Cam selection with manifolds

[TAG]

Pretty all-around question, but lets take a hypothetical situation, where you have an 355 sbc and by law its mandated to keep stock exh. manifolds, log-style.

Okay, lets carry on further. When you look for generic grinds offered, manufacturers states what they want from your whole package for the cam. Pretty much they want headers from everything over stockish cams. Were talking about flat-tappet hydraulics, not a race-engine. Street

Now. If ive understood right, theres some things you can do to prevent exhaust reversion back to chambers, and building exhaust well with x-pipe, you can further diminish the loss vs. headers.

So now i come to LSA. If ive understood right, wider LSA is good with manifolds to lessen overlap, since log-style manifolds hardly wont offer any scavenge effect. Most topics, and articles, touch the subject on point with identical opening times. But how does longer opening time equate to this? For the sake of comparison, lets choose Lunatis 262/268 adv and 112LSA, versus their 268/276 and 110LSA.
Im at loss there, no matter how i read. Seems like when they get longer duration, LSA gets smaller. How for example that aforementioned generic 268/276 work with 112LSA?

Please, keep in mind exhaust manifolds ( logs) are mandated. I gave some generic ideas, but would like to hear talk how does it affect cam selection for street use, on flat-tappet hydraulic design. Please dont carry on headers and rollers, its not in the subject.

[cardo0]

Okay I'll try this but I'm no exhaust manifold expert let alone a cam expert. The stock log manifolds like you say have no scavenging to help the cylinder breath. But the ram horn and LT1 sbc manifolds are said to get you 2/3's of what a simple header will give you for reduced backpressure and some scavenging. Reading John Lingenfelter (RIP) sbc book he shows an 2nd gen LT1 build making 500hp through stock exhaust LT1 manifolds.

So the cam will make more overlap with more duration on the same LSA but the scavenging effect of LSA pretty much remains as before. The way to treat higher exhaust back pressure is with at least 10 degrees added duration to the exhaust lobe which allows more time for the exhaust to blowdown (per D. Vizard I recall).
So what I'm trying to say is while you and duration w/o changing the LSA the cylinder sees more backpressure/reversion but added duration on the exhaust lobe will give that cylinder more time to recover and blowdown.

For a stock type sbc head a 108 LSA is said to be optimal. That and 55 degrees of max overlap to keep the motor in streetable limits your cam will need to be less than 108x2=216*, then 216*+55*=271 degrees max duration on a single pattern cam. But you want a dual pattern cam for the added exhaust duration which will make an even shorter cam for street use. But keep in mind anything goes for a full race only motor.

Hope this can help.

[steve316]

wider lsa you will have less overlap with the same duration. There is no rule that fits every application.

[BOOT]

Ok this doesn't help with your cam selection but there is a place that ports sbc manifolds, brenz something long n hard to spell google it. And they also claim y-pipes will make better power with manifolds, doesn't have to equal length. I know, I know dual pipes or death, may I burn for bringing up y-pipes LOL


Far as cam section, top end is gonna be restricted with manifolds so in my opinion I'd pick something that has good low-mid range.

[novadude]

Subscribing, as I am interested in this. For a simple street engine with a hydraulic cam around 270 deg adv duration, as discussed above, how would LSA and int/each split differ for manifolds vs headers?

[MadBill]

Hopefully someone like Camking with actual experience will weigh in, but pondering from the basics and building on above replies:
Working from what would be a good cam choice for the application with headers, and depending on how bad the manifolds and exhaust are for restriction and runner lengths, you might want:
1. A later IVO to reduce overlap, due to the loss of exhaust scavenging.
2. An earlier EVC for the same reason.
3. An earlier EVO to aid venting against higher backpressure.
4. I don't see that IVC needs to change.

A 270° single pattern cam, 106° LCA, advanced 4° would give 33°-57°IN and 65°-25° EX., 102°/110°CLs events.

The effect of 1. & 4. would be to shorten intake duration by perhaps 4° and retard the ICL by 2°, thus 29°-57° on a 104° ICL.
The effect of 2. & 3. would be to increase exhaust duration by perhaps 4° and advance the ECL by 4°, thus 73°-21° on a 116° LCA.

Combined, we get a 266/274° cam on a 110° LCA, 6° advanced.

I love doing math in public...

[RevTheory]

I had a similar thought process as you, MadBill. I left the intake alone and added 8* to the exhaust opening. 111 LSA +7, if I remember right. That was just in my head while looking at this thread.

It'll be interesting to see what some of the dirt track guys who have to make these compromises have to say on the subject.