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A tale of two carbs, and FI

Paul_G

Paul_G

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My car is a 4000# 72 Charger with a mild built 360, about 250 HP, 330 FT#, at the wheels.

I have been fiddling with two different carbs, both Holleys, one a 670 Street Avenger, the other a 750 with vacuum secondaries. The engine likes to idle pretty rich in the mid to upper 12 AFR range. Both carbs will set up and idle well in that range, but any leaner and the engine idles rough and just doesnt like it. But then they differ.

I can tune the 670 to run very well, idles well in the upper 12 to low 13 AFR range which the engine likes, cruises nicely in the upper 14 AFR to mid 15 AFR range which gives good economy. It's easy to drive and runs pretty well. Will run full throttle in the mid 12 AFR range, pulls hard and and just a nice carb. But it just doesnt make the power the 750 will make at any point in the range. Just barely will crack the tires loose from a stop.

The 750,
when tuned for performance it is much better performer than the 670. It will crack the tires loose from a stop easily, pull steadily to 5000 RPM and about 120 MPH in overdrive. The car is fun to drive with this carb.

This carb idles in the mid 12 AFR range but has stinky exhaust, any leaner and it stumbles, shakes and just doesnt like it.

I have tried to tune some economy in to the 750 since it is a better performer. I have read that when cruising at a steady speed the carb is actually running on the idle and in to the transition circuit. In an effort to lean out the cruise AFR's, I tried restricting the idle feed circuits with small wire. It really didnt like it, stumbles, and cruises rough. It cruises well in the high 12 to low 13 AFR range, but gas gas mileage is terrible.

WOT it runs in the mid 12 AFR range. I have tried to tune in some economy many different ways. Jet changes wont do it, jet sizes have to be set for WOT performance along with the PVCR. I have been using small wire dropped in the PVCR along with primary jetting to get the WOT AFR in the mid 12 range where it pulls the hardest, and at the same time get it to stay in the mid 13 AFR range during mild acceleration on the street, the range where primary jets are in play. Thats where primary jetting and tuning the PVCR have to work together.

I have just given up trying to get the bast of both worlds from either of these carbs. The 750 is tuned for performance right now and does that very well. The 670 runs really nice and smooth with good economy, I just dont expect any performance from it.

Which brings up fuel injection. Can the new self learn units from Fitech and Holley do both? Can they get good economy like the 670, and give the great performance like the 750 does?
 
Last edited:
They might? I'm at the some place. You can try some more timing, it may help. I have a fitech 600pa I am going to try next. P
 
Simply put, yes. Both Holley and fitech I believe would perform bette than you’re carb and will give you “the best of both worlds” in terms of economy and performance. Both have tunable software which you could play with. Of course there is the self learn which is fine but you could tweak it to your likings once it has a solid base tune “learned” fitech 1200 going on my stroker
 
66chargerpat said:
They might? I'm at the some place. You can try some more timing, it may help. I have a fitech 600pa I am going to try next. P
Click to expand...

I have to limit timing to 32 degrees on 91 pump octane or it will detonate when hot. I am in Phoenix and it's always hot.

I talked with Fitech and Holley. Timing can be controlled in the FI unit, if I pick the right unit, and adjusted on the fly which I like. I can dial in a little more for better performance in cooler weather, and pull it out when it gets real hot.
 
On the 670 Avenger, is the idle rich to cover a lean transition circuit? I have had two Avenger carbs, and both ran better after enlarging the idle feed restrictions. I think I may have also gone slightly larger on the Power Valve restriction too.
 
Paul_G said:
My car is a 4000# 72 Charger with a mild built 360, about 250 HP, 330 FT#, at the wheels.

I have been fiddling with two different carbs, both Holleys, one a 670 Street Avenger, the other a 750 with vacuum secondaries. The engine likes to idle pretty rich in the mid to upper 12 AFR range. Both carbs will set up and idle well in that range, but any leaner and the engine idles rough and just doesnt like it. But then they differ.

