Exhaust dimension vs engine displacement

[Greg Foster]

This is something I have been pondering for a while.
I have 2 elans;
67 s3 SS with dcoe 40s
71 s4 with cd 175s
The 67 has this teeny weeny tail pipe and exhaust ( 1 3/8" just a guess) from steel tube headers back. The engine has normal valve diameters and a mild cam, don't know any more than that.
The 71 has a larger bore straight pipe, maybe 2" all the way back to a dbl. tail pipe and has a cast exh. manifold.
They are both fun to drive but the 71 stromberg head car seems to be quicker in throttle response and acceleration..mind you this is only recorded via seat of the pants. Others who have driven the cars state the same.
I am wondering whether installing a big bore exhaust would change the dynamics of the 67 in a positive way.
ALSO
Is there a chart /rule of thumb determining exhaust runner size, length, size of Y joint and exhaust to tail pipe in consideration of engine displacement, valve sizing and carburetion. In other words to maximize the dimensions of exhaust to engine size with variable ie carbs, cam, porting etc? I have seen theory charts for V 8 engines and exhaust size but cannot find anything regarding 4 cyls. OR are there just too many variables?

[JimE]

I'm no expert but I had a big bore exhaust made up and fitted to my +2s 130 and not only was it very loud it spoilt the performance and the engine felt down on power. Back to standard fitment now. Miles Wilkins states in his book on the Twin Cam that Lotus put a lot of effort into the design of exhaust systems to get the optimum from the cars so possibly best to leave all well alone. Others may disagree. Jim

[seniorchristo]

I too have been leery of the smallish exhaust in my S3 but without an increase in gas flow via ports, valves etc., I would be reluctant to make big changes. John Passini (Weber carb author) says reducing back pressure without forethought can substantially reduce performance.

[661]

Perhaps the Stromberg's strong performance rather than the exhaust system?

[USA64]

I would echo Graeme. Strombergs good and little point in the exhaust being larger than the port.

[Chrispy]

Here is a calculator for header design.

http://www.mezporting.com/exhaust_length.html


I just finished reading Wilkins' book the other night and the comment on the exhaust grated with me a little. While I agree that chucking on a large diameter exhaust with little consideration may not result in the desired effect, saying that anything than factory is going to be a disaster is scaremongering. The exhaust after the headers/manifold is just a straight bit of pipe with a muffler on the end. I very much doubt that Lotus spent the time (money!!) on developing highly tuned mufflers, especially at the end of a dirty old bit of pressbent pipe.

Most interesting to me in your case is you are getting excellent performance out of the cast manifolds (or possibly just poor performance from another cause on your weber car - compression, tune, etc etc)

[rgh0]

everything you want to know about exhausts

http://www.burnsstainless.com/

The sprint style tubular headers and exhaust were pretty good for the big valve state of tune. There were a compromise to fit into the space and probably to keep the cost down also. The earlier dual down pipe cast headers used on SE engines were OK but not as good as the tubular headers. The earlier still single down pipe headers that were used on non SE engines up until the change to the Big Valve engines were not much good

Once your past the headers its all about getting out the end with the lowest pressure drop and bigger is better and straight through mufflers better but noisier.

cheers
Rohan

[Chancer]

reducing back pressure without forethought can substantially reduce performance.

I've heard that fairy tale trotted out many times over the years by those who have no comprehension of the subject.

RGHO's advice is bang on the money:


Once your past the headers its all about getting out the end with the lowest pressure drop and bigger is better and straight through mufflers better but noisier.

The reason he says past the headers is not about back pressure, their function (if they are correctly designed) is that the inertia of the spent gases aids the scavenging of the cylinders during the valve overlap.

A good reference book on the subject is Practical gas flow by John Dalton, also this one which is missing from my bookshelf and I now realise has not been returned by the borrower https://www.amazon.com/Scientific-Exhau ... 0837603099

[UAB807F]

My S3 moved from the CI manifold to tubular headers a few years ago, but I also changed to sprint cams at the same time so I can't honestly point to an advantage with the exhaust system change. This year I fabricated a set of tubular headers for the Europa which was on the same CI manifold as the Elan used to be, and that was interesting.

