I'm currently running a 1970 Z28 (is this the same as the LT1 intake??) intake manifold that the previous owner bolted in. Performance has been very good overall but I've had problems with stalling before fully warmed and hesitation off idle. The main contributor to this problem is a large 110 deg. overlap cam (also choosen by the PO) that, at this point, I don't wish to change. Once the engine is warm the hesitation virtually disappears and I have no stalling problems. I realize that tuning the choke will cure much of this but I get the feeling that fuel atomization isn't the greatest. My question is, will one of the new high performance intakes make a noticable difference? What kind of effect could I expect on drivability and performace?

The LT1 was a hot package but how much has 30+ years of design made a difference??



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'69 350/350 conv.

What kind of choke does the car have...divorced style, hot air or electric? Many if not most of these engines with the stock Q-Jets and intakes used a hot air choke that gets heat from the hot exhaust passing through an opening in the intake under the carburetor. If that hot air passage has been blocked off, the choke will be inoperative, no matter how you adjust it. If you have a divorced style or electric choke, you may simply have an adjustment problem.

As far as intakes go, todays intakes are generally much better designed than those of 30+ years ago. Computer design has taken much of the work out of designing an intake that can be tailored to your needs and driving style. Factory intakes, by design, are nearly always compromises to fit the driving and performance needs of the general public...good torque and upper end power, but not the best for either. Intakes today can be tailored to exactly your specific needs and driving style. The same goes for carburetors. You can't get something without giving something up elsewhere. That being said, the intakes and carbs for Corvettes were generally designed for better performance than the standard sedan engines anyway.

I'm well aware of the increased flow characteristics of today's intake manifolds. I agree that 30+ years had made a difference. BUT, the '70 LT1 intake was considered to be ahead of its time; how much of a difference has 30 years made on the low end where fuel atomization has been a concern?

The carb itself is a brand new Holley avenger with elecric choke. I have fine tuned it (pretuned carb is a bunch of BS!). The accelerator pump has been bored to .040 and I'm running a MSD electronic ignition. I still have a slight off idle lag. I'm looking to improve performance but the choke is the least of my issues.

BTY, for anyone interested, the divorced choke worked better than the newer electric choke!!


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'69 350/350 conv.

If you're experiencing any off-idle lag, the carb may be running a bit too lean at idle. If that's the case, moving up to the next size primary jets (maybe even two sizes) for the carb should take care of it.

I do agree with you about the LT-1 intakes. Stock Vette intakes are very good on their own. Since the LT-1 was a very high performance engine, I doubt if changing to an aftermarket intake would yield any appreciative change, unless your specific needs (such as using the car strictly for racing) requires it.

It's interesting that you would mention the primary jets. I have yet to pull apart the worn out '70 Z28 Holley carb the previous owner put on the car but richer jets would explain why I picked up 4 mpg by changing carbs & ignition (I wish I knew which one made the difference).

The off idle lag isn't that bad so I'm still hoping to solve it by playing with the accelerator pump. Do you think this is possible or will I have to change to the richer jet?

My understanding is that the jets don't make a difference coming off of idle.





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'69 350/350 conv.

I'm not that familiar with Holley carbs...mostly with Carter AFB/Edelbrock clones and some Q-Jets. Different carb makers design their products to do essentially the same jobs, but sometimes by different design methods.

Holley definitely has the name in performance carburetors, but I've heard so many "knowledgeable" people complain about their constant need for tuning, etc. These are the same people trying to sell Demon carbs it seems. I don't know the truth about Holleys or Demons. The only Holley carbs I have are on my '69 427/435 and they were professionally rebuilt by Holley Custom before I bought the car and haven't needed to be touched since.

I do know the primary jet size has a lot to do with the engine's idle and take off without bogging down. The jets have to be carefully matched to the camshaft profile, intake characteristics, etc. If the cam, intake, or any combination has been changed appreciably from stock, engine vacuum is affected along with the idle. While the carb itself may still be OK, the jetting may be mismatched to the new cam/intake profile. I know this from personal experience (usually the best way to learn).

A previous owner of my '78 L82 put a bigger cam in the engine, but left the Q-Jet alone. The engine wouldn't idle well at all and bogged down at take off. I had the carb rebuilt but neglected to tell the rebuilder about the cam change. When I put the carb back on, the car ran much better, but still wouldn't idle. When the tech found that out, he checked and verified the carb was running lean at idle with the stock jets. He installed one size larger jets and that completely eliminated the problem. Until then, I had no real appreciation of the necessity for how the camshaft, intake design and carburetion needed to be carefully matched for proper performance. That's why Edelbrock, Holley and others design and recommend matched kits they sell. They have their names on the line and don't want anyone having problems like mismatched components ruining their reputations.

