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Diy Intake ...re-visited

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I had a couple of questions for the gurus on this forum.

1. Is it possible to see better gains in HP/TQ when intake pipe is reduced in diameter?

2. Is it possible to see better gains in HP/TQ when intake pipe is reduced in length?

3. Which should you see better gains in HP/TQ; when the intake is built with a smaller or larger cone-style filter?

During some research I read that if you go too large you will lose substantial low-end power but sort of make up for it in the top-end and vice versa. It seems that a lot of people on both this and the CL forums say that the car feels slower off the line but around 3000 RPM it hits hard. This leads me to my final question...

4. Would you suggest perhaps mixing intake pipe sizes for optimal low-end and top-end performance? For example, use an Apexi power filter (3" dry-type filter...supposed to be the best for filtration AND flow) on a 3" pipe then connect it to a 3.5" pipe to the throttle body.

What do you guys think? Will the gains be too negligeble to notice?


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What are your power goals? What mods do you plan on doing in the future? It all comes down to that. Honestly, an intake mod isn't going to make much improvement, regardless of it being a 3 or 4" intake. A/R and engine air flow/match is going to be a factor but how much power are you aiming for?

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Future mods to be added very soon:

S-AFC controller - because right now my car IS running lean and throwing off check engine lights and sensors. Stock ECU doesn't know how to add MORE fuel for the increased air. I am going to have a local shop that I just found run a baseline dyno, then install the S-AFC, then run more dyno's to tune. I feel that my car COULD be running slower than stock right now due to the lack of adjusting of the stock ECU.

3.76 gears - Although costly for the bolt-on Torsen type unit ($1599) I feel that the results are that of the PI torque converter or better however without the excess wear on the tranny (I understand gas mileage will decrease). Also, I just finished reading a post from a sorry Lexus owner that had his tranny blown from a faulty PI Torque converter. (Eventhough it is rare to happen I cannot take that chance.)

If the stock GS4xx produces 300HP at the crank I would like to see 350HP at the crank but FEEL like 375HP because of the greater torque feel of the 3.76 gears. I have researched and been told that changing the stock gears to 3.76 reduces 0-60 by about .5 seconds. If these cars SUPPOSEDLY run 5.9 (0-60) and 14.2 (1/4 mi.) doesn't that mean the new 0-60 will be around 5.3-5.4 and 1/4 mi. 13.7???? If not...why not? Please advise.

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I don't understand why your stock ECU can't compensate for the additional air seen through your CAI. I mean if your car was getting insane amounts of air and caused your MAF sensor to max out then it would pose a problem. I doubt it that your intake would cause the lean condition. Maybe your MAF sensor hotwire is coated with the oil from your CAI filter?

If it was my car I would save the money and not do the 3.76 gears. I'd invest that money into having the head street port and polished. If you're DIY you could even port/polish the head at home and save money there. Then look into having someone custom tune your car.

I just think the actual wheel hp gain would be far greater than crank hp you're aiming for. Since your car is an auto, you would lose somewhere between 20-25% hp from the crank. So that extra 50 hp (at the crank) would be more like 37 hp to the wheels. Not very noticable because of the weight of these cars.

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"I don't understand why your stock ECU can't compensate for the additional air seen through your CAI"

Why not...why do you think everyone on these forums re-set their ECU from time to time? In the beginning your ECU will re-learn your driving habits and intake and such but after a little while your ECU will go back to stock settings. IT IS TRUE! Right now my car is hesitating (almost like turbo lag) when I hit the gas. Whereas when I re-set my ECU the car feels and responds so much better - it is FACT!

I cleaned the MAF sensor once before last summer, (before I added the ram air port in my bumper leading to my intake filter) it needed to be cleaned as dirt and oil build-up was present. However, just 2 weeks ago I checked the MAF and it's clean. So a dirty MAF is NOT the problem.

If I had the heads ported and polished then my after-market warranty is out the window for my engine. I would rather NOT have my tranny covered than the engine! Plus I heard that the gears make a big difference... why does it seem like you think otherwise?

I see that you own a BMW, not sure if you own Lexus also but if you did you would realize that the GS models are running 3.23 gears and still manage to be pretty quick off the line. A lot of the new cars like Infinity G35, Nissan 350Z, Maxima, Altima Etc. are all running with 3.7x gears THAT is HOW they are ALL able to hang with the GS4xx ... stock vs. stock! (I know they are a little lighter as well) but still it makes me wonder IF I should even BOTHER modding my GS400 to KEEP UP with the newer technology! Now I'm upset :-(((

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I wouldn't say that your MAF can no longer adapt to the increased air thus maxing out. It seems like your MAF is slow to respond.

Yeah, 3.76 gears are going to make a difference off the line. You might want to consider your daily driven vehicle is going to see freeway useage.

Yes, I've owned 3 Lexus' in the past. I have 2.93 gears and I can hang with you. :D

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Yeah, maybe NOW you can probably blow my doors off after $10K of engine work/turbo using your 2.93 gears. Stock vs. stock a GS4xx will take out a 528i anyday! BTW thanks for your advise maybe I will consider doing some port and polish work on my engine in the future. Unfortunately this whole thread did not help my situation...I am still showing a lean code. My mechanic won't even change out my o2 sensor because he feels that it might not fix my problem. I really don't know what to do at this point. If any1 else would like to contribute your responses will be greatly appreciated! Nice modding on your Beemer BTW.

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