The first and most important, where do new users go for the answers to the simple questions? Right here!

For the love of God - INCLUDE THE YEAR IN YOUR POSTS!

This is a living document. As suggestions are made, they will be updated within this page, so users don't have to surf. I would like to see posts kept to a minimum, or, at the very least, deleted every so often.

Within the text will appear words in bold print, that means it is a topic covered in a section within the FAQ.

This is NOT the place for questions, just comments and suggestions to improve the FAQ! Thanks!

Your C4 Corvette and E85

This has been added due to the increase in questions about using E85.

Q: What is E85?
A: It's a newish fuel blend designed to lower our dependency on foreign oil. Will it work? I donno. E stands for Ethanol, the 85 is the percentage of ethanol, the remaining 15% is regular gas. It's designed to give the fuel color, odor and flame.

Q: Can I use it in my Vette?
A: In a word, no. More accurately, not without work. The ECU must be tuned to deliver enough of the new fuel, because it requires much more E85 to burn than it does the gas. Also, ethanol is very caustic, so the fuel delivery system needs to be able to handle this.

Q: What about MPG?
A: As stated above, it requires more fuel... so you can expect to see a 30% or more drop in economy.

Q: Where can I get it?
A: Mostly in the mid-west. There are, at current writing, only about 600 stations selling it. I have been informed that there are several stations around Ft. Meade, MD, that sell E85. This is due to military/governmental usage of the fuel. Perhaps that means more stations will appear around military bases across the country?

I hope this clarifies the point!

Model year changed and milestones

1984 - First year of the C4, introduced in March, 1983, as a 1984 model. This was done due to QC problems. US DOT gave GM permission to sell the car as a 1984, because all requirements for the model year had been met. The only engine available was the Crossfire, L83, HP.

1985 - Tuned Port Injection introduced. 230 hp.

1986 - Aluminum heads introduced mid way through the model year. Chassis modified to resist cracking problems at the firewall and b-pillar. ABS introduced. First year of 1-piece rear main seal. Aluminum heads introduced at roughly #18,000 on Coupes and all verts had it. If the car came with aluminum heads, it has HP.

1987 - Carry over of 1986. Beginning of the B2K, Callaway Twin-Turbo option. First year of the hydraulic roller cam. Introduction of the Z52 option. Aluminum 128 heads were standard.

1988 - 35th Anniversary - two-toned white/black top/white wheels and interior. First year of the D-port 113 head, which carried over to 91. Two sets of wheels offered; a 16" wheel came with the base suspension and a 17" came with the Z51 and Z52 suspension. Five HP increase for coupes with 3:07 rear axle.

1989 - First year of the ZF sourced manual transmission. Last year of "Atari" digital dash.

1990 - Redesigned interior, drivers side airbag standard. Start of ZR1, HP, unique rear fascia with squared tail lights and standard front end.

1991 - Redesigned exterior, ZR1 style back end, with new, rounded front end. Last year of B2K option. Stock L98 produced 250 hp.

1992 - LT1 introduced, 300 hp, 330 lb-ft torque. Interior revamped with new digital display in speedometer and finish changes (Dul-cote) on plastic surfaces. Introduction of ASR (traction control); standard on all Vettes.

1993 - 40th Anniversary, "ruby". Special Ruby colored interior, exterior and on verts, top. ZR1 up to 405 hp. Cam change on LT1 added 10 lb-ft of torque. Midway through the year, ZF 6spd manufacturing move to USA.

1994 - Redesigned interior, passengers side airbag. New ECU, added sequential fuel injection. GM changes the automatic transmission to become electronically controlled (4L60E).

1995 - Revised Optispark. J55 Heavy Duty Brakes made standard. No other major changes. Last year of ZR1. OBDII hardware installed, but not functional.

