: Rochester QJ and N* Throttlebodies



Submariner409
06-25-07, 09:30 PM
A few interesting calculations for the performance seekers. An 850 cfm Rochester QuadraJet has primary bores of 1.375" and secondary bores of 2.25", for a total of 10.88 square inches of "hole" area. Your desired N* 80mm throttlebody comes out to just over 8 square inches of "hole" area. About 27% less than the monster QJ. BUT, the QJ supports engines from 400 c.i.+ to over 600c.i., while the N* throttlebody supports a 280c.i. unit. (This is exclusive of butterflies and shafts) One of the most common complaints of the carburetor performance crowd, after installing some huge carb (bigger is better) is a pronounced flat spot off-idle and a sluggish midrange. (Sound familiar?) Why? Too much airflow exactly where it's not needed --- low to mid-rpm. If you want to win in the stoplight GrandPrix, try a smaller throttlebody for higher air velocity at the port. Remember the Venturi principle: as area decreases, velocity and pressure increase. As the venturi widens, flow and pressure decrease rapidly, actually creating a vacuum downstream of the butterflies, which is one of the reasons a carburetor "works". Think about it.......

Maxb49
06-26-07, 06:29 AM
A few interesting calculations for the performance seekers. An 850 cfm Rochester QuadraJet has primary bores of 1.375" and secondary bores of 2.25", for a total of 10.88 square inches of "hole" area. Your desired N* 80mm throttlebody comes out to just over 8 square inches of "hole" area. About 27% less than the monster QJ. BUT, the QJ supports engines from 400 c.i.+ to over 600c.i., while the N* throttlebody supports a 280c.i. unit. (This is exclusive of butterflies and shafts) One of the most common complaints of the carburetor performance crowd, after installing some huge carb (bigger is better) is a pronounced flat spot off-idle and a sluggish midrange. (Sound familiar?) Why? Too much airflow exactly where it's not needed --- low to mid-rpm. If you want to win in the stoplight GrandPrix, try a smaller throttlebody for higher air velocity at the port. Remember the Venturi principle: as area decreases, velocity and pressure increase. As the venturi widens, flow and pressure decrease rapidly, actually creating a vacuum downstream of the butterflies, which is one of the reasons a carburetor "works". Think about it.......

Interesting thought, but we are working with fuel injection, not carburetion.

jadcock
06-26-07, 07:40 AM
FI or carburetion -- the concept is the same. That's why you don't see a 100mm TB on the Northstar.

The "big" Q-jets were also used on smaller engines. The reason they worked is because the secondaries were either vacuum or mechanically operated, and phased in after x% of throttle travel. Through 40% of throttle travel or so (I forget the exact amount), the secondaries are closed, and your throttle opening is simply the two primary bores. But open up the secondaries, and it'll sing.

I don't think there was a Q-jet smaller than 795 CFM, and those were on everything from 4.1L V-6 engines (enlarged Buick 3.8L) to Olds 403s (I'm thinking emissions era now). My '84 Cutlass had the 307 and had the computer-controlled Q-jet. Now THAT engine compartment was a mess of vacuum lines, let me tell ya!

eldorado1
06-26-07, 10:37 AM
One of the most common complaints of the carburetor performance crowd, after installing some huge carb (bigger is better) is a pronounced flat spot off-idle and a sluggish midrange. (Sound familiar?) Why? Too much airflow exactly where it's not needed --- low to mid-rpm. If you want to win in the stoplight GrandPrix, try a smaller throttlebody for higher air velocity at the port. Remember the Venturi principle: as area decreases, velocity and pressure increase. As the venturi widens, flow and pressure decrease rapidly, actually creating a vacuum downstream of the butterflies, which is one of the reasons a carburetor "works". Think about it.......

Not quite, but close. Installing a large carb is a bad idea because it kills velocity through the carb. Venturis need a minimum air velocity for gasoline to vaporize. Now what happens if you install a 1000 cfm carb on a 750cc bike engine? Won't run. Why? Not enough velocity.

However, stock GSXR 750 engines have 50mm throttle bodies. 4 of them, actually. That's equivalent to a single 100mm throttle body, on a 750cc bike. That would be like a 250mm throttle body on a N* if you go area/displacement.

As you can see, it doesn't work with fuel injection. ;)

A throttle is only needed to throttle the air flow through an engine. This is what gives the engine a linear response to throttle input. At wide open throttle, it is more desireable to have 0 resistance to air flow through the throttle and the rest of the intake. However, go too large a throttle body and your linear response becomes more exponential. Cracking a 5" throttle open a little bit is letting in the same amount of air as half throttle on a 2" throttle. At some point you reach diminishing returns, and going from a 5" to a 6" throttle gains you nothing in terms of HP or torque, just an "on or off" throttle response.

GM tends to undersize throttle bodies for that reason - smooth delivery.

However, with their new e-throttles, I don't know why they wouldn't have used significantly larger throttle bodies, when they can have the PCM control the opening rates.

