Have been told that when you run a supercharger you don't run high compression pistons, also that basicallly the motor is stock accept for teh supercharger ? How true is this and what doe the rest of the motor need to live with a supercharger ?
Forged pistons, low compression. Depending on how much power you plan on making, it goes up from there. I always recommend the best rods you can get. I don't think anything in an engine takes as much abuse as rods. 4-bolt mains are also a good idea You'll need very stiff valve springs, since now you have something above them actively trying to push them open (hence, "forced induction.") A keyway cut into the crank snout to keep the lower cog from slipping too. -Brad
cam overlap is also a commonly overlooked issue... the above info is definetly good info, but if you're only going with a small blower, 144, 177 baby blower, they are suitable for a good stocker motor too...but if the engine is weak anywhere, even a baby blower will find the weak spot... best said that it depends on what you're gonna run..and how much ya want to twist it.......
Almost every blown street motor I've seen has 8:1-8.75:1 static compression. And, the motor CANNOT be stock, especially if it is a SBC.
run them stock many times....but hey don't take my word for it...take Weiands..........http://www.holley.com/data/Products/Technical/199R10244-1.pdf scroll down to the Technical Information just about middle of the page..second paragraph..."will install on your motor with little or no modification..... and yeah, a sbc will live longer then ANY OTHER MOTOR with a baby blower on it,,,,,,,,,, it ain't braggin if ya back it up...............
Gee, I wish someone had told me that about 30 years ago when I started playing with GMC blowers.I have run several different 6-71 setups and a 4-71,street driving,some ocasional"horseplay".They have all been on basicly stock motors,with cast pistons and stock heads.They were all rebuilt by me in my garage with no fancy machine work.The 4-71 is driven at crank speed,but the 6-71 is underdriven slightly. Bear in mind we are not talking drag racing here,just traditional looking,effective power adders.
It depends on what you want out of it. I have a 327 with forged flattops stock bottom end but with all ARP hardware (main studs and rod bolts head bolts) humper heads,mild blower specific cam, rollertip rockers, 471 @ 6psi with many miles on it makes good power and gets 17MPG with 3.42 gears and muncie M20 its a good street combo. You only see boost when you are "on it" so most of the time its like a N/A motor. Thats why I like to run a bit more static comp ratio with a MSD boost retard box. As long as its tuned right you will be fine but if you lean them out or spark knock them you will hurt them no matter how built up they are.
ok,,getting a better picture of it now,,and was thinking drag motor So its not the just bolt on affair i've been lead to believe
All of what you've been told is true in this thread.......just the lines are a little shaded here.......... You can run a blower on a stock engine, IF, You don't overrev it, your milage is low so your clearances are still tight.....a few other things have to adressed......got to time it some. Blowers make the engine 'THINK' its bigger than what it is.....its taking in 3 or 4 times the fuel and air and burning it.......this puts strain on the crank, rods and pistons. 1. A good blower engine is gonna have these parts beefed up. 2. Heads......your intake is pressurized..... free flowing heads aren't as critical. The dyno shows gains in setups where there's some turbulance in the intake runners and valve shapes, adding to the atomization of the fual/air mixture. 3. Carbs....gotta have them to flow all the air flow, and they can't run lean or you'll melt down your pistons. Try to avoid backfires at all costs! 4. Reason for low compression is 1-heat,2.-pressure from the 7 or 10 psi of pressureized fuel/air mixture raises your compression pressure to unbelievable levels while the engine is putting out power. A stout 350 horse 350 Cu.In. SB Chevy can pound out over 600 Horses with a 6-71 on pump fuel with a 15% underdrive. And this is with 7.5 /8.0 to 1 compression. Higher compression can be used....but only under extreme racing conditions with appropiate intercooling, timing, yada, yada. When we run these on the funnycar engines......we're really turning these puppies up and putting out incredible boost pressures.......we're o'ringing heads to decks, running special drylube systems in the blowers, alot of times we've toasted or have blown pistons and valves apart making a run.....we tear down between every pass. On the street.....this isn't necessary.....GMC the makes the roots type blowers can run 100,000 miles plus on he old trucks as long as the maintenance was kept up............