1996 - Last year of C4. Special editions: Collectors Edition, all silver with either gray, black or red interior; GrandSport, 1000 made, all came with Admiral Blue paint, white stripe, red hash marks on front left fender, black ZR1 style wheels, rear fender flares (coupes only), interior black or red/black - All GrandSports were LT4/6spd cars. All manual cars came with LT4 (HP), automatics came with LT1. ODBII fully functional.

Definitions and Abbreviations

F-Body - Referring to V8 equipped Camaro Z28 and Firebird Formula or TransAms. They used a version of the LT1, same heads, 2 bolt main.
ZF6 - Six speed manual transmission used from 1989-1996.
4+3 - Four speed manual transmission, with planetary overdrive, available from 1984-1988. Made by Doug Nash.
Frisbee - A metal chunk on the water pump pulley on 1988-1991 L98 cars. It was designed to retain the belt better, sadly, it weighs a ton and robs the motor of 5hp! Remove it!
Optispark - This is the distributor used by LT1 and LT4 engines. It is mounted below the water pump. It is prone to water damage, and the water pump can leak on it, causing all sorts of damage. For more detail, see the Optispark section.
OBD - On Board Diagnostics. There are two levels of OBD - I & II. Level one is basically everything before level two came out! Level one changed many times over the years, as it refined and defined itself. Early cars have only one O2 Sensor, or no knock sensor, etc... As time went on, OBDI cars gained a second O2 and knock sensor. But OBDII is most sophisticated. I has the ability to compute values of unknowns based upon known values. For example, there is no "sensor" for the catalytic converter, but because there is an O2 sensor before and after the cat, the computer can determine if the cat is working OK.

Misc Specs & Cautions

4L60 is a renaming of the old standard 700r4, they interchange. There are harness differences between the years, though.

The 4L60E is the electronic version. They are physically interchangeable, yet require a computer to make it shift.

Somewhere about 1990/1, the Society of Automotive Engineers (SAE) changed the rules regarding the composition of wire insulation.

What does this mean? Newer wire is better. It is better able to deal with heat and harsh chemicals found under the hood.

For those of us who own a Pre- 1991 Vette, we have a problem. The heat under the hood is slowly eating our wiring away. The insulation starts to get brittle and break, causing possible shorts and grounding conditions.

If you are having a problem with your early C4, and all other logical fixes ain't fixing it, get out a test light and a DVM and commence chasing the harness. It sure isn't pretty, but it might be your only fix.

At this time, I am unaware of aftermarket harnesses being made. Additionally, I do not know if any new parts from GM are NOS, or newly made, meaning they have new wire. If push comes to shove, a new GM harness will last 20 years or so!

Fluids

Doug Nash 4+3 - use standard 75-90 wt gear oil, and Dextron III ATF in the overdrive.

Automatics - Dextron III.

Rear differential - 75-90 wt gear oil, 2 bottles of GM Limited Slip Differential Additive.

ZF 6spd - The GM oil (available from GM with part number 1052931; superseded by GM part number 12345349) and the Castrol Synthetic Formula TWS 10W60 (available from BMW with part number 0751009420). Other types have been used/suggest/recommended, including ATF, but the Forum consensus is that the two listed above are the SAFEST solutions. Click for the [URL=http://www.c4guru.com/articles/ChangingZF6Fluid.pdf]C4 Guru Tech Tip - Changing ZF6 Fluid[/URL]

Coolant - Prior to 1995, all Corvettes came with GREEN coolant. Do not mix Dex-Cool with conventional green coolants, or it will cause the mixture to become jelly. It's a real mess to clean up. There are new coolants that are compatible both ways, they are OK.

Engine oil - It does not matter the year, but any of the good quality Synthetics are worth their weight in black gold. Mobil 1, Valvoline Synpower, etc; 5W-30 is the recommended weight.

Oil Filter - Premium brand, K&N, AC Delco Gold, Mobil 1. The K&N has a nut on the end for easy install and removal.

Brake/Clutch Fluid - Valvoline Synpower. There are better options for racing, but Valvoline is a great value and works wonderfully on the street.

