I played Street Rod 1 though and it enjoyed it very much, but never managed to finish Streed Rod 2.
I'm going to go out on a limb here and say that it probably depends on how the game was coded - in real life that might have been the case, but the game developers might not have implement all of those aspects into the game.

Totally off topic - to this day I see people sometime install these huge exhaust outlets on their cars - makes a lot of noise, but I doubt whether you actually gain that much more performance from it (I can just listen to how long it takes the car to rev up to maximum rpm as it pulls away). My logic tells me that you need to make enhancements to the entire exhaust system (right from the exhaust manifold straight through to the end to achieve better performance from that perspective). However, you also need to have at least some back pressure. In the end, there are a lot of aspects that influences performance on a vehicle, but in the end, there is only so much that you can do (apart from throwing on a turbo charger or replacing it with a larger engine under the hood) until you reach a point of diminishing returns.

1. Glass pack is not really going to be lighter but it should offer less backpressure.
2. Differential. It does matter in some races where you need the acceleration more than top speed.
3. Headers - not just under the car. Headers goes straight to the heads and will decrease backpressure AND should even out the backpressure between cylinders. Stock exhaust manifolds on older vehicles are pretty much crap.

As the Street Rod 2, it has been years since I played it but any of those upgrades should make a difference. If I remember correctly, some of them help a lot more when you have the higher end stuff.... Likely some of them won't matter at all if you do those upgrades before the rest of the engine is upgraded.

The differential should be easily noticeable with lower power engines, especially between 2 and 4. With the lower geared one, it is a lot easier to blow your engine because it revs higher.

So I have been playing a bit but I haven't been able to get any performance increase with :
- Glass pack muffler
- remove muffler
- Upgraded headers

However with the diff yes, I think different gear ratio for different races:

Drag race is a no brainier Highway ratio for best top end since straight line.

Mullholland drive is a tricky one because each new game gets a different track generated, so I came to the conclusion you either need the lowest ratio 4x or mid ratio 3x

4x ratio for a track that has the wet cement slick or whatever it is the grey part of the road that u need to slow to 60mph to.
* if the wet cement is near the end of the track I think 4x ratio is needed to get the best acceleration from 60mph to catch up before the track ends

If the slick is in the middle or near the start I think 3x ratio is good because you have more time to catch up to opponent while having a slightly better top end speed and slower acceleration.

The aqueduct race is like the drag race with right technique simply go for best top end acceleration.

And I just learned a new trick on youtube, when racing the king race with only 1 gallon petrol for drag race and 2 gallons for Mullholland. Means after winning each race you don't have enough petrol and can go back to garage, change parts or even the car before refuelling. This is where changing the diff is the key, drag race king with 2x diff, go back garage before Mullholland race and install 3x or 4x diff, only have 2 gallons. After winning Mullholland once again no petrol for aquaduct. Go back garage and install 2x diff.

Ofcourse the game can be finished without doing all this but it feels like completing it 101% this way

Hated the wet cement, I always assumed it was gravel but thats based on 0 facts, whatever it was it sucked so went back to the original after a few aqueduct races
Your now giving me the urge to give it another try

Yeah I am not sure what its supposed to be, wet cement or gravel are probably the only 2 things that come to mind.

I think this is the only reason i keep playing this game because this cement/gravel has always been such a hard obstacle. Its a challenge that keeps calling you back.

Slowing down to 60mph and going down 2 gears seems to be working best for me to get through, but it is still a hard process of slowing down just at the last moment because computer can drive through at 100mph