From: Per Axelsson
I have a 1987 Plus 8 and I have decided to change to a twin exhaust system. Is there a difference in the performance between cast and tubular manifolds?
John Sheally II
The answer is "no two things are the same even if they appear identical". That being said, a cast manifold holds heat longer than the tube type. Also cast is heavier and in a competition situation lighter is better and the car is faster as a result. But these are still minor points. FLOW is what you are looking for. The idea today with an exhaust system is to get rid of the exhaust as quick as possible. In the past, there was a tendency towards intricate exhaust systems that twisted and turned in an attempt to tune the performance of the engine and dump the exhaust out. Those systems looked good but have have been found to not be as effective as good short tubes that get the exhaust out of the engine quickly, simply and as fast as possible.
This type of simple effective
system is not always easy for a street application where you are required
to hang mufflers, cat converters and smog pumps on the end of your tubes so that the engine is forced to further "re-digest" what it is trying to get rid in the first place.
In sum, if you are into COMPETITION you want tube exhaust (headers) with about a foot to foot and a half straight out or turned down tubes with only a noise restrictor at the end of the collected pipes (like a Super Trap) and that is it.
For the STREET, tubes would also be the first choice with a small free flow muffler (to cut down noise from a legal stand point) to get the best results. On the other hand a cast system such as the SD1 units used on the Plus 8s work very well also (heavier, hold heat longer but still flow well) again with a small free flow muffler installed. However, even with this information your final decision may turn out to be a function of your local laws of your city, state, or country. On my 1983 Street Plus 8 I use the SD1 exhaust system with a straight pipe attached to the header tubes which runs all the way to the rear of the car. The pipe passes behind the rear wheel between the chassis rail and backside of the tire and is topped off at the end of the pipe with a Ansa exhaust tip (twin pipes and tips). I end up with good flow, performance and a mellow exhaust note. For the racing Plus 8 it is tube exhaust with one and one half feet turned slightly down and back towards the cars rear with four pipes joining one collector pipe with a Supertrap on the end and that's it for maximum performance.
Best to you. JHSII
We have found that the tubular headers sold by Morgan work extremely well if the junctions of the tubes are cleaned up to remove the overlapping pipes and projecting edges on the inside of each of the junctions. We connect the headers to one 2" inlet +4 muffler on each side of the car. We then install a 2" OD tailpipe going all the way to the rear of the car. This system is very free flowing and is capable of handling an engine with a power output of over 280hp with little or nor restriction. In engine dyno tests, the tubular headers made more power than the later cast header setup at all engine speeds on a 4.3 l. engine. It is certainly superior to the early cast manifolds and crossover pipe to a single exhaust.
By the way , we use a piece of stainless flex tubing to connect the header to the muffler on each side of the car so that the headers are not stressed by engine movement and don't consequently crack.
Regards, Greg Solow
There is a substantial difference in performance (AND SOUND !) with a dual exhaust system provided you use the tubular 4 branch manifolds/twin branch down pipes and then "straight through" silencers (just one each side)...probably up to 10% increase in power. (This system is in fact an amalgam of the manifold extractor system used on +8 between 1977 and 1985 together with the silencers and tailpipes used from 1971 to 1976.).
Do remember that the fitting
of a dual system where a single system was the original fitting, involves
a lot of work, as the valances (inner wings) have to be recut and flanged
to allow the manifolds to exit correctly...also on later cars with a brake
servo (1993 on) the fluid tank from the master cylinder sometimes needs
to be moved up to the bulkhead area (in effect making it a remote tank)
in order to keep it clear of the heat from the manifolds...this is not
a problem with non-servo cars and only rarely with LHD cars anyway. In
either case it is worth the trouble in order to regain the traditional
V8 sound and the extra performance!
The most potent stock system fitted to any Plus 8 was that used from 1977-1986. Morgan, watching engine power drop with the never-ending emission regulations from the 1970s, was inspired to fit a made-for-Morgan branch twin system for that short period. It had headers designed by a friend of Maurice Owen and though the systems was mediocre from twin pipes back, a replacement of that area of a variation of the Plus 4 Super Sport's mufflers and 2" pipes made it a powerful road system indeed with power up by 8-12% and torque more. I have fitted two of these systems to later single exhaust systems (a 1990 Plus 8 and a 1995 Plus 8) without much trouble. The only trick is to open holes in the valences (inner wings) and edge them properly (while closing the single hole made by the old system). https://www.gomog.com/restoration/valence2.jpg