Power Steering Fluid - Valvoline Synpower.

Reading and Documentation

The best manual is the Helm's/GM factory service manual. Whenever the name "Helms" is used, it means the factory service manual. You may get a copy from [url]www.helminc.com[/url], the parts for sale section or eBay.

Corvette Black Book, Michael Antonick

Cor-vette Specs 1984-1996, Michael Antonick

Corvette From the Inside, Dave McClellen

Corvette Fuel Injection and Electronic Ignition: 1982-2001, Charles O. Probst

101 Projects for your Corvette 1984-1996, Richard Newton

Heart of the Beast: The story of the ZR-1, Anthony Young

Performance Mods that Don't Work

This is a short list of things that don't work... and could waste your hard earned money!

Air foil
Relocating IAT
Throttle switch

Performance Mods that Do Work

Roller rockers
Headers
High flow cats
Remove frisbee
Electric Water Pump
Beam Plates
Expensive engine rebuilds

Wheel Offsets

This section has been enhanced, due to info from CentralCoaster.



Power Steering Trivia



C5 Brake Adapters

Forum member Eike sent these links to me. The PDF contains the CAD drawing for C5 Brake Adapters. Enjoy!

The files are a little slow to load, it's coming in from Germany! When I have a moment, I will save them locally. There is a pic, but it needs resized!

CAD Drawing as PDF file.

Fuel Injection Definitions

Tuned Port refers to the L98 setup. It alludes to the long runners of the intake manifold.

Sequential Injection fires an injector in time with the spark plug.

Batch fire is when all 4 injectors on one side fire at the same time, followed by the other side. On earlier C4s, all 8 injectors fired at the same time, once per crank rotation.

The advantage to batch is at high rpm operation - IE, racing.

The advantage to SFI is all other times... better MPG, better idle, better drivability, and better tunability.

On-line Resources

GM Parts Direct - They have much of the GM parts catalog available on-line. One warning - they charge shipping like they are sending you gold bricks. Be cautious.

Ecklers - A great place to find pictures of parts and hard to find parts!

C4 Suspension Chart

Nathan Plemons' LT1 Performance Page

Gordon Killebrew - Gordon is a true genius. He helped write the Helms manual. If you are on your last straw, a call to him is worth whatever is might cost.

Suspension Specifications This page lists all the suspension specs, spring rates, et.al. for all years of C4.

Rebuilding the Chevrolet LT1 Engine By Doug Anderson

Crossfire Information This site is dedicated to supporting Crossfire engines.

Engine Casting numbers and locations - This site looks to have a ton of great information on GM engine castings.

VATS Bypass - From member GIJoe. EVERYTHING you could ever want to ask about the VATS system. It is currently focused on the earlier years, but all years will be covered soon.

CentralCoasters Speedo Correction Page - Helps you calibrate the speedo readings when changing rear gears!

Tools

A good set of tools is required for working on these cars. A nice 100+ piece Master Mechanics set from Sears (Craftsman) is a great start!

Also, you should have:

A small hand pump for removal and addition of fluids

Digital Volt Meter (DVM) and test light for debugging electrical glitches.

1/2" drive, 17mm allen head socket. This is used to remove the drain and fill plugs on a ZF6spd. These can be VERY tight.

Breaker bar

Pry bars

Differentials

This topic has been fodder for fights for a long time now! I hope we can clear the air on this subject once and for all.

In 1984, only the Dana D36 was available. It was used on both manual and automatic cars.

In 1985, the Dana D44 was released for use with the manual, ONLY. No automatics came from the factory with a D44 rear end.

The difference between the two is ring gear size, and the associated strength that comes with a larger ring gear.

The D36 has a 7.5" ring gear; the D44, an 8.5" ring gear.

The maximum ratio that will fit in a D36 is a nominal 3.73. After that, your taking your rear ends life into your own hands. The D36 has a power limit of about 450hp... but it won't last long on the strip, even then...