I've read an interview with one of the designers of the LT-1 engine, and he said that motor had a camshaft profile that was really useless for street use, as it was designed for high horsepower at hign rpms for racing. That may be true for the intake as well...designed for peak torque and horsepower at well beyond the rpm range you drive at. It's been said we talk horsepower numbers, but we drive torque. You need to determine what kind of driving you do and tailor the components to that. That LT-1 intake may not do what you need.

Just for perspective, Holley still sells their copy of the LT-1 intake, the 300-36 Street Dominator. It's a great intake for high performance street cars. The modern air gap manifolds might have something because of cooler temps, and the regular performer better low rpm performance, but if you have a package designed for 3000-6000 rpm power, you can't beat the chevy intake, and stil fit under a vette hood. Joe

anybody know if a 69 427 390hp is a good intake, i bought my 74 454 last spring and it had it on it. It came from the factory with an iron intake, of course this is aluminum, and it sure seems to run pretty well with it in. Would it make a difference in hp on my car?? Anyone out there have one??? Thanks

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I replaced the carb primarily because I was tired of the rough idle caused by the loose throttle shaft and the rigged linkage. I also decreased the venturi size from 780cfm to 670cfm in an effort to gain throttle response. Truth be known though, outside of the idle the car actually ran very well with the old Z28 carb. I've been meaning to tear the old one down in order to make note of how it was built (jet sizes, power valve, etc.) but I haven't gotten around to it yet. After talking carbs here though, I'm thinking that I need to spend more time tuning the carb to the cam and less time worrying about the intake manifold.

As for the cam itself, it's waaaaaay too big for the street. I too have read that the LT-1 cam design was useless on the street and was designed for high revs on the track (or it gave GM bragging rights for horsepower numbers). So, when the previous owner went looking for a cam he naturally went for one that was even bigger than the LT-1 cam (higher and longer duration). Too bad he didn't understand anything about torque! Combine this with domed pistons and it only compounds the problem!

So, bottom line, I've spent most of my time trying to improve drivability, torque, throttle response, & octane requirements.





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'69 350/350 conv.

Regarding that big block intake for Blackbigblock...I don't think I would worry about a difference in performance between the '69 and '74. There may be only one difference between the two...one is aluminum and was is cast iron. If that's the only difference, you're saving some weight for otherwise identical perfomance. The air doesn't care what the intake is made of if the design is identical.

That '74 454 would possibly have some more smog controls originally installed on it that the original intake may have had provisions for. Either way, if it's running that good, don't be concerned about it...just enjoy it. You've likely got the better end of the deal anyway.

i wasn't sure if there would be a difference because it was a higher performance 427 intake or not. Thanks for the feedback. And you're right, i have absolutely no smog controls, but little does it matter in minnesota!!!

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Go to Holley.com for assistance in tuning your carb. It takes time, but you have to balance your primary jets with the power valve, and the accelorator pump system. If your jets are too lean, you will have hesitation. If your accelorator discharge nozzles are too small, you will have hestitation. If the accelorator pump or pump cam are not adjusted properly, you will have hesitation. Also, check the vaccum of your engine at idle. Large profile cams will reduce vaccum. When your vaccum drops, the power valve opens. This needs to be sized for the engine vaccum. If it is not opening, it can cause a lean condition. If it opens too soon, it will make the engine run rich at idle. Once you get everything tuned to the cam shaft, you should not have to touch the carb again. I run a 750 vaccum secondary. Once I get it tuned for my engine, I don't touch it. I have used Holleys on many engines. Holleys allow greater tunability (if that's a word) over some other carbs. They can be frustating due to this benefit. As someone stated earlier, a copy of your intake is still produced by Holley. It is a dual plane for idle to 7200 rpm range. You may check the power range of the cam shaft. You may have to install a higher stall torque converter to take full advantage of the cam shaft profile. Many people install too large of a cam shaft for the steet, but we love the way they sound!!
Good Luck & take your time,

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sstanford '76 L48

Hey sstanford, great response!

I'm thinking that you are probably right on, I've underestimated the amount of tuning that my new Holley is going to require to maximize performance and driveability. I'm going to tear down the old Holley that ran pretty well and I'm going to email Holley all of my specs including the vacumn readings.

I think the cam is already pretty well matched to the intake. On top of that I have the larger valves, headers, solid lifters, and hardened valve train parts that should allow me to take advantage of the high rpm horsepower. It's just too bad I don't drive there most of the time!

By the way, my stall point is completely dependent on my left foot!!!

Thanks everyone for the input!



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'69 350/350 conv.