steven mack

Run the AFR heads with the valves they come with. Put in a 383 Scat rotating assembly .A comp cams 268 HR cam and you will get what you are looking for in performance.

ddahlgren

What free mods to go with the cat back? Who's cat back did you use?

thurman_merman

Oh let's see, it's been a while. Removed frisbee, cut airbox, gutted AIR pump, TB bypass. I think that's it. I was still running both precats and the main cat. The catback was put on by the previous owner, it has flowmaster mufflers, that's all I know. It might even be just mufflers with the stock piping, in fact, that's probably what it is. That's how it made 225, it would guess 215-220 ish stock.

esp42089

I took it off and tried to get an electric one for a 55 working, but the motor was toast. I don't have any wiper motor on it at the moment but have been playing with the idea of putting the vacuum wipers back on with a cannister and check valve.

This thought had occurred to me but I was concerned the price would balloon beyond my budget way too fast (TPI would be really restrictive, need new injectors, get into the whole mess of cleaning and machining the block. I feel like I should be able to get enough power out of 350 cubes.

ddahlgren

Read it and the dyno sheets especially. What is the value of the dashed red line in the original test as the scale is not shown. If the same as the same colored line in the after test at what looks to be 12.5 to 13/1 it would mean in the before test there was a pile of power left on the table just with tuning.

The other thing I can't reconcile is another poster said stock 205 rwhp and that line up with history for a broad range of cars with an 18% loss. Using that and 310 rwhp for your mods put a L98 with stock tpi and heads at 378 crank hp so 38 more than a LT4 with a few basic mods and a cam and LT1 exhaust manifolds. Cam springs rockers and retainers seem to be the only big buck ones. The rest seemed to be well under 100 DIY things. I did notice it dropped 20 FT.LBS. but carried it much higher due to cam no doubt. Makes me wonder if a shorter cam might be a better compromise with an automatic that I have. I own a ZZ4 cam and with springs retainers rockers and finding some elusive LT1 manifolds that I might end up with 285 rwhp with a fatter torque curve at the bottom. I do have the 3.06 gears so would help some there.

Not trying to start a rake fight like the last hp discussion I just walked away from but curious. The tuning seems like everything if my observations with the before dyno test are correct.

thurman_merman

I checked the sheet and it is the same value, 13:1.

That's the stock gm tune for you. There is definitely some power left on the table but as a percentage of the base number, it can't be much. I'd be surprised if it was as much as 8-10. Every stock C4 would benefit from a custom tune, it's a modification just like anything else. The tuners on the forum could probably answer this better than me.

I can't answer questions about other cars or even what mine would have done before the exhaust and free mods. I talked a lot with comp cams about cam selection for my application. I originally wanted to turbocharge this motor so I wanted to bring the power band way up in the rev range to attempt to gain low speed traction. There are other intakes and cams for making power in a more useable rpm range for the street but that's not what I was after. Not to say this thing is a dog, it has plenty of power through the rev range but it really wakes up over 4k rpm.

As far as comparing it to an lt4, from what I have read stock lt4s dyno from 290-310 on the norm. They we're underrated from the factory and that would put mine right there with a stock one. Exhaust and cam on one of those and they would leave me in the dust.

As far as the zz4 cam, I can't really help. I'm no cam expert, I use calculators and talk to cam companies when I'm choosing a cam. Be realistic about where you want your power to be in the rev range and don't worry about peak numbers. LT headers, bigger exhaust, bigger TB, under drive pulleys, and things like that can add power over my set up if you want to spend the time and money.

esp42089

Can an L98 stock bottom end spin to 7000rpm reliably, given my intent to bulk up the valvetrain or is there more work to be done?

eutu1984

At that rpm I would want an all forged bottom end.

thurman_merman

IMO that's asking a lot. 6k is usually pretty safe on a healthy motor in my experience, anything more than that is asking for trouble.

esp42089

Alright, I was thinking about going single plane intake, but I don't think it's the best way forward.

thurman_merman

I really like the idea of a single plane but it's hard to get height in the manifold without flattening out your TB elbow under the stock hood. You really need a smooth transition to bend the air. That, and there isn't much of a selection for single plane efi manifolds. I was looking to go down that path before I decided on my miniram.

thurman_merman

Of course it's really over kill for anything close to stock. Big inches, cam and heads are the way to use a single plane properly. Unless you're going deep into modding the motor you would be better served with a super ram.

ddahlgren

Read your combo again and really 1 question. I have a Y pipe I bought from a member here years ago and don't have the slightest idea if a L98 or LT1 pipe. It bolted on but had to slot the bolt holes to fit L98 exhaust manifolds. I will measure the pipe diameter tomorrow if that is a clue. The only other thing is the Y pipe has no CATS but did fit the main CAT just fine with zero mods. If there is one thing I have more than body work it is exhaust work. I found some manifold for more than I really want to spend but curious if a direct bolt on to L98 exhaust system. Asking you as you have installed them so all for real real and no guesses.