Submariner409
06-26-07, 10:47 AM
:rolleyes: Yes, we're working with FI. The throttlebody butterfly controls airflow and the attached rotary sensor sends throttle position to the PCM. Mass airflow, which controls part of the mixture signal, is sensed by the MAF/IT unit ahead of the throttlebody. For proper fuel tuning, you need to address several sensors, not just airflow. (If you snoop around you can find QJets with different sized primaries. 750 and 850 cfm, wide open. The problems occur when a "tuner" reduces the secondary air valve opening spring pressure too far, which causes a nasty bog and midrange stumble. The secondaries begin to open mechanically at around 60 degrees of primary, and everything is wide open at full pedal, BUT the air valves over the secondaries control airflow according to engine vacuum and demand. (The air valve also lifts two metering rods to control WOT mixture...)) Eldo1......Your post just showed up.....yep, a larger throttlebody, exclusive of other fuel delivery mods, will make throttle opening and position a little squirrely.........

AJxtcman
06-26-07, 10:59 AM
Q jets ran from the 400 range to 1000. Production carbs range from 450 or 500 to an 800? I think it was 800, but it may have been 850. They came on GM, Ford, and Chrysler's. I have a book with the listings and the sizes some place.
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Just think of it as a push or pull mentality. If you push the air in you need to be big on everything. If you are pulling the air in you don't want to stall the air.

jadcock
06-27-07, 08:21 AM
AJ, are you talking about Doug Roe's book, Rochester Carburetors? I have it also, excellent book.

I played around with the secondary metering rods once on my Cutlass. I bought up a bunch of old carbs for 2-3 dollars apiece and added to my collection of secondary rods. Very interesting how changing the size of the metering rods would directly affect the WOT performance. This was certainly an area were MORE wasn't necessarily BETTER.

I bought an '87 Regal with the 307 engine and Q-jet for $500 as a winter beater. It had an incredible bog when going to WOT. The problem turned out to be the vacuum pull-off on the secondary air valve...it was in-op. The secondaries would just snap open at WOT. Once I installed a new pull-off, it was much smoother. That roller-cam 307 was one of the smoothest "old" engines I've driven. Almost had the characteristics of fuel injection. On warm days, you didn't even have to pump the gas to "cold start". Those high-velocity intake ports (only 1.3" square, compared with the 1.3x2" ports of the older 307 design) really helped with torque. That was a nice engine.

Maxb49
06-27-07, 08:23 AM
Not quite, but close. Installing a large carb is a bad idea because it kills velocity through the carb. Venturis need a minimum air velocity for gasoline to vaporize. Now what happens if you install a 1000 cfm carb on a 750cc bike engine? Won't run. Why? Not enough velocity.

However, stock GSXR 750 engines have 50mm throttle bodies. 4 of them, actually. That's equivalent to a single 100mm throttle body, on a 750cc bike. That would be like a 250mm throttle body on a N* if you go area/displacement.

As you can see, it doesn't work with fuel injection. ;)

A throttle is only needed to throttle the air flow through an engine. This is what gives the engine a linear response to throttle input. At wide open throttle, it is more desireable to have 0 resistance to air flow through the throttle and the rest of the intake. However, go too large a throttle body and your linear response becomes more exponential. Cracking a 5" throttle open a little bit is letting in the same amount of air as half throttle on a 2" throttle. At some point you reach diminishing returns, and going from a 5" to a 6" throttle gains you nothing in terms of HP or torque, just an "on or off" throttle response.

GM tends to undersize throttle bodies for that reason - smooth delivery.

However, with their new e-throttles, I don't know why they wouldn't have used significantly larger throttle bodies, when they can have the PCM control the opening rates.

If this is the case, can you explain how a throttle body in any scenario can provide more power?

jadcock
06-27-07, 08:29 AM
However, stock GSXR 750 engines have 50mm throttle bodies. 4 of them, actually. That's equivalent to a single 100mm throttle body, on a 750cc bike. That would be like a 250mm throttle body on a N* if you go area/displacement.

As you can see, it doesn't work with fuel injection. ;)

Well, I think that may be a bit of an apples-to-oranges comparison, no? I would imagine a 750cc motorcycle engine would turn RPMs in the neighborhood of 12-15k, and have NO torque below 4000 RPM. I imagine that if you were build a 12,000 RPM Northstar engine, you'd need a throttle body larger than 76mm, or whatever our stock throttle bodies are. No?

In other words, the loss of linear throttle response isn't an issue with that small engine because there's no throttle response way down low anyway. And the sheer number of revolutions per minute necessitate a throttle body larger than what you'd "normally" find on an engine of such displacement. :confused:

eldorado1
06-27-07, 09:25 AM
If this is the case, can you explain how a throttle body in any scenario can provide more power?
Not in any scenario..... only when there's a restriction. If you measure a restriction at WOT, then there is power to be gained by going to a larger throttle body.



Well, I think that may be a bit of an apples-to-oranges comparison, no?

So why would it not be able to run with a 1000cfm carb, but run perfectly with 1000cfm throttle bodies? Seems like a perfect comparison. What applies to carbs doesn't apply to EFI.

