#41  
Old 01-16-2022, 11:51 AM
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Originally Posted by Cliff R View Post

Over the past 20 years or so I've back to back tested dozens of carburetors against my 1977 Pontiac Q-jet. It is completely "stock" aside for a slight recalibration and outfitted with the high performance Q-jet parts I offer for them, never once has a grinder or sanding roll touched it. On the dyno or at the track it's never once been outran by anything else back to back testing........and believe me the dyno facilty I use HATES that carburetor and pulls out everything in their arsenal to outrun at every possible opportunity!

https://www.youtube.com/watch?v=6zVdoLR-VzM
Hi Cliff. That's great information, thanks. It's funny because one of the engine builders I talked to (not the one I'm using) flatly refused to build me an engine with the stock intake and Qjet. Just said no. Now, MY Qjet is obviously not YOUR Qjet... Clearly they must have known that.

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  #42  
Old 01-16-2022, 12:24 PM
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Not to Hijack........ It's just nice to see a recent post from Cliff. Carry on.

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Old 01-16-2022, 12:32 PM
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Not to Hijack........ It's just nice to see a recent post from Cliff. Carry on.
Agreed!

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  #44  
Old 01-16-2022, 02:26 PM
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"IF tuned properly. I have rebuilt and tweaked my fair share of them, and found the hotter the engine, the bigger the pump shot needed so it does not bog down when you nail the throttle. It really depends on what the set up is that a Q-Jet is sitting on - in my case I run a old ported Holly Single Plane Manifold (back in the day that is what Herb Adams used), Race Ported RA3 heads (Nunzi), and a HO HC-03 Cam with 1.65 rollers, and could never get the Q-Jet to like that set-up. Prob cause I could not get the pump shot and tip in tuned correctly,"

The pump shot although important and a "player" in that deal really only plays a minor role in the issues you were seeing. If you are experiencing hesitation, stumble, bog or noticeable transition going quickly onto the secondaries the cure is going to be in the secondary POE system, choke pull-off release time, secondary air door spring adjustment and hanger height/metering rod being used. "Bog" is more times than not a momentary lean condition when the engine gets too much air and not enough fuel. The Quadrajet out of all the carburetors out there has the best set-up and fully adjustable in seconds, but most folks just don't know where the problem lies or how to address it........Cliff

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  #45  
Old 01-16-2022, 02:44 PM
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"Hi Cliff. That's great information, thanks. It's funny because one of the engine builders I talked to (not the one I'm using) flatly refused to build me an engine with the stock intake and Qjet. Just said no. Now, MY Qjet is obviously not YOUR Qjet... Clearly they must have known that. "

Last time I looked it was 2022 and we should be past all that by now, but sadly many parts including the Q-jet still get a bad reputation when it comes to these things. Usually the same folks who bad mouth the factory carburetors do the same for stock type distributors and use some sort of "bug zapping" unit in their engines as well, which don't make any more HP anyplace than a well set up stock unit.

A recent and good read being that stories and real life experiences are helpful with these things, so here goes:

I got involved with a "stock appearing" early 70's Chevy project that was pretty much going to be a "max-effort" deal but look completely stock. It was a big block 454 and used the factory "flat" iron intake and Cowl induction set-up.

The owner hired a very good engine builder who owned a machine shop and dyno. Common with this sort of thing during the planning stages he mentioned that a "quadra-junk" and that "POS " stock flat iron intake those engines used "will not feed my engine". I hear that a lot so really didn't pay it much mind as I've been involved with quite a few of those set-ups that are raced in FAST and some run deep into the 10's and a few even dip into the 9's when the sun, moon and stars line up well. Anyhow, I went ahead and built a carburetor for the 496CID high compression 454 he was building. It would use the factory heads but they got bigger valves and ported, plus a custom cam, and all of it fitting under the stock valve covers and completely factory appearing in every respect. The target level for power was around 560-600hp and hopefully at least that much torque.

