VVVROOMTX

Evo_Joe

if you're looking for more power, I think A&A secondary drive would be more cost efficient

Unreal

Close to none unless you pulley down to make up for the loss of boost.

VVVROOMTX

How is the secondary drive going to provide me more power? Less belt slip?

VVVROOMTX

Makes sense

TurboLX

Um, it doesn't really work that way... Boost is a measure of restriction. Airflow makes power and creating boost pressure saps power from the crank. Removing a restriction (properly porting heads) will improve flow, increasing power. Even if the boost gauge goes down, you should see the MAF (and power) go up. The original question was by how much. That answer is "it depends", basically, dependent upon how much restriction there was and how much was improved while allowing for clean combustion, all in the hands of the porter...

g23crawler

Every build is very different but on my twin turbo build, from all the research I did it was about 40-50 rwhp for stock ls3 ported heads. Which I couldn't justify the $1,000 to port them. If I really want the extra 50hp ill just turn up the boost alittle.

Turbo-Geist

Unless you are looking for every last hp, I wouldn't spend the money on ported heads especially with a SBE. It's too easy and inexpensive to just add 1 to 2 more psi. Even my stock 853 stamped LS1 heads performed fine with forced induction.

Unreal

Yes it depends, but on a blower car, with the blower at a set speed, the minor differences in efficiency from lower boost typically are almost unnoticeble on dyno. Now boost may go from 12 to 10psi, then you pulley down to take advantage of that flow, and can pick up big power. With a turbo the WG automatically does that. So as I said, very little on a blower application until you adjust the boost to make up for the restriction. The blower is moving the same speed, moving basically the same amount of air.

BLOWNBLUEZ06@RKT Performance

I agree with Nick (Unreal).
With all due respect, it does really work that way. You're right. Boost is a direct reflection of restriction but consider this: You turn a centrifugal 'X' speed and move 'X' amount of air, creating 'X' amount of boost. Keep the impeller speed the same, change the flow efficiency of the engine and your boost drops as a result. Yes, you make the same power at less boost with maybe a small bump because of the decrease in discharge temps and parasitic loss. With a turbo, after the head changes, the impeller speed increases, moving a greater volume of air to compensate for the changes. With a turbo speed sensor, what I'm saying would be a lot more easy to see.

The answer to the original question is 20-25whp per lb of boost. If you port the heads and it goes from 12 to 10psi and you change pulleys to bring your boost back to 12psi, you can expect 40-50whp as a result. That's of course with a comparable tune, on a comparable dyno and so on.

Bret

TurboLX

Sorry, nope. Like any other pump, a centrifugal compressor has a pump curve that defines the relationship of output flow and pressure at a given input power and speed. Decreasing the output pressure required (boost) allows the pump to increase flow for the same input. Don't believe me, take a look at a compressor map from Vortech:


Notice the diagonal lines of constant speed. ("45,000" for example) If the compressor is not required to deliver such a high pressure ratio (boost level), it is free to deliver more mass flow. THIS is where the extra power can come from. Even better, the lower pressure increase (again, at a higher flow) comes with less heating so our spark can also be more advanced to unlock more power again. A smaller side benefit not shown here is that the torque required to turn the compressor also drops with pressure ratio, which again takes a small amount of loss off the crankshaft.

If none of this were true, we wouldn't bother putting larger camshafts or free flowing exhaust in race cars either. If you have ever done a cam or header swap (without any other changes) on a blower car, you probably saw the same thing: lower boost, higher MAF and more HP even at the same pulley ratio. Sure, we can change pulleys and pick up even more flow by restoring the boost level, but that doesn't mean the flow improvements didn't work.

Turbos aren't much different other than the regulation of compressor speed/boost. Since they are not mechanically linked to crank speed, the wastegate merely maintains the same target pressure after removing a restriction like heads or cam events, resulting in an increase in shaft speed (and flow). This effectively makes two changes at once, compressor speed and total flow. It takes two discrete changes with a belt driven compressor, but the aftermarket loves to do them together.

Unreal

Worked building turbos for years. Generated compressor maps. Done this for a long time. Real world and omenty of cam header etc etc swaps on a belt driven blower back up time and time again the answer is not much. Drops boost because of the less restriction and gains a slight edge in efficiency but the big gains are then from pulling down. He isn't going to see 50-60+hp by swapping heads because it isn't going to move that far on the compressor map since the rpm of the blower will be the same. Been there, done that tons of times.

ZR1-ab

Porting heads doesn't help ? Since when ?

silvrhand

I can show you the dyno chart if you want to see before and after the 102mm TB, same power, less boost.. Unreal is correct.

Turbo-Geist

No one said it doesn't help but the gains vs money spent aren't healthy to your pocket book. As mentioned if you need every last horsepower to win the class you are racing in or if you are chasing some type of record, then it might be worth it. Otherwise, a pulley swap on a blower is way cheaper and easier.

My formula has always been address the known weak points for the platform and add forced induction. Continue to add airflow and fuel until you reach your goals or break something. I would not pay money swapping to ported heads on the combo being discussed in this thread.

c5mtl

grocerygetter

Nursing Nevada

I was going to pull the trigger on some ported LS3 heads local to me. For $600. Since I’m already pulling the heads to do cam/lifters.

but seeing this, maybe I should spend the $600 elsewhere. Not worth the 20hp or so I’ll gain?