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So i have a twin turbo 4.8 build with some small godspeed turbos on it. Currently they are too small, boost comes on EARLY, but cuts out too soon and looses power on top end. It wasnt built for HUGE power, 500-550 would be awesome.
Im going to be stepping up to 2 gt35r (similar sized) turbos, .84 AR exhaust. Mainly because they will fit on my existing t3 platform, and use the 2.5 exhaust.
current wheel inducer/ex size 54/71, .63ar, new ones are 61/82.5 .84 ar. I know, i was OFF badly with my first turbos, was given them and decided to mock up, and test with. Had a bit higher hopes but honestly I did expect this to some extent.
So my question,
At what power levels will i notice the 2.5 exhaust holding me back, will I even make it to 500? I know I should of started larger, but I didnt. The gt35 sized turbos can make the power Im looking for, Im just concerned the exhaust isnt large enough. Its 2.5 straight out from turbos.
I have these turbos, and I will be trying them out ideally this weekend. But Im just wondering what i should expect from it.
What does the rest of the exhaust system consist of? Short 2.5" dumps are plenty for 550whp IMO. Put a gauge pre-turbo and look at your back pressure.
I'd think the current turbos would do 500 hp no problem? Denmah and a few others have made 650+whp and 700 ftlb on the small 57mm China t4's anyway. That was on a 5.3 as well.
What does the rest of the exhaust system consist of? Short 2.5" dumps are plenty for 550whp IMO. Put a gauge pre-turbo and look at your back pressure.
I'd think the current turbos would do 500 hp no problem? Denmah and a few others have made 650+whp and 700 ftlb on the small 57mm China t4's anyway. That was on a 5.3 as well.
His build was a part of the motivation for this build.
Its straight down and back prob 6 ft to a magnaflow 2.5 muff and ran over axle and dumped out behind the tire on both sides.
I am replacing the turbos due to blowing a seal on one of these guys, but i think before i do Im going to drop the exhaust after the down pipes and see how it goes.
Parts of me regrets going t3, but if i can work out these bugs and get it running smoothly with ~500 @ 8-10psi, Ill be very happy.
Once I rule out any exhaust restrictions, and get this oil leak under control, I will give myself a few trys at getting the tune right, and if i cant get close Ill take it in. Got a few good shops in the area!
I feel like you will be much happier with them, and are severely underestimating the potential of a twin 2.5" downpipe at the power level you seem to desire (500-700). I have seen 500 come out of a single 2.5" without much struggle in fact.
I must have missed what fuel? If you are stuck with 93 octane, some methanol/water would help make it safer.
The rest of the exh system sounds like it may be pretty restrictive. . Are those chambered or straight through mufflers?
Might look into some electric cutouts. I really like the ability to have a super quiet exhaust system. You could “Y” into a single 2.5” crush bent system to a single restrictive OEM style muffler in the back. Then run 2 2.5” dumps right after the turbos and have the best of both worlds.
The rest of the exh system sounds like it may be pretty restrictive. . Are those chambered or straight through mufflers?
Might look into some electric cutouts. I really like the ability to have a super quiet exhaust system. You could “Y” into a single 2.5” crush bent system to a single restrictive OEM style muffler in the back. Then run 2 2.5” dumps right after the turbos and have the best of both worlds.
straight through design. And i will definitely test that by splitting it at the vband i put about 3ft off turbos, and see what that changes.
Thanks Forcefed & Kingtal0n, your reassurance is nice to know. I thought my design with 2.5 would be sufficient, but i dont see much on it really and everyone kinda cringes when i tell them that i ran 2.5.
I started this project years ago, had it running like it is today over 2 years ago. Bought a house that needed WAY more money and time than i expected and it took a few years to rebound from that. Got engaged and even had a kid in that time frame! But Im excited to get back under the truck and see what can be done with it.
A good general rule for DP sizing is 120% of the exducer. As long as you don’t have a mile of crap bends and restrictive mufflers after, you should be fine with that method. If you have a 54/71 exh wheel 2.5” is about right. A cheapo push to reset tire gauge and some copper tubing plumbed pre turbo can be used to get a peak back pressure reading quick/easy.
A good general rule for DP sizing is 120% of the exducer. As long as you don’t have a mile of crap bends and restrictive mufflers after, you should be fine with that method. If you have a 54/71 exh wheel 2.5” is about right. A cheapo push to reset tire gauge and some copper tubing plumbed pre turbo can be used to get a peak back pressure reading quick/easy.
thats clever, never thought of that.
What readings should i expect, what would be high?
Alot of things play into that but generally 1:1 is ideal. Up to 2:1 is pretty standard for street setups. So at 10lbs of boost you'd see 20 pre-turbo at 2:1. Much over 2:1 isn't good....Over 2.5:1 is pushing it IMO. Cam overlap will also play a big role in how much you can get away with.
i was mistaken, i thought we finished up the over the axle, and forgot we didnt. It goes down, and back, and stops just shy of the axle. My mechanic who has built this with me doesnt think back pressure is an issue. But we are going to be testing it out
He and i discussed it last night, and we are hoping its an issue with a turbo. Occasionally it will surge alittle up top when under boost, and its not consistent.
Sometimes itll pull up through the range with no hesitation and pull great, others it will bog down and struggle to get moving.
