Some serious power losses- thoughts?

[mendelmax]

Hi guys.

Sorry, that I bother you with stupid questions, but I'd like to hear some input from people who knows this stuff, to verify my hypothesis before I'll start fixing not-broken things

My stroker is placed in my friend's 4850 pounds ZJ (heavily modded machine), with HUGE air drag (tire on the roof, 4" lift etc.) but at the time on almost stock tires (235/75R15) makes 0-60 in about 13 seconds. It's too early to dyno it, because it has only 700 miles, but we did this one, short acceleration test, and we were surprised that it took so long. We expected to be somewhere in 11-12 second range. So we are now thinking about possible causes of this behavior.

Specs: 42RE tranny, 0.040" ocerbore, 9.7:1CR, 7.2SCR, Stock 93 Cam (75*AIC), short rods, adjustable FPR, stock 21lbs injectors. Fuel used- 98RON. 5W50 oil.

Now, my thoughts are:
- tranny shifts at 4000- way too early for this engine I suppose. But on the other hand, after turning OD/off it didn't want to rev much above 4000 anyway. Strange with this revver-type timing. AW4 is going to go in, but it has to wait till summer.
- exhaust too restrictive? It is 2.25" diameter exhaust with no cat and only FM44 muffler. To me it looks too tight for this displacement, especially when considering only high-revs power problems. The pipe bent before tranny looks far smaller than 2.25" too. It's the reason I blame most, but my friend didn't have money to make new exhaust now.
- snorkel? It has a snorkel fitted to the box with stock paper filter. Maybe it gives it too much underpressure? The snorkel is necessary in this case, so maybe a K&N filter would help some?
- engine not broken in yet? I know it will loosen up a bit, but I wouldn't expect any huge gains.
- stock throttle body and intake mainfold- these two will be replaced soon (they were planned from the beginning, but we decided to add them later to avoid potential problems), but can it really make that much difference? I expect to gain somewhere between 10 and 20HP from them, so we rather won't break the 12 sec. border.


What I already checked:
- AFR during acceleration stays at the border between rich and optimal on AFR gauge. It may not be the best, but indicates it's not a fuel related problem.
- the o2 sensor is virtually brand new. We installed it 400 miles ago, so AFR should be accurate.
- at startup it spinned the tire (2wd engaged, with truetrac in rear axle, on cold but dry pavement), so it rather doesn't lack power at low revvs. Problem seems to lay at high range of powerband.
-on N it revvs up to 5000 with no problem, which indicates it's not any kind of rev-limiter.

Any input/thoughts/blaming appreciated.

BTW, I must say love this forum, there's so much precious info here Congrats to all the users

[SilverXJ]

To start with the 42RE is just a pos trans. I had a 93 XJ with the AW4 and a 95 ZJ with the 42RE.. the AW4 was quite snappier.. not to mention the 42RE holds no award for reliability. The exhaust could be a bit bigger as well as the throttle body, but shouldn't be a major restriction. Flow masters don't exactly flow as well as an aftermarket muffler should. The air box and snorkel combination are probably where the major restriction is coming from. Snorkels don't flow very well to begin with for a stock engine. Then your stock injectors.. I'm betting they are old so their spray pattern isn't once what it was combined with their flow rate that could be another issue. I also don't see a wideband O2 gauge listed (I see an AFR, but judging from that it seems like a standard one) so you won't know that much from that gauge.

[mendelmax]

You're right, the injectors are old and crappy, but are going to be soon replaced with WJ ones, when we'll swap WJ intake in. They are there just temporarily. I haven't thought about them in this equation

As for AFR, that's also true, it was tight budget build and we just plugged the AFR meter into the stock sensor, just to control whether it runs somewhere in optimal ranges. We are soon going to check in with a wideband sensor on the dyno, but it has to wait some time to let everything break in. And to change the tranny, because measuring torque on 42RE would be quite difficult (BTW, is there any way to force it to stay on 3rd gear? I haven't explored this subject yet)

[SilverXJ]

What do you mean when you say you are going to swap the WJ injectors in? If you are saying you are going to install the WJ 4.0L injectors you are wasting your time. Stock specs for the ZJ 4.0L injectors are 21lbs @ 39psi. Stock specs for the WJ are 22.5lbs @ 49psi.

Now, you will be installing injectors from a 49 psi rail on 39 psi rail, which won't get you anywhere. Doing the math for that situation would yield you an injector that flow 20lbs.

new flow = squareroot ( new-pressure / old-pressure ) x old-flow-rate

20 = sqrt(39/49) x 22.5

[mendelmax]

I have an adjustable fuel pressure regulator, so we will get all the performance needed from the WJ injectors. I'd love to get some bigger ones, but they are pricey as hell here in Poland, so we had no other choice. We got the injectors almost for free, because they were sold with the mainfold.

Dino site specifies that 22.5lbs injectors at stock PSI work, so I guess, that if we'll bring it to higher pressure, like 55PSI it will give us enough fuel.

This way, we'll have 1.5lbs more efficiency and better spraying at low cost

[gradon]

Just don't max out the fuel injector duty cycle--better to have a larger injector and keep it b/t 80-90%. I have a set of the gray top 23.2#s from my 71K 96 if anyone is interested.