Cam LSA for Weber IR setup

[piston guy]

Amen to that Mike!

[BILL-C]

Won alot of races in USA and europe with cam LSA alot tighter than 112 in webber equipped cobras and gt-40's. Proper length stacks and air boxes are very important. I can't think of any reason to go any wider than 108.

[PackardV8]

I guess it's back to the application. The guy who sells the Weber intakes may be making an assumption, "If the buyer has to ask what cam to use, he's not be going to be racing it, so give him the most conservative recommendation."

[RevTheory]

I don't have much experience with this but it seems like stack height and exhaust lengths are absolutely crucial. If you get those correct, an I/R system is much more forgiving of overlap because you don't have the communication of one piston screaming down the bore while another is at TDC/overlap.

I would also suggest that any anti-reversion tricks in an I/R setup would yield more fruit than the same tricks in a common-plenum engine.

[PackardV8]

it seems like stack height and exhaust lengths are absolutely crucial. If you get those correct, an I/R system is much more forgiving of overlap because you don't have the communication of one piston screaming down the bore while another is at TDC/overlap. I would also suggest that any anti-reversion tricks in an I/R setup would yield more fruit than the same tricks in a common-plenum engine.

Yet another reason why those street cars with Webers, 3" stacks and mufflers may need different cam timing than a full race build.



My question was to refresh my memory and confirm we hadn't been breaking any immutable laws of engine building back when.

BTW, who's building full-race cars with Webers these days?

[RevTheory]

Sorry, Jack. I should have said runner length rather than stack length. I can imagine that the same air going past the booster more than once can be a bear to tune around, whether it leaves the air cleaner or not.

As to your last question; other than the nostalgia factor, I really don't know. However, having a finely atomized plume of fuel exit the air cleaner(s) anywhere near an alternator or headers is probably not a good idea so I'd assume that attention to the correct lengths is probably worthwhile.

That's probably an engine combo that would be a prime candidate for a dyno session so a guy could observe (or hopefully not) fuel standoff and deal with it before being installed in his beautiful Cobra replica.

Edit: I guess if you're stuck with a cast manifold that has less-than-ideal lengths and shelf headers in an application that won't allow the right collector length, your only option just may be to fudge the valve timing to prevent standoff.

[hoffman900]

BTW, who's building full-race cars with Webers these days?

V8's?

Bill-C in this thread is - but only if they really have to by rules (read FIA vintage rules).

There are a few other outfits doing V8s as well.

Plenty still are in the inline 4 road racing world, at least on the amateur level.

[lada ok]

who would stick webers on a dog ... if you do, you deserve every flea invested camel known to man

why run an IR system ... because you can pulse tune the runner for better performance, usually over a narrower rev range

you can run webers on any cam timing you like, but the larger the C/L the softer the induction pulses, and peak power drops but what you get is over a larger rev range
same with the exhaust, individual pipes boosts pulsing

I've never seen fuel stand off on weber carbs , I've seen it real bad on high performance 2 strokes, worst was those Yamaha 250cc of the mid 60's > YDS 1 > 2

[MadBill]

I saw it huge on a 2x2 Weber-equipped 1600 cc Toyota Sports Racer. Cam re-indexing helped power but had little effect on the 'fog bank', but (after it was off the dyno) going down 1/2" on the way over-sized collector cut 2.5 sec. off the lap times and the staining evidence of stand off was gone.

[Cubic_Cleveland]

According to a well-known Weber carb expert, cams for the IR setups require a minimum of 112 LSA, "because there's no plenum to contain the reversion."

That statement alone makes no sense, perhaps it was in a specific context.
All else the same standoff would be worse with wider LSA (later intake closing) unless it is caused by the exhaust system. Stand-off is generally bounce back at the end of the intake stroke, above and below peak torque.

If it is caused by the exhaust system then I would think you'd fix the exhaust system rather than change the camshaft. Yes, it's that simple!
If it WAS exhaust related it still that makes no sense, as reversion from the exhaust system(at normal levels) just gets sucked right back in again almost instantly,....and so does not need to be "contained". It is possible for very bad exhaust reversion to shoot fuel out of the carbs at low rpms, it CAN happen, but that is NOT a problem that you should have, usually such a problem is created by highly custom engines with overly large heads and LOW port velocity built by guys who don't know better. The venturi is not supposed to be the MCSA of the system! Some very radical engine will do that if you go WOT at too low rpm but........you just don't DO that, as there is no need. They don't have vac secondaries!

A lot of four bangers with IR webers run 102 Lobe center!

Plenums can dampen cycle to cycle resonance, but camshafts........basically don't.
Really screwy camshafts can CREATE standoff if they are causing extreme intake valve bounce, but once again, wider LCA not going to fix that!

This is quite a good response. You can see standoff or spitback in ANY type of IR engine that you can see the trumpets on. It will also occur in IR engine where the trumpets are covered by an air box, you just don't see it. Take a look at this older video of a Renault F1 engine https://m.youtube.com/watch?v=y2iBbwocYZw
This engine would have the best design intake and exhaust to work with the camshafts that will fit the chassis for the application. Yet it still suffers stand off. Exhaust reversion is a different problem, and as modok said, fix the exhaust if that's the problem!

[enigma57]

I realize this is an old thread, but was reading through it this evening and found the subject of interest, as I will be running Webers on a Chevy inline 6 and when I worked out my cam specs, I did not use an overly wide LSA. Just used what would work well with my engine specs and relatively mild state of tune. Had my cam ground on 108 degree LSA. Overlap ended up at 42 degrees.

Out of curiosity, I dug through my notes and pulled up the factory cam specs for a 1974 UK spec Aston Martin. These were 325 cu. in. V-8s redlined at 6,000 RPMs. The standard engine was fitted with 42 DCNF Weber carbs having 36mm chokes on IR intake and made around 315 HP......



Cams for the standard Series 3 V-8 engine were set up on 110.5 degree LSA and 53 degrees overlap.

The Vantage spec engines were fitted with 48 IDF carbs having 45 mm chokes on IR intake and hotter cams set up on 108.3 LSA and 58 degrees overlap.

Seems to me if an OEM manufacturer saw a need to use exceptionally wide LSA when running Webers on IR intake, they would have done so. That said, I agree that running wide LSA to reduce overlap is unnecessary and must have been recommended strictly to minimize standoff and make tuning easier.

Best regards,

Harry

[lc-gtr-1969]

I have run webers on extremely tight LSA, and whilst it was a challenge, they did work. I ran them on an old engine with an older style road race cam on a 99 lobe separation. When finally tuned right and with modest choke size installed, they were great. The high overlap and resultant poor signal to the carbs below 3000rpm always caused some tuning problems so modest size chokes helped crutch this a little, however, this was more a by-product of the cam than the carbs in my opinion.