spokewrench

Keep what you have and possibly rejet it!

Barry's70LT1

Hi,
Here is my experience on this subject.
I have had a Holley 750DP on mine for many years. In the '80s' (gas crunch) I wanted better fuel economy and put on a Holley 600.

Long story short, the performance was terrible compared to the 750DP. If you never intend to "run thru the gears", or pass another car, then get the 600. I only kept the 600 on for a short while. The h*ll with the milage, a Corvette should work well.!!

Best way to describe the feel of a 600 Holley on a LT-1 is to connect a travel trailor to your Vette and go for a drive. Feels heavy and slow to accelerate.

Check a few posts here on the forum. Lars has done several tests with large carbs on small blocks. Very positive results.

I've been using the same Holley 750DP on my vette for 32 years now. Still completely satisfied with performance and milage.

Do what you must.
Barry

P.S. I still have the 600 Holley, hardly used. Interested ???

Robert N

Tony,

My 70 LT-1 has a 650 DP Holley. It was there when I bought her and since the goal is to keep her running for now, I left it. Initially the car felt as Barry described and she ran lean. I upped the jet sizes (cannot remember what is in her now) and the power is better. I rarely pass 4000 RPM and I am getting somewhere around 11 mpg. As far as passing, at 3000 RPM, just punch it. Below that, the bigger carb would be nicer.

The gas smell still exists in the garage but that is some of the charm of the old raw HP muscle cars.

Gordonm

I run a 750 DP on my LT1 that has been modified. If you are going to stay low in the rpms under 4500 (why would you) the smaller carb is OK. I run mine to 6500+ rpm all the time. The 750 is very strong. The 780s that GM installed were vacuum and they "sized" themselves as to what the engine wanted. The LT1 motors don't really make power until 3000 rpm and really come on strong from there on up. I guess GM figured the motors would be turning lots of rpms so they needed a bigger carb. BTW the Qjet that is installed on millions of SB chevys flows 750 cfm. I say keep the 750 for performance.

tonyk72

I am happy with the Holly, but not happy with the gas stench!
The reason I'm considering the Edelbrock is I hear they are less prone to perculation & gas leak problems...
I have done almost everything on the Holley to fix the problem: phelonic spacer, wiring the heat riser open, sealing fuel fittings with teflon (bubba fix, but it works), tightening up the fuel bowl screws. The gas leaks are gone, but the fuel smell remains...and when you have a garage under your house, it makes it worse.
And so if I do decide to swap my Holley 4150 for an Edelbrock Performer...I should stick with a 750cfm vac secondary...would this be an equal trade off?

ccryder

I have also been up and down this path with my car. I tried a Holley 650 Street Avenger last year because I wanted the electric choke feature (for my climate) and because I'm running headers (divorced choke) and the original Holley 780 carb ran and idled terrible.

Well the 650 Street Avenger just did not cut it for the LT-1, there was a noticable performance decrease, especially beyond 3000 RPM, like mentioned above, this is were the LT-1 really starts to take off.

So I went back to the original Holley 780 this car came with, I'm convinced GM knew what they were doing with the LT-1.

First thing is to make sure it is setup right and go through it. When I opened mine up, the previous bubba's had the wrong size power valves, jetting, secondary spring (go with the lightest on this). Nothing was setup right on it compared to the GM/Holley spec. for this application. Once rebuilt and really tuned properly, (using Lars tuning tips) this is the carb match for a good running and screaming LT-1, especially past 3000 RPM.

I also was able to find an electric choke option that fits in the old divorced choke location in the original manifold, works awesome with the headers and looks great.

My suggestion, run the Holley 780, tuned and setup properly it should run awesome, GM had the right combination for this small block.

gdh

My LT-1/ZR-1 came with a Holley 800, they performed pretty well for there day. I have a Edel Performer 750 on now and it is a great highway cruising carb - by no means is it as capable as a Holley on the track. I am very happy with the Edel. but I will replace the rods to bring back some top end. Where with the Holley I pulled stong through 7k rpm, the Edel (straight out of the box)just runs out of flow around 5500 rpm.

