took my 2x4s off,put the single 4 intake cut out like doc says to do.when i put the the 2x4s on i lost some bottom end.when i put the cut out 4 intake seems like i lost a little more bottom and responds.but mid range and top end it really picks up,pulls way better on the top.with the stock single 4 intake it would just spin the tires way too much.just wondering way it affects bottom end.
took the car back out the bottom end is about the same as the 2x4s.ithink still a little less than the stock 4 intake
Smaller primaries = more off idle air flow velocity... It takes a while for the motor to spin enough to take advantage of the extra manifold volume you created.... Just my uneducated guess...... is your manifold vacuum reading and timing set correctly between the two intakes when dialed in? I don't know if you have to re-jet and calibrate the carb to take advantage of that modification. Todays blended fuel has less energy.
that makes sense. didnt think the timing would change but i will check it. must have to richin the carb with more manifold volume?. i like the way it performs can feel it pulling better. would do again.
I believe the factory shop manual calls for setting the timing at 12 degrees for the 2x4 and 2 1/2 degrees for the stock single 4.... Set the vacuum at highest steady reading...
Just keep on tinkering with it ,,,, you will get it sorted out.... so far everybody that i know of that has done that mod has had good results.... Yes the ign timing will affect the response... also I have my dist recurved.... used a mr. gasket kit.... and i have a 600 cfm holley on my engine...but I had all that before I did the mod to start with....
i got my fresh air hooked up like i had on the 2x4s. alot better. now i will try a 800cfm edelbrock just got it today its used but only a 1 year old. taking off the 750cfm edlebrock which is jetted up 2 or 3 sizes, thinking and hoping the 800cfm is not to big.
It is not to big. I have been running a 950 quick fuel with the annular boosters and a " doc job" on my intake. I really opened up the plenum area and smothed out the turns into the runners. I think that set up is a little more potent than my 2x4. I was going to find out at the chassie dyno but I was tuneing on a back road and at a 10 mph role I floored it and at 5500 and 20 mph the u joint came apart and really caused some problems.ou: That was with the 1x4 set up. Havent had time to fix everything yet
I started out with my original car with a fresh built 401 with a stock carter afb...that came on a 59 Buick.... then Holley started coming on strong with racing, and i bought a 850 cfm and put it on.... then a guy came along and wanted to trade me a 950 three barrel for my carb.....I did it and the car ran so much better with it that I never looked back... it could have used a 1050 three barrell.... no your carb will not bee too big.... but it wont get mileage like my little 600 cfm holley:Brow: :laugh: :laugh: basicly if you dont care about mileage, as in street/strip or you do ,as in cruising .... you can get 2 carbs and in a hour or less change back and forth, easily.....neat set up....:laugh:
the 800cfm is better the 750. jetted up one size over stock. with the cut out intake like doc does and 800cfm might b as good or better than the 2x4s with the 2x 600cfm and eeclo intake.
I bolted on my "Doc Mod" intake yesterday on my bone stock 65 Gran Sport and with no tuning took it out for a few runs this morning, as timed by my G-Tech meter. Nearest I can tell with only 4 runs under my belt, the manifold is good for .2 tenths. First run was 15.5, with mucho wheelspin on cold tires. Next two were both 15.1, one with no wheelspin but plenty of bog, the next with some wheelspin but shifting manually. Next run I matched my prior best run of 14.9, hzed the tires and shifting manually. I was surprised that the mod didnt kill torque like I thought it would, but does feel stronger on the top end. The one dissapointment is that it created a huge off-throttle flat spot. I get the whole "change in venturi effect" thing, and I'll admit I havent tuned for optimum vacuum yet either. But on a square bore manifold, this carb (AFB at factory settings) had immediate throttle response. It almost feels like the secondaries are dumping open too soon, but with out permanent mods I dont think I can change that on an AFB. I could try jetting/rod changes, I assume to go richer but I'm just guessing. One other aspect is it was 54 degrees out, and I put blocked heat riser gaskets in when I did the intake, but I let the car warm up thouroghly, and the choke was fully open. Anybody have any advice? The car still runs very strong but obviously has more in it. Thanks, Rhett
Rhett, it looks like you did a super good job on the mod,,,, I think that the bog you are getting could be from running a cold manifold....... I never block the heat out.... try putting the heat back in and see what that does for it....I tried running a cold manifold with the original car and a 3 barrel and it did not do as good, and I quickly changed it back.... raw fuel will not burn...but the original car did not have the ddp mod... hadent thought of it yet....:laugh: You will get it sorted out and it will run stronger yet......:Brow: 2 tenths is several car lengths on the strip.....and if you can , increase the fuel pump shot,,, you are getting a lot more air going thru the manifold now....
doc is right when i put my scoop on with cold air it would have a flat spot i was running 2x4s. now with the single 4 it did the same thing the first time today because of the cold weather.my heads are filled so when there off i am going drill a small passage. you should try just putting a small hole in the block offs.with the 800 cfm and docs mod i think its faster than the 2x4s. i have to bring it back to the track to find out.