The maximum ratio for a D44 is a nominal 4:11. It will require some minor case machining to make it fit perfectly. The D44 is known to deal with over 650hp with no problems.

I use references to horsepower. This is because most folks can relate to horsepower better than torque. And since the SBC creates about equal amounts of both (neat trick, eh?), it's just as easy to use horsepower. Finally, the real killer to the D36 is drag slicks. They will flat out tear a D36 to hell. The more the power, the stickier the rubber, the shorter the lifespan. A D44 is NOT indestructible. They do have their limits, but they are high, and with care, they will last.

When drag racing, especially when the power levels get over about 600hp, the independent rear end becomes a handicap. At this point, it might be worth looking into a 4-link live axle setup, if this is the case, a Ford 9" is the best piece for the job.

At the time of this writing, a D44 is worth about $2,000.00, complete (differential, casing, drive shaft, C-beam and bat wing).

The best u-joints are the Spicer units.

As mentioned above (see Fluids), use two bottles of Differential Additive before adding the gear oil. This recommendation comes from Gordon Killebrew.

2.59 is the standard ratio for all automatics.

3.07 is an option, G44, but was standard for 85-88 L98 manuals.

3.31 and 3.33 popped up from time to time, too.

3.45 is the standard rear for ZF6 LT1s and ZR1.

3.54 was offered off and on for the ZR1 and the LT1. Beyond that, it's
anybodies guess.

ZF Clutch Issues

This chapter applies only to 1989-96 Vettes with the ZF sourced 6spd. For detailed technical information, contact [URL=http://www.zfdoc.com]ZF Doc[/URL].

The biggest problem involved with doing a clutch on a ZF equipped Vette is the cost - mainly, the cost of the flywheel.

The flywheel is a "Dual Mass" unit. This means, there are two chunks of iron, one that attach's to the engine, and another that is mated to the first.

A few of realities:

1) The springs do not last forever. Expect it to be totally toast by 120k miles of normal use. However, clutches go by 100k miles, usually.
2) Cutting is not recommended due to the space issues inside the bellhousing. Remove .010, for example, and the clutch is too far forward. It IS that close in there...
3) It's HEAVY - ~40lbs. It weight varies depending upon the amount of metal needed to balance it.

The stock clutch uses a SOLID friction disk. This is because the dual mass acts as the springs on a conventional spring disk.

All pressure plates are made by Valeo.

Replacement Options:

1) You can replace with factory parts. A new flywheel is about $700.00. The rest of the clutch is very reasonably priced.
2) You can replace with factory GM parts, using a menu of F-Body parts, to convert to a Single Mass Flywheel. Parts required]Final Thoughts on Clutches[/i]

There are several brands available:

Mcleod; Centerforce; Carolina Clutch; SPEC

The reality is thus: All clutches will blow up if enough stupidity is applied to them.

With that said, there has been a lot of problem with SPEC clutches over the last few years. Yes, some members have had success with them, but they are the minority. I just don't want to see someone else screwed out of several hundred dollars...

Addendum: SPEC seems to have improved QC. I don't have recent experience, so ask the forum regulars for their current opinions.

Optispark

I touched on the downfall of the Optispark up top under Definitions, this is more about how to remedy the Optispark.

History

The Optispark (Opti) was introduced in 1991, with the LT1 as part of the 1992 model year. Why did they do the Opti? Even Gordon Killebrew doesn't have a clue.

The Opti is made up of two sides, one side, the side against the engine, is an optical pickup. The pickup is made by Mitsubishi, and is a very reliable component. The optical system reads a disc that has marks in it. On the outer edge are 360 marks, inside that, are four marks. The outer marks create the so called "High Resolution" signal to control timing, the inner marks, or "Low Resolution" are used to signal fuel delivery.

A common shaft joins the optical side with the high voltage side. The high voltage side is similar to any distributor, except it is much thinner. The unique part, the sides of the cap are sequenced, so all the odds are on one side, evens on the other!

