is the big bore later exhaust a worthwile upgrade?

B

brook shaw

Member
my car has the earlier type of system fitted
small bore

is the larger bore system a worthwile fit,,and above all,,,does it sound any better?? :mrgreen:
 
The large bore system on the 3500S was said to be worth an extra 6bhp over the small bore auto sytem of the time, but the later later auto large bore system has different front pipes so won't give so much of an increase. I don't think it sounds that much different, it's up to you if you think the power increase is worth it.
 
You need to think carefully before relacing any exhaust with a different bore. I investigated exhausts in depth when I supercharged am MG Midget! The mass of gas in an exhaust system has momentum which acts to suck the gas from the cylinder when the exhaust valve is open. Also the smaller the bore, the faster the flow is. The manufacturer will size the exhaust to balance the requirements of the tune of that particular engine. If you fit a bigger bore exhaust, then the gasses will by default move slower. This can result in a reduced scavcenging effect which means that the upwards moving piston now has to push harder to expel the exhaust gasses, which uses energy and reduces useable output power at low revs. Bigger bore exhausts usually give more HP at high revs as they give an easier escape route when there is more gas flowing, but can be detrimental to drivability at low speeds. In reality, the difference in size for the Rover exhausts is not great and the effect will probably be negligible in the real world.

Point in case.. have you ever noticed how small a bore a Morris Minor exhaust has? This is purposely done so that the exhaust is efficient at low engine speeds. The Morris Minor is no racing car so it is optimised for shopping trips and short slower runs. The same engine has a much bigger bore (Relatively!) when used in an MG Midget asn the expected dirving style is completely different.
 
yer i know this well
as well as my p6 rovers ,jags and mk1 escort,i own quite a few mk1 minis-various models
the 850 mini deffo needs its pea shooter to pull properly

i once put a 2" big bore on a mk1 escort 1100 wen i was a lad and made it even slower,,,really slow,,so we learn the hard way

im not on about boy racer big bore,,i refer to the later series 2 v8 had a slightly bigger bore from the factory,
mines an earlier seies 2 with the smaller pipe diameter

its due for renewal at some point,and just wondered if the later series 2 type is the one to go for??
seems theres no big deal in it,,tho the skinny bore one is very quiet,,
bit pants for a v8-dont want noise,but a small throb would suffice lol

the v8 is not a fast car ,so if it gave me a bit more oomph i wud be well pleased
 
You would get much better gains by swapping out the heads for SD1 heads with the larger valves and better gas flow, better power and MPG.

Graeme
 
I concur with the theory above gents; but....

Don't forget you are looking at a 3.5 ltr engine here trying to cope with 150 bhp's worth of petrol. The exhaust size is small in proportion to even Morris Minor standards. Plus the auto exhaust manifolds are known to be spectacularely restrictive. And they are a 4 into 1 design and not a 4 into 2 into 1, which is where you really get an extractor effect on the cylinder gasses.

So my advice is that the larger "S" system is a no brainer. If you wanted to go for something much larger, then that would entail some risk of over egging the pudding, but the "S" system is merely a normal size where the auto's is tiny.

For what it's worth. I have a complete new stainless system including manifolds in stock, which was intended to go with the 240 bhp 4.6 engine. It is sized accordingly and the system size is 2 1/2" with commensurate steps down as the system splits towards the cylinder heads. I shall fit this to Lucky when the current mild steel system expires. That I do recognise will be a risk. I'll report how much of a risk when it's done, as the car will need rolling roading again after fitment, so we will know exactly what the effect has been.

Chris
 
There is that article floating about (i think on this site as well) where the NZ chap did a rolling road compaison of extractors over standard ex manifold and looking at the graphs it was not worth the upgrade, not worth the expense for a marginal return and as has been said any improvement was well up the unusable RPM range and pretty self defeating.

Graeme
 
I'm not sure I've seen that Graeme?

Having driven a 3500S extensively I can report that the flexibility of the engine is not compromised. With a bit of effort you can pull away from rest in top and my Father's and mine default driving style was to pull away in 2nd, then change immediately into top.

The criticisms you and Al level are entirely valid, but only when starting from an already adequately sized system. The factory early auto's system wasn't.

Chris
 
To really know what the difference between the standard and large bore OEM manifolds you would need to fit both sets to the same car with a dyno run after each. The tiny increase at maximum power might well be offset by a reduction at points leading upto peak torque, and that would be detrimental indeed, especially for a Rover with an automatic transmission. If my engine was still a 3.5 I would willingly give away 10 or 20 HP at maximum for an additional 10 or 20HP between idle and peak torque.

Extractors be they 4 into 1 or 4 into 2 into 1 are very much restricted in what they can deliver when they must meet the tight space requirments within the P6 engine bay. In the case of the former, this design moves peak torque higher and boosts top end power as opposed to the latter which shifts peak torque lower with a slight reduction in top end power. The lengths of the pipes are rather important if the most is to be gained from fitting them, and when compromises are made due to space constraints, so the outcome will be less than expected.

The V8 torque curve is typically quite flat, not peaky like a hot 4 cylinder engine, and that is what makes driving the Rover or any V8 for that matter so much more enjoyable. Camshaft profile and duration will be a significant player, much more so than the exhaust manifolds in where the power and torque are delivered within the usable rev range.

