70 LT1 Holley Carb Secondary

[Terry L.]

Hello,
It appears my secondary's are not opening under hard acceleration and I don't get the expected surge in acceleration as if the secondarys were opening. When I run thru the gears shifting between 4500-5000 the acceleration seams very linear with no real surge.
I don't have much experience driving other cars of this nature so I don't have anything to compare it to.
I installed a paper clip on the secondary rod to see if i was getting any movement on a test run and the paper clip didn't move. I checked the secondary spring and it was plain with no color code, I inspected the little seal on the housing and the venturi hole and it was not clogged, the diaphragm seal is good.
I changed the vacuum secondary spring to the lightest spring in the Holley kit, and I am only getting about 1/4 - 3/8" of movement based on the paper clip movement on the vacuum diaphragm rod. I would think it would open all the way, but its not. I haven't ran it past 5500RPM
The engine is bored 60 over, all parts are LT1 spec, cam, pistons, heads, exhaust manifold/exhaust, etc.. original 4489 carb, 4:11 gear. I did change the power valve from I think a 8.5 to I think a 5.5 based on the vacuum at idle. The floats are adjusted correctly, the jets are stock. I do have the TCS hooked up and operational, the distributor is original TI and has been rebuilt by Dave Fiedler to OEM specs. Timing is set at 10 BTC at 750 RPM.
Is this normal acceleration for the LT1? Will the secondarys open more beyond 5500RPM? The engine seems to run great just a little flat when accelerating.
Any thoughts?

[Ken A.]

Re: 70 LT1 Holley Carb Secondary

Remove diaphragm housing from carb. Push linkage upward to simulate open secondaries. While holding rod, cover hole with a wet fingertip & release rod to see if diaphragm is holding rod. If not, replace diaphragm. Make sure you properly adjust squirter.

[Duke W.]

Re: 70 LT1 Holley Carb Secondary

There may be nothing wrong with the carburetor. I don't think Holley secondaries flop open suddenly - they slowly open as primary venturi vacuum increases and may not be full open until near the carb's maximum flow rating at 1.5" Hg manifold vacuum, but the LT-1 without massaged heads won't aspirate much more than 600 CFM at 6500, and the carb is rated at 780 @ 1.5" Hg.

Use a Mighty Vac to pump down the secondary diaphragm and tell us how much vacuum is required to begin opening them and how much is required to pull them full open with the various springs.

The LT-1 cam really isn't that peaky, but if you have the OE spark advance map you're leaving a lot of performance and fuel economy on the table because the map was set up for emission control, not performance and fuel economy, and that's why it may feel "flat".

With a proper spark advance map the ten percent power bandwidth should be from about 5000 and 6500, 80 percent torque bandwidth from 2000 to 6500, and it should feel a lot stronger throughout the rev range than it is now.

Duke

[Terry L.]

Re: 70 LT1 Holley Carb Secondary

Duke, Ken

When i get back home I will test the diaphragm and use the Mighty Vac and see what HG we get with each spring. I'm on a flying trip in Montreal at the moment, hopefully I'll get home on Tuesday for a couple days so I can work on this otherwise it might be the middle of August before I get to it.

Duke, I know you have posted best performance mapping for the LT1 engine in the past, I'll see if I can find it in the archives. I may even massage the heads and install headers and larger exhaust after I have the car judged. Probably a good idea to store the original manifolds, AIR pump, etc.. anyway.

Its still a blast to drive as its performing now.

Thanks

Terry

[Jim T.]

Re: 70 LT1 Holley Carb Secondary

My 1970 has not had the TPS operational since the first month of ownership after buying it new in 8/16/1970. Better gas mileage in the city and lower engine temp in Dallas city traffic. Sure am glad I discovered the benefits of using 100% full time vacuum to the distributor vacuum advance so early in ownership.. Very cheap price of vacuum hose to do this.

[Duke W.]

Re: 70 LT1 Holley Carb Secondary

For a '70 LT-1 you need another very important, but inexpensive part, and then do a simple maintenance operation to accomplilsh a successful conversion.

Duke

[Terry L.]

Re: 70 LT1 Holley Carb Secondary

Duke, Would this be a B28 vacuum advance ?