In 1995, the second generation of the Opti was released. It has a venting hose and is better constructed.

Problems

Codes can present themselves if an Opti is failing. These codes very by year, but for the earlier versions, codes 16 and 36 are bad(Note: these codes only apply to 1992 - 1995, 1996 uses ODBII and has different codes). This means the optical portion has a problem. Most likely, the error is caused not so much a failure in the optical portion, but the bearings having worn out, causing a wobble in the drive shaft.

A common problem is when coolant from a bad water pump leaks in. Coolant will eat the grease inside the sealed bearings. This causes the common shaft to wobble.
http://www.c4guru.com/modules.php?name=RPOcodes
If there is a wondering idle or a high RPM miss, you can blame a bad cap/rotor. The cap/rotor fails due to carbon tracing, caused by moisture getting into the Opti.

Factory Options

New Optis are available from the dealer. Also available is the Venting Hose Kit, that can be retrofitted to the original cap design.

You can also update an earlier engine to the later Opti, by changing the timing cover and various internal parts related to the timing gear.

I cannot advise a cap/rotor change. The labor is the same... and there is no guarantee that a rotor change will result in a long term repair.

If your car is a 1995 or newer, then a cap/rotor change might just be worth investigating.

Regardless of the vintage, removing the cap and sealing with RV is a good idea. It might not be a bad idea to install a drain hose on the water pump, to direct the weep holes output to the side, instead of on the Opti.

I do not recommend the Ecklers or Mid-America cap venting system. It is a single hose, and as such does not create positive pressure. It might suck some stuff out, but I just don't see how it can without creating a complete loop.

Aftermarket Options

There are now a few options available:

Dyna-Tech - This is a totally re-engineered version of the Opti. A good idea for those who wish to maintain a stock look. It is questionable if Dyna-Tech is still around. I will confirm.

LTCC - This uses eight LS1 coils, and the Opti to read the data needed to fire the coils. www.bailey-eng.com

DelTeq - This is similar to the LTCC, but uses a Northstar coil pack.

MSD - They are now making a replacement Opti, and seems pretty good.

Accel - They, too, are making a replacement. It is cheaper, but you get what you pay for. This unit uses oil-lite bushings instead of bearings. I would be leery of it.

Generic replacement Optis are not recommended. They don't include the cap/rotor and are of questionable reliability.

ZF6 Clutch Slaves from GM

This has been an on going problem for several years. Recently, CentralCoaster took a slave apart and found that a seal inside has been installed BACKWARDS. The consensus is that new GM slaves should be checked to be sure the seal is on correctly.

A quick update on this topic, re: aftermarket parts. Three years ago, July 2009, I replaced my clutch. I also replaced the hose and the slave cylinder. I got the cylinder from NAPA. It is a single piece of cast steel. In the past three years and 10k miles, the system has not leaked one drop of brake fluid. This part was made in Taiwan and seems very high quality.

Ignition Lock Cylinder Maintenance

This came from jonszr1:

He was having an issue with the ignition cylinder jamming and not turning properly. He had a conversation with a local locksmith and he gave him these steps to follow on how to clean and rejuvenate a bad Key Lock Cylinder. This is the part that the key goes into to start the car. I also must note, this is NOT the ignition switch; the switch itself is mounted down the column and is accessed via a rod on the cylinder. Modern cars to put the ignition switch on the end of the cylinder, but not car with this style steering column.

You will need a can of electrical contact cleaner. NOT BRAKE CLEANER! Contact cleaner evaporates better and won't take paint off. Powdered graphite and a air compressor with a needle adapter.

Spray about 1/4 of the can into and around the cylinder. This will clean the cylinder and free any debris off the tumblers.

Next, hit the cylinder with a good shot of the graphite powder. Really fill it up. Inside. around the hole where the cylinder meets the column.

Finally, insert the key, gently working the graphite into the cylinder. This will get things moving. Then test. It should move much freer.