The build was completed and I shipped the engine builder a custom early 1970's Chevy Q-jet modified and tuned for what they were doing. Dyno day I get a phone call and it was all BAD. The engine wouldn't make much past 425hp and it was DONE by 4500rpms or so. Along that info I got to hear all over again what a POS the carb and intake set-up was and a little "I told you so" to go with it. So I start asking questions and made a few recommendations one being to crank up the fuel pressure to make sure the carb was staying full, but nothing they did helped. I finally ruled out the carb and asked about the distributor, total timing, when it was be "all in" etc. Very quickly the engine builder snapped back at me that he did the stock points distributor himself and outfitted it with a Pertronix and custom advance curve all in around 2800rpms or so.

Hum, since the engine was basically shutting down at 4500rpms and not making mcuh power I asked him to remove the Pertronix and put a good set of points back in it. The phone went silent for a moment, then he agreed. About an hour late I get a call that the next pull on the engine was 565hp and 585tq and it pulled right to 6000rpm's without much effort! This is NOT the first time I've seen this issue with Pertronix units and other aftermarket ingnition systems/modules.

Of course I didn't get any apologies and wasn't expecting any, but was glad that the sorted it all out.......

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  #46  
Old 01-16-2022, 03:01 PM
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Right on here -
"but most folks just don't know where the problem lies or how to address it."

I did tune it at first when after some minor mods to my engine like eliminating the Emissions stuff, recurve the distributor, etc. The " how to" and Q-Jet parts kits were great for simple tweaks. And the car ran great. That was 40 years ago.

But just saying things get a little more complicated when moving far away from stock, and really needs a pro or dyno for making iterative changes to the Q-
Jet, or any Carb for that matter to improve response and power. I did mess with all of the "secondary POE system, choke pull-off release time, secondary air door spring adjustment and hanger height/metering rod being used", but never could get it right. Lot's of Q-Jet tuning books out there, and I had them all. Like you state, lots of the tuners/pros won't even look at the Q-Jet, like it is some kind of Covid Virus

At some point, it was time to go try plan "B", and in the end, for me at least, tuning a Double Pumper was just friggin easier to get better performance than the Q-Jet.
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  #47  
Old 01-16-2022, 03:38 PM
mgarblik mgarblik is offline
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Originally Posted by Cliff R View Post
"Hi Cliff. That's great information, thanks. It's funny because one of the engine builders I talked to (not the one I'm using) flatly refused to build me an engine with the stock intake and Qjet. Just said no. Now, MY Qjet is obviously not YOUR Qjet... Clearly they must have known that. "

Last time I looked it was 2022 and we should be past all that by now, but sadly many parts including the Q-jet still get a bad reputation when it comes to these things. Usually the same folks who bad mouth the factory carburetors do the same for stock type distributors and use some sort of "bug zapping" unit in their engines as well, which don't make any more HP anyplace than a well set up stock unit.

A recent and good read being that stories and real life experiences are helpful with these things, so here goes:

I got involved with a "stock appearing" early 70's Chevy project that was pretty much going to be a "max-effort" deal but look completely stock. It was a big block 454 and used the factory "flat" iron intake and Cowl induction set-up.

The owner hired a very good engine builder who owned a machine shop and dyno. Common with this sort of thing during the planning stages he mentioned that a "quadra-junk" and that "POS " stock flat iron intake those engines used "will not feed my engine". I hear that a lot so really didn't pay it much mind as I've been involved with quite a few of those set-ups that are raced in FAST and some run deep into the 10's and a few even dip into the 9's when the sun, moon and stars line up well. Anyhow, I went ahead and built a carburetor for the 496CID high compression 454 he was building. It would use the factory heads but they got bigger valves and ported, plus a custom cam, and all of it fitting under the stock valve covers and completely factory appearing in every respect. The target level for power was around 560-600hp and hopefully at least that much torque.

