Wanted your opinion on carb size for new motor. Car is a foot brake bracket car. Wanna shift 6800-7000
Has Power Glide. 1/8 mile ONLY

LQ9 with stock 799 heads.

I called Quick Fuel and they said the Q750.
Filled out Pro Systems tech sheet and they said their 750 that flows 770cfm.

I've used both companies before and liked the quick fuel best. But what do you guys think?

 

All I can give you is this tidbit of info from when I ran the fast EZ EFI TB injection on my small cammed LS3. The TB is rated at 1000 cfms of flow ( I think it is a 750 cfm body, but with no boosters in the way). On my data handheld, it showed a 3-4 KPa restriction to flow !
I know the 799 heads don't move the air that the LS3 heads do, but I don't think you can go TOO big on carb size with these engines, especially on a strip only car where you don't really care about idle quality and part throttle response.

 

I tried a 1000 cfm 4150 carb on my first LS3 head 6.0. It was way to big. we had it jetted 99/99 and it was still too lean. Ended up with a 950 that wet flowed 840 on it.

On the flip side I tried that same 950 carb when I got my LS3 head 408 done and it was way to small. No matter how much jet you took out it was still rich on top.

Put the 1000 cfm 4150 on it and all was right.

 

750 sounds bout right. Quckfuel has lifted there game recently

 

I talked to a Tech at Demon a ways back before the shut down....He stated that a 750 Mighty Demon is WET-flow rated. SO, on a Holley flow bench it would flow in the 930 range. (Holley DRY-flow rates theirs... I'm told)

My LQ4/LS3 combo camed with 7psi fuel pressure shows no problems with keeping up.

76/88 I think are the jets in it right now.

 

My example was pure air flow and didn't take fueling into account. The injectors just matched the available air flow on my example. It is just meant to show how greedy those big port LS engines are for air. 1000 cfms of air flow still showed vacuum in the intake system at the TB . I guess (?) that is one of the compromises a carb system has to make to get the right AF ratio. It would be interesting to see how much vacuum is still in a properly fueled 750-850 cfm carb on an engine with LS3 heads. Vacuum is just backwards boost, so any restriction has to cost some amount of power, I would think.

 

Demon 750 on my LQ4 with 799 heads and I havent had any problems. Runs good and pulls strong.

 

That could have been noise, or error signal. Without something actually calibrated, you can't be sure. The sensors themselves arn't perfect (their scale is not always linear as there are data points and voltage/filtering involved) Sometimes a 1-bar map sensor will not ever read 0KPa exactly, even when first turned on without the engine running you might see 0KPa, then run the engine and shut it off, and it might sit at +3KPa or -3 now that the sensor has warmed up and something minor inside it changed, or in the ground.

 

Ok Guys. I've bummed a couple different carbs to try on the test engine. Should test next weekend. The car currently has an old style Holley 800 DP and I have snagged a Quick Fuel Q750 and a ProForm 850 to try. I'll report back with findings

 

Did you ever test these out?

 

Actually I did last night and not results I was hoping for. I tuned all carbs until I got same AFR going down track at 13:1. I chose that figure since that's where my last setup ran best. Took a few jet changes with each

850 Pro Form went 6.882 @ 100.6
750 Quick Fuel went 6.894 @ 100.4
Old 800 Holley DP went 6.900 @ 100.2

I'm guessing the Holley is actually too small as no matter how far I jetted it down I couldn't get the AFR out of the 11's

 

Do you know anyone with a 750 Mighty Demon you could test ?

I'd really like to see where it falls.

 

LOVED my 850 Mighty Demon

 

No I don't.

Pro Systems now says I need one of their 950's. I'm confused lol

 

Damn....wish you were closer

 

I don't understand this statement. I don't see where carb size should make it go rich up top.

Try disconnecting the throttle linkage to the secondaries. Carb should stay in tune even with just the primaries.

Sounds like more of an issue with the float level, power valve, air bleeds or some sort of internal leak.

 

I may be wrong, but the reason I say this is an engine needs X amount of air. If it can't get that much air it will run rich. I'll give you an example.

On my 408 which by all formulas needed 885-900 CFM, I tried my Pro Systems 950 I took off my 6.0 which flowed 830 CFM. It was jetted 76/86 for the 6.0 and had a AFR of 13.2. On Wideband on 408 it went down the track at 10.8 so I jetted down 4 sizes, went 11.0. Went down 4 more jets and it was so lean on the hit it had a miss, but was still 11.0 on the big end.

That led me to test my 1000cfm carb I got for 408, jetted 75/85 for a 13.2 AFR on 408 and try on 6.0. AFR was in high 15's as it was too lean. Ended up jetting it 99 square as rich as I could go and AFR was still in 15's. No matter how much jet it wasn't enough fuel with how much air was gettin in, so carb was too big.

These are just my personal findings, I may be completely wrong lol

 

Try going up a couple of sizes on your main air bleeds with the Pro Systems and a couple of sizes smaller on the 1000 cfm and tell us what happens.

Carbs transfer to the main circuits somewhere around 2-3 thousand RPM. I set the jets to get the right AFR at this point. As RPM climbs and airflow increases, the mixture can go rich or lean based upon the size of the airbleeds. Letting more air in through the bleeds keeps the mixture from getting too rich as the airflow increases. If your mixture is good at 3K RPM and goes rich at redline, try larger main airbleeds. If it goes lean, use smaller bleeds. Anytime you adjust a bleed you need to revisit the jets.

Think about it. When driving around at part throttle the engine definitely isn't getting all the air it wants. Why would WOT be any different?

At one point I tuned my carb with the secondaries disconnected. That is a good way to do it.

 

"No matter how much jet you took out it was still rich on top" = HSAB
Find someone who knows how to do this.

 

Thanks guys, both those engines and carbs are long gone, but I understand what you're saying about Air Bleeds. If I have the issue again, I'll try that