SOHC carburation and setting up

[Guy Croft]

Andy, sure,

but it's imperative to go one_step_at_a_time. Never 'throw money' at a tuning issue. James is very conscious of this.

What's this about a 'funny noise'. Tell me more please. is it roaring from the intake or drumming/resonance in the exhaust?

Also - James - is there a much fuel standoff at the carb intake on acceleration and low speed/mid range? Is the cam billet (blank) or reprofiled - please advise.

GC

[James Bowen]

Guy, Sump,

Sure, I described the resonance in the 3rd or 4th post I think. The description is recorded again below. Interestingly, since doing the recent work the resonance is over a smaller rev range now, and moved down 200 rpm, as per when the Cam "comes on."

As for the exhaust...., interestingly, and maybe you will think this will add weight to your theory. There is some wicked induction resonance set up at around 3,200 rpm, till 4,100 rpm, most strongly at 3,800 rpm. (sounds like air rapidly slapping itself, "rdprdprdprdprdprrdp" sorry thats the nearest I can get to describing the noise) This is most apparant on partial throttle / high load. At near full throttle, at these engine speeds the resonance is much less.

The noise is from the carbs. When the car was Rolling Roaded last year, the ram pipes were fitted, (though fitting them or leaving them off appears to make no difference) I could see a misting above the pipes, whlst the resonance was making the noise. It then disappeared as the engine came on cam. I assumed the misting was standoff.

The cam is a Steel billet. The company I bought it through, said that Catcam, had some applications made in Steel, and some that were in Iron, it was just luck what you got. The price was the same. Needless to say it isn't a regrind. The alternative I was thinking about using was a Piper 300, but thought it may be to extreme for my usage. (more overlap)

BCD. I measured it and compared with the standard cam. From memory the BCD was not more than 2mm less.

Regards, James

[Guy Croft]

OK > good report.

I think a picture is emerging, proceed as advised!

GC

[James Bowen]

OK.

Firm in Yorkshire are making up some pipes to extend the primaries. I'm looking at cutting off the collector, extend the pipes, weld on the collector.

We're going for 7" extra to make up to 30" primaries.

James

[sumplug]

Sounds like a plan James.

Andy.

[James Bowen]

Hello again.....

Just sorting the manifold. I have found that my primary runners are not 1 1/2" OD Dia..., but 1 3/8" Dia OD.

I have found that P D Gough do that size.

Two questions however....

1. Will the current diameter be affecting engine output seriously?

2. Ultimately the engine will be using sidedraught carbs (not yet). But the exhaust valve remains the same 33mm dia. Will I be better off making the manifold in 1.5 inch Dia OD?

If I will in the end be making up a whole new manifold with 1 1/2" tube, I will go the cheaper route now, and simply extend my current pipes myself, and see if this cures the current problem. If the 1 3/8" pipe is OK, then I'll get P D Gough to do a professional job now, which includes a 90 deg bend in the header for a better fit.

I hope that makes sense,

Regards James

[Guy Croft]

On a 33mmm ex valve the valve throat will be about 29mm - add min 5mm for primary pipe ID = 1.34", so go 1 3/8 - 1.5". I'd go for the bigger size really.

Primary pipe (not incl collector) - measured from head face equal length 30" but go 34" if you have the freedom to do so.

I'd make the tailpipe 1 3/4" if you can, doesn't need to expand into 2" bore pipe there.


GC

[James Bowen]

Great, thanks Guy.

[simon-spanner]

When I made the exhaust manifold and system for my x19 I used 28"primaries(head face to collector) in 1.5"ID tubing, I also used a proper merge collector into a single 2" straight though repackable silencer
It made a 28bhp improvement over the CSC manifold and silencer fitted when i brought the car!

Thinking of selling the manifold and system........

not sure if it will fit the 1500 though

[Guy Croft]

Excellent news, thanks for sharing that important info Simon.

GC

[James Bowen]

Simon / Guy,

Many thanks. I remember you were using a "Race" spec cam, which I presume to have both high lift and overlap, but are restricted to standard valve sizes, and carburation. Would those factors alter the primary runner length significantly?

I also remember that you used software analysis to assist in exhaust sizing?

If its suitable, I'd be interested.........Though I have just sent off a fax this afternoon to PD Gough, for them to make up some pipes to extend the existing manifold.

Regards, James

[James Bowen]

Guy, Simon,

I have been researching "Merge collectors" since seeing the term mentioned by Simon. I was aware of the idea of a proper "merging" of the pipework before,(from a book by Vizard) but never really looked into it.

I realise that I have at the moment, a bog standard, 4 pipes "flat ending" into a collector. I will try to alter, and engineer the inside of the collector, and end of the new pipes to "merge" together. As the idea seems to be sound.

Could the current collector arangement also create unwelcome pressure waves / changes in gas velocity, etc?

Regards, James

[Guy Croft]

James, hi

sorry, just at present I don't know enough to make any comment on the collector layout with the four-square or 'bunched' layout.

GC

[James Bowen]

Hello,

Exhaust manifold has been extended 7". Runner length now 30" long.

I have not changed anything else yet including jetting. Initial drive to test was today. Power seems fairly unchanged, BUT. Resonance from the induction appears to have decreased to almost a neglegable level, and now occurs more briefly, at 2,700 till 3,000 rpm, instead of the previous 3,500 till 4,100rpm. The torque starts coming up at about 3,200 rpm. That is a full 800 rpm sooner than before starting this exercise.

Progression stage still appears sharp. Full throttle appears at the moment no stronger, slight stuttering through 4,500rpm then runs out of steam at 6,500rpm.

Coming off the throttle slowly from WOT and high rpm, the car feels though it slightly pauses or accelerates briefly, as opposed to decreasing speed straight away. Does that equate to needing a leaner mixture? I have got 170 A/C's to try..........

In any case, having chopped into the manifold it is apparant that through the bends the ID of the primaries become quite restricted. Down to 30mm Dia in some places.

From all that has been writted by Guy and others thus far, it seems sure that I could do with a larger dia primary manifold, (possibly Simon Spanners if he were willing !!!) Irrespective of whether the jetting problem has eased or disappeared.

Regards, James

[Guy Croft]

James,

I would not expect the revised header to yield any benefit per se. You have to optimise the jetting because it would tend to be trying breathe properly now but insufficient fule is getting in the right places - one assumes this was not possible before because the header was too short. Unless it's a head/cam issue, it should now be possible.
Remember what I said - If the longer header doesn't make a real difference you maybe better get the head up to me with inlet manifold to flow test it.

Please summarise your current settings, save me referring back to previous posts:
mains
a/c
emulsion tube
choke

GC