Seems to be a trend lately… I went from a 30” to a 28” tire and the problem is noticeably worse. Went from trapping 159 around 6200 to 154 around 6200, same boost levels.

370 LS, untouched 317 heads, LS6 intake/TB, A2A FMIC, S476, 224/224 @ 113 .610 lift cam. (degreed), 3.25 gear/GLIDE PTC 9.5” , E85 for fuel

So at 16lbs I’m trapping the exact same as 21lbs. Same RPM as well. (154 @ 6200)

Fuel consumption is up 20%
Converter slip is 3%
Back pressure is 1.8:1 on both 16 and 21lbs
IAT’s in the 120-130 range. At 28lbs IAT's jump to 160… same result… no power.

Seems to be RPM related at high boost levels over 6k. I think the shorter tires just made the issue more noticeable. I was around 6200 at 159 with a 30” tire. Not I’m around 154 at 6200 with the 28” tire.

Couple theories.

1.) Reached the limit of my 2” hotside?
2.) Valve springs
3.) Spark.


1.) Pretty steady at 1.8:1 back pressure wise above 5k. If my 2” piping reached its power limit, would this show up as excessive back pressure? Or if the gas is exh gas is going supersonic and causing the issue… would that not show up as high back pressure? Figure I’m around 1000 crank at 20lbs. I have a fully 2”divided hotside on factory truck manifolds. 1.25 T4 87/96 exhaust wheel. 76mm compressor.

2.) I was thinking springs because the Engine noses over around 6k in high gear, But will pull clean to 7k in first gear. I have a PAC 1519 .650 lift spring. Still a single beehive. .610 224/224 @ 113 cam with mild lobes. I’d think if it were a valve train issue I’d have problems in 1st gear as well?

3.) Another member suggested spark blow out. I’m gapped at .024 with the D585 coils. BR7 plug. Only spraying 6 gph of water meth total (30%meth). This year I switched coils to the hotter D585. I tried back to back 20lb passes with the water injection off/on and lost no MPH. So I assumed spark was good. I’ll try gapping down to .019 and shutting the meth off and see if anything changes.



Any other ideas? Or things I could try?


Thanks!

 

I would think with spark blow out it would show on dyno or noticeable misfires at least. I am leaning towards springs and also IAT's? 160 isn't good. We had same issue last weekend and it was killing our power big time.

 

Spark blow out should show up in the log somewhere?

 

Thanks, IAT's only jumped to 160 at 28lbs. They are 120-130ish at 21lbs.

 

I doubt its spark blowout. I spray a LOT more water than you do and I don't even close the gap on my BR7s, just throw them in out of the box.

Your cam has negative overlap so I can't see the cam being an issue with reversion from backpressure. I would also rule out valve springs.

Do you have a way to log converter slip? Its possible that it still couples well up top but slips a lot more in the mid range from more torque. Would explain why you're likely making more power but not going faster.

Other than that, hotside size would be my next guess. You're closing in on the edge of 2" hotside pretty quick.

 

As a self admitted rookie to the turbo world, I'd think you're hot side is fine as you're still able to make boost. 20 psi is 20 psi. Seeing that back pressure is still acceptable, I think it's gonna be beyond the intake. The speed difference does coincide with going to a 2" shorter tire at the same rpm. Hmm.

 

For your item 2), you have significantly more load in higher gear which may explain why 1st gear pulls cleanly to 7k but chokes out at 6k in high gear.

If fuel consumption is up 20% its going somewhere. Are you outside of the efficiency range of the turbo at that boost? It seems like you're maxing something else in the setup somewhere. I do agree 2" hotside is a bit small given your power level, but like you said I would expect your back pressure to be worse. Whats the rule, anything under 3:1 is acceptable.

If I were you, and this may sound stupid, I'd get it on a dyno and see the graph in each gear and that should tell you if it looks like springs or if something else is going on in a steady state environment.

 

159mph on that turbo might be all she has. The drop in MPH could be several things though.

 

As has been said...with smaller tyres, how can trap speed be the same but with the same rpm ?

Something doesnt add up there ? 2" in tyre is a fairly substantial change.

And maybe the restriction could be the heads, cam ? If indeed there is one.

