileage you need to advance the timing to 36 degrees total and have a msd boost timing master to retard the timing 1 degree per lb of boost.The cam change will give you way better drivability and 5 mpg, the timing will give you another 2 to 4 mpg and way better throttle response. Good Luck, Perry. :cool: |
That kind of fuel consumption should be fouling your plugs.
My 351W, similar setup, CompCams hydraulic roller 224/224 gets 17 MPG on the highway, and pulls 15" - 16" vacuum at idle. Carb is Holley 700dp (4778). Had to change out-of-the-box primary jets from 69 to 64, and crack open the secondary butterflies to lean the idle out. Ask Crane or CompCams about a street cam with the Paxton, but I'd go hydraulic roller 218 - 224 I, 224 - 234 E, and some jet changes should dial you in. |
cobradave before you change the cam, Check the basics first. Put new plugs in the car and go for a normal drive. Get the motor to operating temp. Shut the motor off and let it cool off and check the spark plugs for color. They should be a medium gray. If they are black you have a fuel problem,EGR problem, or carb problem. What is the fuel pressure regulator set at, with and without boost. Check the float levels, they maybe too high. Egr maynot be seated causing a vacuum leak in the motor maken it run rich. What ignition are you running?? What is the base timing set at. Put a vacuum gauge on the motor and see what the reading is?? Should be 16 to 19 inchs and a steady needle. If you are still having a problem go to a Dyno speed shop that has a air/fuel meter to test the motor. You maybe jetted too big. You need to be rich but there is a limit. Check the basics first than E-mail back if you have not found anything wrong with your car. Rick Lake
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Rick has some great points. However, that cam will not pull more than 8 - 10 inches per the SVO literature that comes with the crate engines. Mine pulls 8 idleing at 800 RPM. If I lean it out any further, the motor wants to die.
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6-8?
I have a 351C set up almost exactly the same as yours (manifold, Ford cam of similar specs 6250-A341, etc) less the supercharger and averaged 15.25 MPG at 80MPH with a 3.75 FINAL drive on a 2000 mile trip. Engine speed at 80 is about 3900. Wonder why your gas mileage is so much lower? I know that the double pumpers are usually a bit rich out of the box.
Vacuum, 16 inches at idle. |
Way too much fuel
A 750 DP is set up to run very rich at part throttle. It is a competition carburetor the air correctors and fixed jetting in the metering plates will keep the part throttle mixture real "fat". Holley has done this to prevent a lean condition in a bolt on application. A good suggestion is to enlist the help of a competent carburetion shop or tuner such as Barry Grant, Brasswell, Reed or others and get the fuel curve changed. They will also possibly reccomend a cam change. They will need all of the specifics of your setup.
Good luck Rick PS: You normally cannot use Brass, or plastic floats with this application in the pressurized environment as they will collapse. Severall other modifications must be done to the carb as well to run in a pressurized enclosure. |
I have a Trick Flow stage 1 roller cam in mine with 221/224 (@.050") duration and 112 lobe separation. Vacuum is 10 in Hg at 800 rpm and the idle is nice sounding but not too lumpy. My KB is pullied for about 6 psi boost. I have never been able to get a good mileage number from the car as I can't keep my foot out of the gas long enough. But with 'fun' driving I get 12 to 15 mpg. It would probably be closer to 20 mpg on a long highway run without the occasional spurt to the high side of 100mph. I am considering installing a more aggressive cam to help the high rpm HP and cut some of the low rpm torque.
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