[Terry M.]

Re: 70 LT1 Holley Carb Secondary

Hello,
It appears my secondary's are not opening under hard acceleration and I don't get the expected surge in acceleration as if the secondarys were opening. When I run thru the gears shifting between 4500-5000 the acceleration seams very linear with no real surge.
I don't have much experience driving other cars of this nature so I don't have anything to compare it to.
I installed a paper clip on the secondary rod to see if i was getting any movement on a test run and the paper clip didn't move. I checked the secondary spring and it was plain with no color code, I inspected the little seal on the housing and the venturi hole and it was not clogged, the diaphragm seal is good.
I changed the vacuum secondary spring to the lightest spring in the Holley kit, and I am only getting about 1/4 - 3/8" of movement based on the paper clip movement on the vacuum diaphragm rod. I would think it would open all the way, but its not. I haven't ran it past 5500RPM
The engine is bored 60 over, all parts are LT1 spec, cam, pistons, heads, exhaust manifold/exhaust, etc.. original 4489 carb, 4:11 gear. I did change the power valve from I think a 8.5 to I think a 5.5 based on the vacuum at idle. The floats are adjusted correctly, the jets are stock. I do have the TCS hooked up and operational, the distributor is original TI and has been rebuilt by Dave Fiedler to OEM specs. Timing is set at 10 BTC at 750 RPM.
Is this normal acceleration for the LT1? Will the secondarys open more beyond 5500RPM? The engine seems to run great just a little flat when accelerating.
Any thoughts?

Terry,
My 1970 came with a List 4555. Has your carburetor been rebuilt? Does it have the correct metering blocks? The original metering block numbers are in the TIM&JG.

If you have the original intake manifold and distributor there will be a chisel mark in the back of the distributor at the junction with the intake. Line chisel mark up and you will have the exact timing that was set when the engine was originally hot fired at Flint Engine. Of course if you have changed the timing set and cam all bets are off. FWIW: my 1970 LT1 was originally set at 14*. Years ago Scott Lindee and I wrote a Restorer article on this subject. Only one caution about 14* -- the fuel available was very different in 1970 than it is today.

How do you intend to use your LT1?

[Mike F.]

Re: 70 LT1 Holley Carb Secondary

My 1970 has not had the TPS operational since the first month of ownership after buying it new in 8/16/1970. Better gas mileage in the city and lower engine temp in Dallas city traffic. Sure am glad I discovered the benefits of using 100% full time vacuum to the distributor vacuum advance so early in ownership.. Very cheap price of vacuum hose to do this.

Just an FYI - I have a 70 L-46 with a functioning TCS system. All you have to do is unplug the transmission switch and the TCS solenoid defaults to the open position giving you full manifold vacuum. In this scenario the system will look complete/installed with the hood open, but you will be getting manifold vacuum all of the time. All that is needed is to adjust your idle screw for the manifold vacuum. When it's time for judging, reconnect the transmission switch and re-adjust idle screw.

[Terry L.]

Re: 70 LT1 Holley Carb Secondary

Terry,

The 4489 is the NA9 California carb, it's been rebuilt and has the correct metering blocks, jets etc..., I guess i should have mentioned this before. I do have the chisel marks you mentioned but they are close to lining up but not matching. I did replace the cam and timing chain during the restoration. The cam is a GM nos cam, If I remember correctly its a 2178 cam. I will bump up the timing a little when I get home.

I plan on having it judged someday soon and I want to drive it. I finished the restoration almost 3 years ago and I have only driven it about 500 miles.

[Terry M.]

Re: 70 LT1 Holley Carb Secondary

I have never felt the need to disconnect the TCS system in part because for the small amount of driving I do the difference in fuel mileage is insignificant, and engine temperature has never been an issue for me. Finding fuel that will support the original advance has been an issue, but I now have access to no-alcohol racing fuel. If one drives the car very much the expense and inconvenience of that fuel may dictate making the modifications suggested or more. I just don't think one can get the Holley idle slots properly exposed when one compensates the carburetor adjustments for the full-time vacuum operation. BWTFDIK?

[Duke W.]

Re: 70 LT1 Holley Carb Secondary

Duke, Would this be a B28 vacuum advance ?

