different blower sizes and pulleys

ADRENLN · Aug 3, 2007

i was wondering about boost levels in a given situation.

say you have 2 different size blowers such as a regular vortech and the novi 2000.

if you put each blower on the same car with the same mods with the same size blower pulley, would one make more boost then the other?

are the blowers themselves built differently, spec wise in anyway that would effect boost levels in a different mannor with the same size blower pulley?

thanks for any info...this was a bit hard to explain what i mean. i hope you guys understand.

Gearbanger 101 · Aug 3, 2007

Yes, different blowers will make different boost levels based on pulley size. Larger blowers will see more boost, with the same size pulley, since they're moving more air, per revolution than a smaller blower and visa versa.

Stan Weiss · Aug 3, 2007

Remember there is more to it than just boost numbers. There is blower efficiency and compressed air outlet temperature to look at also.

ADRENLN · Aug 4, 2007

ok so blower size will effect boost levels givin the same size pully, but do blower efficiency and compressed air outlet temperature effect boost with the same size pulley or are you saying that they just effect the overall blower performance in a different mannor?

Gearbanger 101 · Aug 4, 2007

ADRENLN said:
ok so blower size will effect boost levels givin the same size pully, but do blower efficiency and compressed air outlet temperature effect boost with the same size pulley or are you saying that they just effect the overall blower performance in a different mannor?

Generally, larger compressors are more efficient on the big end and visa versa for the smaller blower. The smaller compressors generally reach their adiabatic efficiency rating a lower air flow levels. This is nice for an entry level street car, since this also means cooler ACT's down low in your general driving range and less horsepower needed to drive the unit. The draw back to running the smaller blower is that since it's physically smaller in it’s capacity, it needs to spin measurably faster (this is done by changing the pulley ratio) in order to reach the higher volume airflow levels that the larger compressor would be able to see with the larger pulley. In order to change the pulley ratio, you need to decrease the diameter of the blower pulley (which can often result in belt slip), or increase the diameter of the crank pulley (which would also increase the speed of your other accessories). Once you’ve succeeded in changing the ratio and increase the rotation speed of the blower, you now have to worry about elevated ACT's because of the increase blower RPM. Once you've reached the blowers adiabatic efficiency rating, charge temperatures get harder to control. Hotter temperatures require the computer to pull timing to keep everything happy up top and your engine together. The result….power drops off rapidly at the top end.

The larger compressor on the other hand doesn’t need to turn near as fast in order to see the same boost level across the board. They're often rated on the high end of the efficiency scale. Meaning that the ratio of over all volume of air produced, in comparison to the charge temperature increase is more desirable the faster they’re spun. This sounds great and you'd wonder why anyone would want a smaller blower after hearing this. The problem is, is that first off, the larger blower does eat up a little more horsepower than the smaller blower under general driving conditions. This may not be a big deal for some, since power can be made up in other ways. Second problem, is that since their adiabatic efficiency rating is that much higher, running them below that level for any length of time often results in elevated ACT's as well. This leads to doggy low and part throttle performance on the street. Big blowers love to be run hard!!! But running them in a car that’s not going to take advantage of their capabilities is like using a Transport Tractor to tow your utility trailer. What’s the point when the family station wag will do the job just as well.

Stan Weiss · Aug 5, 2007

If you are running 10 psi of boost at 29.92 inches of mercury (barometric pressure) you have a pressure ratio of 1.68. You have this pressure ratio no matter what your compressed air temperature is. If the air temperature is 79 and your compressed air temperature is 175.5 you have a density ratio of 1.425 but if your compressed air temperature is 135.5 you have a density ratio of 1.521. That is a 6.74% increase in air density at the same boost pressure. Remember it is how much oxygen you can get into the engine that counts.

ADRENLN · Aug 5, 2007

very intereseting and information here. thank you guys for your replys. i did go with the bigger blower...the novi 2000. i hope it dont feel to doggy on the street during normal driving. i will let you guys know.

hotmustang331 · Aug 5, 2007

Do the blower companys release compressor maps like what we use for turbo compressors? That would give you a good idea as to what would work....just then have to learn how to read them . But its not too hard.

