Sarg

iTrader: (6)

Interestingly enough, in further conversation with Geoff, he stated that necking down after a 2.5" collector is probably not the best idea. His idea was rather to have a header or manifold with the correct size collector to start with. But he said that with an auto and the turbine wheel that I have the effects of a 2.5" pipe will be minimal on the overall performance of my Borg Warner S480.

alexcoreas

iTrader: (1)

Thank you phil,

Sales@Tick

iTrader: (3)

FWIW I think Jon@huron speed is building my kit with 2.5" cross over but I could probably have him change it to 2.25" if I wanted to.

Your thoughts Phil? This is a RS275 car with a 370 and a S476R T6 turbo that I'm going to try with the 1.32 AR housing first and if it drops off hard like I think it is going to at 6800-7000rpm I'll switch to the 1.58 housing.

Tjabo

Hey guys, I haven't seen any talk of crossover sizes for some really "low HP" setups like 600 crank HP? In that case would 1.75" or 1 7/8" OD tubing with a 16 gauge wall thickness come into play (for the crossover)?

I'll do some calculations of the actual internal area of those tubing sizes, but I know that is really only the first part of what we're looking at here because this is about velocity. I need to search around some and see if I can find the formulas, I don't have Corky's book handy...

MIGHTYMOUSE

iTrader: (59)

.... which leads us to my newest set up

Phil99vette

iTrader: (11)

Honestly, thats what the velocity calculations say to run but man its scary to think about it.

Phil99vette

iTrader: (11)

Honestly, it would be interesting to see how it works with a 2.5" crossover and a 2" crossover. what headers are on it now, gonna make 1000-1200hp?

Phil99vette

iTrader: (11)

Is that Keith Berrys car?

Sales@Tick

iTrader: (3)

I actually just talked to Jon and he is going to do a 2.25" cross over for me.

I'm going to run truck manifolds so 1x5/8" primaries with a 2.5" collector I believe.

Shooting for 1050rwhp to the wheels.

MIGHTYMOUSE

iTrader: (59)

if martin is going 2.25 then im going 2"!

Sarg

iTrader: (6)

And the guy with the 4.8 is going 1.75!

67Firebird455

iTrader: (27)

I'm going 2.25" and I'm sticking to it!

Tjabo

Phil,

I can do the math to get from CFM to ft/sec based on an actual internal cross sectional area of a tube, but I was wondering if your calculations start with the premise that CFM of the engine is roughly equivalent to 1.5 times the peak BHP of the engine/setup?

If so, then I'm in business to mess around with various exhaust velocities in various parts of the exhaust system based on what portion of the total CFM is flowing through that part of the exhaust system.

If I'm doing this correctly, then I do see that the tubing sizes can be startlingly small compared to what we ordinarily think of for exhaust tuning on normally aspirated engines. Even if you shoot for keeping the velocity under 450 ft/sec as I've seen somewhere...

Using this method I'm getting 1063.63 Ft/Sec through your crossover pipe.... Is that right?

Thanks!

Phil99vette

iTrader: (11)

Close enough. I am using an estimated VE %, CID, RPM, Boost PR, that will give you an estimated airflow number based on CID, VE, PR and RPM. 1063 sounds close enough.

Tjabo

Thanks Phil, this is extremely interesting stuff!

I'm baffled by the relationship between the hot and cold side calculations though since the exhaust should be MANY times more volume (cubic feet) than the intake mixture..... I'm thinking they should be the same mass, but vastly different volumes, and therefore extremely different gas velocities.

Of course, the exhaust has a much higher speed of sound since it is at so much higher of a temperature. Good lord.... Fun to ponder, but confusing! Haha

engineermike

If I'm understanding this right, it won't yield the right answer. The standard calculation for engine cfm is on the inlet side. To calculate it for the exhaust, it must be corrected for temperature and pressure. The exhaust density causes the acfm to be very different from inlet acfm, as Tjabo said.

Edit: The speed of sound is very different with temperature and pressure, too. That's how a bullet can be propelled down a barrel at speeds much higher than sound. Anyway, as 1500 deg F, the speed of sound is around 2200 fps.

engineermike

You're right.

Also, FWIW a typical industrial gas system rule of thumb is to keep velocities below 150 fps.

Tjabo

engineermike, do you have any idea what the volume relationship is between the intake and exhaust side?

engineermike

It's going to be something like this:

exhaust acfm = inlet acfm x ((egt+460)/(iat+460)) x ((boost+14.7)/(drive pressure+14.7))

Tjabo

Thanks!

So is it correct that it really comes down mostly to the ideal gas law, and the ignition of the air/fuel mixture really does nothing more than add temperature, thereby causing the increase in volume?