An observation Ive noticed, started it up drove to work after lunch yesterday, and the whole ride up the thing seemed like it couldnt get out of its own way. I changed nothing, fired it up and drove home later in the day, and i chirp the tires going from 2nd to third and its pulling like crazy! Drove great again this morning as well.
So this weekend Im going to work out variables, tear it back and go over everything.
Here is a list of the stuff done to the truck.
2000 Silverado 2500LD
05 LR4
799 heads
PAC 1211x valve springs
ARP bolts all around
Ligtenfelter HP/HV oil pump
LS7 lifters
Comp Cam push rods
LS9 Cam
LS9 head gaskets
new map and knock sensors
TR6 plugs
converted CPS to gen4
SD 80lb injectors
FAST rails
aeromotive FPR
twin wally GSS342 pumps
performance 4l60 trans build
tial bov
4" intercooler
custom hot and cold side piping. only cold side couplers are at turbo, intercooler, and intake. everything else was welded.
I feel like you will be much happier with them, and are severely underestimating the potential of a twin 2.5" downpipe at the power level you seem to desire (500-700). I have seen 500 come out of a single 2.5" without much struggle in fact.
I must have missed what fuel? If you are stuck with 93 octane, some methanol/water would help make it safer.
I am using 93 octane now, and i did pick up a meth injection kit, but after watching my IATs for a while, im only 20-25 degrees over ambient at most. In the evenings its even cooler. Would there be another reason besides air temps to do that?
LS9 cam has very little overlap so that’s a good thing if you have high back pressure. That cam will make a 4.8 feel pretty wimpy below 4600 or so, then come on strong to 7k or so in a 4.8 I’d guess. That’s how it acted in my 5.3 anyway.
Have you looked into the knock retard? Verified it’s not pulling a ton of timing?
I am using 93 octane now, and i did pick up a meth injection kit, but after watching my IATs for a while, im only 20-25 degrees over ambient at most. In the evenings its even cooler. Would there be another reason besides air temps to do that?
Yes. Read up a little bit about meth/water. I am pretty sure water has a better heat capacity than methanol (it carries away more energy when it evaporates) and if that is true, water is the better candidate for controlling IAT. This should give you some clue as to why methanol injection has very little to do with IAT adjustment. Instead, typical water/meth is injected as a liquid to the plumbing close to the chamber, and a majority of it will stay as liquid until it hits the combustion chamber and changes state, where a large amount of energy is taken up by the phase change, protecting the engine by helping to prevent a spike in combustion pressure, which would damage gaskets/pistons and so forth. In other words, you are reducing the temperature peak which helps control the reaction (to be more slow) thus reducing power of course (its like pissing on a fire so you will always lose power) but now you have the headroom for all this extra air, since adding all the extra air wont cause an explosion anymore thanks to all the water/meth you added. So in this way it increases your headroom for power, provided you have the means to get the extra airflow through the engine (you can raise the boost if the compressor has flow left in it) most just think of it as an octane improver so they can use more boost without actually having to purchase real race fuel or tune for E85 or other nonsense options, keep the same old tiny fuel pump in the quiet tank and just inject an extra 150~ horsepower worth of fuel from a supplementary unit as opposed to re-tuning the entire setup or buying a bunch of huge pumps etc...
Yes. Read up a little bit about meth/water. I am pretty sure water has a better heat capacity than methanol (it carries away more energy when it evaporates) and if that is true, water is the better candidate for controlling IAT. This should give you some clue as to why methanol injection has very little to do with IAT adjustment. Instead, typical water/meth is injected as a liquid to the plumbing close to the chamber, and a majority of it will stay as liquid until it hits the combustion chamber and changes state, where a large amount of energy is taken up by the phase change, protecting the engine by helping to prevent a spike in combustion pressure, which would damage gaskets/pistons and so forth. In other words, you are reducing the temperature peak which helps control the reaction (to be more slow) thus reducing power of course (its like pissing on a fire so you will always lose power) but now you have the headroom for all this extra air, since adding all the extra air wont cause an explosion anymore thanks to all the water/meth you added. So in this way it increases your headroom for power, provided you have the means to get the extra airflow through the engine (you can raise the boost if the compressor has flow left in it) most just think of it as an octane improver so they can use more boost without actually having to purchase real race fuel or tune for E85 or other nonsense options, keep the same old tiny fuel pump in the quiet tank and just inject an extra 150~ horsepower worth of fuel from a supplementary unit as opposed to re-tuning the entire setup or buying a bunch of huge pumps etc...
If his IAT's are only 20-25 degrees above ambient as stated, then water will not evaporate enough to do any good for IAT's, and will only serve as a knock suppressant, meaning it will kill his power. In such a situation, 100% meth would be a lot better option. Model airplane racing fuel (meth/nitromethane mix) would be even better. However, none of this is actually necessary for his current build.
Please re-read my post, as it literally says the fact, quoted: " ...water is the better candidate for controlling IAT. This should give you some clue as to why methanol injection has very little to do with IAT adjustment. "
likewise it says "... you will always lose power"
Finally, a helmet isn't 'necessary' but still serves as significant protection in case of an accident. I would NEVER run a turbo build without meth injection on gasoline, on any car, anywhere in the world, provided it was available and easy to implement.
08-18-2016, 09:58 AM
twin turbo 4.8 exhaust questions 