71coupe

ccryder - quick question: What brand and size are your rims? They look great. (and what tire size also)
Don't mean to hijack this thread - but IM & email is turned off.
My email: jetspecialist@hotmail.com
Thanks

Solid LT1

My 72 LT-1 has a 4777 Holley 650DP, the choke tower is milled off. The car has run a best of 12.28 @ 117 MPH with this carb on a factory hi-rise with an uncut plenum divider. The motor is not stock with max ported heads and good camshaft (Ultradyne solid) but, I ran 12.50's with early Vette rams horn 2.5" manifolds but the bottom end torque and power is majorly improved with the 1.75" dia Hooker Super Comps on it now. I just dialed in a Pro Form venturii and it is 12HP more rear wheel horsepower than the 650 Holley that ran the 12.28. The Holley 650 is a great carb and not as sensitive to jetting as a larger carb (mine was 2 jet sizes larger than stock at 70 primary and 76 secondary jetting.)

tonyk72

I've been reading all the posts on carb sizing & have a question as to why GM placed a big 780 on the '70 LT-1 and 750 on the 71-72 LT-1's...?
From all the stuff I know & read, this carb is clearly too big for the engine. So why did GM engineers go with this big carb size???
I still have my Holley 750 sitting on my LT-1. It runs great, but am considering another carb (Edelbrock) to try and cut down on the fuel stench left in the garage...my dilemma is do I stick with a 750 cfm size? I don't drive up in the 5k RPM range...so I assume a 600 cfm carb would be just fine. But I would expect a drop in performance once I go beyond 5k + range...right?
My point is, I think GM knew what they were doing in sizing the carb for the LT-1...and I'd like to stick with something that will flow as much as the stock application...but common sense point me to something smaller. What do I do?

mandm1200

When mathematically calculating the cfm for an engine, one of the factors is the rpm's of the engine. Some have posted they are turning 7000+ rpm's. A healthy 350 will need 700cfm, but may start to suffer and limit the power at that rpm or above. It is obvious that going up one size will eliminate the starving effect and probably add a couple of horsepower (perhaps 1 to 3). This was a high performance engine. The market Chevy was selling to was not the person who drove around at 1400 rpm. They probably assumed the owner was going to have a heavier foot and put on a carb that suited the car and it's intended application.
In my opinion, if your engine is not built and does not turn 7000+; but rather 5500 rpm then there is no reason for a 750 cfm. A 600 or 650 should be more than adequate. The extremely small performance lost going to a 600 will be made up by the small gain in drivability/responsiveness when putting around at 1200 rpm in bumper to bumper traffic.

Mike

69autoXr

I have a '70 spec LT-1 in my '69, with a Holley 650 dp. It'll spin to 7200 rpm all day with no problem, and plenty of power.

Solid LT1

My LT-1 will run right to 7500RPM with a 650. The power comes from the head, manifold and MOST IMPORTANT camshaft combination. My motor made 426HP on a conservative engine dyno with the port matched but stock plenum diver heigth hi-rise intake and 4777 Holley (choke tower removed.) I use the factory AC chrome top air cleaner with a K&N 3" tall element and continue to use the same.

Ganey

For a 350, 600-750.

If someone has a mild over 300 hp 350, then all they need is 600. A Holley 600 dp for example is good for over 400 hp.

Remember a carb is a responsive device & something else I have mentioned many times is :
The formulas are really for single plane intakes. A 2 plane can run more.

As the performance level is increased & the emphasis is placed on high performance, rpm & racing, then the carb size can be increased. So a 750-780 was a very good & correct choice for the LT-1.

Take a very close look at the HP/TQ nos. in this.
CARBS Tests TQ/HP http://www.corvettefaq.com/c3/carbcomparo.htm

For that engine the Carb Shop selected & set up a Holley 650 dp & won, note both HP & TQ. JET Perf. selected & set up a Holley 750 3310 & came in close behind along w/ the Speed Demon 650 also selected for that engine.

It's nice that Solid LT1 :cheers: gave his example before I posted this.

:cool: