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Car Making Peak Power Under 4500RPM, Things To Check?

2K views 53 replies 13 participants last post by  Meangrn92GT 
#1 ·
Guys I'm having an issue with my car making peak power real low, like 4300rpm low. Here is a picture of my last dyno, this was rich and low timing but you can see how it made peak power at 4300ish RPM and just fell off after. I've since got a new tune from Bob Kurgan with a leaner AFR and more timing and it still "FEELS" like it's making peak power early then falling off after. I'm going to get to the dyno again by next weekend but it still "feels" the same as it did before so I'm making this thread to see what I can check in case I'm looking over something simple. I was going to add more boost to get it faster but it should be making peak power a lot later than it is so I need to fix the problem first. Here is the combo:

Stock block/bottom end
TFS TW heads
1.72 Roller Rockers
Stock Cam
Exploder Intake
Stock TB
Hellion Turbo kit with a Master Power T70(.68 exhaust housing) making 10psi
Timing now is at 21* total but I have a 22,23,24* total timing tunes I've yet to try(waiting till dyno day)

Plugs are new and there is no check engine lights and no mechanical issues/problems that I know of. Anyone have any suggestions? Thanks in advance.:salute:

Last Dyno Graph(hard to read, sorry but you can figure it out if you look at it for a sec):
 
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#2 ·
i would start by putting it on another dyno..
 
#4 ·
Hmm, I'll see if I can find another one in the area to slap it on. It just feels like it doesn't pull up top on the street either.

I dont know ,but that stock cam with tw heads cant be helping the problem to well.
Most stock cam cars pull up top pretty good, if I knew that would of been an issue I would of put a F cam in it, everyone said it doesn't really need a cam change being a stock block.:snore:
 
#7 ·
Car doesn't have an air filter on it, just a single layer screen. I don't think it's the cam, at least everything I've saw in the past would show that it's not the cam at all because the stock cam is considered a "decently good" turbo cam.

Any other suggestions, there is something causing it not to pull up top and I want it fixed!!!!
 
#8 ·
I'll start.. I think its the tune LOL :)
 
#9 ·
Don't start with me Bob! Hahahahaha, the poor tuner always gets blamed but not from this person, it's something though. The car may be going a little rich, I plan on pulling a lb or two of fuel pressure out on the dyno if need be but it shouldn't stop pulling like that with a 11.5ish AFR should it? It's 11.6-11.8 throughout the curve for the most part but could go a little richer right before a shift.

The exhaust housing on the turbo shouldn't be an issue either should it? I'd like to try a .96 housing on it just to see how much it actually does affect it. Anyone have any feedback, need to get this thing fixed so it pulls like it should!
 
#10 ·
.96 would definately move the power band UP
 
#12 ·
Took the car to the dyno again, with the air fuel around 11.8 the entire pull the car still falls on its face at 3800-4000rpm. Turned the boost up to 13psi and the car made 449hp at 3900rpm. I won't be keeping the boost this high once I figure out why it's not pulling up top like it should but I want it to make around that hp number on 10lbs and I don't think it'll have a problem doing so.

Thinking I will do a exhaust housing swap though, maybe that is actually what is killing it up top. For the money I have to try it, buddy has one I can use but I'll probably just order one to try and see how that helps the car.
 
#13 ·
question is.... do you have a log of your boost?? does it hit 13psi at 3800 and fall to 10psi by 6000? might be your wastegate..
 
#14 ·
I don't have a log of the boost gauge but it was holding steady on all boost settings based on my gauge. Everyone seems to think it's the housing and I've been wanting to try the bigger housing for awhile because it'll work better with what I have planned in the future anyways, I think I'll be getting one just to try it out.
 
#15 ·
I cant see the dyno here at work but "usually" big down low and nothing up top is turbo running out of breath, turbo housing too small, exhaust restriction, intake restriction.... something along those lines.

Personally I like running an AFR close to 10.8-11.2 I dont like going much higher just from a safety standpoint.
 