Ron.
 
ghce said:
You would get much better gains by swapping out the heads for SD1 heads with the larger valves and better gas flow, better power and MPG.

Graeme
Click to expand...

shame,cos its only just head head work,skim and composite gaskets and head bolts fitted
bummox
 
just found in the mountain of spares that came with the second v8,,a large bore system, inc well used condition ,shame its for a manual..mine are autos unfortunately,,arrgh :mrgreen:
 
hi, I agree with Sidneyroverp6b. From what I remember years ago reading all
about engine tuning true extractor manifolds need all the primary's to be the
same length and the first silencer/expansion box the right distance from the
exhaust valve. Even then the effect is limited to a narrow rev range. So they
are only really effective in combination with other engine modifications ie.
competition engines.

So for a road car any exhaust that is free flowing is best.

Colin
 
I'm not sure I've seen that Graeme?
Click to expand...

Chris; this Kiwi P6B owner published the dyno graph of an ( expensive ) 4-2-1 custom tubular header set, compared to the standard exhaust + concluded it wasn't worthwhile for "Street" use and only worthwhile for the racetrack ;

http://oldsite.p6club.com/improvingthep6.doc

Here's a link to a clip of this car going around a track ( great sound! )

http://s187.photobucket.com/albums/x289/japanparts/OldV8/?action=view&current=SlimsRover.mp4

GW
 
Thanks for that Graeme.

That confirms what I thought, that this thread is trying to compare apples and oranges! The original poster and myself are talking about upgrading a standard Rover V8 Auto manifold and exhaust sytem to a standard Rover V8 S manifold and exhaust system. Ron and others are talking about abondoning a standard Rover system of any type and upgrading to a large bore system.

There is absolutely no debate that it is worthwhile going from the Auto size to the S size. The auto size is dramatically restrictive; arguably, so is the S, but less so! Neither are anywhere near the point at which the diminishing returns discussion kicks in.

I do subscribe to the theory behind the discussion engaged in over loss of torque etc. But it does not apply at this sort of size of exhaust. It should be noted also that the discussion applies to the whole system, not just the manifold. A huge manifold with a pea shooter on the back is just as reatrictive as the pea shooter alone. Likewise, removing sources of restriction, such as the back silencer, also has the same effect on torque as an over large manifold.

Have no doubts, an S manifold and system will improve your P6!

Chris

PS, later Auto manifolds and systems (perhaps including the one you have upgraded to a lrge bore system, Ron?) are built to S sizes anyway
 
TokyoP6B said:
I'm not sure I've seen that Graeme?
Click to expand...

Chris; this Kiwi P6B owner published the dyno graph of an ( expensive ) 4-2-1 custom tubular header set, compared to the standard exhaust + concluded it wasn't worthwhile for "Street" use and only worthwhile for the racetrack ;

http://oldsite.p6club.com/improvingthep6.doc

Here's a link to a clip of this car going around a track ( great sound! )

http://s187.photobucket.com/albums/x289/japanparts/OldV8/?action=view&current=SlimsRover.mp4

GW
Click to expand...

Thanks for that Graham, I had lost that link! rebook maked, do you know of any other videos of that car in action, sounds and looks impressive.

chrisyork said:
Thanks for that Graeme.

PS, later Auto manifolds and systems (perhaps including the one you have upgraded to a lrge bore system, Ron?) are built to S sizes anyway
Click to expand...
Any idea of what year cars might have this fitted, would be nice to think that my late model car was so equiped.

Graeme
 
Thanks for that Graham, I had lost that link! rebook marked, do you know of any other videos of that car in action, sounds and looks impressive.
Click to expand...

You're welcome! Gentleman that owns this car is Simon Craig, he lives North of Whangarei; PM me + I'll try + find his email address if you want to talk to him directly
I exchanged a couple of emails with him ages ago to try + find out where he had got his bonnet louvers done ( ! ) + to thank him for his finely written article

GW
 
His car was the one I copied for my bonnet louvres. Only subsequently did I discover that they are a genuine Rover idea, used on the P7!

Chris
 
chrisyork wrote,...
PS, later Auto manifolds and systems (perhaps including the one you have upgraded to a lrge bore system, Ron?) are built to S sizes anyway
Click to expand...

Hello Chris,

S manifolds were extremely rare to start with as only 250 Rover 3500S were imported into Australia, so it is my understanding. Later Rovers may well have had the larger bore S manifolds, but I don't have one to compare mine to, so I don't know. What I do know however is that the down pipes that come off my manifolds are 47mm in diameter, so a fly's leg off an 1 7/8".

The P6B cylinder heads have rectangular exhaust ports. They have a width of 20mm and a height of 30mm. The P6B exhaust manifolds have ports which measure 22 to 23mm in width and 33 to 35mm in height. The exhaust castings are not especially accurate, hence the variations along a single manifold. The position of the manifold once bolted to the head though is very significant if the most is to be gained. There needs to be a step between the two, and this needs to be at the top, not the bottom, which should be as near flush as possible between cylinder head exhaust port and manifold port.

Ron.
 
My S1 V8 1970 auto needs a new exhaust, if I understand correctly I might as well go for the 3500S system.

The slightly larger bore will leave me in a good place if I ever get round to a 3.9 or 4.6.

I'm going to ask a local garage to install it, presumably its a simple swap? Straight forward at the manifold end? Mounting brackets the same?
 
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