=====================================================

Final Thoughts

It is important to understand that the LT1 still needed the harmonic balancer (essentially the crank pulley) to be balanced and "keyed" to the right place on the crank. If you ever remove the harmonic balancer, mark the location on the crank CORRECTLY or it will be a problem. Sadly, the crank is not keyed, this means the pulley can go on anyway it wants to.

Before you condemn the Opti, make sure the coil, ignition module and coil wire are in good shape.

=====================================================

As I said, this is a living document. Please comment on errors/omissions.

Credits and Thanks!

A lot of people helped make this happen! Thanks to you all!

I know there are link errors, I will fix them soon!

Hell of a post, bro!

Glad to see it again. :tongue:

I'm glad you saved it.

It's a valueable resource that will bring many to the forum.

Since this is an area of the LTx engines that differs greatly from other SBCs, even those of us who were familiar with SBCs had a learning curve when it came time for some internal mods. I hope this helps. And since I'm doing it from memory, please feel free to correct or clarify and I'll make any needed changes.
First the electric water pump. It's popular for several reasons but for here I'm mentioning it because it's the only way you can added a true double roller timing set to your engine, if that's something you desire. This is because the factory water pump is run off of the cam gear by a smaller gear/drive. This doesn't allow the room under the timing cover for a double roller set-up. As a side note, can you install an EWP without going under the cover and eliminating the drive? Yes you can, but I don't recommend it. Without the mechanical pump at the other end of the drive to support it, I don't think it would be long until the drive seal in the front cover started leaking. Remove the drive and plug or weld up the hole.
The early opti used in '92-'94 Vettes uses a different drive set-up than the later one. I'm not entirely sure if there's a double roller set-up made that works with it or not. I would definitely appreciate input from anyone who knows this. But for sure you can upgrade to the later opti and use the popular Cloyes 9-3145 timing set or similar. The later opti runs off of an extended dowel pin for the cam gear. On an early LT1 or other SBC this pin is only long enough to locate the cam gear properly to the cam. To use the later opti, this pin needs to protrude about .620" measured from the front of the cam (with the gear off). You'll also need the harness for the later opti which plugs into the engine harness under the right side fuel rail cover on the intake. You'll need to buy or make the vacuum harness the later opti utilizes to expel moisture. (This is one of the design improvements to the later design) You'll require the later style '94-'95 timing cover too. The '96 cover can also be used. Just plug the 3/4" hole for the crank sensor.
It's also my understanding that on a '96 LT1 or LT4 that this sensor can be eliminated with a programming edit. Again this will be necessary if you want to use a double roller because you need to eliminate the trigger wheel for that sensor to make room for the DR crank gear.

Rebuilding the Chevrolet LT1 Engine, Doug Anderson, Automotive Rebuilder, September 1999
http://www.automotiverebuilder.com/ar/ar99928.htm

If you ever remove the harmonic balancer, mark the location on the crank CORRECTLY or it will be a problem. Sadly, the crank is not keyed, this means the pulley can go on anyway it wants to. SORRY BOGUS BUT I HAVE TO DISAGREE


THIS IS INCORRECT THE 3 HOLES ARE OFF SET THE BALANCER WILL ONLY GO ON ONEWAY AT LEAST ON LT1 ENGINES ANYWAY. :thumbright:

Quote:
Sorry but you're speaking of the way the balancer bolts to the hub. The statements you referenced refer to the hub installation onto the crank. So guess who's wrong now?

The LT1 balancer has an arrow... the hub does require marking...

you are correct and i now see the error of my ways ....Tim

I mentioned this to you once before.

In *ALL* batch fire Corvette applications (L98), the injector drivers are internally wired together in the ECM, so all 8 fire at the same time. Not bank to bank as you suggested.

I didn't remember it was you, but I remember someone mentioned it.

I have to disagree.

