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Discussion Starter #1
This was asked on a different thread that went in different directions so I'm reposting..

I currently have a sum-1103 (214/224 duration) cam and ordered the Quickfuel hr680vs. I plan to change the cam in near future. They are on a L48. Intake is performer rpm and heads are cast 041, gears 3.42, th350. Three questions: Will the current cam and carb work ok together? What cam would you recommend for this carb that doesn't require a special stall converter? I'd like to do a retro roller but not mandatory.
 

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You are currently pushing the upper limits of a stock converter. Depending upon the lobe separation angle (LSA) you may not need a change at all.

The wider the separation angle 112° to 114° degrees the less likely the overlap will require an aftermarket converter. With a 107° to 110° LSA you might be happier with a converter out of a Corvette. They don't cost an arm and a leg compared to the aftermarket versions and have an additional 400 RPM over the stock 1,800 RPM stall of a stock converter. Get above 230° degrees of duration at 0.050" of valve lift with an LSA of 110° degrees and you will be needing an aftermarket converter to keep from stalling the car when you put it in drive.

That is discussing need. If you have an aftermarket converter of say 3,400 RPM (a common off the shelf setting for a SBC) you will enjoy the drive a lot more. You will need a large transmission cooler (or a couple of long thin ones hidden under the car) to save the tranny from melting down. You would really enjoy that drive with a looser locking converter with an Over Drive tranny such as the 700R4 or a 200R4 if you don't want to cut the drive shaft.

Larger Dave
 

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Discussion Starter #3
That's good to know on the cam. I think I will just leave the cam alone then and stay with the stock stall converter. As for that Quickfuel 680 carb I'll just plan for a little smaller primary jets if needed.
Anyone run a similar set up?
 

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Discussion Starter #4
I looked online and I can't find the primaries jet size for this carb. Quickfuel tech is closed. Anyone know off hand the jets on hr680vs?
 

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If it ain't broke don't fix it. That carb was tuned to flow the correct amount of fuel with that CFM rating.

The carb is the perfect size for your combo so I would try it out first. Then tune it only if it doesn't mix the correct ratio of gas to air (for which you would need an air fuel gauge and a wide band oxygen sensor in your header's collector). Unless you can read plugs and take it to the track to tune it (for maximum mph).

Big Dave
 

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Discussion Starter #6
Oh for sure. I don't intend on changing anything if it runs good. Reading on Quickfuel website HR series is intended for cam duration of 220 or under, dual plane, headers and less than 2000 stall. In contrast the speed demon I took off was duration 220-240.

I found the jet sizes (hard to find) and primary 70 secondary 74. This carb should work well as it is so I anticipate some good luck. Thx for the help.
 

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You are good, as they are only concerned with he intake duration. Though exhaust duration can also affect overlap the effects are not as pronounced. The vacuum signal o the carb is what Quick Fuel is talking about. With a long exhaust duration the only down side is you are pumping raw gas out the exhaust pipe because the exhaust valve is left open longer.

Larger Dave
 

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I boils own to this
u match up all the go components.. carb cam , dynamic compression ratio, vally cover , header design, final gear ratios and torque convertor to what u want out of the engine/ preferably for how it will be used for 90% of the time
The dial it in to establish the tuning specs.

u end up with a milder engine than intended but one that out performs other with wilder cams, carbs added on basically as bragg factor.

If u then take that engine, thinking a bigger cam , or carb what ever will give it a bit more, u 'break' that nice balance and will either do as well at more discomfort IF it is re dialed in , but mostly likely drop off in performance

If u want a bit more out of the engine.. dial it correctly in for that potential to be realised.. spend the money on a data logger...sit down do a bit of reading.

What Dave is saying above is spot on stuff...I know this because I have messed with custom designed cam profiles, converters .. overlaps lifts rate of lift durations... althu my current cam is close, it does still have a smidgen too much lift, and overlap.. but because dialed in it doesnt warrant the capital expense to resign, cut and install a new one...I still run quicker at lower rpms, cruise at far better economy than others with brag right carbs cams ign systems etc.
 

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Discussion Starter #9
I know my combo is pretty common and that's fine. Better than stock, reliable and fun works.
I was just afraid of having to do a bigger cam if carb was too big. After learning about jetting I feel ok if tweaking has to be done but I think it will ok now. I'll know in two days for certain.
 

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Discussion Starter #10
Two thumbs up on the Quickfuel! I had to go back to the 1 inch spacer as it's shorter than a Demon. it fired up with minimal tweak and ran very smooth. I replaced plug, cap and wires with the change. Plugs were fouled on one whole side for some reason. To fit the air filter... Drop base k&n 1.25, 2.8 inch element and k&n Xtreme lid. All fit and ready to roll.
 
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