I was wondering if I could get some expert advice since my mechanical knowledge is very limited.

I have a 91 CRX HF, all stock except for the DX transmission which the previous owner installed. I have heard that the engine is most efficient (and creates maximum torque) at 2000RPM, but with the DX trans, 2000RPM = 47MPH. I do mostly highway driving, and normally drive 60-65mph which is between 2550-2850RPM.

I understand that increasing the width of the exhaust will shift the torque curve so that max torque is at higher RPM. The exhaust is original, and at 170,000 miles, it's leaking, and well overdue for a change.

My question is this:

If I buy a wider exhaust (such as an SI exhaust which I believe is a direct fit and looks snazzy with its dual tips), will the shift in the torque curve increase my fuel efficiency while cruising above 2000RPM?


Thank you in advance for helping out a beginner.

 

I have a much better alternative to get precise results that you can change as needed/experiment with... Adjustable Cam Gear.

Advancing and Retarding Camshafts = Although there are always exceptions to any rules, if you advance a camshaft you can expect the torque and power curves to move to the left, with a slight drop in peak power, but a smoother curve. (Left being lower RPMs and right being higher RPMs.) If you retard the cam you can expect the curves to move to the right with a lower peak torque and maybe higher peak power figure; once again the curve will be smoother. We are not talking big degrees here (i.e. 4-6*). The way to visualise what happens is for the standard cam without any advance or retard, the rpm versus power/torque curves will not have a smooth curves. Advancing the cam will tend to smooth the curve and bias to the left side of the bumps, while retarding will smooth and bias to the right side. (Strictly dyno related though.) Retarding the cam, makes the exhaust valve open later, but also has a negative effect in raising the engine temp and exhaust valve temperature. On the other hand this could be beneficial in your case, for scavenging or the likes.

With that being said, I would retard the mechanical timing around 4* (or 2* at a time if you have the time) and use at least a half tank of gas for your results. The optimal thing to do would be utilizing a dyno and logging software, but I believe that is out of the current scope of things. The only problem with the "try it out" method is to see real results you need to be able to keep your driving pretty consistent - weather plays a minor factor too. Sure tuning by exhaust can always be done, BUT that gets extremely pricey and time consuming from Research and Development.

P.S. - There is plenty of reading you can do on the Internet that will support my claims.

*** I forgot to add...
Be sure that before you change your mechanical timing to have everything set properly with your ignition timing. One would hope the rest of your engine is in order as well.

 

You have opened up a whole new world of interesting things I know nothing about. Let the research begin!

I'll see if I can get my cousin to help me out with this. He is a bit more comfortable under the hood than I am. Also, I apparently need to finally fix my bouncing idle before I check/adjust the ignition timing. I've been putting it off for far too long.

Thank you very much for your advice. I'll give it a go and let you know the results. I was really hoping for a reasonable excuse to install a dual tip Si muffler instead of that dinky looking one I have now!

 

Yes, you can NOT have idle problems be it high or bouncing; you will never be able to set the ignition timing or idle correctly. Also, while it is true that "tuning" exhaust helps move the curves around, the easiest "exhaust tune" for a DIY enthusiast is for automatic transmission "shift points". This can be accomplished by widening, narrowing, lengthening, and/or shortening of the exhaust.

Another thing you may want to look into is scavenging and how it helps fuel economy. Especially before you start changing ANY exhaust... you may be fascinated by what you find, because Knowledge is Power!!! (Cheesy, yes... I know....)

 

^Good info