what is so bad about gen ii engines?

[19Cutlass94]

For an N/A motor, I take it ported 3500 heads would out perform ported 3400 heads?


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[Superdave]

It's really pretty simple... For Gen 3 engines 3500 is the best. 3400 would come in 2nd and 2000 or newer 3100 comes in 3rd...distant 4th is the small port 3100. Mixing/matching is done all the time but it's usually putting 3500 heads on a 3400 or 3500 UIM on a 3100/3400. Like The Raven said, there are gains with the 3500 UIM on the older models so don't go backwards.

If you want the most possible power from a combination, start with a 3500 and put in whatever cam you want. Even without porting it will still perform great. There aren't that many people that really know the 3500 heads, If you have the funds to get them professionally ported then talk to WOT-Tech. If not then I wouldn't touch them. Not the best head to learn on.

could i use the 3400 plenum on the 3500 manifold without major loss in performance? basically i just want a cam, injectors, and a tune. but i want a somewhat mean cam so i will have to do porting.

Why would you do that? Why not use the matching 3500 UIM?

People swap the 3500 UIM onto their 3400 for the additional plenum volume, so why would you go the other way?

[SwiftDrift]

could i use the 3400 plenum on the 3500 manifold without major loss in performance? basically i just want a cam, injectors, and a tune. but i want a somewhat mean cam so i will have to do porting.

[Superdave]

Not that much bigger... but better is the key. The intake ports were raised and re-shaped plus have a few small changes that promote great flow stock. Exhaust ports are a little wider but the inside is quite a bit different.

here is a 3500 LIM gasket on a 3400 head:
You can see how much they raised the 3500's intake port.


Same gasket on a 3400 LIM:

[SwiftDrift]

i take it i should stop referring to the 3100/3400 as the "sfi motors" lol and thanks for the pics, i never knew how similar they were. i made a post in pushrod general about the 3400 and 3500, no one responded. :/ maybe u got pics of those too? i like the 3400 sfi, but i am starting to feel like the 3500 makes it obsolete, and i would be better to get a 3500. but how much bigger can the ports be?

[Superdave]

Remember, SFI is Sequential Fuel Injection.. just an idle/low RPM mode that made the Gen 3 engines idle smoother and save a little bit of fuel.

After around 2K RPM they revert back to MPFI (Batch fire) just like the Gen 2's were and the later Gen 1's. 80% of the swaps end up running the Gen 3/4 engines in MPFI mode with either OBD1 ECM's or aftermarket like Megasquirt.

[Superdave]

Overall dimensions are the same between the 2 generations.

The exhaust port spacing is identical, bolts are the same size/location (I've used Gen 2 logs on gen 3 heads). The exhaust port shape is different (D vs O).

Intake ports have different shapes but are in a similar location. The LIM bolts are all in the same place for a Gen 3 VS Gen 2, but there is no way the ports will line up. Coolant ports are the same. Combustion chambers are smaller but the same design, valves are at the same angle but the intake valves are bigger in the Gen 3 heads IIRC.


Here are some pictures..

This is the top side of a ported 3500 LIM vs an un-ported Gen 2 LIM:


Here you can see the exhaust ports on a Gen 2 head:


VS Gen 3:



I'll dig up some intake port pictures... I know I have some. If not i've got gaskets from both generations on the wall so it's easy enough to get an idea of port shapes from that.

[SwiftDrift]

i wish i could get side by side shots of a geniii and a genii head, i know they are different but i just wanted to see if the geniii was based off of that design. i like looking at older and newer stuff and seeing what got changed over time. do the genii heads share some of the same casting/dimensions as the sfi heads?

[Superdave]

The latest Iron head 3.1 (1992 Fbody) and aluminum head 3.1 (1994 Cavalier) were pretty similar in output overall.

Gen 1's are limited by the crappy head design, Gen 2's are limited mostly by the upper/lower intake design, and secondly by the casting quality of the ports (they did get better over the years).


It's always better to start off with the best foundation you can get, gen 3's requite less work to reach the same power.

[SwiftDrift]

so are first and second gens about the same hp/lbs wise? or does the genII actually have noticeably more power than the gen I? i know they are both a little on the granny side of the performance spectrum, but i have limited knowledge on the gen IIs. and i want to know as much as i can about these 60*s, ive looked into the sfi motors quite a bit+the dohc, i know about the iron heads, but never really looked at the genIIs, i just figured they must be somewhat decent if they have aluminum heads and splayed valves. im probs not gonna build one, but i just dont like not knowing about something like this.

[robertisaar]

i'd like to re-state gen2 intakes. gen2 heads aren't really bad of a starting place from what i've seen(disclaimer: i am in no way qualified to make such a statement), but the intake set is one hell of an object to try and work around/be compromised by.

so, you can either use gen2 heads and be stuck with a stock intake/port what you can or go nuts and make a fully fabricated intake......

and all of that might bring you up to the level of a small port gen3. just doesn't make sense to spend so much time and money on something that has been superceded and requires less money and work.

[geoffinbc]

There are 2 camps most drivers fall in. FWD Gen II owners and RWD which are all iron head. When you want more power on a FWD vehicle you jump right to a 3X00 because it is basically a bolt in and the gains are great for little cash. I did a 3400 swap with a friend into his 90 Z24 and the swap was very easy and it made that old girl into a pretty quick car. The RWD guys like me sometime swap up to a 3X00. It is a nice swap but for a lot of us it takes too much time and a lot of parts purchasing. It is just SO EASY to swap in a 3.4L F-Body motor or a GM Crate. You can start on Friday after work and drive back to work on Monday. The stock EFI or carb will run the motor and the swap is unnoticeable to the untrained eye which helps in places like California. Heads don't get swapped much because none of the RWD intake manifolds will bolt to the aluminum heads. So you either stick with the old iron and do what you can with it or take the leap up to a 3X00.

[SwiftDrift]

i almost want to build one just because no one bothers XD but ya makes sense. i just hate to see any 60 degree go to waste.

[caffeine]

Building the iron heads for N/A is really pointless if Gen III is an option. They will simply never match up performance wise.

The reasons I see for people using iron heads is typically to
- avoid having to do a hybrid swap
- keep the stock look
- avoid any wiring

When you think about it, the vast majority who use iron heads are using them because that's what their vehicles came with. If you have a front engine, RWD vehicle that came with an iron head engine, a Gen II swap is the same amount of work as a Gen III swap. So why would anyone bother?

You'll probably see more people sticking with the Gen IIs in the cars that they were available in.