This is where I learned how to really read a compressor map....and centri types should be the same way.

http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html

Gearbanger 101 · Aug 5, 2007

ADRENLN said:
very intereseting and information here. thank you guys for your replys. i did go with the bigger blower...the novi 2000. i hope it dont feel to doggy on the street during normal driving. i will let you guys know.

It's usually more apparent with the Positive Displacement style units. Perhaps "doggy" was too strong of a word. Just a little less "snappy" in the lower boost ranges. To really take advangate of the big-uns, you want to be pushing them through their paces....not doddling around in traffic.

the98stang · Aug 5, 2007

i bet 4.10's would go nicely with that novi2000....hehe

showstopper2 · Aug 5, 2007

you may find the Novi to be lazy in the lower RPM range... centri's build boost according to RPM so in order to feel the power the RPMs need to be up. not often do you get to run WOT everywhere your driving so the power will be according to what RPM your driving at.. in the lower range my car doesn't even feel like its blown but as soon as I get it up to the 3500rpm range and above it starts pulling like a freight train!

ADRENLN · Aug 6, 2007

showstopper what blower do you have? also what pulley are you useing? i hear that the smaller pulley you use on the blower the sooner or lower in the rpm ranch the boost will kick in.

i dont know if this is true. i hope so because i stepped down to a 3.0 pulley size. ill see how that does. i would really like not to feel sluggish in the around town driving as that is what everyone does most of.

that is one reason i chose a vt stroker block to go with this set up. it comes at 9:2.1 compression but my tuner says he dont feel comfortable with that compression and would like it a little lower so i could run higher boost.

he wants to go with thicker head gaskets to bring the compression down to 8:9 or something like that.

however, i dont want him dropping compression down to where the car feels sluggish and has no bottom end. i didnt pay extra for the stroker so he could suck away power!!!!!!

what do you guys think?

Gearbanger 101 · Aug 6, 2007

ADRENLN said:
it comes at 9:2.1 compression but my tuner says he dont feel comfortable with that compression and would like it a little lower so i could run higher boost.

he wants to go with thicker head gaskets to bring the compression down to 8:9 or something like that.

however, i dont want him dropping compression down to where the car feels sluggish and has no bottom end. i didnt pay extra for the stroker so he could suck away power!!!!!!

what do you guys think?

Meh....9.2:1 isn't so bad. I know guys running higher boost levels through built engines without any worry. The difference between 8.9 and 9.1 is negligable and running an excessivley thick head gasket will probably shorten your engine life quicker than a slightly higher compression ratio. If you really want to take advantage of it while keeping things together, spray it with some water methanol to cool the charge and add octane to your fuel. You'll then not only be able to run additional timing, but increase the safety factor as well.

What kind of horsepower are you shooting for?

ADRENLN · Aug 6, 2007

not sure if the 3.0 pulley will put me where i want to be. in the end id like to be around 550-600 rwhp.

however, i think with this 3.0 pulley and no boost pipe when things first get installed, id like to break 500.....i guess we'll see.

yea, i dont mind the higher compression. vt said i could run 15psi on it with no problem. they actually said they perfer the compression that high on a centri blower because of the lack of bottom end with that blower.

id like to keep that compression, but i guess he is thinking about the future, and if i want to raise the novi boost high some day. i dont know. if there isnt a significant differance between the 2 compression rations mentioned, then i guess it dont really matter. he did mention wanting it lower, but i dont think im going for that though. especially now that you told me it could shorten the enging life.

Stan Weiss · Aug 6, 2007

Gearbanger 101 said:
running an excessivley thick head gasket will probably shorten your engine life quicker than a slightly higher compression ratio.

What would be an excessivley thick head gasket that would shorten engine life?

Gearbanger 101 · Aug 6, 2007

I've heard stories of the Modular Aluminum heads "shifting" when used with thick gaskets. Also from what I'm told (although I'm not sure how much stock to put into it) that cam timing is effected if the heads are raised or decked to any great extent. In order to increase a modular engines relatively small combustion chamber enough to decrease compression nearly half a point, I could imagine a fairly thick gasket would need to be used...if one even exists.

