Power Question for the Gurus

[project 92z]

Ok, after doing the swap (94 dohc into 91 Z34) and fracturing a few speed limit laws, it seems to me that my 93 was so much faster and more powerful. It's possible that my timing might be of a tooth or 2 but is there another possibility? Is there a difference on 91s shift points, gearing, or rev limiting? Was the 94 dohc de-tuned? Or is there a Lil incompatibility between 91 and 94? It's not a huge difference but from 70 to 116mph took almost 2 miles to hit @ WOT on a cool evening. My 93 did that and more in less than 1.

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[Schurkey]

If anything, the '94 should have more power based on the sequential fuel injection. I think you aren't able to use the sequential FI on your application. I'm not aware of any reason for reduced power other than the normal issues that would affect any LQ1..

What is your cranking compression?

When your original engine died, did it poison/plug the catalyst with coolant?

What is the scan-tool showing for data-stream anomalies?

[carbon]

If anything, the '94 should have more power based on the sequential fuel injection. I think you aren't able to use the sequential FI on your application.

Sequential fuel injection does not give the engine more power.

I'm not aware of any reason for reduced power other than the normal issues that would affect any LQ1...

What is your cranking compression?

When your original engine died, did it poison/plug the catalyst with coolant?

What is the scan-tool showing for data-stream anomalies?

This ^^^

[project 92z]

Well the new 94 motor had been recently rebuilt (a lot of the internals I saw were aftermarket or very clean for a 94). The compression were all between 130 and 170 and the original engine didn't totally die- the #3 cylinder was at or below 70psi with no improvement when oil was added to it. Being in a OBD1 vehicle, I can only paperclip for info.

NEXT?

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[robertisaar]

Being in a OBD1 vehicle, I can only paperclip for info.

being an OBD1 vehicle, you could acquire a tech1 or some of the better aftermarket scanners, buy an ALDL cable or make a DIY version of one starting at a couple dollars.

[project 92z]

I just looked on eBay and they're a few ALDL cables available. I'm guessing the cable is useless without the program disk to read it. One cable looks like it's OBD1 male to OBD2 female. What's recommend? Also if memory serves me, the OBD1 plug on the car had only 3 wires. Do I need to mod it?

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[robertisaar]

what you need is a cable with the standard 12-pin connector on one end and either a serial port or USB port on the other. as for specific cables, I got kind of fed up with current offers back in 09 and built my own. the Moates cable is nice, but kind of pricey, the aldlcable.com version got really flakey after some use and some people have had issues with the reddevilriver cable that require modifying the cable or the connector to fix. beyond those three, I can't remember any that are still being sold.

3 wires in the ALDL connector is normal(you'll have A, B and M). depending on the vehicle, down to 2 is normal(A and M or A and E), some have significantly more.

the only software you need is tunerpro and the correct ADX file to go with it, which I've posted on more than a few forums.

[Schurkey]

Or just buy a used, professional-grade scan tool with the appropriate cables and software. eBay has several at any given time. Prices are very reasonable for the ones with older software, but the prices increase as the software handles newer vehicles.

The Snap-On "red brick" MT2500 was a popular unit. Mine goes as new as 2006 for VIN-specific readouts, and of course I get generic OBD II on any vehicle new enough to have OBD II. Also covers body computers, ABS, etc.





Compression from 170 to 130 is barely within spec. Some folks would say it's out-of-spec. I'd have hoped for better, especially since it's "rebuilt".

[caffeine]

GM manuals usually say anything over 100 psi is within spec Lol.

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[robertisaar]

there is usually a max percentage between the highest and lowest cylinders as well, I want to say 25% is common?

[Heartbeat1991]

I thought it was more like 10%. Either way, that's a wide gap.

[robertisaar]

looking at the 94 specs, GM says 100PSI minimum with the lowest cylinder no more than 30% lower than the highest cylinder is a pass.

[Schurkey]

there is usually a max percentage between the highest and lowest cylinders as well, I want to say 25% is common?

I thought it was more like 10%. Either way, that's a wide gap.

looking at the 94 specs, GM says 100PSI minimum with the lowest cylinder no more than 30% lower than the highest cylinder is a pass.

Cranking compression varies based on cranking speed, altitude, camshaft grind, compression ratio, and many other factors. A grocery-getter engine can get by with wider variation than a performance engine.

The GM spec above is less about proper engine performance than it is about keeping warranty costs down. The more variation and the lower the pressure they allow, the more customers they can push out the door without fixing the rings/valves/pistons etc.

Since MY emphasis is getting reasonable power and smoothness out of an engine, I get concerned about compression variation where the lowest cylinder is more than 15% lower than the highest; and overall I'd prefer more like 130 minimum. I see 150--160 as typical "stock" results on older emissions-controlled V-8s, maybe a little higher with more-modern engines. An engine comes to me with 100 psi cranking compression, I start looking for problems no matter what the service manual says.