drive pressure

S

smokeisfun

still learnin
ok...i know drive pressure is the pressure in teh exhaust manifold...but...what does it hurt other than having exhaust back pressure...and dont you have to have it in order to have boost and drive the turbo...which brings me to another question...if there is 50+ psi in the manifold b/c of the turbo restricting flow...then why does it matter if you have 4" exhaust...


im jsut needing edjucated on drive pressure more or less...thanks for your time
 
The ideal drive pressure is 1:1 meaning that at say 40psi of boost you have 40psi of drive pressure. If your drive pressure is to high then you run into high egt's and excessive cylinder pressure which can lead to head gasket failure among other things. You will always have drive pressure since that is what drives the turbo but like most things in life you don't want excessive drive pressure. Also, if you have really high drive pressures then your motor isn't working very efficiently because it's trying to push more exhaust out than your turbo can handle which can/will reduce power. The 4" exhaust is just to help the escaping gasses get out as fast as possible but really it comes down to your wastegate and how much it can flow that determines drive pressures. If your trying to cram 40+psi out of an hx35 at 400hp you could have an 8" exhaust and it wouldn't matter because the wastegate on the 35 just physically can't move that much air fast enough. This is just how I rationalize drive pressure in my mind. Hopefully some more educated members can give you a better explanation.
 
thanks...ya that was pretty much how i was looking at it...i just didnt know the "ideal" drive pressure...im getting a 18cm housing for my hx35...that should help low drive pressures right??? or am i goin to have to get an external was gate some where....

also...i here jim fulmer saying he has "devoriced twins"...what does that mean
 
I probably know what divorced twins are but I've never heard them called that so I can't say.

Also, drive pressure has to do with your exhaust wheel as well. To small of a wheel can't suck the exhaust out fast enough so it builds up and creates high drive pressures. The 18cm housing will help some with drive pressures at the expense of spool up but in reality it's just a bandaid. An external wastegate would help more with drive pressures as would upgrading to an s300 based turbo.
 
One thing I noticed about switching to a turbo with a low drive pressure wheel is once boost came up power kept building, instead of leveling off and dropping as soon as boost is built. Just my $.02
 
How do you measure drive pressure? Is there a gauge that you use-like the usual boost/egt gauges, or just going off of something else? Just never seen a Drive Pressure Gauge before...
 
The one I'm borrowing is an air pressure gauge that is connected to some copper tubing that screws into the EGT pyro hole. You measure drive pressure in the exhaust manifold. Most people just use their egt hole since it's already there.
 
Drive pressure is pretty consistent so it just needs to be checked for tuning purposes. There are better options, but I ran copper tubing from the manifold, through the passenger window for a quick test run. I haven't tested drive pressure with this new turbo, but the looser feel to it along with the difference on the dyno tells me drive pressure is lower.
 
I imagine pushing a HX 35 up to 45psi is somewhere around 75psi drive pressure! Man this Adrenaline fuels hard!
 
Big Blue24 said:
I imagine pushing a HX 35 up to 45psi is somewhere around 75psi drive pressure! Man this Adrenaline fuels hard!
Click to expand...

when I still had my stock turbo I was pushing it past 55 psi and everyone was telling me I probably had drive presures greater than 100 psi.
 
I had an HX35-18 on my red 98 when I sold it. with 215 injectors... at 35psi boost, drive pressure was in the 45psi range IIRC

with the 12cm gated housing on it, drive pressure burried the 60psi gauge at 35psi
 
My dp on my twins was 90psi with 60psi in the intake. Making a 38mm internal wastegate on the Hx35/14 dropped about 10psi off thats about it. Ill get at it and fix it some day.
 
Why dont people run the steam rated preassure gauges like you would find on a boiler?

Or do they?

Ive never seen anyone test this im just thinkin so you can hear it. Im thinkin if im using a stainless steel braided hose meant for steam but most steam in say a food processing plant only goes up to 450 or so at max in the lines so I would still need a way to cool the exhaust gas down by 950 deg. or so to keep one of those gauges alive right?
 
tristan21 said:
Why dont people run the steam rated preassure gauges like you would find on a boiler?

Or do they?

Ive never seen anyone test this im just thinkin so you can hear it. Im thinkin if im using a stainless steel braided hose meant for steam but most steam in say a food processing plant only goes up to 450 or so at max in the lines so I would still need a way to cool the exhaust gas down by 950 deg. or so to keep one of those gauges alive right?
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there's no heat running through the gauge... I run 1/8" copper with a couple coils right off the manifold, then union over to nylon at the driver's corner of the firewall before it goes to the cab.
 
Ok I remember hearing people concerned that had hooked up a full time gauge that were worried about melting wires and so forth and alot of broken gauges, etc. I didnt realize that with a few coils you could slow the heat to the gauge by that much.

No need to reinvent the wheel then.
 
nah, really I just run the coils for overkill. the copper only discolors RIGHT next to the manifold. If there were actual hot exhaust running through it, yeah, it would be an issue, but all you're doing is measuring the pressure inside the manifold, so there's no actual flow. The heat stays very close to the manifold
 
so what would you say i would run for drive pressure with hx35 18cm housing and bht3b twins with 4" exhaust and a stock manifold...just a ball park
 
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