black98ws6ta

iTrader: (77)

I am limited to a t4 size turbo on my car and got a good deal on a 408. I know that as I turn up the wick backpressure will rise. Should I stay with a semi stock cam and heads to keep the comsumption on the motor low? Or should I stay with a decent set of heads and cam and just find my stopping point with backpressure? TIA

smokinHawk

iTrader: (48)

well to help with back pressure issues have your downpipe as big as possible and a very free flowing exhaust.

engineermike

Why don't you just put a restriction orifice behind the throttle body? That would have the same effect as restricting the cam and heads. . .

The poor backpressure/boost ratios offered by T-4 turbo's on large displacement engines is the nature of the size of the cylinder. Good heads and cam will only increase power at the lower boost levels and break even when you max out the turbo at 25+ psi.

black98ws6ta

iTrader: (77)

It will have a very free flowing exhaust only a foot or so of open 4 inch downpipe. As far as my comment about good heads and cam, I was reading over a post where people said making the motor more efficient would cause higher backpressure. Where as if you had a 408 that didnt move as much air it would help some, and just run more boost to get the power where you need it.

black98ws6ta

iTrader: (77)

This is the post I was refering to by JOSE:

Addressing bacpressure starts on the exhaust side. Making the engine breathe even better will make it worse actually. The further the PR gets away from 1:1 the worse it will get and the more hp you will lose.

DrTurbo

iTrader: (3)

That is true, the futher the pressure ratio gets from it efficient state the hotter the discharge air from the turbo will get. Now of course I'm sure he is talking a max effort setup here. If you ask him about pump gas he will change the answer. For pump gas setups he will tell you larger cubes, cause you can get to 700rwhp easier with a 408 @ 12psi, then a 346 at 16psi with 93.

black98ws6ta

iTrader: (77)

I was thinking the same, I would like to make around 800rwhp on pump and then be able to turn it up somemore. The amount I turned it up might just depend on egts, and backpressure. Anyone think I would make 1000 rwhp on racegas, without crazy backpressure.

PurEvl

iTrader: (2)

i have a t4 flanged t88 with a 3 1/2 dp to a 3" exhaust...havent had any problems even when 820rwhp on pump. Its a 408 as well with victor intake setup/afr heads/ very small cam

black98ws6ta

iTrader: (77)

Has the boost ever been over 15 lbs? Do you have a backpressure reading at 15 psi? What afr heads?

PurEvl

iTrader: (2)

225 out of the box heads, Never got to push it past 15psi, i need a metal outlet for the filter. At 15psi it starts collapsing the rubber on me

Havent had any signs of heat or back pressure related issues so never tested it. (yet)

engineermike

I hate to disagree with Jose because he's helped me out a bunch and is very knowledgable. However. . . the backpressure/boost ratio is little decieving here. Boost pressure is measured in the intake manifold plenum. When you make the cam, heads, and intake flow better, you are doing a better job of transporting the air from the plenum into the cylinder. Since more air is exiting the plenum and entering the cylinder, the pressure in the plenum is lower. This makes the backpressure/boost ratio look worse (on paper at least), when you're actually getting more intake pressure in the cylinder and that's where it counts.

engineermike

Just FYI, we recently dyno'd a 377 / T-76GTS and got only 28 psi backpressure at 18 psi boost. This 1.55/1 backpressure ratio is uncharacteristically good for a T-76 on 350+ cid. I attribute this to the 3.5" downpipe going to 4" dyno exhaust. It just goes to show what is possible with a T-4 and large displacement.

JZ 97 SS 1500

iTrader: (2)

Mike you can't have one without the other. IF you are transporting more air into the cylinders (ala better flowing heads, cam, plenum) that flow still has to exit the engine through the same path, which means through the turbine it goes. We have seen some killer engine combos die out early due to backpressure and small turbine wheels. Also by making the engine flow better (402-408 LS1) is the same as having a lower flowing 454 or say 502. You wouldn't want a 502 with a 76GTS would you?? Also your backpressure #'s were well below the average, but also you had optimum piping size, optimum space contraints and optimum length exhaust. Most cars can't fit 15 ft of 4" pipe, and most don't want to run a 4" downpipe that is only 6 ft long with a turndown at the end. Yes these are the best setups, but not typically doable on daily driven cars. Our best and fastest drag radial cars we work with have high flowing 360ci engines 18psi and big GT55's with 1.08A/R's, and they still see 32-35 psi in backpressure.

Pro Stock John

iTrader: (34)

The more power you make with your engine, and the more air it flows, the lower the boost you will see at a given hp level. So a built 408ci could hit 800rwhp at a much lower boost level than a 348ci with the same heads and cam... maybe 15psi vs 21, who knows.

I've been told repeatedly to keep the backpressure at 1.25:1 and lower.

JZ 97 SS 1500

iTrader: (2)

John 2:1 is ok, theortically the best is 1:1, but that only happens with thumper units. On a street car we shoot for 1.5:1.

Jose

black98ws6ta

iTrader: (77)

So my 408.....should I stick to a smaller cam and unported heads to keep the motor flow down, that way it seems as if it is a smaller motor. I will have very free flowing exhaust about 2 ft of 4 inch downpipe, I would love to make 1000rwhp without backpressure issues.

Pro Stock John

iTrader: (34)

Almost seems like the opposite would be true right? If you run a bigger cam you will make less boost, and thus run into less backpressure issues.

Willem @ Fuel Injection Concepts

iTrader: (2)

less pressure on the intake side.

the better you make an intake flow= more exhaust gas getting backed up at the manifold before the turbo

engineermike

I don't see how this is a bad thing. More exhaust pressure in the header means more dP across the turbine and thus more exhaust flow. More flow = more power.

If you build an NA 572 and an NA 350, then force them both to exhaust through a single 2.5" orifice, then which one will make more power? My bet is that the 572 will, albeit with more exhaust pressure. That would hold true up until you reach sonic gas velocity through the orifice, or "choke", at which point both would make the same power. "Choke" is also known in the turbo world as the absolute limit in flow of a particular turbocharger.

smokinHawk

iTrader: (48)

youd start to damage your 572 then. lift gaskets, warp ringlands ect.