 

Cast or billet S476? What's the pressure drop across the A2A?

 

Trust me it can be spark blowout and not really see or feel anything crazy. He is also more prone to it with the small cam and low rpm

 

And exactly why it revs in low and won't in high. When I get blowout I get raped in performance, a full second off 100-150mph.

Can't really feel anything strange other than slow. Can see it in the log sometimes and other times hardly at all


Still consumes the fuel. Just labors to make any rpm. And always hits on the shift when when the converter is coupled and falls to low rpm and very high load

In the log it is pronounced and you can see it

 

I’ll def. gap down and report back. I’m all for getting the cheap easy stuff out of the way. Hard to test 6k+ on the local streets at big boost though! May have to wait till the track…

Guess my PAC’s were rated at .650 lift as well. Here are the specs… hard time believing it’s floating these springs. Also have a 7.35 push rod in it.. BTR suggested their .660 dual spring kit.

I’m right at 3150 race weight with untouched 317 castings. So I was hoping I didn’t max the combo yet. We know it has the power to trap 160 at least…so being limited to 154 with the smaller tires tells me something else is up. It’s the big 87/96 Borg wheel in a 1.25 T4 with a Billet 76 compressor. Not a known forced inductions comp wheel. It was from “Apex Racing” ,so I assume cheap china copy of some sort. Blades from hub to tip were longer than my cast wheel. Was good for 5mph gain on my 5.3 at like boost. If the compressor was done, wouldn’t it be heating up the air more in the 17-22lb range? Similar to how it behaved at 28lbs and the IAT’s jumped 30* to 160*? Also if the hotside were maxed would that show up as excessive back pressure?


Trap speed isn’t the same between the 2 tire sets. Was the same at different boost levels on the small tire. Smaller tire trapped lower at the same RPM. Hitting 6200 at 154 with the smaller tire. 6200 with a 30” tire I was hitting 159-160. Seem like the engine is petering out at the same RPM point regardless of the tire. So the taller tire was quicker because it ran out of steam further into the run… (close to the end I’d bet).

-thanks!

 

I would guess it's turbo limit, but check spark. The springs are wrong style, are they set up short?

Kurt

 

For us back yard mechanics, what do you mean by "setup short"? These are the drop in factory replacement style springs with the OEM retainers/keepers. I just "dropped them in". Why/how are they the wrong style?

-thanks!

 

I doubt the hotside is restricting you. I went 160+ at 3400lb with 2" hotside and a 96mm turbine wheel on 383ci.

Where are you measuring boost at?

 

I like hearing that! what kind of rpm? how much boost? t4? t6?

Measuring BP off the #1 runner where the emissions tube is.

 

I doubt the turbo is done by the data you provided.

I was compressor limited and when I approached the limits it just quit making boost. Could put more amd more pressure on the dome and not get any more boost or very little

 

If you put them in without shims they are low on spring pressure, not sure if this will hurt you now. When you buy springs for .660 lift, but only have .610 lift you will never get to the open pressure rated, as your cam stops short
Normally springs have safety in them that makes the problem worse! If I set up a .610 lift cam, I buy .600 springs or set up the .660 short. If possible you would take your stacked number (coil bind), add .050, than add your at valve lift. That would be your set-up height, it will be short of what the box says. Setting up that way will give you good open pressure and help calm spring surge when running near the limits. It will add seat pressure also, but I don't worry as much about that being high, just don't run low! Loft at higher rpm can cause performance problems without audible misfire.
If you can't check your springs adding shim that is equal to your missing lift should be safe, so .050 for you should be ok. I would try and find someone local that can measure them, or splurge on the tool.

 

Ah, I see. So after looking at the actual cam card it's .612. So a .038 shim should get me back in the general area of acceptable. The 7.35 push rods would make it even worse. Add the thicker LS9 HG in there and who knows what kind of actual valve lift I'm getting... I'll chk the push rod length and shim them up.
-thanks

Also noticed my #2 plug wire end is discolored. Looked like it was firing on the business end,but I don't recall ever seeing that dull discoloration. Plugs were .025 which appears to be what the new set I installed are gapped at out of the box. Dropped them down to .019.