compression ratio + pump gas=wtf

[alex12gt]

so im trying to decide on what comp ratio to get when i pick up my new motor options are 9.5 or 11.0 (only options on aluminator) my goal is around 700rwhp and possibly nitrous on top ive currently got a roush 2.3 but that may go to a 2.9 if needed and i plan on staying on 93 as e85 isnt an option.....

so whats the thoughts seems theres guys building motors but not
a whole lot of talk goes on about it

 

[F8L SN8K]

Pump gas + a positive displacement supercharger = lower compression ratio for more safety margin.

The positive displacement blower is nearly instant boost at any engine speed from 1,800rpm up. The dwell time at the lower engine speeds increase the potential for pre-ignition of the fuel.

Higher compression is good on the centrifugal blowers because of the lack of the low end cylinder pressure(in comparison to the PD). When a centrifugal blower gets up to its efficiency range the engine speed is much higher and the dwell time at Top Dead Center is limited allowing more of a safety margin against pre-ignition/Detonation. The higher compression also helps make up somewhat for a softer low and mid range power. (And a nitrous engine prefers higher compression also)

That being said the better of a naturally aspirated engine you build the better of a supercharged engine it will be. PROVIDED the Fuel can support it.

Since you said pump gas and Positive displacement blower drop the compression.

 

[alex12gt]

also would like to know if i drop cr does it use more fuel or less fuel per lb of boost....for example stock the fuel system with a bap can support around 700hp but if i drop the cr does that number change?

 

[F8L SN8K]

X amount of air requires X amount of fuel.

If you do not change pulleys or change air flow in any way it does not require any more or any less fuel.

You will lose power if you do nothing but reduce the compression ratio. Now using the formula that calculates how much fuel is required to support X amount of horsepower would show that you would need less fuel for less HP if the BSFC remained the same. However, your BSFC is not remaining the same it is actually going down because the engine is not as efficient with the lower compression.

Now to make the same power you will need to spin the blower harder or lessen the restriction in the system. If you lessen the restriction on the inlet side or speed up the blower it will cause the boost Pressure to rise. This will cause more pressure pushing against the injectors requiring more Fuel Pressure which restricts fuel flow from the fuel pump hence lowering what the pump can effectively support HP wise.

Now reducing restriction on the engine side will reduce boost pressure(remember boost pressure is just a measurement of resistance) there by allowing lower fuel pressure and hence more HP support it can flow.

Again though to make more power you either need to make it more efficient or by stuffing more air/fuel in.

 

[alex12gt]

so a set of blower cams should be able to bring the motor efficiency back up correct or at least make up for some of whats been lost.....or am i better off to just spin the blower harder? yeah spinning the blower harder will increase chances of heat soak but at the same time will save me $1700 that could purchase a fuel system

 

[F8L SN8K]

If you're going to make 700rwhp on pump gas you better make the engine as efficient as possible. You will run into a ceiling for cylinder pressure on pump gas and can only stuff so much into it.

 

[alex12gt]

any other thoughts on efficiency seems head port isnt exactly a large bang for the buck im wondering how good of results id see with the jpc regrinds over the comp cams i suppose i could always add a little meth in as well

 

[Bud]

I guess it depends on what you plan to use the car for. I don't street race and don't have a lot of ways to use 700hp on the street without getting in trouble. If I was in your shoes I'd do lower compression to be safe on 93 and just pulley the blower to a safe level. Then when it comes time for the track dump in race gas, pulley the blower down and let it have it. Best of both worlds with a quick pulley swap, may take the 2.9 to make that happen though. Also you will start running into belt slip once you start pulleying down a blower to make up for lower compression.

Not sure how I feel about meth for street duty, lots of people run it with success but it is just another possible point of failure.

I did 11 to 1 on my build but totally different circumstances (turbo, e85, and elevation kills me up here)

 

[alex12gt]

Yeah am thinking the 2.9 is pretty much going to be set in stone and meth and street use is fine of its tuned right and doesnt fully rely on the meth wonder what amount of boost it would take to make around 700 on the 9.5 without cams

 

[F8L SN8K]

Really tying your hands on making 700 rwhp while strapping it with the handicap of pump gas. Not saying it can't be done just makes it harder and more expensive. There are many options of many different ways.

I've never been a fan of Methanol injection myself.

Cams... The nice thing the re-grind option offers are they are lighter has the factory uses pressed on lobes not billet blanks like aftermarket.(you can have the aftermarket ones gun drilled to make up that). And they are cheaper. The down side is the shimming and some have issues with grinding down the base circle takes away some of the surface that was hardened from the Heat Treat process. Some early Modulars would develop hair line fractures. In my eyes Regrinds are a choice only when an aftermarket unit does not exist.

I agree that 700 rwhp is not mandatory all the time and it would be far cheaper just to strap a different pulley and dump some race gas in the tank and have at it. Depends all on how you plan to use it and what kind of budget you have to work with. You can make 700 with a TVS.

 

[alex12gt]

Maybe ill just go 9.5 and pulley the tvs up and when i feel like getting stupid through a 75-100 shot on
That way if and when i do decide to push up to 7-800+ i can go e85 and not worry about the limit of the cr

 

[Bud]

Maybe ill just go 9.5 and pulley the tvs up and when i feel like getting stupid through a 75-100 shot on
That way if and when i do decide to push up to 7-800+ i can go e85 and not worry about the limit of the cr

That sounds like a winner

 

[alex12gt]

And nitrous plus pd blower normally make some pretty nasty results and if im
Not spending cash on a 2.9 might just throw some cams in to help with the lower cr drop

 

[bigriff85]

You'll need to be making at least 20lbs and on a 6 rib that will never happen...get the 2.9 and be done with it or you'll be fighting with a bunch of parts and tuning headaches like me I'm on a 9.5:1, get the 11:1...14 psi on tvs made 575 with horrible belt slip all over ...11:1 and I'd be at 700 on less boost with no slip or minimal

 

[alex12gt]

You'll need to be making at least 20lbs and on a 6 rib that will never happen...get the 2.9 and be done with it or you'll be fighting with a bunch of parts and tuning headaches like me I'm on a 9.5:1, get the 11:1...14 psi on tvs made 575 with horrible belt slip all over ...11:1 and I'd be at 700 on less boost with no slip or minimal

yeah spoke with brenspeed and they put a 9.5:1 in and regret it im thinking may just stick with 11:1 just checking into all my options and i swear i saw eaton came out with a 3.3 tvs rotor pack but who knows how long before we see that

 

[atistang]

if building an engine for boost then lower compression is the way to go, plain and simple

1 psi of boost is more efficient power than .5 in/hg of vacuum


so giving up compression for more boost is the way to go in any situation

 

[D.T.R]

yeah spoke with brenspeed and they put a 9.5:1 in and regret it im thinking may just stick with 11:1 just checking into all my options and i swear i saw eaton came out with a 3.3 tvs rotor pack but who knows how long before we see that

I'd not lower the compression. You can still make pretty close to 700whp on pump (might be able to actually make 700whp on pump) with your stock 11:1
More compression will make more power per PSI ... more throttle response, less lag if turbo or centri blower... there are so many benefits to compression...
If 700whp is not enough, then throw e85, or some other higher octane fuel.

 

[alex12gt]

Looking at staying 11:1 with the 2.3 or going 9:5 with 2.9 that gives me a little room to grow as well

 

[sambandit]

11/1 regardless! FWIW, I am at 685 RWHP on pump right now with a very conservative tune and thats with the 2.3 TVS....

 

[alex12gt]

wish this was simple lol