I can tune the 670 to run very well, idles well in the upper 12 to low 13 AFR range which the engine likes, cruises nicely in the upper 14 AFR to mid 15 AFR range which gives good economy. It's easy to drive and runs pretty well. Will run full throttle in the mid 12 AFR range, pulls hard and and just a nice carb. But it just doesnt make the power the 750 will make at any point in the range. Just barely will crack the tires loose from a stop.

The 750,
when tuned for performance it is much better performer than the 670. It will crack the tires loose from a stop easily, pull steadily to 5000 RPM and about 120 MPH in overdrive. The car is fun to drive with this carb.

This carb idles in the mid 12 AFR range but has stinky exhaust, any leaner and it stumbles, shakes and just doesnt like it.

I have tried to tune some economy in to the 750 since it is a better performer. I have read that when cruising at a steady speed the carb is actually running on the idle and in to the transition circuit. In an effort to lean out the cruise AFR's, I tried restricting the idle feed circuits with small wire. It really didnt like it, stumbles, and cruises rough. It cruises well in the high 12 to low 13 AFR range, but gas gas mileage is terrible.

WOT it runs in the mid 12 AFR range. I have tried to tune in some economy many different ways. Jet changes wont do it, jet sizes have to be set for WOT performance along with the PVCR. I have been using small wire dropped in the PVCR along with primary jetting to get the WOT AFR in the mid 12 range where it pulls the hardest, and at the same time get it to stay in the mid 13 AFR range during mild acceleration on the street, the range where primary jets are in play. Thats where primary jetting and tuning the PVCR have to work together.

I have just given up trying to get the bast of both worlds from either of these carbs. The 750 is tuned for performance right now and does that very well. The 670 runs really nice and smooth with good economy, I just dont expect any performance from it.

Which brings up fuel injection. Can the new self learn units from Fitech and Holley do both? Can they get good economy like the 670, and give the great performance like the 750 does?
Click to expand...
Sure they can. These units are dynamic in their operation meaning they are not really sized by CFM but will operate at the A/F that is required at any given driving requirement. On TBI you set the pump squirt ( accelerator pump setting on carbs), cold plus medium plus warm A/F's separately, WOT (wide open throttle) A/F, cruising A/F and idle A/F. All you have to program is the engines' Cu. In. and desired idle RPM. Drive them under different conditions and they learn based on selected A/F ratios. You can put them on any size engine up to their HP max, even 6 cylinder engines. It's a "one size fits all" senario without the BS of csrbs. Got them on both cars of radically differen sizes of engines, I'll never look back.
 
451Mopar said:
On the 670 Avenger, is the idle rich to cover a lean transition circuit? I have had two Avenger carbs, and both ran better after enlarging the idle feed restrictions. I think I may have also gone slightly larger on the Power Valve restriction too.
Click to expand...

Are you running them on a big inch engine? In stock form, my 670 can be adjusted for idle mix with a pretty broad range just using the idle mix screws. Anything above 13 AF ratio and this 360 gets very rough at idle. I suspect the cam as the reason, 250 HP and 360 TQ at the wheels. What kind of cam does that?

Increasing the size of the IFR will make the cruise ratio richer too, and might cure a lean transition stumble. I like the lean cruise the 670 has stock. Didnt want to drill the IFR. I did upsize the squirter to cover an off idle bog it had. The 670's in general have that off idle bog problem, along with a lean stumble at cruise, probably because of the primary jetting from holley is so lean, 65 main jets stock.

The 670 on my 360 ci engine, ran lean in stock jetting, mild acceleration was in the 14 to 15 AFR range on the street, I want it in the mid 13 range, but WOT was still very rich, in the upper 10 AFR range stock. Holley sends them out very rich at WOT for safety. I had to upsize the main jets to 68, and restrict the PVCR with a small wire to get the wot ratio in the mid 12's.
 