Correct, and you better pick one up now as they may be out of production, but are still available in the Airtex brand. There have been recent threads on this issue.

The OE centrifugal curve is maximum 26 @ 5000, so it's very lazy. Install some lighter springs to get it in by 2500 to 3000, and set the initial timing at 12-14. If it doesn't detonate, then you're good to go.

I don't recommend headers, and the OE exhaust system is low restriction, but even the modest OE system backpressure will mostly negate header wave dynamics, so you don't end up with that much more power, and then there are all the installation and heat issues with headers.

A few years ago a "327 LT-1" was tested on a lab dyno with both manifolds and headers, and, of course, large exhaust pipes and no mufflers. Headers improved peak torque by about 8 percent, but only yielded about 3 percent more top end power. With the OE exhaust system at least half this increase will disappear, so the increase is barely above noise level.

The AIR pump can be "gutted" (and the distribution pipes blocked) so it's merely an idler pulley.

Massaging the heads will yield about 300 SAE corrected rear wheel HP, which is about 350 SAE net at the flywheel and close to 400 honest SAE gross with a useable power bandwidth to about 7000 all while maintaining original appearance, excellent torque bandwidth, and all around pleasant driving characterics in normal road use.

The '70 LT-1 was the ultimate development of the Gen I SHP small block. and it doesn't need much work to extract maximum output without fouling up its good behavior and drivability in normal road use with no judging deductions.

Duke

[Terry L.]

Re: 70 LT1 Holley Carb Secondary

I picked up a couple of B28 canisters last year. Thanks for the info. Ill keep you posted

[Timothy B.]

Re: 70 LT1 Holley Carb Secondary

Hello,
It appears my secondary's are not opening under hard acceleration and I don't get the expected surge in acceleration as if the secondarys were opening. When I run thru the gears shifting between 4500-5000 the acceleration seams very linear with no real surge.
I don't have much experience driving other cars of this nature so I don't have anything to compare it to.
I installed a paper clip on the secondary rod to see if i was getting any movement on a test run and the paper clip didn't move. I checked the secondary spring and it was plain with no color code, I inspected the little seal on the housing and the venturi hole and it was not clogged, the diaphragm seal is good.
I changed the vacuum secondary spring to the lightest spring in the Holley kit, and I am only getting about 1/4 - 3/8" of movement based on the paper clip movement on the vacuum diaphragm rod. I would think it would open all the way, but its not. I haven't ran it past 5500RPM
The engine is bored 60 over, all parts are LT1 spec, cam, pistons, heads, exhaust manifold/exhaust, etc.. original 4489 carb, 4:11 gear. I did change the power valve from I think a 8.5 to I think a 5.5 based on the vacuum at idle. The floats are adjusted correctly, the jets are stock. I do have the TCS hooked up and operational, the distributor is original TI and has been rebuilt by Dave Fiedler to OEM specs. Timing is set at 10 BTC at 750 RPM.
Is this normal acceleration for the LT1? Will the secondarys open more beyond 5500RPM? The engine seems to run great just a little flat when accelerating.
Any thoughts?

I would suggest installing the correct power valve in the metering block and while you are in there double check the primary jet size to be certain it's the right size. Proper operation of the Holley vacuum secondary is not supposed to give the feel of surge or bog when the rear throttle blades open.

The secondary pick up is located in the primary venturi on the passenger side. This passage gets a low pressure signal from the high air flow through the primary venturi. There is another pick up located inside the secondary venturi that bleeds some of that signal off to delay opening the secondary's until the primary signal overcomes the secondary bleed. At that point the diaphragm lifts and starts to open the secondary blades. As the blades open air flow through the secondary venturi changes the pick up from bleed to vacuum and it adds to the primary signal to open the throttle blades faster.

I bet there is nothing wrong with the carburetor, run it up to the red line once and see if the paper clip moves the complete distance. Many of these carburetors with the stock plain spring don't even open the secondary blades as the air flow through the primary side is enough to keep up with engine demand.

Does this carburetor have the little brass end cut on a 45* at the primary pick up tube. I have read that installing this will greatly improve the signal for the secondary. I think somewhere along the line Holley stopped using this little part.