6-71 blower on a 340

hi everyone

can I run a 6-71 blower on a 340 with about 10-1 compresstion?

Run a cam with a tight LSA to bleed off some boost and having some overlap will keep the combustion chambers cool. Of course limit the total timing.

If you want to run very much boost go ahead and drop your compression ratio. Unless of course you're gonna run race gas or inner cooler or something along that line.

Your going to get about 7 lbs of boost running 1 to 1 pulleys. That is going to be a problem with that much compression. If you go with a meth injection system you may be able to get away with it. Otherwise I think it is going to be race gas or retard the timing way back.

Jack

I'm curious about this too. I have access to some free 100LL aviation gas. I've been half tempted to drop a 6-71 and run the 100LL and just see what happens this summer. One can dream!

Keep in mind avgas will damage O2 sensors

Coyote Jack said:

Your going to get about 7 lbs of boost running 1 to 1 pulleys. That is going to be a problem with that much compression. If you go with a meth injection system you may be able to get away with it. Otherwise I think it is going to be race gas or retard the timing way back.

Jack

Click to expand...

Hi there, Are you sure about this?

I have never run a 6/71 first hand but know they displace approx 411" every revolution (360 crank degrees)
a 340 mopar engine motor inhales 340" every *second* revolution. (or 720 crank degrees)

Therefore hypothetically it be would trying to cram 822" into a 340" motor at 1:1 drive ratio.

Now to relate this to "boost" pressure.
A good hypothetical for figuring out the maths involved is to use Atmospheric pressure at sea level, which is approx 14.7psi from memory. It's this pressure that causes air to fill your cylinders when naturally aspirated.

Adding "boost" increases this pressure, adding an additional 14.7 PSI of boost would DOUBLE an engine's power output. (everything else being equal. heat etc)

This would become 29.4PSI absolute pressure at sea level. (atmospheric pressure + boost pressure combined)

Now back to our 340 with a 6/71 and a 1:1 drive ratio

822" blower output divided by 340" engine input = a ratio of 2.417

Now if we multiply our atmospheric pressure by 2.47 we get an absolute pressure of ~35

Boost pressure = absolute pressure minus atmospheric pressure.

35 - 14.7 = boost pressure of 20.3PSI at sea level.

But wait, we're not done yet!

Blowers aren't 100% effecient like that.

Let's be a bit realistic and consider that these blowers are not race units, a 10% reduction in volumetric efficiency to compensate for leakage past the seals. Bringing it to a factor of 0.90 (90% efficiency)

20.3 x .90 = 18.27 PSI boost at 1:1 drive ratio

As with everything, real world scenarios are subject to a lot of variables relating to engine configuration and efficiencies.

Here's a chart I found for those who hate maths. It suggests that a 340 at 1:1 would be about 16.5PSI
So it's good to know I wasn't too far off!

It is all good to quote the charts. They are a good basis to start from. The problem is they are not real life. There are a lot of factors when it comes to measuring real boost. Two of the more important are the cam profile and the flow ability of all the parts of your engine. Remember, when we measure boost we are just measuring the amount of resistance your engine is giving to the flow of air. If there is no resistance, even a 16-71 won't produce much boost.

My engine is a 340, +0.030 with heads that have lots of porting done and a healthy cam. When I run around the street, most of the time my top pulley is a 37 tooth and the bottom is a 33 tooth. That gives me a .89%+/- underdrive and my boost gauge tells me I see 6 lbs of boost. That is the reason I say at 1 to 1 my 340 will see about 7 lbs of boost. Another 340 configured differently will see a change in boost.

Hope this helps to explain my original statement.

Jack

Coyote Jack said:

It is all good to quote the charts. They are a good basis to start from. The problem is they are not real life. There are a lot of factors when it comes to measuring real boost. Two of the more important are the cam profile and the flow ability of all the parts of your engine. Remember, when we measure boost we are just measuring the amount of resistance your engine is giving to the flow of air. If there is no resistance, even a 16-71 won't produce much boost.

My engine is a 340, +0.030 with heads that have lots of porting done and a healthy cam. When I run around the street, most of the time my top pulley is a 37 tooth and the bottom is a 33 tooth. That gives me a .89%+/- underdrive and my boost gauge tells me I see 6 lbs of boost. That is the reason I say at 1 to 1 my 340 will see about 7 lbs of boost. Another 340 configured differently will see a change in boost.

Hope this helps to explain my original statement.

Jack

Click to expand...

I agree about charts being a good start. There's no hard and fast rule but there are guidelines.
These charts are put out by the manufacturers of the superchargers, such as hollley (weiand), dyers, littlefield etc. So they are indicative of typical results.

Your motor is way way... way outside of those guidelines. I've never known anyone to drop from 17psi to 7psi from a cam and some port work. so your results seem atypical to me.

I layed out all the basic maths involved in determining an approximate boost level to confirm it's relative accuracy within a pretty narrow margin. These equations can be applied to any positive displacement blower on any engine.

That being said, I would recommend anyone considering a blower swap contact a blower specialist/manufacturer when trying to determine drive ratios to suit their compression ratio, displacement etc.

thanks everyone!