Is is possible it's just a bit lean at the colder temps? You could try increasing the accelerator pump shot. Maybe opening up the idle mixture screws will richen up the off-idle too....but I'd have to hit the books to be sure if that'll work (in theory). Oh yeah, I had a similar problem with my Q-jet......turned out the float level was too low. Might want to check that.
Thanks to everybody for your quick responses..Before I take the manifold back off, I'd like to try the increased pump shot idea...but how do you go about it? Modify the existing pump?Different part number? The way the car runs, it feels like just a little extra fuel off throttle might do it...the rest of the run is perfect, been shifting the car at 5k and no fuel starvation throughout.. Next question - are the metal intake gaskets a single use item? The ones I took off the car looked perfect and didnt seem to have the ridge crushed. They are cheap enough, but I might just wind up switching back and forth fairly often.. I ran the car in mid-80 degree weather this summer and it cost me .4 - I remember thinking it was at least partially due to heat soak. People ice down manifilds for a reason, right? Thanks again, Rhett
Rhett, in the past, well, in 1970, I was racing the same basic set up that you have , but I did not have the ddp but was running a holley 950 cfm three barrel with a cold air pkg and heated intake manifold....after 3 back to back runs , hard runs , I could tell that there was heat fade.... but i never did cool the manifold down....heck, I did not even cool the fuel before going into the carb.... i dearly wish that I had thought of the ddp mod back then.... :laugh: the problem that you are having is at the low end , not the upper end,,,, I would say, put heat into the manifold first,, if it eliminates the problem, great....then on the top end, feed it cold air from outside of the engine compartment...consider tapping into the ww chamber either like chevy did on the 1968 z28 camero, or like I did , using drier duct..... The cold air pkg is worth 8% of total hp, according to Buick....I have done it and it really works good.....and doesnt fade away....but I still got some heat fade after back to back runs....I think that a way oversize radiator along with a 165 deg. thermostat and good fan set up would be a problem solver along that line,,,, because heat is the enemy, the intake needs enough heat to make the cat pee fuel ''flash'' off of the floor and sides of the manifold and vaporize as much as possible..... I took temp readings on my 425 manifold and got an average of about 125 to 127 deg..... which is cooler than I thought,,,,but the engine running hard in a race generates enough heat to add to that basic heat thru the hot air coming out of the rad. and off of the exhaust manifolds... in short , any way that you can get heat out of the engine compartment is good,,,, this is why when you look at vintage rod books you see louvers in the hoods... hope i have made sense here.....
Doc - That all makes sense, and I've put alot of effort into general cooling, including 4 core radiator, overflow tank and 16 lb cap, 7 blade clutch fan and a 165 thermostat. So now we're back to open gaskets, thankfully I ordered both last time so I dont have to re-use the old ones..but as I did the mod I also blocked the heat ports at the carb flange on top of the manifold with some JB Weld. I should be able to knock it out easy enough, but should I? Is heat to the manifold enough, or does it need to circulate under the carb to fully heat? Rhett
Move the actuating rod to the innermost hole. That'll increase the length of the stroke. If it's already in that hole, maybe you can drill another hole. You'll have to adjust the length of the rod (by bending it) to compensate. The goal is to maximize the stroke. First, measure the full up and full down position of the acc pump...measure from the carb top to the top of the pump. With engine off, test it....watch the fuel spray as you open the throttle to max.... does the spray start as soon as the pump starts to move down? then push the acc pump down the rest of the way with your finger...do you get more fuel? You should be able to determine at what point the pump starts to spray fuel, and what position it stops.....then adjust hole position and actuating rod length to use that whole stroke. Changing the spring on the plunger affects how FAST the pump shot happens, not the volume. Changing it may help or hurt you. P.S. a low float level will also affect how much fuel is in the pump shot! Make sure it's right. Reusing metal gaskets....you can give used ones a coat of copper spray-on gasket seal. Just install them on the same side they came off of.
Great advice, I'll have to at least take a look tonight. It'd be nice if that took care of it, but I get the whole flash aspect as well, I assume the exhaust channel below the carb flange is to prevent icing, so I wonder if I have to open that back up as well. You think I have questions now - wait until I do the distributor! Rhett
making the carb richer you might just have more raw fuel thats not burning it starts going into oil, smell the dip stick.
