I have a 69' Coupe with a 3.70 gear, M-21 trans and a ZZ3 crate motor rated at 345 horse and 380 torque.
I'm running stock dual exhaust out the back with the factory cast iron exhaust manifolds and a 750 CFM Holley double-pumper (square bore) carb. I have the stock L-46 distributor with a Pertronix II module and I'm running the recommended 10 degress initial advance.

I'm a old big block guy (Olds 455's etc) and the power of the ZZ3 seems to be less than what I would expect for a car equipped the way it is. The low end torque is not overly impressive- I would think with 380 ft/lbs and a 3.70 gear, turning over the tires would be fairly easy. The only way I can get the tires to turn over is if I dump the clutch at 3 grand or above, and even then, it's nothing I'd write home about (Note: I don't make a habit of 'Pulling Holshots').
Can anyone tell me if this is to be expected or if this sounds weak? The motor otherwise pulls strong, but it just doesn't seem to be 345/380 strong.
I plan on swapping the 750 Holley for a 650 Holley w/Vac secondaries, but I'm not sure how much that will help.

Any insight, suggestions or ideas?
Thanks!
Elm.

I think you're on the right track with the carb swap. That 750 square bore may not be giving you enough fuel velocity for low end, and may be creating some bogging down. I think the 650 will be more responsive down low, but at the possible expense of some top end.

I would make any changes in stages. Try the carb swap first. I would think you'll notice some improvement. If it's not enough to suit you, then start looking at reducing back pressure in the exhaust...either through less restrictive mufflers or headers or both.

Although I don't have a crate engine, I think my hp numbers are in the same neighborhood as yours. Dumping the clutch above 2k results in roasted tires. I think the 650 holley would probably help. A cheaper first step might be to look at recurving the distributor. I put lighter springs in mine (full advance by 2k) and it was a huge impvovement. Every Combo is different as you probably know.

I to have experienced the big block Olds torque!

Dave

SundayMoney said: Although I don't have a crate engine, I think my hp numbers are in the same neighborhood as yours. Dumping the clutch above 2k results in roasted tires. I think the 650 holley would probably help. A cheaper first step might be to look at recurving the distributor. I put lighter springs in mine (full advance by 2k) and it was a huge impvovement. Every Combo is different as you probably know. Dave

I agree with tuning the timing first.

also remember the first gear of an m-21 is only 2.20:1 which makes you lose some of that low end.

the carb may help the low end.. but I personally prefer mech sec carbs with manual trans...

cthulhu said:... but I personally prefer mech sec carbs with manual trans...

enquiring manual transmission owner wants to know:

I've read this before on other posts, etc. What is the reason you prefer the mech secondaries?

now this is from a stret/strip perspective, from a street perspective its different and similar results can be had if you dont mind swapping secondary springs based upon how your going to drive it that day.

A manual trans is all about control.. you determine where to shift. When your running a manual car hard you will experience wild variations in the vacuum, it will be low at the bottom and low at the top, it both of these cases your secondaries may be closed with vac sec. when you add a gear and the load goes up the vac secs have to reopen.

If you miss a gear (it happens, especially if youre like me and have 3 diff cars with sticks all having different clutch strengths and shift patterns) with a vac sec carb that hesitation has probably cost you your secondaries. you might as well pack it in.

using a very weak secondary spring on vac will protect you from this..

with an auto you dont miss gears, you always have a load and therefore your performance will be predictable with a vac sec carb.

another consideration for hard use. when you break traction the engine load drops, this is when you want to put it in second and haul... if your load has dropped enough.. secondaries will close and youll stumble on the shift.

all of these factors change depending on vehicle weight, diff gearing, trans gearing, and cam duration.

If memory serves there is an article somewhere on holley's web site that lists the conditions where you want to use each.

Vac Sec
High gearing (lower numerically)
Heavy Vehicle (not a C3)
Auto Trans
Low duration cam

Mech Sec
Manual Trans
Light Weight Vehicle
Lower Gearing (higher numerically)
High duration cam

the weight and gearing are basically the load issue

the trans specific is due to vacuum fluctuations at shifts.

the cam is strictly a vacuum issue.

swapping sec springs on a vac carb is realtively easy with a quick change kit. A coin and the new spring and your ready to go. Vac Sec carbs will usually (depending on app) get better mileage (progressive link mechanicals do pretty well though).

I tend to think of the auto as the street trans and the stick as the street/strip trans... many people use autos at the track with the reasoning that if you adjust your trans right it can shift faster than you can. doing this to the trans makes it annoying for street use and turns it into a one trick pony.

as for which is better for RAW power..
the 69 350/350&300 used a vac sec 4bbl (4MV)
the 427/390 used a vac sec 4bbl (4mv)
the 427/400&435 used three dueces which is like having two sets of mech secondaries

The mighty L-88 used a holley 4150 double pumper (mech secs)

you be the judge..

|UPDATED|6/24/2004 8:29:49 PM (AZT)|/UPDATED|

Very accurate Ben.
As I said before, I prefer the Q-jet for street, but your accessment for strip I completely agree with. There I would perfer something else. Same true of other racing.

The Manual does give more control. But many folks forget the torque convertor in the auto trans also multiplies touque. My turbo 400 when at maximim stall speed provides a 5.2:1 first gear ratio. This can be a real plus. And it does shift faster than you can shift a manual. From the factory in the 70's and 80's the automatic was quicker than the stick in the 1/4, all other things being equal. But not all other things are equal. Ratios make a big difference.

But for racing on a track you can have better control with a stick, unless you modify the auto, and then it's limited in it's street use. The stick is never limited to one type of use. Point and counterpoint.

Besides, manuals are a lot more fun. Despite the fact mine is an automatic.

kstyer said: Besides, manuals are a lot more fun. Despite the fact mine is an automatic.

thats probably what ruined me... my mother couldnt drive a stick and father wouldnt drive an auto.. guilt by association.

just because you have an auto now doesnt mean you cant do a t56 swap.. then we can compare notes and come up with the ultimate howto !!!

I agree with recurving the dist. maybe 24 degrees total in dist all in by 2800-3000 rpm, with an inital of 12-14 degrees, that will give you a total of 36-38 degrees at 2800-3000 rpm.

cthulhu said: Vac Sec High gearing (lower numerically) Heavy Vehicle (not a C3) Auto Trans Low duration cam

i forgot to mention the case where a mech sec carb actually gets better gas mileage and works better on a heavy vehicle.

this example illustrates how the circumstances are key, and you have to factor in everything.

there may be other situations but this is the most obvious one.

in a truck where you do alot of hauling the engine will always be under a heavy load. if you use a vac secondary carb the secondaries will always be open. Which reduces your gas mileage and can cause rich conditions.

the simple answer to this is the wrong one. One might be tempted to put a strong spring in the carb.. but this would adversely affect the low end and you might not be able to get the truck and cargo moving without more responsive secondaries.

just food for thought.