There's loads of info out there, some contradictory, some confusing, but after trying to learn about the design I ended up closely following the advice in Dave Vizard's "Tuning TC Fords". I stuck to 4-2-1 rather than 4-1 and made the primary pipes just over the recommended 13" so I could get them the same length & fitting in the space. The secondary pipes are 15" IIRC and then it goes into a fractionally larger tailpipe to the exhaust.

The change was part of an upgrade program for different cams over winter and I didn't expect any improvement from the exhaust.

Now it might just be in my mind but I do think the car picks up from lower speeds better than it did and crazy though it sounds I do think it revs better overall. It doesn't rev any faster and I don't believe it's generating any more power, it just seems zippier in getting there.

I put that down to matching and increasing the primary lengths which I did get anorak-ish about, the CI manifold can't really be considered as a matched set.

Brian

[rgh0]

In a race engine, a well designed and made Burns Stainless 4:1 system with a venturi collector is worth about 5 to 8hp at the top end versus a TTR 4:2:1 race exhaust based on dyno tests. Unfortunately you cant fit the 4:1 system in an Elan without taking out the alternator or going through the passenger footwell ( both of which are not allowed in my class ( the one tested is used on a S4 Seven).

If your building a high torque spread race engine with a little less top end HP they are similar
In a road engine in real life no real advantage in a 4:1 system I would believe.

cheers
Rohan

[Spyder fan]

Your S3 has the standard 105bhp twin cam and your S4 has the 115bhp SE engine?

[nigelrbfurness]

I would echo Spyder fan's comment re most likely source of apparent performance difference. The stromberg engined cars always felt a little quicker and more responsive to me. Regarding exhaust bore, I can quote no science but personal experience, increasing downpipe bore and silencer bore slightly over standard seems to improve low end torque, maybe at the expense of maximum power around 6500 rpm on a standard TC. It is more noticeable on smaller capacity engines like minis and sprites. Increase bore beyond a certain point though just produces more noise which to me implies power wastage.

[Greg Foster]

Alan,
Not sure if the S4 is SE as there are no markings indicating this, unless the engine spec was already to SE tune as standard. As stated, my SS has steel headers coated with JET HOT and possibly a one groove cam profile but only on the exhaust side if I recall correctly.
I do know that I will be removing the head from my SS as the last valve job was incorrectly done. The guides were reamed with too much clearance and the engine smokes/ drops oil into the cyls. from the valves. When parking the car I try to leave the front end slightly down hill to help drain excess oil sitting on the followers. Most of the time this seems to work but that first start up oft times fills the air with bountiful blue bellows from oil seeping into the headers.
I will check out the recommended reading and at some point measure the OD size of all my exhaust components and post.
Regards!
Greg

[StressCraxx]

Things to consider for exhaust dimensions:

Purpose of the engine: Road / Rally / Race
Engine state of tune Std / SE / Sprint / Level of modification$$$

Exhaust header:
Primary tube diameter
Stepped primary? y/n
Primary tube length
Anti-reversion device y/n
4 into 2 into 1 for road/rally use for broad torque curve
Collector design - long gently blended collector - developed by empirical testing

4 into 1 for race
Collector design/blend angle is critical. - developed by empirical testing.

The most important thing: The entire system must be considered, from the airbox inlet to carbs & jetting, to the intake porting, exhaust porting, piston, cams, combustion chamber, header primaries, collector to exhaust outlet.
Some very valuable lessons learned racing Formula Ford. Some of the lessons learned were very productive, particularly the collector design to optimze managing the acoustic pressure waves for scavenging +2-3HP. I learned the position of the header ports to the exhaust ports in the head was worth 2HP on the dyno.

Your engine builder's knowledge and willingness to develop the performance is vital.

[Spyder fan]

I always like to go for the simple reason or answer. Your S3 has been modified in that it wouldn?t have left the factory with tubular headers, maybe it produces a little more top end bhp as a result, but was it ever set up correctly with alterations to the advance curve and fuelling to tune it to the new exhaust, probably not. Your S4 is more stock and less likely to be out of tune with its exhaust components for which it was designed.

I doubt that in a drag race or showdown on some twisty bits given equal drivers that either car would come out on top significantly.

Timing, advance curve and fuelling need altering when changing an exhaust from stock if you want to keep drivability as well as improve top end BHP.

Apart from all that, I would recommend having a good read of the link that Rohan posted above. If you have the time and a little money available then making a few changes will be very rewarding.