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6-71 blower tuning

26K views 20 replies 8 participants last post by  lemonice 
#1 ·
Dears I recently build 496cu 8.5 cr on 114cc iron head with 6-71 long nose supercharger, dual Holley carburetors 750cfm hp,6AL MSD,coil MSD BLAST3 .
Since this the first time I deal with blower .how I can did the timing for this? It is the same like regular engine? Or different.
 
#14 ·
well actually I am using 93 octane.
Is it safe to assume that you're not living in USA? I can hear a French accent as I read what you're writing...

Just so you are aware, octane ratings are expressed differently in different countries. For example, throughout the Oceanic countries, 98 octane is roughly equivalent to 93 octane in USA.

How that relates in Europe is something worth checking. It'd be a shame to run mid to high 30s advance only to find you don't have the quality of fuel to support it
 
#5 ·
Blown big blocks on the street like high initial but do not require a ton of total.
I would shoot for 20* initial and a total of 32-34* all in by 2500 rpm.
You can run more total if you install a boost retard type ignition.Say 38* at no load cruise..but pulls out a degree of advance for every 1lb of boost.
For example..if this combo could muster up 5 lbs of boost..timing would lay back from 38* to 33* with spark retard.
As for the pulley combo..how well the engine moves air thru it will dictate stacked up pressure in the manifold.You will need a vac/boost gauge installed below the blower and monitor it at wot/max rpm.
As big as that 6-71 is..its not ideal atop a free breathing 496.The 8 series would be better suited..buts lets work with what you have.
If the underdriven 6-71 suits your needs..dial in the tune up and enjoy the few lbs of boost she offers.When that gets old,flip the pulleys and run 10 over.Boost pressure will not be the issue..intake charge temp will be.
if you can stay out of detonation on pump swill,fine.If not..water/meth injection is an option as well as an e-85 conversion.
And yes..boost referanced primary power valves is a wise choice for a blown street unit.There are times when the the engine requires additional enrichment to the main well..but the blower is still pulling x# of inches of vacuum on the carb base.External referancing allows the power valves to operate as per design.
 
#11 ·
+1. George gives good advice on the timing. you are at 10% under drive which will only give you ~4.4 lbs boost with a 6-71 on a 496. Flip the pulleys around for 10% overdrive and make ~8.6 lbs boost. I wouldnt overdrive the 6-71 much more than that on pump gas. If you want more boost, than that, you should go to an 8-71 on that 496.

mike
 
#7 ·
:( I installed the boost gauge and I set up the timing starting from 32 to 36 and I test the car every change of timing but it doesn’t show much different only the sound of the blower are increased and the gauge boost doesn’t move at all the RPM times and the car test like any car with stroke it look like that the blower not working at all because there is no that much power like any car with blower.
I really don't know what the reasons ,but I am thinking of the pulleys teeth numbers.
And how about if I want 8 PSI for example what is the pulleys teeth# I have to install? Or there is other than that part I have to change.:confused:
 
#16 · (Edited)
:( I installed the boost gauge and I set up the timing starting from 32 to 36 and I test the car every change of timing but it doesn’t show much different only the sound of the blower are increased and the gauge boost doesn’t move at all the RPM times and the car test like any car with stroke it look like that the blower not working at all because there is no that much power like any car with blower.
I really don't know what the reasons ,but I am thinking of the pulleys teeth numbers.
And how about if I want 8 PSI for example what is the pulleys teeth# I have to install? Or there is other than that part I have to change.:confused:
Ideally..you want a dual purpose vac/boost gauge installed in the manifold below the blower.
Typically at idle rpm and no load cruise..you will see inches of vacuum,not boost.
A roots blower will not show boost on a gauge at idle rpm,or no load cruise with minimal throttle angle.You need to drive the car under load at wide open throttle to measure max boost pressure.
I would think you could expect 4 lbs of boost with a 10 under 6-71 atop 496 cid.Possibly 9 lbs if you swapped the pulleys around.
Don;t look at 9 lbs as being a good thing..thats an artificial 1.5 lbs of boost,brought on by superheated intake charge.Good example of why you want to run a bigger blower at reduced speed for a given boost target.
Not doubting you..but I have a hard time imagining a 6-71 not registering some semblance of positive manifold pressure under load during a road test.
Are you certain you did not install a vacuum gauge only under the blower?
And if it is a boost gauge only..do not look for it to read lbs of boost with out some engine load/throttle angle atop 496 cubes.
 
