DOHC yearly updates and changes?

[TurboGt]

I have been doing some searching and am still not to sure on the differences from year to year in the 3.4 DOHC.

What changes form year to year?

Are some years better than others?

Things I found were;
96-97 has different intake manifold, heads, and is possiably a interfearence motor. There are compression changes depending on year and auto or manual.

The motor would be installed into a fiero and I am wanting to go the mega squirt route so I dont care if it was obd1-2 or if it is multi-port or seq. injection.

[TurboGt]

Nobody????
Second question
Besides compression ratio being the main factor are some years better with boost?

[Juglenaut]

no

[SappySE107]

96-97 is better for a manual trans for sure. Im still not sold on the 96 intake setup for an auto trans even though GM thought it was a wonderful idea. Manuals got different pistons for the .25 bump in compression. 96-97 heads will raise your compression though no one is really sure on how the heads performance stock vs stock with 91-95 heads.

As for boost, neither is high compression so without someone testing it out, we won't know if there is an advantage to either setup over the other.

[Mike W]

91-93 did not have sequential injection, later years did. 96/97 had exhaust port changes for improved flow. Although these engines advertise at 215 hp, my guess is that they were possibly a little fatter under the power curve due the the heads, cam, compression and intake plenum and runner revision. I've never seen a hp and torque graph from the factory on the earlier engine. They seem hard to find. I got gone from a friend at the proving ground for the late engine (poor quality) but don't have anything to compare it to. The latest also had obd II.

[TurboGt]

96-97 is better for a manual trans for sure. Im still not sold on the 96 intake setup for an auto trans even though GM thought it was a wonderful idea. Manuals got different pistons for the .25 bump in compression. 96-97 heads will raise your compression though no one is really sure on how the heads performance stock vs stock with 91-95 heads.

Why would the 96-97 intake be better? Shorter runners than the previous years?

[SappySE107]

Larger ports, better transition from the runners to the plenum, better transition from the TB to the plenum. Runner length is about the same between the years.

[TurboGt]

Thanks for your help guys

[Scott205]

Lets see.. 91-93 were all very similar, I think just a minor plenum change.

1994 added a MAF sensor (which also changed the temp sensor and air box), and that remapped the the computer, the plenum was slightly different, intake still the same, altenator was change and the brackets that bolt it to the engine. The engine itself was identical from 1991-1995. 95 I believe had a slight change to the throttlebody part of the plenum.

1996 and 1997. New engine, different cast on the block, updated starter (same teeth and bolt holes though), heads were changed, with the different exhaust port, cams, valves, head flow, intake slightly changed, majorly different plenum, MAP sensor was moved, Vacuum came from a different place on the plenum, Throttle body was different, coolant routing is different both through the basic layout of the engine and the throttlebody, as well as the heat core lines running together instead of seperately.

[TurboGt]

Wow that was great thanks man

[dohcfiend]

quite a few little changes. here is what i know. most of it is already mentioned but a few things may not be. (since the author doesn't care about ecm/sensor changes, i will not mention them)

91-93 the beginning comp=9.25 for auto and 9.5 for manual. manuals also have that A.I.R injection in the exhaust too whereas the auto's do not.

94-95 i hear there is a plenum modification internally but i think it is heresay. slight difference in cam timing i hear and different(better cam cogs) as in removing and adjusting them is alot easier. revised injectors(quieter)

most of the generational changes in the 94-95 models were in the ecm wiring

96-97 same cam timing but advanced on the intake and may be the same on exhaust(there is a technical term but i cannot recall it at the moment) different intake and plenum. i am with ben, i really do not think the 96 plenums are much better than the previous, only feature they posess that makes them desirable is they have removable tb's whereas the previous years had integrated tb's. different HEADS and i say much superior heads, different pistons, differnt exhaust valves, different combustion chamber design, different exhaust manifolds, also these years have A.I.R injection again, comp ratio is 9.7. ok on the block, it may have a different casting number but the block is the same as a 91-95 i believe. the difference is over the generations of lq1 there were many factory recalls or revisions to the block if you will. mostly they were concentrated on the oiling system mostly. the deal with the 96-97's is all those revisions were already in the block and the previous years had to have them done. supposedly the 96-97's had adjustable fpr's on their intake setup. lastly, the valve springs are a little weaker than the 91-95 years but it may just be for a 97....not 100% on that.

edit: oh the drive cog and cam cogs have a different amount of teeth i think. don't hold me to it but i think you can use 91-95 idler pullies and tensioner if you tried on the timing belt setup.

97's recieved different cams and lifters(nodifference in timing just design). the lobes are a little smaller(according to yourid) and the lifters are 33mm and all the rest recieved 35mm lifters.


my opinions: i touched slightly on the intakes already. next i will go into detail on the higher comp ratio of the 96-97 engines. it was obtained by the heads and the pistons. the 96-97 heads, they have the same intake ports as the previous but much wider and a little shorter exhaust ports than the others, the exhaust valves have dimples on them and the valve is shaped a little differently, whereas the previous valves were flat. the 96-97 heads have the clover shaped combustion chamber(plus it is smaller than the previous) where the previous ones have the hemispherical shaped chambers.

i think the 96-97 heads would be much better for boost b/c they kill the previous ones on exhaust flow. ironically the previous years seem to flow better on intake. but intake flow should not be a problem on boost as getting the air in is no problem.

in comparison and opinion. i have had both intake setups on the same motor and the 91-95 had noticable more bottom end and the 96-97 had a little more on top. also you have to remove the plenum every time you want to change plugs or wires on a 96-97..........ack

SHORT VERSION: i think you should use a 96-97 motor(but adjust the cams to be the same as the 94-95's) with a 91-95 intake.

edit: wait there's more. the 91-93's had oil cooler and tranny coolers and the rest did not. the 96-97 had a different coilpack location where they are up top near the intake and the 91-95's had them underneath the exhaust manifolds(real smart GM) ok i'm done

[Scott205]

Correct me on this DOHC.. but there is a different Alt on the 1996-1997 with a radically different bracket system. Did they change this minor part on the casting in 1994? becuase I had to buy a new CS-130 alt and the brackets to bolt it on the new engine from the 1992 alt.

I know the 1994-1997 altenator was the same.. but how did they do it?

The oil cooler on the 1991-93 is some what of a joke.. works but nothing like an aftermarket one. Seems to do a great job on the 96 engine though. Everyone should have a tranny cooler on the 4T60E if you want it to last.

[dohcfiend]

good shot, i completely forgot about that. the alt design changed in 94. for the better i might add.