Age old question: 70 or 75mm?

[csledd]

After searching through the forums to find an answer to this question I have come to decide that everybody has an opinion on this. Some say 70mm TB and plenum for N/A and 75mm for S/C. Others say 75mm won't hurt a N/A car. So I decided to read what some retailers had to say.. same thing. One said to use 75mm on a "pretty heavily modified" car but never describes that as FI, another said it's good for any 96-04 4.6 2v engine.

The reason why I'm asking this is because I want the Accufab TB and Plenum for my car. If you read my sig my exhaust is pretty opened up, so why not open up the induction But I don't want to hurt my power by going with 75mm. But if 75mm will not hurt at all then I'll pay the extra $20 for it no problem. One of the retailers described the 70mm of adding power across the whole band, does the 75mm do this also? I would like to hear something from the people who have 75mm on their N/A car, or have gone from 70mm to 75mm.

 

[Dark Knight GT]

There was a test done at Modular Depot about the plenum and throttle body combo's. The 70mm on most bolt-on n/a applications made more power across the band than the 75mm, though the 75mm has slightly more "peak" power in high rpms. If you are staying n/a, the 70mm should do you fine. If you're going to go with a supercharger in the future, then I'de get the 75mm throttle body.

 

[csledd]

link to the article? I've seen the plenum showdown part 1 and 2.. but haven't seen the 70 vs 75 comparison.

 

[Dark Knight GT]

Ok. Now I'm doing some reading. I found a thread over at Modular Depot at Matt(the guy who did the testing with his car for all plenums) makes a odd statement in this thread.

http://forums.modulardepot.com/show...age=20&highlight=Plenum showdown&pagenumber=4

Read the next to last post. He says he got an overall bigger gain with the 75mm but a overall higher net gain with the 70mm. Now I'm puzzled.

 

[Dark Knight GT]

Here's a thread from Modular Depot with a guy asking the same question as you. Read the posts and come to your own conclusion.

http://forums.modulardepot.com/showthread.php?s=&threadid=50442&highlight=Plenum+showdown

Its really going to be up to you. I honestly don't see where a 75mm throttle body is going to give anymore real world power over a 70mm. Its only 5mm difference. Especially on a n/a vehicle with the stock lower intake manifold. However, that is only my opinion.

On a side note, my brother just ordered the BBK Upper intake plenum to match his BBK 75mm throttle body. I'm going to port and polish this plenum he's getting to make it mirror smooth inside. As for me, I have a stock intake plenum here I've been working on porting and polishing on and off all year. Its looking really good. I'm going to combine it with a FRPP polished 70mm throttle body I have here and run that on my car.

 

[csledd]

Its really going to be up to you. I honestly don't see where a 75mm throttle body is going to give anymore real world power over a 70mm. Its only 5mm difference. Especially on a n/a vehicle with the stock lower intake manifold. However, that is only my opinion.

Honestly I'm not worried about the extra power gains right now, I'm more worried about it hurting in some way.. people have mentioned hurting down low with the 75. But if I'm going to buy one I want one that's going to grow with me, so if it doesn't hurt and doesn't help then why not for $20 bucks when it has potential in the future (while staying N/A)?

That link helps somewhat, I'm gonna guess that nobody has dyno chart proof of the 70mm vs the 75mm posted because well, maybe there's not a difference in slightly modded NA stangs.

 

[90RanGer]

I say just go for the 75mm, the plenums are all 75mm inside anyway, minus well take full advantage of what you bought, plus when you start doing bigger mods it will be worth it.

 

[Dark Knight GT]

Honestly I'm not worried about the extra power gains right now, I'm more worried about it hurting in some way.. people have mentioned hurting down low with the 75. But if I'm going to buy one I want one that's going to grow with me, so if it doesn't hurt and doesn't help then why not for $20 bucks when it has potential in the future (while staying N/A)?

That link helps somewhat, I'm gonna guess that nobody has dyno chart proof of the 70mm vs the 75mm posted because well, maybe there's not a difference in slightly modded NA stangs.

Honestly, I don't think its going to make a difference either way unless if you with a supercharger or turbo. As for staying n/a, should you decide to go all out n/a, then you'll probably need to look into an entirely different intake setup, like the one from Fox Lake or something.

 

[starchman]

I don't see how more air coming into an engine will hurt power, but make sure the plenum and TB are port-matched; if you bolt up a 75mm TB to a plenum with a 70mm opening, I think that would cause turbulence (and a bottleneck) and hurt power. My Dragon setup was port-matched; I don't know about Accufab.

 

[LI98GTStang]

There is something called velocity of the air coming into the engine right? Just like you can have a set of ported heads that are hogged out too much, I would think that you could have an intake that is also too opened up.

 

[GDawg]

I would say for the cost difference you might as well go for the 75mm. It's not going to hurt you and if in the future you do decide to go with a SC, you won't have to replace it. That's my plans fwiw...