elle88

I feel that my engine is not powerful and crisp as it should be when i accelerate in 2nd or 3d gear . At high rpm is a beast but i believe it can do better at lower rpm and i start to think the problem may be the holley pump cam i'm using

here is the holley pump cam chart and my engine specs




441ci , ZL1 cam , 11:1 CR , 4.11 rear end . 510hp dyno. 38deg total advance
carb :
Holley DP 850 annular boosters
discharge nozzles : 35
primary pump cam :pink
secondary pump cam : brown with 50cc acceleration pump

i increased the squirters from 31 to 35 but no big effect.

if you see the graph , you see that the pink cam is probably too much progressive and slow... is it reasonable to think that the green cam may work better in my situation?

am i wrong or the pink cam seems more intended for a vac secondary carb , where there's no secondary acc pump so the continuus and straight slope of this cam supplies fuel till wot ?
would instead the green cam be more suitable for a double pumper ?the green cam supplies all the fuel quickly then quits when the secondary opens ( with its own pump)
before spending lot of time trying these cams , i would like first to understand

scottyp99

It's pretty easy to replace an accelerator pump cam, why don't you just give it a try and see what happens?

Check out this link:

http://forum.grumpysperformance.com/...hp?f=55&t=1790

Scott

elle88

thanks for the link

for various reasons i cannot make any carb test before september...and i would like to hear forum's members experience and toughts about how these cams work

TCracingCA

What intake manifold?

elle88

GM dual plane with a thin carb spacer that allow some communication between the 2 planes

MelWff

looking at the original 4296
.035 pri and sec nozzles
218 pri cam in #2 position
6.5 power valves
stagger jetted pri 78C/82T sec 82C/80T C=Choke, T=Throttle

gkull

I think that you are looking in the wrong place. by the way I have the pink cams on my 825 demon

The squirters and power valve only come on at the hit of the throttle to enrichen the mixture.

You might need to replace the little rubber squirter plugs. They do go bad every few years.

bigger diameter squirters only serve in masking the real problems and all you end up with is a big blast of black smoke on the hit of the throttle. The arms have to adjusted so tight that on the smallest move of the throttle you see wet dribble looking in the bores with the engine off. Anular booster like what you have are better for a linear A/F ratio, but they do cut down on the total CFM. Wood 1/2 inch 4 holes give the best signal and work as a thermal barrier. That little piece of aluminum does nothing



Annular boost

Rebelrob

elle88, I had a Holley 850 DP. I just could not get it tuned right, but then again, I probably did not try hard enough. I upgraded to a 50 CC accel pump on the primary side which matched the 50 cc pump on the secondary. I found that if you used any other cam besides the yellow or maroon ( I think it comes with) then you have to bend the accel pump arm. That's when I got into trouble and F.... up the arm. I did put the pink cam on once and the throttle was very crisp but ran out of juice after around 3000 rpm. I went with a vac sec 750 for now until I get some good miles on the engine. I will try again.

HBZ_81_C3

What cc pump is on the front? What is the measurement between the pump arms and the adjuster.
I'm betting the cam isn't the issue.
If you have a 25cc pump on the front swap them.
Don't know what heads you have, but I bet that big block would like some more timing too.

gve

It might not be your carb at all, if you have rectangular ports they make there hp at higher rpms which is where you say it has power, also it depends on how your cam is degreed.

GDaina

Your carb has nothing to do with your low end performance...you have a cam with hardly any low end, you are running huge square port heads with hardly any low end....it is what it is...once you hit the upper RPM range, as you say, it is a beast....

You might want to try different transmission gearing if you are running a Muncy close ration...

TimAT

MY question too- what RPM are you trying to accelerate from in 2-3 gears? Your cam may not really hit it's mark until about 3000 (Guessing a bit- I have the factory ZL-1 cam and it starts coming on about there). What RPM range did the cam manufacturer say? Mine's as soggy as a wet sock until about 3k- and then it wakes up.

dmaxx3500

is your intake divider cut out in the top of the manifold,i remember talking to a GM engineer at elkart lake race track 20+ years ago[i was pitted next to his ''can am car''] and he was showing me the zl1/l88 factory orignal engine,and they were using a 3 barrel 850 cfm carb[they got better top-end],and they cut the divider in between the left/right sides down 1-1.5'' ,,i'll try to get a few pics of an orignal L88 manifold i have on the shelve,

elle88

@Tim
i think you are right.The cam is what it is
i mean accelerating when i'm between 2000 and 3000 rpm

i will try a couple of different cams anyway , possibly the white ( as MelWff says it's in the original L88 carb ) and the green or blue.

If i'll see no big difference, i'll go back to the pink that seems the most general purpose one

Tim , i think you sold your LS7 ....do you recall which pump cams on your carb?

@Gkull
which are the rubber squirters plugs? i don't see them . carb is new
no space enough for 1/2 wood spacer
now i have about 3/8" spacing counting all the stuff sandwiched between the carb and intake.
so there's about this 3/8" gap that puts in communication the 2 planes of the intake

@dmaxx3500
so as you can read i don't have the single plane L88 intake. choice made to avoid too few low end TQ

So
the original L88 carb had the white cam on hole 2
Gkull has the pink one on the 825 Demon

other forum members?

GDaina

if you're going to try different cams, look at comp cams Ar288 street roller...power throughout the entire RPM range and idles at 850 rpm

elle88

i meant accelerator pump cam tests.... the engine cam is and will remain the ZL1

TimAT

I still have it. Same as always. I'll look when I get home to see what pump cam is on- may not make any difference- I'm running a Demon 850.

I am building a new engine- pump gas friendly, more cubic inches, more cam. I figure what I lose by dropping compression, I'll more than pick up with the other parts. Block is at the machine shop waiting on lifter bore bushings.

gkull

I was reffering to the Umbrella check valve. If it is old or torn you won't get a good pump shot.

http://www.holley.com/types/Accelera...ce%20Parts.asp

GDaina

That won't solve your low end performance....all that will do is prevent off idle stumble, and if you don't have a stumble starting from a dead stop, why bother changing?

TheSkunkWorks

I'll get back with you on this...