Also, the 750cc engine is using a trick..... It's using independent throttle bodies. This allows you to run with much larger throttle [area] than you normally could, and still maintain streetability/linearity. 4 smaller throttle bodies have more control than 1 large throttle body.

Submariner409
06-27-07, 01:45 PM
Good words and good ideas. The whole purpose of this thread was to get people to think about more than just slapping a bigger hole in front of the intake manifold, carbureted or FI. (jadcock......secondary rods are good tuning aids....there's about 60 different sets of tapers.....)

AJxtcman
06-27-07, 02:41 PM
The whole purpose of this thread was to get people to think about more than just slapping a bigger hole in front of the intake manifold

That is why I listed the bore sizes in the T-body thread I started. I had a Thread going in the Fiero forum and then Told every one they were idots for hogging out all the intakes and heads.
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I installed a set of "ported heads" on a 02 Eldo and it was slow.
In my opionion the air will stall if you open up every thing. You need to have Volumeteric Effecency of the intake system meet you goals, but not over that.
If you want your power down low you would tune it for that range. If your T-body and intake system can flow enough CFM for 10,000 RPM and it will bog out at 1200 RPM's then it will not work in a Caddy.

AlBundy
06-27-07, 04:39 PM
Ya know I love it when you guys post such good info. Keep up the good work.:thumbsup:

Mountie
06-27-07, 10:32 PM
Way back, in '73, I had a 1970 Pontiac GTO. I had the Quad blueprinted, by an NHRA Pro-Stock carb builder. The transition from primary's to the Secondary's was super smooth. The difference between a good/ stock Quad, and this bluprinted Quad was like installing a little turbo charger, for the performance improvement.

High 13's in the 1/4 mile ....stock. ( with a modified automatic tranny)

Boy, do I miss that car.

eldorado1
06-27-07, 10:33 PM
I installed a set of "ported heads" on a 02 Eldo and it was slow.


Heads are very dangerous things to port. At least the intake side is, the exhaust not so much...

The intake ports have features built into them that cause the air/fuel mixture to tumble and swirl once it enters the cylinder. Haphazardly grinding and porting can eliminate these features. Our "guru" once mentioned a northstar race engine that had ported intakes and was running 12:1 and on the verge of a rich misfire because of the stratified charge. Normal northstars will run 10:1 perfectly fine, and in fact MUST run 10:1 to save the pistons from melting after extended WOT time.

IF anyone considers porting the intakes for better flow, I would consider it a necessity to measure the swirl and tumble and make sure your porting does not change either by much. I don't think even CHRFab has done this, so caveat emptor.

AJxtcman
06-27-07, 11:56 PM
I don't think even CHRFab has done this

I have never seen any thing good come from them!
any thing!
I have seen a waste of money.

fubar569
06-28-07, 12:40 AM
Heads are very dangerous things to port. At least the intake side is, the exhaust not so much...

The intake ports have features built into them that cause the air/fuel mixture to tumble and swirl once it enters the cylinder. Haphazardly grinding and porting can eliminate these features. Our "guru" once mentioned a northstar race engine that had ported intakes and was running 12:1 and on the verge of a rich misfire because of the stratified charge. Normal northstars will run 10:1 perfectly fine, and in fact MUST run 10:1 to save the pistons from melting after extended WOT time.

IF anyone considers porting the intakes for better flow, I would consider it a necessity to measure the swirl and tumble and make sure your porting does not change either by much. I don't think even CHRFab has done this, so caveat emptor.

i'll assume you meant intake ports..and not the intake itself which you and i have both done...

AJxtcman
06-28-07, 06:39 AM
This is from the web site that the Heads came from for the Eldo.
Cadillac Hot Rod Fabricators Northstar powered '32 Ford

AJxtcman
06-28-07, 06:41 AM
This is also from the site

eldorado1
06-28-07, 01:49 PM
This is from the web site that the Heads came from for the Eldo.
Cadillac Hot Rod Fabricators Northstar powered '32 Ford

The heads are good on paper, but I would ask for before/after testing of MY heads, and include swirl/tumble measurements. Not every shop is equipped for that. I would not buy their heads without some numbers.


i'll assume you meant intake ports..and not the intake itself which you and i have both done...

Yes.

AJxtcman
06-28-07, 02:10 PM
The heads are good on paper, but I would ask for before/after testing of MY heads, and include swirl/tumble measurements. Not every shop is equipped for that. I would not buy their heads without some numbers.


Why oh why would you need tumbling in a Northstar?
I am not sure how important swirling in the intake runners is on a Northstar.

eldorado1
06-28-07, 02:18 PM
Actually in DOHC heads, tumble is more prominant than swirl. These are in cylinder measurements, not in runner.... i.e. after it passes the valves.

airfuel2001
06-30-07, 12:21 AM
When my motor was down for timeserts, I had the option of porting my heads I was thinking of cleaning up the intakes...just the casting flash and recontouring the exhaust for more flow. I wish I had-but it still runs strong.