Jeff20B

iTrader: (13)

REPU needs a new exhaust system. Figured I'd ask in here since it's staying NA. I have an RX-4/Cosmo collected header from RB. Should I leave the 2" collecter and expand after to 2.5" or do like the FC header from RB and collect directly into 2.5"?

By the way the 2" collecter already had an O2 bung added so it's more convenient for me to simply expand to 2.5".

Will it hurt or help power in the end? What kind of RPM range will be most useful? Keep in mind it's a truck. So low end is welcome.

Jeff20B

iTrader: (13)

Here is a picture of the RX-4/Cosmo header. http://www.racingbeat.com/resultset....rtNumber=16007

And here is a picture (generic) of the FC collected header and its 2.5" outlet. http://www.racingbeat.com/resultset....rtNumber=16130

The owner wants a 2.5" system so that's what I'm working with. Current engine is streetported old school 4 port 13B. Hitachi carb but could get a Holley or Edelbrock. Future plans are an NA FC EFI engine and MegaSquirt. We figured since RB sells an FC header with 2.5" collector, we'd build a 2.5" system for the REPU so it'd be ready by the time the FC engine goes in it.

diabolical1

iTrader: (3)

go with the FC-style collector.

Jeff20B

iTrader: (13)

What I've read recently on this forum suggests an expansion of some sort after the collector is better.

rotarygod

iTrader: (1)

It's actually hard to draw a conclusion from info around here if you start digging. If you are referring to the thread where the guy claims to have picked up over 50 hp by changing from an RB header to an old Mindtrain header with an expansion chamber, I'd say that his results need to be looked at more closely. First off he went from a full 2" system using the RB header and muffler to a new system which collected at 3", went through an expansion chamber and then through a 3" muffler. Since 2" can not flow the required about of air, I'm not that surprised that he picked up lots of power. He also retuned it. If you look at Kahren's results, he's easily topped 180 with the RB header for the FC and no other fancy exhaust work such as an expansion chamber and he's hit that with a stock ported engine. His results with that header are within the margin of error of the one with the expansion chamber using a Weber style intake.

Now I look at those 2 and think that there are other differences between their setups that could account for variances that we can't see. However I also have to notice that each system produced a lot of power and Kahren's RB header did far more than the other guy claimed it could do. He probably had the 2" outlet header that you have as opposed to the 2-1/2" outlet version.

If there is a benefit of one over the other, it's not going to be across the board. There is always a range as well as the "but" and "what if" scenarios that pop up. Unfortunately no one has ever just tested various systems on the same engine to show powerband differences between them. I don't even care about what the ultimate numbers are. All I want to see are how the powerbands look laid over each other. Maybe that would be a good project to work on.

j9fd3s

iTrader: (3)

i think its pretty proven that the FC diameter makes more power. the 1 7/8" ID of the early stuff is just small.

i mean the factory did it too!

any more than that, and you're right, its gets conflicting

Jeff20B

iTrader: (13)

Thanks for the level-headed response rotarygod.

Yeah I got to take a look at Adrian's setup in person the other day. Too loud for my tastes. He mentioned that if he could do it over, he'd have gone with a 2.5" collecter instead of 3" but leave everything else the same. Hmm.

This REPU used to have an RB collected header for REPU that is no longer made. The differences in bends, curves and lengths between the REPU version and the RX-4/Cosmo version is very slight and well within any margin of error.

Unfortunately I never did any dyno testing on the old header. All I have are some vague memories of how it used to perform and how it performs now on the current exhaust manifold. The biggest difference was it gained power from idle up to 2k but lost power above 4k with the manifold. I blame the rest of the system because I've swapped in manifolds on other cars and they perform well up to 7k, stock carbs of course.

Interestingly enough, the RX-4 header was previously in an RX-4 with a 2" presilencer and RB power pulse muffler ('83-'85 model with single 2" pipe modded to fit the 4). The engine has a basic streetport but a stock carb. The owner had no plans to swap to a bigger carb, plus the weight of the vehicle and tall geared rear end means keeping the stock carb for best driveability. I swapped him over to an FB exhaust manifold slotted to fit a 13B and it has a wide powerband from idle to 7k and the noise is no louder than before. Manifold to straight pipe 2" thickwall to RB muffler. Works perfectly.

My Cosmo is a very similar story. Beleive me I struggled with wanting to install the header and presilencer and weighing my options. The Cosmo is a heavy car with a tall geared rear end, stock carb and milder ports (74 spec). The stock muffler is still good so I kept it, and the stock resonator right before the rear end still works. I again used a manifold and 2" thickwall straight pipe. The result is even quieter operation. Still has good power from idle to wherever (tach is broken lol).

This freed up the header for the REPU and the 2" presilencer for another project.

Speaking of a good project to work on, I could start with the 2" collector, dyno it, then oval a 2.5" pipe and weld it on where the collector width matches like RB does, dyno it again and see what the differences are. The rest of the truck's system is worthless however.

I think I'll start out with a 2.5" collector and keep the bunged 2" piece for something else. Thanks everyone.

speedturn

iTrader: (1)

On my PP race 13B, I use twin 1.88" ID primary pipes, merge the collector throat down to 2.3" ID, then I gradually expand it back out to 3" pipe.