I found in the book, Corvette Fuel Injection and Electronic Engine Management: 1982-2001, by Charles Probst (deceased), on page 10, Overview, paragraph 3.2, reads:

Multiport Fuel Injection (MFI), used in Corvettes from 1985 to 1993, delivers fuel to multiple injectors at the intake ports, ganged as left bank/right bank.

Later on the same page, caption for figure 1-15 reads:

The Corvette Multiport Fuel Injection systems deliver fuel to each individual cylinder. From 1985-93, Corvettes pulsed the eight injectors in two ganged banks of four. Sequential Fuel Injection pulses each cylinder separately.

Also, schematics on page 354 (1992/93 LT1 ECM) show two ECM ports, D11 and D10, each with four injectors. On page 359 (85-91 L98 ECM) show two ECM ports, D15 and D16.

I don't know what else to recommend.

If my source is wrong, I will notify Bentley Publishers for a revision/investigation.

It doesn't say the banks are pulsed seperately.

From what I typed, which is word for word from the book, gives me the strong feeling they are pulsed separately.

I really want this cleared up. If we can come to an agreement, I will contact a friend who works for Bentley and forward him the results of this discussion.

I assume at some point they will have to revise that book, as it stands, Charlie passed just as it was going to press, so there is no chance he will be that person(s).

I have seen some other errors in it, but I don't remember where... so I am not saying it's right, I am saying it's been my source for many of my discussions on the EFI systems used in the C4.

Quote:



The books say they are wired separately. They are, two pins.


Inside the ECM, On 1986-1989 applications they are bridged together.

Here is a logical diagram of the '165 ECM found in 86-89
L98 cars (Camaro, Firebird, Corvette)

http://www.lt1intake.com/images/InjectorDrivers.jpg

On 1990, 1991 applications using the '730/'727 ECM they have two injector drivers. Here is the logical diagram out of the 1991 ybody FSM:

http://members.cisdi.com/~anesthes/fuel-control.gif

So since we have the capability in hardware for a '727,
why would it run batch (all 8)? Well here is a dissasembled
BIN from the '727 ECM:

http://www.geocities.com/ecmguy.geo/software/anht_hac.pdf

(ANHT, 1990 Corvette, auto)

However, if you read the code you'll find that the injector PW is a single integer, rather than a bank to bank pulsewidth. In fact, the $8D code only has two firing modes - single fire, and double fire. NOT bank to bank.

This is also why $8D still has a 100% DC limit at 6,000 RPM of 8 milliseconds.

A good read:

http://www.thirdgen.org/techboard/diy ... .html?highlight=730+batch



-- Joe

My 85 shows two seperate circuits, two seperate ecm controls, not joined in the ECM.

Do these all pulse at the same time? By the vibrations in the fuel lines and guage, I'd guess yes.

Quote:

The '870 is not very well documented, so I'd be lying to say I new for 100% certain. With that said, with my experience with the other ECM's I'd be surprised if the '85 had a technology edge over the ECM's that replaced it.

-- Joe

Maybe it was too weak to drive all 8 at once.

Regardless I don't think there's any edge to firing 4 injectors twice vs 8 at once. It's not as if those 4 all happen to be on the verge of their intake stroke.

Amen See How to tune and modify Chevrolet fuel injection by Ben Watson pub 1997 page 39 1st paragraph for his explanation.

"The ECMcontrols the enegergizing of the injectors... On the PFI and TPI apps., the injectors are synchronized to primary ignition and open once every crankshaft rev." Watson.

Perfect!!!

Sorry for my lack of attention to this thread... I have been real busy...

I will update the tech faq and let the publishers know what is what!

It appears that the "wheel offset/front spacing/rear spacing" needs some attention. The math is flawed. The correct math for front or rear "spacing" is done from "TOTAL" wheel width and not "advertised width" bead to bead. I've seen these figures posted on other forums "as fact" and never actually knew from where they came. Someone PM'd me the thread and I thought I'd check it out.

