Team Camaro Tech banner

1 - 20 of 32 Posts

·
Registered
Joined
·
12 Posts
Discussion Starter · #1 ·
What size carb should I be running on a 302 built up motor? Has .546 cam, dart heads, and 11:1 compression. Also running a Muncie 4 speed, and 4:11 gears.

What size carb should be on this? I have a 750 double pumper and feel this is way too much. And I am leaning towards a 650. I am not racing this and will be just cruising around a few times a month.
Your thoughts?
 

·
Registered
Joined
·
5,198 Posts
780CFM was the original size carb in 1969.
True, but I always thought that was a little big for a 302 unless you were really spinning it up. I'd think a 650 would be a better fit for cruising... maybe even a 600.
 

·
Premium Member
Joined
·
3,676 Posts
With a slightly modded 302, 12.0 pistons -140 cam, and some port work on stock 186 heads and 4.88's back in the 70's I ran a 750 dbl pump and loved it for the street. I recently ran a 4777 650 dbl pump on a stock spec built 70 LT1 in my Z and loved it. Only problem you really need to worry about is the price of gasoline. In the 70's gas was cheap, today it's a lot more and you will use it up fast with dbl pumpers.
 

·
Registered
Joined
·
2,013 Posts
I tried a 650 on my 70 Z28 in '85 when the motor was 9:1. The engine ran out of air/fuel at 5,000 rpm, so I used a 3310 750 Holley and it was fine. I currently have the stock 780 Holley with 10.2 compression ratio with 4.10 gears and it runs fine too. The 650 is too small for the 302 or even a 350 with the Z28 cam.

You could borrow a friends 650 if they have one to try.
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #10 ·
Ok. I think I'll stay with a 750. I bought a quick fuel 750 dbl pumper with a electric choke. I'm having very low vacuum though and can't get it to idle at 759-800. Any suggestions?
 

·
Premium Member
Joined
·
3,676 Posts
Have you just tried the stuck idle RPM? I'm thinking 900 or 950 should work fine. I'm running right at 1000 RPM idle with an over the counter 4053 and my engine like it there. That of course is the 70 LT1 spec 350 that is currently in my Z now.
 

·
Premium Member
Joined
·
4,596 Posts
302 12.5:1, 500/300 range cam 8in/hg vacuum 650 Demon double pumper works fine.
 

·
Registered
Joined
·
1,747 Posts
Get the timing up to 16* initial, 36* total. It should get the vacuum up to 10"-11" Hg.
stock idle speed on a 69 Z is 900 RPM... 800 is too low. can't hold vacuum with the overlap.
What power valves do you have in the carb?
If they are above the vacuum reading or close then the idle drops/loss of vacuum are causing raw fuel dumps everytime the vacuum drops off.
Power valves help the transition from idle to main fuel metering circuits.

Power valves should be 1.5-2" below the reading at idle, if it (vacuum reading) dithers up and down then the reading needs to be below the lowest reading you see, if it only hold 8" then get a 6 Power valve.
* Double Pumpers have a power valve on each metering plate.
 

·
Registered
Joined
·
2,013 Posts
I didn't think a power valve could kick in when your just idling, being on the idle circuit? I'll have to verify that in my Holley tuning book.

Holley in the past used the method of -2 from your vacuum reading at idle but have since changed to a more accurate method my taking 1/2 of your reading. If it's 8" at idle then I'd be using a 4.0. http://www.holley.com/data/TechService/Technical/power_valves.pdf

Mines 10 and I'm using a 5.0 and it works fine in my 70 LT-1 roller motor.
16* initial / 36* total
 

·
Registered
Joined
·
13,315 Posts
Basically u are trying to run a hi rpm oval track car on the street at low rpms on pump fuel...

Has .546 cam, dart heads, and 11:1 compression.
For a start that compression and cam is definitely not suitable for cruising street

I am not racing this and will be just cruising around a few times a month.
Running that compression on modern low octane gas will need some serious mods in the VA/ and cent curves of the dizzie, regardless of the carb

Add to that, the cam has so much lift/ duration/ overlap at low idle and cruise rpms the engine will be running so inefficient, with very little vaccuum.. to get stable idles at lower rpms, unless a lot of idle (VA+intial ) advance.
U may also find with the bigger carb secondary diameters, also taking into consideration all of the above u wil not be able to get the idle speed and mixture screws to within the +/- 1/2 turn of the 2 1/2 turns out spec without closing up the sec butterflies onto the bore...this causes bent butterfly rods, worn butterfly bushes.

If only cruising, and top rpms say 6K drop a 600 on.. this will give far better tuning, better idle, make the engine far more efficient at the rpms u will be using, better vacuum, Significant improvement of bottom end torque, show off stuff, and even encomoy

Even running a 600, secondaries adjusted own and mixture/ idle speed screws set in spec, u can expect around 7 to 10" vaccuum at idle, around 14 to 16 at cruise..and get around a 700 to 800 idle speed.
 

·
Registered
Joined
·
12 Posts
Discussion Starter · #17 ·
If I put a smaller cam in this would I be able to keep the 750? And have a smooth running car?
 

·
Registered
Joined
·
13,315 Posts
Drilling holes was an old school 60s /70s thing before holley made modes to carbs
very obsolete and a myth thay still hangs around.
Drilling hole in effect opens up the "vaccuum leak"...which is why secondaries are adjustable ....

Yes a more streetable cam change will make a big difference
But 1st adjust the current secondaries so the idle speed and mixture screws are in spec with a stable but will be somewhat 'lumpy' idle
If do change the cam u will still have a way over carbed engine running on modern low octane pump gas
keep in mind also higher the octane, the less power per gal... which a carb meters by, but more power per lb.



The secret to a good performing engine design is simple... choose ALL the 'go' components for the load and rpm range the car will be used for 95% of the time.
Cam headers carb....match the lot
Then dial the engine in .. carb and timing
result u have one of those cars that is rather mild in brag part yet goes like the clappers and yet economical on a road trip.
Our daliy driver 69 runs a 13.8 @93mph under 5000 rpms and gets around 30 to 34 mpg open road...mild bottom end torque cam, 450 cfm carb, cast iron truck rams horns...
 
1 - 20 of 32 Posts
Top