Last edited:
I could be wrong, but I think it is the altitude here in Denver. The idle/transition is more like a controlled leak of fuel (no venturi effect like the main jetting), and there is just less air pressure (pressure differential or engine vacuum) to push the fuel through the idle restriction.
I used some billet metering blocks to tune with that have the screw-in restrictions, and made adjustments that way, then drilled the stock metering blocks the same as what I used in the billet blocks.
 
I had the chance to put the car on a chassis dyno last weekend at the HEMI Show in Phoenix. So I brought both carbs. Horse power was very close, but surprisingly the smaller 670 actually made a little more HP, and more torque than the 750 did. I was really surprised at that because the 750 feels stronger. But the dyno dont lie. The 670 had a bad dip in power right on the hit of full throttle. The dyno graph plainly showed it. The 750 had a much smoother pull, clean power curve all the way across the graph, probably why it felt stronger.

On the graphs around 5000 RPM there is a dip, I could even feel it. Felt like a stumble. Valve float? Ignition?


The 670

March_3_2018_670.jpg


The 750

March_3_2018_750.jpg
 
Did they log O2 (air/fuel ratio?) First thought is it may have gone lean, what is the fuel system? I think ignition or valve train would not have as smooth a dip, but also don't know what filtering was selected in the dyno results display?
 
He did not have any o2 sensing. It is using a stock style mechanical fuel pump. I should have been watching my o2 gauge after I felt that stumble the first time. I dont normally run the engine up to 5000 rpm, but next tme I do I will watch the o2 gauge.
 
I was at the show as well and ran my car. No O2 hookup, no RPM hookup. Didn't ask gearing, final drive, tire diameter, etc. Not a very accurate dyno but they did get a bunch of people through :/
 
threewood said:
I was at the show as well and ran my car. No O2 hookup, no RPM hookup. Didn't ask gearing, final drive, tire diameter, etc. Not a very accurate dyno but they did get a bunch of people through :/
Click to expand...

I asked Cris, the dyno operator about that. He stated that because he has the vehicle at a known RPM, 2500, and in the 1:1 gear, the dyno will record the torque accurately using those two known values. For what that is worth?

You can see on the graphs that my torque number is more than 100 points higher then the horsepower number. Why would that be I wonder? Could it be due to the gearing? 3.91 gears.
 
Paul_G said:
I asked Cris, the dyno operator about that. He stated that because he has the vehicle at a known RPM, 2500, and in the 1:1 gear, the dyno will record the torque accurately using those two known values. For what that is worth?

You can see on the graphs that my torque number is more than 100 points higher then the horsepower number. Why would that be I wonder? Could it be due to the gearing? 3.91 gears.
Click to expand...

Makes sense. My printout was jacked as it showed I was letting off at 4k when I took it to 6k. I don't know how the derived the HP number because my torque number is over 200 higher.
 
Wow. My car tracked RPM pretty acurately. Thats what prompted me to ask Cris how the machine knows engine rpm.
 
Should have stopped by the Coronet, we could have talked FiTech all day and you could have took my 318 for a spin...

Bottom line you won't get fuel economy from leaning it out. If you look at the curves you will see the fuel usage curve as a function of AFR is at the bottom of a bathtub. A lot of change in AFR = very little change in fuel usage. The biggest thing for fuel economy is the vacuum advance and getting the timing advanced... yes I understand the pinging. I have 12 initial, 34 full mechanical and about 45 full vacuum and the FiTech. I got 15.1MPG from Tucson to Phoenix and back last weekend. The factory setting was 2 ATDC and a 36 degree mechanical advance to get the magic 34. With the FBO lockout plate and medium tension springs allowed me to get the 12 BTDC initial and 34 mechanical. I got about 12MPG and really hot running.

It will just barely ping under light acceleration on the freeway once it gets good and warm.

P.S. I should have said the First Place '68 to '70 B-Body Coronet :)
 
I am disappointed that I never got to meet other forum members at the show. Congrats on the win Jim! I hope you guys had a good time.
 
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