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89 Posts
Discussion Starter · #1 ·
Will a 1227165 ECM in an '89 L98 correctly time injector
pulses for use with a cam that has the 7 and 4 lobes swapped?

My vote is that it will.

Cylinder location:
2 - 4 - 6 - 8 Chevy
1 - 3 - 5 - 7

Firing order:
1-8-4-3-6-5-7-2 Chevy (OEM)
1-8-7-3-6-5-4-2 Chevy (7/4 Swap)

Summarized from 'Corvette Fuel Injection' Probst, Charles O.
Bentley Publishers, ISBN 0-8376-0861-9

The L98 uses an electronically controlled HEI distributor.
Instead of weights, springs and vacuum; Electronic Spark
Timing (EST) controls spark advance. The rotor, cap and
wiring routing determine which plug is fired by the secondary
windings of the coil-in-cap.

As for the injectors on the L98

"For every two revolutions of the crankshaft, each gang of
four injectors opens and closes twice, alternating left
bank (four cylinders), right bank (four cylinders). GM
-speak: "Double-Fire​

Comments, opinions, links to previous threads?


89 Posts
Discussion Starter · #3 ·
The test by Hot Rod that I'm aware of used a fairly radical BBC
with a single plane intake and a carb instead of a SBC with EFI
and a TPI, SR or HSR.

Testing Lunati Cams - Swap Cam Test
Can Changing Your Firing Order Actually Make Extra Power?
writer: David Freiburger

The engine is the same 492ci big-block Chevy long-block you last
saw in our story, "The Great Bore vs. Stroke Shootout," (June '05). It
uses a bore and stroke of 4.560x3.766 with a compression ratio of 10.87:1.
Airflow Research 335 CNC heads are sealed with Cometic gaskets, and the
induction this time is a Holley 1,050-cfm Dominator atop a port-matched
Edelbrock 454-R manifold. We ran MSD ignition and Hooker 2-inch headers
with 18-inch extensions and Dynomax 311/42-inch Bullet mufflers. All our
testing was done with Lucas synthetic 10W30 oil and Rockett Brand 91
-octane gasoline.​

This engine recorded a worthwhile dyno improvement in the test's operating
range for the modest increased cost of the 7/4 swap . TQ was up 8-11 lbs/ft
in the 5700-6000 range. (What the dyno results do not show are changes in
the engine's willingness to accelerate.)

There were anecdotal remarks in the article about 'no gain' results
with SBCs. But there were no details about those engines other than
that they were only tested above 6K and had billet cranks.

If you have more detailed information or links to the test you are
referring to, I'd appreciate receiving these.

But as to the point of the original question, if the ECM fails to actuate
the injectors in a way that accomodates the lobe swap, trouble and/or
further expense is the result. I am not familiar with the workings of the
later iterations of EFI in the C4 model run but I vote that the swap will be
transparent to the 7165.

Ken R.

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