Were it my set up, and I absolutely had to drop the compression a little, I'd probably grind down the emissions masks in the combustion chambers rather than tear down a perfectly good stroker engine. That'll increase chamber volume a little and allow a little more timing as well. As far as what gaskets I'd suggest, I've got a set of the multi-layer steel gaskets in my engine. I'm told they're better suited for the use of the aluminum heads on the iron block because of the different expansion rates of the two metals.

Stan Weiss · Aug 6, 2007

Gearbanger 101 said:
I've heard stories of the Modular Aluminum heads "shifting" when used with thick gaskets. Also from what I'm told (although I'm not sure how much stock to put into it) that cam timing is effected if the heads are raised or decked to any great extent. In order to increase a modular engines relatively small combustion chamber enough to decrease compression nearly half a point, I could imagine a fairly thick gasket would need to be used...if one even exists.

Were it my set up, and I absolutely had to drop the compression a little, I'd probably grind down the emissions masks in the combustion chambers rather than tear down a perfectly good stroker engine. That'll increase chamber volume a little and allow a little more timing as well. As far as what gaskets I'd suggest, I've got a set of the multi-layer steel gaskets in my engine. I'm told they're better suited for the use of the aluminum heads on the iron block because of the different expansion rates of the two metals.

Thanks. As the heads is raised or lowered it will change the cam timing unless they are redegreed. In a 281 ci engine to go from 9.2:1 to 8.9:1 you need to add 2.66 cc's in volume. In the stroker engine this amount will increase.

ADRENLN · Aug 6, 2007

wow, thanks guys! im not sure what to think at this point to tell you the truth. 2.66 cc plus for the stroker. hmmm, that isnt too bad is it? is that think enough to cause problems?

should i just absolutely tell him i dont want the compression lowered at all? i dont want to cause any problems. i also want him to be comfortable tuning what he is used to as well. what do you guys think is the best way to go.

i dont really want to mess with the combustion chamber.

Gearbanger 101 · Aug 6, 2007

ADRENLN said:
i dont really want to mess with the combustion chamber.

If you really insist on dropping the CR for the least amount of money, taking off those emissions masks would be the easiest and cheapest method of doing it. You'll have to measure the volume of the chambers, but assuming Stan's math is correct, then taking off those bumps might just be enough to bring it down to your tuners liking.

As long as you (or your engine builder) are careful with the dremel, you should even be able to do it without disassembling the heads. Just keep the filings out of the cam valley as much as you can and wash the heads in Varasol and blow them off with compressed air when you’re done. Not only that...but it'll save you a ton of labor compared to tearing apart the whole bottom end again. The downside to doing this will be a slight drop in low speed efficiency, which will probably make your car a little harder on gas around town, but then with the ability to add a couple degree's more engine timing up top without the need to increase the octane of your fuel, you'll see a nice boost in power in the mid and upper ranges.

All this is of course assuming you really think it’s necessary to drop the compression. I mean….what is it that you’re wanting to do, make big power, or run big boost. With the blower and engine combination you’ve got, and some decent form of aftercooling you probably won’t have any trouble at all making big numbers without pushing the engine all that hard. I don’t think 500rwhp on pump gas is out of the realm of possibilities with your current compression ratio. Much more than that and you’re going to really have to start addressing other area’s (fuel, ignition, drivetrain).

...just something to think about.

ADRENLN · Aug 7, 2007

thanks gearbanger. i am wanting big power over big boost. however, it will probably take decent boost to get the numbers i want. this will all have to be done on pump gas...its a street car for the most part.

i do have the paxton intercooler going in, with 60# injectors, and cobra duels with a bap on the 8 rib set up. that is pretty much the total set up for now.

i do want over 500rwhp. i figure i might as well take a little advantage of buying the bigger blower and intercooler set up. i dont think the compression will really bother anything if i dont change it. people seem to think i could run 15psi at 9:2.1 compression on pump gas.

its just my tuner that wants it a little lower for some reason. if i do have to lower it, i would just do a slightly thicker head gasket as it seems to me the easiest. theres no way the block is getting torn down. i ordered the stroker shortblock from vt so they can build it perfectly and i would not have to deal with that part of the build. i just dont want to damage anything with the head gasket or have a sluggish bottom end from lower compression.

different blower sizes and pulleys

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