Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

What changes were made to the 90-92 LT-5 to generate the additional HP?

I am looking to purchase a 92 but I love the "get up and go" of my 95. The 95 has had the smog stuff removed, headers, porting chip etc. Is it possible to perk up the 92? It is a low mile car and all original so I am hesitant to mess with it. I also have Collectors addition 1996 LT-4, my guess the cars are about equal in "feel" in their stock trim?

The 92 will also be exempt from emissions inspection in another 2 years, so I could do more with it without concerns.

Are there any problems specific to 92's I should look for before purchase?

Thanks!

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

The higher hp cars are a tad heavier. Most of the extra 30hp comes from backpressure reduction in the exhaust.

From Graham Behan(current LPE Engineer-former Lotus of GM Engineer)
Cylinder Head Differences:
The production cylinder heads for the LT5 engine were originally produced by Birmal, all engines right up to the last few of the 1995 MY were fitted with Birmal heads. The machined port matched and hand blended heads were all Birmal parts.
Originally when I returned from being Resident Engineer at Stillwater the changes for increased output for the 93 MY were intended to be increased lift and duration on the secondary cam profile from what was known as the 'B' profile to the 'RB' profile which had a further 10 degrees duration. This resulted in engines which achieved the power target but paid the price in terms of reduced low end torque, something for which the LT5 had always been criticized. Since this was now my responsibility we made a change in the design direction ie. we would stick with the same cam profiles, optimize the cam timing and devise a simple machining and hand blending operation for increased port flow. So the 1993 MY engines started to have a revised throat cutter diameter an increase in the primary and secondary ports and a simple blend at the junction of the two machine cuts. I would not really call the operation CNC porting because to me that implies some change in the fundamental port design which this operation certainly did not. Now the time frame for all of this was early 1991, Birmal advised us that they did not want to produce heads for the future model years, infact they wanted to get out of the semi-permanent mould business. So we looked for alternative sources. A.L. Dunn were selected and the tooling was transferred from Birmal. At this time because of the original production schedule and the cut back in the volume of cars produced there were sufficient Birmal heads for a large portion of the remaining engine builds. We decided that since the port core box needed refurbishment, we would take this opportunity to re-design the port so as to eliminate the need for machining and hand blending at Mercruiser. The final cast port actually flows better and more consistently than the machined part. This change was incorporated with the production fix to the right hand side chain tensioner reservoir for engines late in the 95 MY build. All A.L Dunn heads had these features incorporated. There were about 200 head sets cats by Dunns and I believe about 130 engines were fitted with them.

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By Greg Van Deventer
LT5 Polishing and Performance

The fact is, the cylinder heads were the only castings to receive any "fettling" as Lotus referred to the process of removing metal from the ports with a hand grinder. They only got a very small area of the intake ports blended just above the valve seat in the short side radius on the inside wall where the bowl transitions into the runner. Yes some could have received a little more attention than others, but believe me, there wasn't enough time to get carried away, and no way to do a "special set" to go on an engine as they were picked randomly to build the engine many steps further in the process. This "hand fettling" process wasn't a huge factor in the final engines performance.
A more likely culprit of poor airflow would be core shift especially in the injector housings. Core shift in other castings could have had a part in the losses also.
Another suspect cause of variability that has already been addressed is the cam timing. The production cam timing method did have more variability than "degreeing" them in using an indicator and degree wheel.
This leads me to respond to another statement regarding the Dynomometer testing on the LT5 engine. Every LT5 was dyno tested and met the tolerances before it was shipped. The dyno sheets and tons of other data were given to the museum, in hopes it would be shared with the car owners.

From Marc Haibeck
http://zr1specialist.com/HAT%20Web/presentations/LT5%20Power%20Upgrades.pdf


The LT4 car may feel similar to the 90-92, but some 1/4 mile ET's and MPHJ comparisons will illustrate the additional output of the LT5.

There are many changes as the C4/ZR-1 went forward during its production run. I have seen a list of all the improvements per year, but couldnt locate tehm in a quick search I just did. There awere some 400 changes to improve the NVH (Noise Vibration Harshness) characteristics of the C4.

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

I don't think you can compare a modded LT5 to a stock any years. My 91 with most the mods you have on you 95 (+heads and TB) will just walk away from my 93 with chip & exhaust. I'm sure if they both had the same mods it would be an even run. Guess it shows it does not take much for the 375hp cars to match or exceed the 405 cars. I would guess a 375 with ported top end and good flowing exhaust will match hp with a 405.

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

don't forget the later cars are carrying more weight. The lt4 isn't going to run with a lt5

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

From my readings you never know what kind of power you are getting from these engines from the factory. My '92 put down 342whp on a mustang dyno. Thats ~402 flywheel HP, way more than the 375 its rated at.

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

Shhh, you are not supposed to tell!!!!!!!!
:confused:

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

Marc does say that most 90-92 cars actually produce 400HP.

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

Thanks for the reply's! It would seem I could easily get some decent bang for the buck by just porting top end and adding headers/exhaust. In a year I could lose the cats.

What would a low mile 1992 in excellent condition sell for? I know color combinations play into all that but on average?

Re: Looking to purchase a 92, why HP Difference Between 90-92 and 93-95?
 

Truly, you can very easily eclipse the 93 LT5 performance, including the few 95s w/ Dunn heads, by top end porting alone. Then adding long tube headers and a free flowing exhaust and adding a tune you will be close to the same HP at the WHEELS as the 93-95s made at the crank in stock form. By the time you get to the 430 range at the wheels, the 405 "majic" is totally absorbed in the process.

The LT5 is starving for air! Give it what it wants, and be amazed at the rewards!