THE LAST Z

iTrader: (5)

I tryed a search but found nothing. I am reserching turbo kits & I see people using stock heads on their turbo motor & getting good results. Is this because of velocity? How does it work? Would a stock head work better than a ported head on a street turbo motor?

Louie83

iTrader: (3)

I would go ported.

You want to force in as much air as possible, right? So larger ports should help aid this. Take a stock headed turbo 346, then add AFR 225 or TFS 225 and you will gain some good power.

engineermike

Turbo's are funny. Any given turbo will support a certain amount of hp and it will make as much boost as it needs to get there. For instance, the popular T76GTS has made just under 1000 rwhp on a 355 at 22 psi boost, and been just over 1000 rwhp on 180 cid at 45 psi boost.

So, with any given turbo, you can make the same amount of power with stock heads as you can with ported heads, but it will take more boost to get there. Conversely, alot of guys, including myself, are trying to maximize power on motor alone using head flow, cam selection, intake manifold, and cid, so that you can make more power at lower (safer) boost levels.

Mike

chuntington101

also boost means heat! and you have to get rid of that heat some how to make the thing reliable!

Chris.

LILS

What is the relationship between increased head flow and decreased boost required to produce a given HP? For example, would a head that flows 20% more require (as a first-order approximation) 5/6 (inverse of 120% = 6/5) the original boost, or is the relationship more complex?

MrDude_1

iTrader: (4)

its gets alot more complex.

just one example would be the compressor efficiency... you change where that compressor is at, and the efficiency changes... so the charge may be hotter or cooler, so the density changes... ect...

LILS

Yes, efficiency would change some with the change in boost, but if the difference in charge temperature were offset by a corresponding change in charge air cooling, how close would we come to the "required boost is the inverse of head flow" rule? Are there other significant factors that would similarly have to be compensated for in an effort to isolate the effect of changed head flow, all else being as nearly equal as possible?

engineermike

You first have to estimate the hp the engine would make naturally aspirated with both ported and unported heads. Then, It would be something like this:

FIhp = NAhp*(boost1+14.7)/14.7

FIhpported = NAhpported*(boost2+14.7)/14.7

So, if FIhp = FIhpported, then:

NAhp*(boost1+14.7)/14.7 = NAhpported*(boost2+14.7)/14.7

Solve for boost2:

boost2 = NAhp*(boost1+14.7)/NAhpported - 14.7

Example: If an LS1 makes 380 hp with stock heads and would make 410 hp with ported heads. Your initial boost level is 10 psi and you want to add ported heads and reduce boost to get the same power level.

boost2 = 380*(10+14.7)/410 - 14.7

boost2 = 8.2 psi.

So, if ported heads add 30 hp over stock heads, and you're running 10 psi, then you can port the heads and lower the boost by 1.8 psi and get the same power.

Of course, these are gross approximations.

Mike

FUroundeye

iTrader: (6)

So engineermike, are you saying that the guys who jump up and down about a 408 making too much backpressure for a T76, and therefore will be outperformed by a 347 w/ a T76, are wrong?

ddnspider

iTrader: (26)

no i dont believe thats what hes trying to say.It comes back to efficiencies and octane as well.Its pretty well known that so long as your not WAY out of the efficiency range more cubes=more power on pumpgas.But in certain cases a 347 can make more power than a 408 because the setup on the 347 is much more efficient=cooler air=less backpressure=more power.But if for example the larger ci motor isnt putting the turbo way out of the efficiency range then compare the area under the curve for hp and torque.The bigger motor will spool the turbo faster and make power sooner and so long as backpressure isnt too high it can carry that power and not drop off too much at higher rpm.

FUroundeye

iTrader: (6)

I meant all other things being equal i.e. head flow, etc, w/ the only theoretical difference being displacement.

chris93266

iTrader: (5)

I may be wrong, but i see turbos not as boost makers but air blowers. it's not how much boost a turbo makes, it's how much air it can throw. Isn't it true that each horsepower made needs the same amount of air to be made regardless of engine size? which means if a turbo is flowing as much air as it can with unported heads or lower ci, then it will not flow more air with ported heads or bigger ci? Could ported heads reduce heat by increasing efficiency therefore increasing power?

-also... the smaller motor probably makes more power because it is lighter than the 347 and both pump the same amount of air.

Mike454SS

iTrader: (4)

However when you look at the compressor map, given the same airflow, one pressure ratio is DRAMATICALLY more efficient than another pressure ratio which is why there's more to picking a turbo than just saying "how much power do I want to make?".

engineermike

I really believe that people exaggerate the effects of large displacement on the T76GTS. I do believe that in some cases a smaller motor can make more power than a large motor with this turbo, but it's not because of the backpressure ratio or even the efficiency range. That's a more complex topic. If you're going for max total hp regardless of boost level, then smaller cid is better, but if you're going for max power on pump gas at, say, 15 psi boost, then you'll do better with large displacement.

Note that I was running 383 cid with my T76GTS, but then switched to 363, and then my current motor is 388.

Mike

On 3 Motorsports

I subscribed to this thread, some very interesting and useful information.