The easiest way to "correctly" establish "front spacing" is to first calculate "rear spacing". Front spacing is the "simple result" of subtracting the "rear spacing" from the "TOTAL" wheel width.

The math when "advertised offset" is known:

(offset)/25.4 = X + (total wheel width)/2 = "back spacing"

"front spacing" = (total wheel width) - (back spacing)

It is generally accepted fact "total wheel width" is "advertised width" + 1"(1/2" each bead flange"!

A simple example: C4 9.5/56mm

56/25.4(2.205) + 10.5/2(5.25) = 7.45 (BS)

10.5 - 7.45 = 3.05 "front spacing"

The math from "advertised back spacing":

(back spacing)- (total wheel width)/2 = offset

The same C4 9.5/7.45:

7.45 - (10.5/2)5.25= 2.2 X 25.4 = 55.88 (offset)

round up 56mm




This math is using "generally accepted" allowances for 1/2" bead flanges. There are "minimal" differences wheel to wheel! I didn't do the math for all wheels but it appears that all were calculated in the same fashion so I anticipate the same eroneous information.

I'm quite comfortable with my numbers and my math.

From these sources:
http://carfiche.com/fiche009/c4/index.pl?7:c:20
http://carfiche.com/fiche009/c4/index.pl?7:c:24
http://www.idavette.net/hib/c4/sustunch.pdf

(*) Required some interpretation, you should look at the fiche yourself

(**) Note discrepancies in the 85-87 info: Fiche gives same spring codes but Hib gives different rates.

Coupe

Year	RPO	Front Code (carfiche)	Rear Code (carfiche)	Front Rate (Hib)	Rear Rate (Hib)
84	FE7	BMB	BMH	102.0	87.5
84	non-FE7	HMZ	BMF	63.5	72.0
85	FE7	HMZ	NYU (*)	63.5 (**)	57.2
85	non-FE7	HKX,HMP	NYR (*)	54.0 (**)	39.9
86-87	FE7	HMZ	NYU (*)	66.5 (**)	57.2
86-87	non-FE7	HKX,HMP	NYR (*)	51.8 (**)	39.9
88-91	FE7 w/o FX3	FHB	NYU	115.5	57.2
88-91	FE1 and/or FX3	FHA	NYR	93.1	39.9
92-93	FE7	FSK	NYU	90.1	57.2
92-93	FE1	FSS	NYR	73.2	39.9
94-95	FE7	FSK	NYU	90.1	57.2
94-95	FE1 w/o FX3	FSS	RHZ	73.2	39.9
94-95	FE1 and FX3	HA	RHZ	60.0	26.0
96	FE3	FSS	RHY	73.2	33.0
96	FE1	P/N 22173028, no code given	RHZ	60.0	26.0

ZR-1

Year	RPO	Front Code (carfiche)	Rear Code (carfiche)	Front Rate (Hib)	Rear Rate (Hib)
90-91	all	AA	NYU (*)	96.2	39.9
92-95	all	FSM	RHY	75.4	33.0

Convertible

Year	RPO	Front Code (carfiche)	Rear Code (carfiche)	Front Rate (Hib)	Rear Rate (Hib)
86-87	all	HMN	RCA	54.4	39.9
88-91	all	FHC	RCA	93.1	39.9
92-93	all	FSR	RCA	73.2	39.9
94-95	non FX3	FSR	RCA	73.2	39.9
94-95	FX3	JA (*)	P/N 22112254 (*), no code given	60.0	26.0
96	all	FSR	RCA	73.2	39.9

Great post!!!! I just replaced my opti-spark.

Quote:

Just noticed this. The math is not flawed. The "front spacing" I calculated is from the hub to the outer BEAD as stated. With this info the reader can compare a new wheel's published dimension to the existing one on their car.

Yes the bead is about 1/2" in on the wheels I've seen, but varies. If a wheel manufacturer publishes backspacing rather than offset then the reader should subtract 1/2" from the backspacing, and then they can calculate offset from that.