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Discussion Starter · #1 ·
Have a 6-71 on a 383 with Track 1 heads and 9.o compression, 8 psi boost, 32 degrees timing, 2-750 QFT carbs, mild cam, 1 degree timing out per pound of boost. Originally dyno'd @ just under 600HP running Holley 650 blower carbs with choke horns. Tried dialing in the new carbs just for a good baseline idle but noticed when you hide some light from the coated headers they were glowing a dim red. Richened it up a 1/4 turn on all the idle screws, started idling ruff and running rich. The idle screws started out 3/4 out from bottom with a good idle to 1 turn with a ruff rich idle. How sensitive are these blowers to tune at idle? Is the dim header glow from being too lean? The QFT carb have 74/84 jetting with power valves in the primary's and 31 high speed air bleeds. How close will these "out-of-the-box" carbs be? They seem very responsive but sure seem very sensitive as they huff a pretty rich mixture out the tail pipes when you snap the throttle up to 4500. They only have 30cc accelerator pump compared to the 50cc's that used to be on the 650's. Also have a Boost cooler that needs dialed in. Boost Cooler claims the boost AFR needs leaned out to 12.0 in order for the cooler to work right and make power with the added timing you are suppose to be able to add back in. Any tips would be appreciated.
 

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Have a 6-71 on a 383 with Track 1 heads and 9.o compression, 8 psi boost, 32 degrees timing, 2-750 QFT carbs, mild cam, 1 degree timing out per pound of boost. Originally dyno'd @ just under 600HP running Holley 650 blower carbs with choke horns. Tried dialing in the new carbs just for a good baseline idle but noticed when you hide some light from the coated headers they were glowing a dim red. Richened it up a 1/4 turn on all the idle screws, started idling ruff and running rich. The idle screws started out 3/4 out from bottom with a good idle to 1 turn with a ruff rich idle. How sensitive are these blowers to tune at idle? Is the dim header glow from being too lean? The QFT carb have 74/84 jetting with power valves in the primary's and 31 high speed air bleeds. How close will these "out-of-the-box" carbs be? They seem very responsive but sure seem very sensitive as they huff a pretty rich mixture out the tail pipes when you snap the throttle up to 4500. They only have 30cc accelerator pump compared to the 50cc's that used to be on the 650's. Also have a Boost cooler that needs dialed in. Boost Cooler claims the boost AFR needs leaned out to 12.0 in order for the cooler to work right and make power with the added timing you are suppose to be able to add back in. Any tips would be appreciated.
I have been to the dyno 5 times with mine and friends SBC-400 or larger and made over 100 pulls....First one of the best carbs and holley 3310 vacuum opening change them over to secondary jets metering plates with #76 jets,leave the front at #70,put #35 shooters in,I made 748 Hp with a 248-260 street roller,,,,,,Holley blower carbs are REAL POWER LOSERS,We dropped the jets 8 sizes and picped up 60 hp.....Pro system blower carb are 40hp loser that can't be helped thru jetting..........The best out of the box where HP 950s...If I remenber right I ran 34 timing with 10# of boost on pump gas....If you pm me,I will look up in my records more info_Oldhead
 

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NOTE: THE ABOVE INFO HAS MISTAKES<USE THIS ONE>I have been to the dyno 5 times with mine and friends SBC-400 or larger and made over 100 pulls....First one of the best carbs and holley 3310 vacuum opening change them over to secondary jets metering plates with #76 jets,leave the front at #70,put #35 shooters in,I made 748 Hp with a 248-260 street roller on C16 fuel,running 14% overdrive ,,,,,,Holley blower carbs are REAL POWER LOSERS,We dropped the jets 8 sizes and picked up 60 hp.....Pro system blower carb are 40hp loser that can't be helped thru jetting..........The best out of the box were HP 950s...If I remenber right I ran 28 timing with 10% under drive on pump gas with 680 hp at 6700 rpm..If you take that retard box off you will go up in power..If you pm me,I will look up in my records more info_Oldhead PS I now run a Vorteck YSI blower on pump gas makes 809 hp at 14# of boost using 28 degs(No cooler) and over 1000hp when turned up on the same engine
Going to the dyno soon with 2 Dominators on a friends 421 SBC....I don't think they will do anything...Oldhead
 

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ANOTHER THING,About the only thing Red pipes mean is the engine is running......Lack of air blowing on them causes this ,so turn you idle screws back in....Oldhead
 

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do you have a boost vacuum gauge?adjust the idle screws to get more vacuum[leaner] the idle should smoothout.where did you get the boost reference for the carbs,in the intake right?
later in your tunning make sure the boost cooler line to activate the pump does not get pinched or it wont work.
you are sure you are in the idle cercuit?
it will just take some time to find what is going on.
 

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Have a 6-71 on a 383 with Track 1 heads and 9.o compression, 8 psi boost, 32 degrees timing, 2-750 QFT carbs, mild cam, 1 degree timing out per pound of boost. Originally dyno'd @ just under 600HP running Holley 650 blower carbs with choke horns. Tried dialing in the new carbs just for a good baseline idle but noticed when you hide some light from the coated headers they were glowing a dim red. Richened it up a 1/4 turn on all the idle screws, started idling ruff and running rich. The idle screws started out 3/4 out from bottom with a good idle to 1 turn with a ruff rich idle. How sensitive are these blowers to tune at idle? Is the dim header glow from being too lean? The QFT carb have 74/84 jetting with power valves in the primary's and 31 high speed air bleeds. How close will these "out-of-the-box" carbs be? They seem very responsive but sure seem very sensitive as they huff a pretty rich mixture out the tail pipes when you snap the throttle up to 4500. They only have 30cc accelerator pump compared to the 50cc's that used to be on the 650's. Also have a Boost cooler that needs dialed in. Boost Cooler claims the boost AFR needs leaned out to 12.0 in order for the cooler to work right and make power with the added timing you are suppose to be able to add back in. Any tips would be appreciated.

Not enough timing.
 

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Discussion Starter · #7 ·
The carbs have the boost/vacuum line for the power valves coming from the lower intake. This line also is tied in with the boost retard and Boost Cooler. I will go ahead and set the idle for best vacuum like I had it before, just never headers start to glow on me before. The guy wants to wrap them to help it keep the engine compartment cooler. I will be test driving it next week to see where the mixture looks to be. Just by the ruff guess I have now, it looks to be a jet or two too rich. When it was dyno'd, they said do not go any higher than 32 degrees for timing since compression was a little high at 9.0. we'll work on getting more timing back in it when we work with the boost cooler.
 

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My first blower motor was in 1993, as follows:

BBC 462
9.5:1
B&M Powercharger 250
7 PSI
93 Octane pump gas

I used a pressure switch to enable a nitrous timing retard (MSD didn't have the 6BTM out back then), to retard 4 degrees of timing. Static timed (locked out) at 36 degrees.

Never had any detonation issues. I still think you don't have enough timing. Try backing the boost retard down to .5 degrees per psi and see how that works.
 
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