The build was completed and I shipped the engine builder a custom early 1970's Chevy Q-jet modified and tuned for what they were doing. Dyno day I get a phone call and it was all BAD. The engine wouldn't make much past 425hp and it was DONE by 4500rpms or so. Along that info I got to hear all over again what a POS the carb and intake set-up was and a little "I told you so" to go with it. So I start asking questions and made a few recommendations one being to crank up the fuel pressure to make sure the carb was staying full, but nothing they did helped. I finally ruled out the carb and asked about the distributor, total timing, when it was be "all in" etc. Very quickly the engine builder snapped back at me that he did the stock points distributor himself and outfitted it with a Pertronix and custom advance curve all in around 2800rpms or so.

Hum, since the engine was basically shutting down at 4500rpms and not making mcuh power I asked him to remove the Pertronix and put a good set of points back in it. The phone went silent for a moment, then he agreed. About an hour late I get a call that the next pull on the engine was 565hp and 585tq and it pulled right to 6000rpm's without much effort! This is NOT the first time I've seen this issue with Pertronix units and other aftermarket ingnition systems/modules.

Of course I didn't get any apologies and wasn't expecting any, but was glad that the sorted it all out.......
Good info there and a happy outcome. I am one of the lucky guys who has never had any real trouble with Petronix stuff. But a big heads up for people who use them. Be aware that a Petronix distributor or module called the Petronix III has a built-in rev limiter factory set to 4000-4500 RPM. Had a student's 383 MOPAR on the dyno recently and it went flat and wouldn't make over 310 HP. But it made 415 Ft. Lbs. at 3800 RPM's . Stock intake, carb, and exhaust manifolds. Sounded good to about 4200 rpm, then lightly started to break-up. Followed the instructions and re-set the rev limiter to 6500 rpms to take it completely out of the picture. Next pull, pulled right to 6000 rpm. Made 364 HP, which was just above our target of 350. Peak power@ 5600 rpm.

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Old 01-16-2022, 08:41 PM
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NEVER had ANY dyno operation attempt to tune a qjet…….that would almost be laughable. I couldn’t imagine a dyno shop busting into his qjet tuning kit complete with specialized drills, rods, jets, hangers etc…etc..

Bring your own qjets and parts if you want to go down that road…..and frankly, I’ve not had success translating dyno tune to the track. Seems once I get on a track, the “dyno tune” goes out the window.

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Old 01-16-2022, 09:19 PM
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Originally Posted by mchell View Post
NEVER had ANY dyno operation attempt to tune a qjet…….that would almost be laughable. I couldn’t imagine a dyno shop busting into his qjet tuning kit complete with specialized drills, rods, jets, hangers etc…etc..

Bring your own qjets and parts if you want to go down that road…..and frankly, I’ve not had success translating dyno tune to the track. Seems once I get on a track, the “dyno tune” goes out the window.
Imagine that! get into the real world and the theoretical has to be tweeked.....

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Old 01-16-2022, 09:23 PM
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Originally Posted by Formulabruce View Post
Not with the cam choice, and while he "may", He also might be retarding the timing some too.
10:1 is NOT needed for over 450 ft lbs of Torque, and THATS what moves you.
Have seen many of these type builds built to impress others and the lack of vacuum makes the brakes kinda "iffy"
Please yourself. Can you go get ice cream comfortably?
Forged Pistons are HARD on the block...... Do you want a good running and long reliable engine?
Are you suggesting that he use cast pistons? I've not heard of forged pistons being hard on the block......

