So I put together a mild 370” LS engine. 224/224 @113 cam .612 lift. Factory untouched 317 LS heads. 9.5:1 compression. Factory LS6 intake manifold. Verified valve spring closed pressure of 151 and open pressure right around 400. Should be plenty to rev to 6800 or so cleanly.

Decided to try the factory exh. manifolds and a 2” divided hot-side for 1000 HP goal. The 16G 2” tubing used had an actual ID of 1.87”. Didn’t really stop to consider that until the kit was complete. Initial testing was awesome. Boost came on very quickly compared to similar LS kits I’ve built with 2.5” piping. But seem to have hit a wall and I believe the 1.87” ID piping may be the cause. Running E85 fuel, no knock, plugs look great. Tried bumping timing a few degrees and it didn’t help. Using the hot GM D-585 coils. Tried .019 to .025 plug gap. No changes. Exhaust consists of a 4" open pipe about 24" long.

Currently the motor just lays over around 6k. With a 30” tire I’m running 8.70 @ 159 in the quarter (3100lbs). Thru the traps about 6200 rpm. I dropped down to a 28” tire so I could rev the engine more thinking I’d pick up, and lost 5mph. Trapping 154 at the same 6200rpm. The engine laying over at 6k was much more evident with the smaller tire. Hits around 6k and just stops pulling and hovers around that area. In 1[SUP]st[/SUP] gear I can rev it out to 7k easy. But didn’t pick up any over my usual 6500 shift. (Power glide trans)

Was seeing 2:1 back pressure at 20lbs with a 1.24 T4 S476 turbo. I was measuring back pressure at the #1 runner.

Figured I’d eliminate the turbo as the cause first. Installed slightly larger 1.32 T6 480 turbo. Both had the same 87/96 exhaust wheels. Back pressure remained roughly the same at the #1 runner around 2:1. There is no merge in the system, so I moved my back pressure rig to the turbo inlet. (one side of the scroll) Back pressure dropped to 1.4:1. Street testing revealed the same thing. Power still dropping off around 6k. Blew the trans my next track visit first pass. So no more data till next year.

Not sure I should try pushing this setup any further? Really wish I could have got some hard data that last track visit. That way I could justify building a new hot side this winter.

Think I’ve found the limits of the hotside? Or should I look elsewhere for problems?




Few pics.

Hotside.

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Rolling into throttle, Building boost on brake.
https://youtu.be/egA3BrfxsN8

Original back pressure tap location.
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New location on housing.
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I've not made the power you're making, so I can't speak from experience. But, thinking about this from an air pump standpoint you can come to some possible root causes. At the 6k lay over point, you don't have any dips/dive/changes to the tune, correct? timing, AFR, DC all stays in check? Then is the compressor big enough? at 6k you're seeing the same boost, it's not falling off, correct? So operation cram air in seems OK. Air in must = air out plus heat. A couple points here though, you have the hot side, and the DP side. DP side seems to be open well enough, leaves the hot side and layout. If you are divided all the way then layout shouldn't be a major concern. Leave hot side size as suspect. Could be manifolds, could be pipe, could be turbine.

At least that's how I'm rationalizing you building a new hot side.

 

I'm thinking the same. The 2:1 vs the 1.4:1 would be enough for me to rework the hot side. Even going to 2.25 would be a 25+% increase in area.

 

Thanks for the input!

I was thinking the same but a guy over on YB is making 2000ish crank hp with a 414 over 7k on 2.125" ID piping and a single 94mm gt55 deal. He was telling me they gained 7/10ths at 9 less psi going to collector style 1 5/8 manifolds VS the factory manifolds as well. He thinks I must have issues somewhere else. Also since there's no merge the combined area of the 2 1.87" pipes seems like alot. Not sure if the exhaust is going sonic or what. I mean if you divide the 370" engine into 2 separate 185" engines... 1.87" piping doesn't seem so small... 500hp thru each pipe seems reasonable? Makes me think the new $99 "GM" LS7 lifters may be pumping down or something. That's the only other thing I've changed.

I get the 2" probably isn't ideal for as far as I've pushed things, but I've got another project I'm trying to finish this winter. So making another hotside was something I was hoping to avoid if possible.

 

Can you tap in a couple transducers to measure the back pressure?
A buddy is making about 1500hp (E85) with twins on a 600ci BBC using 2.25" od piping (these are pretty short pipes tho) and I'm making 700 with 2" hot side on my TT 4.6 (gas) if this helps any. All this being said I think your piping is a tad small especially if you start adding bends to the system and the pipes are long.

ks

 

If you read the first post I have tapped 2 locations and measured back pressure, posted pics too.

 

AARGGG.. Sorry about that!

ks

 

I would definitely upgrade the piping to at least 2 1/4", if not 2 1/2".

On the Subaru's, they upgrade the "up pipe" to 2", and that's only a 122 ci engine, which using the same logic is (2) 61 ci engines. They claim better response and HP. I don't see how you can lose.

Al