I'm going to start digging into a pretty much stock Holley dp 850 carb. I see a lot of stuff from the aftermarket for this carb. I need to base line this carb in the rebuild with the best parts right off. This is my 1st Holley and I want to do it right. This is going on a 455 /10to1 total sealed ring motor with a 290-oh8 cam SP-1 intake port matched to a set of milded ported&polished ST1 cast heads. Headers with a total free flowing X-piped exhaust. What would be the best way to go with a power valve set-up.?
First off, buy a BG (Barry Grant) rebuild kit. The parts are better quality than the Holley stuff. Whatever you do, don't buy the off-brand rebuild kits from an auto parts store. Your power valve number will depend on your vacuum. Start with a 65 and work from there. Set all four butterflies so they just uncover the bottom of the transfer slot. You want the primaries and secondaries to be open the same amount (or close) at idle. Set the idle mixture screws 1 turn out from lightly seated. Adjust after running. I prefer to use the clear float bowl plugs and set the float level a little over the bottom of the hole. Make sure everything is clean!!!! Have fun.
It's based on engine vacuum. The actual one that works best for your application may vary slightly. Holley usually ships the 750's and 850's with 65's, meaning they open at 6.5" of vacuum. All PV's flow the same amount of fuel, the number just tells when they open. The idea is for the PV to remain closed at part throttle cruise and open when you go to WOT. It essentially richens the main circuit at WOT which allows smaller jets for better gas mileage at cruise. If the number is too small there may be a hesitation during transition to WOT. If it's too large a number, the PV will open prematurely causing poor gas mileage and a slightly rich condition at cruise. BTW, if it's a race only vehicle, put solid plugs (sold by Holley or BG) in both PV holes and jet up about 6 or 8 numbers. Have fun.
5"-6" at idle is pretty low for a street driven vehicle, so I would say you would end up with about a 25 or 35 PV, or you may just want to block it off and jet up 6 or 8 numbers. You're probably not concerned with gas mileage. Your PV size will depend on other things, too, like torque converter, gearing, etc. You need to do some test and tune to get it right. Have fun.
It is kinda low right?is that normal?i have a manual trans so torque converter is not an issue and i have 3.73 gears out back,the cam is TA Perf. and intake is TA Sp1,cam specs are: Lift int.:525 Lift. Ex:520 Advertised Duration:290 Intake and 308 exhaust @50:238 int. 248 ex. Lobe sep.112 Thanks
Also the float level is suppost to be just a bit lower than the hole,does that go for the rear flaot level as well? Thanks
?????????????????? Where on the power valve can you read it's number.The one I got has 5s&6s on it. It sound like I'm going to have a similair motor as stage1gspower but with a auto.
As far as what? 850DP SP1 290-0h8 cam iron heads with mill port work& polich chambers with ST1 valves 10to1 motor [no overbore] total sealed rings complete free flowing exhaust 3:73 gears 200r4 with 2800 stall 3700pound 79 REGAL with A/C
yea basically the same,only difference in mine is the .38 overbore and the JE lightweight Pistons,i have an 800 DP,maybe alittle more compresion than u and the Aluminum stage 1 soon to be on. Thanks
Looking forward I wanted a cam that pushed the limits of this motor.Didn't want to be cam limited with the future plans of maybe a set of those same alluminum heads your looking forward to. Once I get this set-up tweaked on gas and get the ET's up to12.00'sI'm hoping.It will be time to start adding N2O to it up to 125shot. I think this cam can carry with efficiancy what I want to do. Also I like the idle music.
Re: ?????????????????? It's usually on the flat face around the hole for the diaghram. Sometimes it's on the "flats" on the sides (where you would put the wrench). There are other numbers, but there should be 2 numbers by themselves, such "65" or "45". If they're on the front face there is a space between the two numbers. Does that make sense? Have fun.
Been looking thru the JEGS&Summit catalogs and I'm thinking on going with the fuel bowls that have the external jet changes made right in them. They go for about 80. for both and than a standard 34.BG rebuilt kit and I'll never have to remove the bowls. Also thinking about buying that power valve preventer blow out kit. Holleys home page says the stock 850DP comes with 65 power valves.With this cam 290-oh8]I'm going to run 45's for break-in of the new motor and go up from there after that. The carb has 82 jets all around. I hear 78's are good to start with on the fronts and 82's in the back. Is this a good base line?
Imo if the carb was "square-jetted" to begin with, I would leave it that way. Front to rear jet stagger is for NO rear power valve, like my 800DP. 82F, 92R jets, 2.5 front power valve (not original jets or pwr valve!!), no rear pwr valve in my case! Sounds like your 850 has BOTH frt and rear pwr valves.
Being that mine is a 4780-3, they were 71 & 85, now they are 76 & 90 (I just went out and checked, I thought I said it wrong earlierou: ) There are at least 5 different versions of model # 4780 800cfm. Look here for specs: http://www.holley.com/HiOctn/ProdLine/CarbList.pdf HTH:grin:
