Camshaft question - guys with 6-71 blower experience

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My427stang
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Camshaft question - guys with 6-71 blower experience

Post by My427stang »

Background - I just finished a 347 Ford with a centrifugal supercharger, I considered it mildly cammed for the peak I wanted, but it just kept climbing as the blower fed it. We ended the pull at 580 HP or so at 5900, it still had more in it, but with only a 215@-050 lobe I didn't expect that. We didn't find the top end as although it was a strong bottom end, it exceeded all the goals and we decided to give the stock block a limit, both on the dyno and in the EFI controller as a rev limiter. Heads flowed about 280 and intake was a ported Vic Jr with equalized runners at about 315 cfm each. Used a Sniper EFI in nan enclosed Vortech top

Current build - 355 Chevy, good parts, older RHS heads flowing about 290, but using a 6-71 blower and big carbs, old school induction, so very different, currently set up as 11% underdriven. This owner doesn't mind a little more RPM, but it also is built for a street car, 4 speed, 3.55s or 3.73s (TBD).

My question is: Will a positive displacement blower like a 6-71 extend the hp peak higher in rpm like the centrifugal did? I anticipate it will build boost sooner, but how does it behave up top in terms of changing the peak location (in general terms). My initial plan was a hyd roller, 229@.050 intake, 243 @ .050 exhaust, .550-ish lift, 67 degrees overlap.

After seeing the peaks on the first blower motor, wondering the rules of thumb guys may use for the 6-71.

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Re: Camshaft question - guys with 6-71 blower experience

Post by Orr89rocz »

I dont own one but tuned one centrifugal car and been in/driven positive displacement blower cars. I have friends that have used both often. It does seem centri build boost with rpm so as boost keeps increasing rpm peak is pushed out further. The positive displacement stuff seems to be instant boost and holds more consistent. A little bit of boost increase with rpm but doesnt seem to be as drastic as centri.
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Re: Camshaft question - guys with 6-71 blower experience

Post by rebelrouser »

I have a mopar small block on the street with a procharger and it acts like you describe. My cam is 218 @.050 on the intake and 224 @ .05 on the exhaust .462 and .470 lift 110 centerline. When I pulled it on the dyno at 6,000 rpm it made 550 HP with 7 lbs of boost. which was the peak HP, did not want to float the hydraulic lifters so did not pull it harder. It has a set of the Indy X cast iron heads so they flow pretty good. The electronic EFI system was rated at 650 HP It also get good MPG if you keep your foot out of it. I think from memory the torque peaked at 4,500 rpm
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Re: Camshaft question - guys with 6-71 blower experience

Post by gordon »

On a Roots blower it will build boost like right now and carry it in a linear fashion until about 9000 rotor rpm. This will vary a bit depending on weather it is set up with sealing strips or not and if it is a standard helix or hi helix . Also the newer blowers with altered inlet and exit openings will affect the boost curve greatly.
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Re: Camshaft question - guys with 6-71 blower experience

Post by CamKing »

Like said above, the 6-71 will come in right away, and hold the boost, so you want to run a bigger cam, to make the engine more efficient at the higher RPM's The larger duration will make it less efficient at low RPM's, but because it gets up on boost so quick, you won't notice.
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Re: Camshaft question - guys with 6-71 blower experience

Post by mag2555 »

One issue to keep in mind with a 6-71 that I almost found out the hard way before I had a boost gauge in the car is how fast you need to have more Intake valve spring pressure .

With a 2.02" Intake every 2 psi of boost will lob off just about 6 psi of seat pressure.
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Re: Camshaft question - guys with 6-71 blower experience

Post by F-BIRD'88 »

Just roughly looking at it with the limited information
I'd say that with the generous port flow / CID and the generous extended camshaft exhaust duration that this one is going to want to make peak horsepower well up there in rpm.
The generous exhaust valve events allow the engine to keep breathing to make top end horsepower.
It may well be able to RPM past the RPM limitation of typical street oriented Hyd Roller lifter valvetrains.

From testing you'll have to make rpm limit practical decisions on that.
I tend to bypass/ avoid the whole HYD Roller limitation "rabbit hole" thingie on roots boosted engines.
Go right for either SFT or (street) Solid roller setups.
Its just what I do.

Some people that do end up coming up against a hard rpm performance limit with HYD rollers end up swapping for sold roller lifters on their "Power Adder" engines.

You sure don't want the valves bouncing around at rpm when under boost or on nitrous. It gets ugly.

I think this one is going to want to rev up.
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Re: Camshaft question - guys with 6-71 blower experience

Post by My427stang »

Thanks guys, I was out of town and couldn't respond.

I am likely pulling back on the cam, I think my peak is going to be above the intended use of the engine, didn't order a cam yet so I can make any changes I need to. CamKing, your advice is good and understood, but although it's a strong bottom end, I hate to leave it in the hands of an owner that will take it to the top

When planning this out, I mistakenly thought a Roots came on quick but petered off, while a centrifugal kept climbing. That being sid, since this is a small engine with plenty of blower, it makes sense it'd carry high

I also agree that hyd rollers would be tough if I continued as planned, but I am going to try to get the peak down a bit, with light parts and watching the lobes I have had some very good luck letting the HRs spin, at least at the levels I am looking at here
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Re: Camshaft question - guys with 6-71 blower experience

Post by blown265 »

On engines where we've added a positive displacement supercharger to an existing good naturally aspirated street/strip combination, the rpm peaks and curves havent changed greatly- no more than a few hundred rpm.
Power increased, but the rpm ranges remained much the same.

Reconfiguring the NA engine to suit the blower (mostly compression and cam events) didnt change the situation a lot either. More power again, but not revs.

Changes to headflow and longer cam durations DID increase the torque and horsepower rpm peaks higher, and had a much larger effect than (only) adding the PD SC.

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Paulie
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