#13 ·
Doesnt matter if it is a solid cam. People have different opinions about locking out the timing on a street car. You will get recommendations for doing it and for not doing it. I like a little bit of a timing curve for the street, especially since I have manual overdrive and sometimes lug the engine a little. Nevertheless, a blower engine prefers to have less of a timing curve than an NA motor, and as George points out, they really like a lot of initial timing.

mike
 
#15 ·
I am living in KSA and we have fuel 91 & 95 octane which 95 Octane is equal to 93 fuel in US,
And you know I test the car by C12 fuel race it show that only faster respond of the throttle but stall the performance of the engine not that much improving.
Mike I'll flip the pulleys to be overdrive for more PSI and I’ll give you the status.

Salman
 
#18 · (Edited)
Not suprised the engine coolant temp rose at wide open throttle when you swapped from 10 under to 10 over with no other modifications.
The additional boost has led to higher cyl temps that are now being absorbed by your cooling system.Normal and no need for fear as long as coolant temps are not exceeding 210* F.
The intake charge temp also rose with the pulley swap.You;ll need a thermo sensor in the blower manifold to register this.
Not all boost is created equal.Two identical engines operating at 10 lbs of boost..but one having a lower intake charge temp..will NOT produce the same power.The engine with the lower charge temp will come out on top.
Your ok though with only 10 lbs.Sounds like the 496 is not a heavy breather that truly needs an 8 series at this point.
Down the road if you need more power..consider some form of intercooling.Chemical would be your best bet..beit water/meth injection or e-85 fuel.
You could pulley an 8-71 slower..make the same boost (@ 10 psi) and drop the charge temp significantly = more hp.
Fwiw..I always thought a 1-1 6-71 was ideal atop a 427 bbc.
When your crowding 500 cubes..the 8 series is preferred at the same drive ratio.
 
#19 ·
Just for reference and by no means for the street;


A dual stripped, 8-71 on a 500ci. alcohol engine with very good heads, ie."Big Chief", will push around 34lbs. of boost,@ 47%over, and 7400rpms.,while using a gal. of fuel per second,with the thingy on the floor....if you can keep the wick lit.

A retrofit or high helix 8-71 will push the same amount of boost around 34%over.But, that blower adds a couple of grand to a regular 8.

You will never gain more power at any setting than you will, from the initial underdrive to the same overdrive. As in 10% under to 10% over. Mainly because of the power it takes to turn the blower and the heat it creates, by turning it faster.
I've been told, it takes over 300hp. to turn a 16-71, 37% over @ 8000engine rpms. The blower is spinning close to 11,500 rpms.
 
#20 ·
Just for reference and by no means for the street;


A dual stripped, 8-71 on a 500ci. alcohol engine with very good heads, ie."Big Chief", will push around 34lbs. of boost,@ 47%over, and 7400rpms.,while using a gal. of fuel per second,with the thingy on the floor....if you can keep the wick lit.

A retrofit or high helix 8-71 will push the same amount of boost around 34%over.But, that blower adds a couple of grand to a regular 8.

You will never gain more power at any setting than you will, from the initial underdrive to the same overdrive. As in 10% under to 10% over. Mainly because of the power it takes to turn the blower and the heat it creates, by turning it faster.
I've been told, it takes over 300hp. to turn a 16-71, 37% over @ 8000engine rpms. The blower is spinning close to 11,500 rpms.
You summed it up best.Just for referance and by no means for the street.

There is a point of diminshing return with an overdriven undersized roots on the street..but I thought that was already covered.;)
 
#21 ·
ok, i have just been reading all of this and absorbing it. i to have a street car that i am putting the 671 blower back on. i removed it first because i never could get it to start very good after it sat for a week or a few days. would always foul the plugs, and i would have to go buy another set to fire it off. so first may i ask what plug would be a great plug for my application. i live in wa state,motor is a 8.25 to 1 496. eldebrock performer rpm heads,671 weiand blower, 2 pro-systems 780 carbs blower cal. msd ready to run distributor,
there is no choke horns. also its hard to read my timing light. is that because of the msd dist.
i just got it all back on now and about to try to start it tomorrow. ill try what i read here with the timming at 32-34 and int of 20.
do i need the vaccum lines to the carbs from dist.
 
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