Restrictive Exhaust Collectors!!!

[Gary Sullivan]

I have been searching the links for posts on the head pipe. In replacing a head pipe on a 75 Ascona I took apart a stock joint where the 2 feed pipes join into one. It doesn't flare out to permit maximum flow. On the contrary, the opening necks down into a small opening. The widest opening coming out of those 2 feeders, necks down to a 1 5/16 ID. I was shocked by that. I'm certain there is more I don't know. The stock pipe coming off of that is close to 1 5/8 ID. Man that seems restrictive. I'll try to get a picture. I didn't believe it.
Anyone had any experience with an aftermarket collector or building their own?
Seems I could get some power back by opening that up considerably or is there something I am missing (that is....other than my marbles)
Gary Sullivan

[Aardvaark]

Originally Posted by Gary Sullivan I have been searching the links for posts on the head pipe. In replacing a head pipe on a 75 Ascona I took apart a stock joint where the 2 feed pipes join into one. It doesn't flare out to permit maximum flow. On the contrary, the opening necks down into a small opening. The widest opening coming out of those 2 feeders, necks down to a 1 5/16 ID. I was shocked by that. I'm certain there is more I don't know. The stock pipe coming off of that is close to 1 5/8 ID. Man that seems restrictive. I'll try to get a picture. I didn't believe it. Anyone had any experience with an aftermarket collector or building their own? Seems I could get some power back by opening that up considerably or is there something I am missing (that is....other than my marbles) Gary Sullivan

Do you still have the stock fuel injection air flow meter? If so, opening up the exhaust really won't help since the air flow meter is undersize on the Opel
L jetronic and is the limiting factor to any engine mods. A higher capacity
air flow meter may have been found that works on the '75 intake manifold and
maybe someone can tell you about it.

[opelnut10]

That subject has been flogged back and forth for a number of years. If the car is going to be used as a street driven vehicle, then an exhaust header ran with a muffler is of little benifit below 5000-5500 RPM and they come with a lot of baggage (rust out, warped flanges, blown out gaskets and so forth). My prefered system is a 75 FI (Sprint) manifold with a modified headpipe that requires an exhaust shop with a tubing bender to make. (1) increase the size of the down tubes to 1 3/4" into a y'ed collector with 2" outlet and 2" tubing back to a low restriction muffler (2" inlet/2" outlet) then y'ed again behind the muffler and 2" over-the-axle pipes back to the "dimples" if it is too loud for your taste, use two packed tips to cut down on some of the noise. This is not a cheap system because it requires time, material and labor to build but done right it will be there a long time and perform much better than a "stock" system (IMHO)

[opelnut10]

The system I told you about is for a GT, in the case of a Ascona/Manta you do noth need to have the Y behind the muffler, just run the single over-the-axle pipe and use the packed tip if it is too loud for your taste.

[GTJIM]

Originally Posted by Gary Sullivan Seems I could get some power back by opening that up considerably or is there something I am missing (that is....other than my marbles) Gary Sullivan

The entire Opel exhaust system is designed on the 'interference' principle that deliberately damps exhaust pulses. This reduces peak torque and hp slightly in return for a broarder power curve that is more useful on the street where the motor rarely operates at high revs.

As you move away from stock specs in camming, valve size and carburation some extra power can be unleashed by opening up the exhaust system behind the exhaust manifold.

[Gary Sullivan]

Thanks for the replies. Very helpful.
I haven't tinkered with the internals. I have stock fuel injection, stock heads, etc. (relatively low mileage car)
I am now doing the math on the amount of the restriction at the center of the Y (on the head pipe) and it still blows me away. The reduction in mass at the Y is astonishing. But it sounds like I want to be careful how much tinkering I do if I still want around town drivability.
This car is a driver. Never raced. Only used around town.
I have some take offs head pipes I can experiment with and was considering trying a 2" flowmaster collector at the Y end of the head pipe. That would open up that connection hugely and should allow it to breathe better. Anyone think that is likely to make the car less smooth running and docile for daily driving?
Gary

[BQS4]

Gary;
I have a 75 Sportwagon, pretty much stock, except for Pertronix ignition. Although I converted it from auto to 5-spd, the exhaust was modified. It's stock till it gets to the front muffler, it's a MIDAS Gold off the shelf, then over the axle, eliminating the rear muffler/resonator and using a straight pipe out, with a MONZA dual tip. Sounds great, not like a ricer, but, does have a good "tuned" sound.