#16 ·
We unhooked the catback from the car on the dyno last night and it didn't gain any horsepower but it did make peak power a few hundred rpm higher. We know the turbo isn't too small, exhaust isn't a problem either or it would of gained a lot of power when we unhooked the mufflers. Intake restriction, nothing there really to cause a restriction, sure it might not flow as good as a bigger intake/bigger TB would allow it but it's not hurting it TOO much I don't think, def nothing to cause it to drop power like that.

Turbo housing will be bought tomorrow, everyone seems to think that's it.
 
#17 ·
Changed the turbo over to a turbonetics T76 with a .96 exhaust housing on it, took it back to the dyno to see how it done. Same ####, different day pretty much. Car made peak power at 4000rpm and then the power went bye bye.

Did find three boost leaks though, one HUGE one where the throttle body shaft comes through the throttle body. I have another stocker TB laying around that I'm going to put on it but it may be time to order one. :( The other one is where my boost gauge connects, gonig to redo it...it's very small but I want it taken care of. The third one is where the MAF electronics mount to the actual MAF housing. What would be the best way to fix this one, use RTV or gasket paper?
 
#18 ·
if the boost leak is as big as you say, then im sure thats a huge issue.. i would also look into the rocker lash.... possible valve float??
 
#20 ·
Well here is a way to put the boost leak in prospective, using a boost leak tester and a air compressor that powers a shop it took 5-6 seconds for the pressure to go from 5psi to 20psi in the piping.

What does valve float sound like or do when it happens? I've never been around that before so I can't say for sure on that one. The rockers are quiet, they don't make a peep but I guess that doesn't mean they are right, I'm up for anything.

Ignition is stock right now, I had a Mallory Hyfire box and it went out. Need to send that thing back for repairs, running 3923 autolites gapped at .032 I think it was.
 
#21 ·
i would say the boost leak and stock ignition.. ive never had luck over 6psi with stock ignition gapped as low as .018..
 
#22 ·
If your running stock ignition its definatly a problem. ESPECIALLY with a .032 gap
My car breaks up horribly over 4k without the ignition hooked up..
Pick up a 6a and I bet it makes a different
 
#23 ·
Maybe that's it, not sure. It's funny because my Mallory box died a day or two literally in front of my first dyno date. Things always work out for the best don't they. I'm going to send this box back to Mallory and get a quote on fixing it. They want an invoice and I bought it new from Ebay awhile back so I'm going to have to dig to find it. Not sure why they want an invoice if it's not covered under warranty anyways, oh wells.

I'm going to fix the boost leaks and get the ignition box fixed and try it again. Even though it's a stock cam, it should still make power at least to 5200-5400 shouldn't it? I was wanting to shift the car around the 5400-5500 rpm range, hence the reason I went with the stock cam and 1.72 rockers.
 
#24 ·
Bob, if the engine was having that much trouble moving air don't you think the MAF curve would be flatish?

I am with you on another dyno, will the engine make it to 6000?Backpressure? Is the turbine housing free of debris? I had a turbo a while back come out of the box with a piece of tubing in the scroll and it was "shy on power"...

Jay Meagher
 
#29 ·
So you're saying if it was valve float and it was doing it that bad then it wouldn't be able to rev as high as it does? That gives me some hope, haha. I sent the ignition box out for repair and am currently looking to buy a TB. Redoing my boost gauge tonight because it was a tiny little leak where it connected. Also waiting on a buddy to find the T20 bit to take my MAF apart so I can put a gasket/RTV under it to hopefully seal it. That will take care of the ignition and the boost leaks.

Is .032 too big of a gap for the plugs even with a good ignition box and coil?
 
#31 ·
Jay,

That's what I've said all along about the ignition not being the issue because the power curve is still smooth. I guess having the ignition box fixed and put back on def isn't going to hurt anything though.

Not sure how you figure out "pound per hour" of air but the car is making 10psi. We turned it up to 13psi and it made 449rwhp at 3900 or 4000rpm. Obviously I want it running right because it'll make that much power at 10psi no problem. I want around 460ish rwhp to be happy I think, just looking to get this thing going. Would valve float cause the power to fall off like that and still have such a smooth graph? What are some signs of valve float?
 
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