A 500 hp blown small block engine is pretty much a street motor. Assuming you want to build a 350 you would want good heads, a steel crank, forged pistons about 8 or so compression, decent rods (not the best, just decent), etc. You won't need to spin it very fast, so pick a moderate cam. Probably need to make 8-10 psi boost.
Then you'll want to make it a bit more wild...add more cam, build the bottom end and valvetrain to hold up to 7000 rpm, and run some more boost.
Yes...main cap studs will help, steel crank, good Hi-Perf. rods, forged pistons. All blown engines that are built to run on the street.....limit your RPM to about 5000, 5500 max. Just a little more than that in a 600 Horse blower motor, you'll snap your rods like twigs! As your rpm goes up, several hundred RPM, you torsional and compression stess goes up exponentionally. The smaller the engine....the the less power that will be made. For a 283, it'll have to be a little hotter than the 350. Its not uncommon to get 750 to 875 horses out of stout blown streetable big block. I know of several guys that prowl the streets in Texas putting out well over 1000 horses on pump gas....and thats before they let it suck on the bottle! All these parts you can get used....as long as they're in good shape. Pistons, and usually rods....its best not to take a chance on them on a fairly hot motor....go with new parts. Cast pistons and cast rods.....limit you rpm to 4500 to 4800 occassionally. Forges piston, rods, cranks....5000 to 5500 all day long. With blower motors if your not making it by a certain rpm.....you spinning it another 500 to 1000 rpm ain't gonna matter. All the horses can be made up in carburation and drive pulley size.......
As your rpm goes up, several hundred RPM, you torsional and compression stess goes up exponentionally Not exactly. The difference in inertial tension between 5,500 RPM and 6,000 RPM is exponential: it goes up by 19% (6,000^2÷5,500^2). However, peak inertial load is just ATDC on overlap, and is reduced by blower pressure since vacuum adds to the tension on the piston. The compressive load does not change at all by RPM, and it's lower in amplitude than the same power developed by N/A methods.
You know what I mean dude........putting a big strain on your rods and pistons....when you get close to their limit.......justa sniff at a little higher rpm will cause them to fail. A normally aspirated engine doesn't have the load on the rods and pistons that a blower engine does........but on a big block Pontiac.........you dance on up to 5000. 5500 is pushing it. 6000 you got a perforiated oil pan. A blown Pontiac at 5000....OK.......5500....you got a grenade on your hands! LittleWings....just keep your RPM down..... better safe han sorry.
Run a chevy if you want to go over 5500, they aren't like pontiacs... Also the load on rods due to RPM and the load due to a blower are completely different...RPM tries to tear the big end apart, while a blower adds compressive load. There are different ways to get more power out of an engine. One is to increase RPM (adding a long duration cam, improving breathing), another is to add torque by increasing cylinder pressure (such as with a blower). If you start out with way too small of an engine, then you probably want to do both.
Yes....I agree if you start out with a small engine you gotta do both. But if you've got a 350 with a 6-71....if its done 1/2 way right.....your looking at 500 HP at 5000 with about 12% underdrive...........why go higher? I understand the dudes got a 283 that'll wind its ass off......thats not the point......he can go higher than 5500rpm,,,,,,safely....I was using the long stroke Pontiac as a example......dawm good engines......I've had 4- blown 400's, 2- blown Super Duty 455's, and 2 blown 421's....they're not speedsters on the rpm range......but great powerful engines. Heavy rods + long strokes with cast rods and cranks = low revs on Pontiacs. Yep, a chevy will go higher, and so will a ford.....
There are different ways to get more power out of an engine. One is to increase RPM (adding a long duration cam, improving breathing), Your supercharging a engine....your pressurizing your intake system to 6 psi or higher.........just a few degrees of duration is gonna give you all you want plus some...................long duration isn't what you want. Normally aspirated.............yes
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