thurman_merman

Did you have to slot the flanges inward toward the pipe? If so it's a y pipe made for LT1 manifolds. If you had to slot them outward away from the pipe I don't know what it is. Piping should be the same diameter either way. Mine was for l98 manifolds and I had to slot mine outward to fit the lt1 manifolds because the stud spacing is wider on the lt1 manifolds. As far as I know that is the only difference between one for l98 and one for lt1. It bolted back up just fine.

ddahlgren

Yep ground inboard around 3/16 of an inch if memory is working right as it was 7 or 8 years ago. Did the CAT back and high flow main cat at the same time. I guess this will be the time to pull off all the AIR pump parts and build the idler bracket at the same time.

GREGGPENN

I think you should get the tune locked down before thinking mods. In 1st/2nd gear, you should already get what you're looking for.

esp42089

I need to back off my statement that everything is great. I think that the engine isn't running at full advertised power right now but it is usable and consistent in it's behavior. I'm sure some of that is down to my amateur tuning, but I'm also not confident the top end of the engine is actually in great shape. Here's what I've checked:
-vacuum leaks on manifold with aerosol starter fluid - no surging or signs of big leaks
-fuel pressure and bleed down (regulator and pump appear to be functioning correctly)
-checked lash, most of the rockers were quite loose and required adjustment
-lost spark, and ended up replacing the plugs, wires, coil, cap, and ignition module which helped a lot
-IAC wasn't working reliably, I replaced it, and it appears to function but this leads into my next comment:

My idle always starts at 650rpm when the engine is cold but after coolant is warm the idle will end up resting at some point between 800 and 1200rpm. It will reliably return to that same point for the rest of the drive until I shut off the engine and it cools down. The idle point changes drive to drive but it doesn't hunt for idle while running; it is stable. I can start the engine, unplug the IAC, plug in a tester switch and run the engine idle up and down which is why I think the IAC works. I'm not certain about why it's doing this, but I suspect a manifold leak somewhere.

The engine also falls flat on its face at 4500rpm and sounds like the world is ending. I routinely shift it at 3500 because that's pretty much when it starts sounding strained and stops making good power. I know LTR intakes run out of breath, but I think it's running out a bit early.

I also can't figure out my base timing, because the balancer marks are all over the place every time I put a light on them, leading me to believe that my balancer has separated. I chose my base timing through trial and error with dizzy adjustments in conjunction with IAC steps to arrive at 650rpm and mid-range IAC counts in the tune. Right now, my tune thinks I'm set at 6degrees advanced.

Like I mentioned before, the bottom end seems solid based on the oil in the sump and compression checks (160-170psi on each cylinder). I thought about taking apart the intake and reassembling it to see if that helps, but that lead me into thinking I should consider also replacing the balancer, but then I need to rediscover my timing, which is easiest with the heads off. And while I'm in it that far, maybe I should do a timing chain replacement, which is a prime time for a camshaft replacement... And then we arrived at a 3k budget and the thought to redo the entire top end

ghoastrider1

Btw

ghoastrider1

I like this project. To bad you don't live closer to Indianapolis.

ddahlgren

I have worked on a bunch of different aftermarket ECUs over the last 20 some years and your problems all sound like tuning. I have a pretty much stock L98 that is also a '91 and it runs way better on the factory ECU than what you describe. First you set the timing by unplugging the EST wire from the ECU then set it at 6 degrees BTDC it should stable damper slipped or not though will be wrong if damper has slipped. This can easily be determined with a TDC stop. Pull all the plugs and insert the stop in #1 plug hole and slowly and getly turn the engine over by hand until the piston hits the stop. Make a clear mark there and turn the engine in the other direction the same way gentle and slow until the pist hits the stop and make another line. Halfway in between the 2 marks is TDC #1. Set the timing using that as a reference and just use a timing light with a dial back feature setting it to 6 degrees and when the 0 marks line up you will have 6 degrees BTDC then plug in the EST wire and yes the timing may move around if the ECU uses idle timing to stabilize speed if not it should be pretty stable at any given RPM.

Your idle speed is either the wiring from ECU to IAC is wrong or the gain factors are all wrong and lacking access to those you might check to see if there is a table for IAC position vs. engine temperature and adjust that accordingly for correct idle speed or at least a good starting point for the ECU to use as an initial position to start making corrections. Lacking any hard info you might download the bin files and trial software to read them for a L98 and get your timing #s and injector pulse widths as a starting point to work from along with warm up fuel enrichment tables etc.

In all honesty you would have proably have been better off using the factory ECU with a chip burned to eliminate error codes as needed for things not installed.