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Old 01-16-2022, 11:03 PM
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Originally Posted by 72LuxuryLeMansLa. View Post
Are you suggesting that he use cast pistons? I've not heard of forged pistons being hard on the block......
Fir a nice, long lasting, driver, with good power, a KB piston is the way to go. Which is a 12% silicon piston, which is cast, can be fitted tighter, and yes, much less wear than forged.
Nearly all new engines use a version of silicon cast aluminum pistons.
So yeah, Forged pistons are harder on the cylinder bores.
They are harder than the block casting...
Forged are Great for racing, and they have their place.
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Old 01-19-2022, 12:49 PM
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Hi pal … for me since not a daily driver I’d go for the gold!! (Actually I did with mine)
Love it I have too much gear (3.73) too much compression too big a carb and all the cubes I could afford (463) too much cam rattles windows rear ground pounder AND
I Wouldn’t change a thing !!

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Old 01-19-2022, 12:53 PM
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I have 2618 pistons in a mild 505. With an aftermarket block I'd probably be leery about hyper pistons, but I would do 4032 alloy if I could do it over again. I don't mind the noise but I don't really see the point of 2618 pistons in a basic pump gas street engine. For a factory block street engine I would feel just fine with a hyper piston, as long as the compression ratio is safe.

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Old 01-19-2022, 06:11 PM
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My only word of caution if considering using hypereutectic pistons is their very poor resistance to detonation. Being very hard and brittle, they have little tolerance for detonation and if your combination ever experiences audible detonation, it will result in piston damage over time, sometimes in spectacular fashion. First the ring lands generally crack and can sometimes break the rings, then as the damage progresses, the skirts tend to break right under the oil ring groove. Things can get ugly at that point. Most factory supplied hypereutectic pistons are used in applications with factory detonation sensor systems in place to keep these things from happening. If you keep your compression low, have your fuel and timing correct for the fuel, then they can be a good choice. They do run quieter than forged, but for old muscle car engines, with point ignitions and carburetors, I like the extra safety forged pistons offer.

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Old 01-19-2022, 09:24 PM
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I'm on the same page. Been a forged piston guy for decades and build everything with them, which are primarily street cars. Today with pump gas what it is, never knowing what you're getting from one tank to the next, I prefer the piece of mind that I have a piston in there that'll take a little abuse, just in case.

I've never had an issue with forged pistons, even for well over 100k miles. Never made any noises for me, never had a block with issues because of them.

Nothing wrong with other options. Having those options is great for budget minded builds.

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Old 01-20-2022, 07:07 AM
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"NEVER had ANY dyno operation attempt to tune a qjet…….that would almost be laughable. I couldn’t imagine a dyno shop busting into his qjet tuning kit complete with specialized drills, rods, jets, hangers etc…etc.."

I've been on the dyno (engine) with Q-jets scores of times and they are a LOT easier to tune than anything else out there. All that you need is a complete set of fully machined secondary metering rods. NEVER touch the primary side of the carburetor when dyno tuning. You may get some odd looks from the dyno operator when they are doing a "run-in" on the engine and it shows "lean" at light load. They will NOT be used to seeing that since most of if not all of their experience will have been with pretty "generous" aftermarket carburetors.

NEVER let anyone yank the top off the Q-jet and start changing or drilling out the jets. Once you start making pulls make secondary metering rod changes only. If you have a set of machined rods from the same cores with the same tip length you will NOT have to mess with hangers. If your arsenal of secondary rods are from stock Q-jets you may need to change hangers with secondary metering rod changes to go the intended direction and get the results you are looking for.

Track tuning? I've chased that deal to DEATH and don't really see big changes messing with fuel curves. This assumes the carb was pretty close right off the bench. I've tuned carburetors from "pig" rich to so lean the engine started to go "flat" and surge slightly and you'll be lucky to see a tenth and a half or so and less than 2mph difference all the way across the board. I used to play around with that deal a lot back when I was trying for every last possible hundreth of a second and best MPH, but ended up running the same rods most of the year and only leaning the carb up on those 100 degree plus days when the air was so thick you could cut it with a knife........FWIW.....

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Old 01-20-2022, 02:17 PM
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Well stated I could agree more I’ve never been on an engine dyno tuning only street and track time
(best dyno anyway is ET/MPH)
I ran q-jets on dirt a million laps when the were allowed

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