intake air reversion

[chessterd5]

Hello, I've been reading Leo Santucci's book. I know its about L6 cheverolets but I'm thinking some of the principles are the same. In particularly, I'm interested in them talking about air intake reversion between cylinders 3 & 4. They also concluded in the book that a change in firing order solved the problem. Does anyone know what that firing order would be?

 

[CNC-Dude]

This only applies to the Chevy engine that has siamese intake ports and does not apply to engines that have individual intake ports for each cylinder like these Ford engines. It not only takes a firing order change but a camshaft change to alter the intake and exhaust lobes position in the firing order as well. It is also not necessary to do this except it the extreme racing conditions where the camshaft overlap is so huge that it really becomes a problem in siamese port engines. Our cams were over 286@.050", and mentioned in his book as being the highest HP engines with siamese head ever produced and we didn't change our firing order. We did however have the intake and exhaust events on each cylinder advanced progressively as you go from cylinder #1 to #6 on the camshaft though. This condition was also experienced in Rob Harrison's Comp engine that has individual intake ports, but again, the overlap of the cam is so huge, that only then does it really become an issue, and most of us will never see engines or cams large enough to experience these problem mentioned in his book.

 

[J.R.]

OK, then… maybe off-topic... or not: Ford supposedly found they had better driveability (at some RPM range) or some other benefit(s) that caused them to grace the ’87-’96 4.9L EFI motors with a dual throttle body vs. a larger single TB, as well as an internally ‘semi-split ‘ plenum. Anything to do with intake flow reversion, avoiding sequential firings, or… (?) to do with those choices?

If so, in a stock-to-mildly modded 4.9L street engine, would those assumed benefit(s) show themselves at lower engine speeds(800-2000 RPM), or would they be about giving greater oomph at the relatively higher engine speeds (2500-4300 RPM)?

What predicted differences, if any, would the effects be for a turbocharged EFI engine with reference to keeping the factory EFI plenum/runners vs fabbing a shorter inlet tract with a single large TB feeding an 'open' plenum? Will a turbo engine (8-9 psi) still show low-RPM torque gains by keeping those long intake runners?

J.R.
SoCal

 

[CNC-Dude]

One thing that comes to mind about the smaller dual TB vs. a single large one is the two smaller ones will likely create more air speed through them than the single large one will, giving better throttle response.

 

[chessterd5]

Thank you for your replies.I am also curious if alternative firing orders will have any effects on inherent crankshaft vibrations over 5500 rpm. In particular, I found a list of firing orders for straight sixes elsewhere on the internet & wondered about this one 1-6-5-4-3-2?

 

[CNC-Dude]

Yes, you can hurt an engine with having the wrong firing sequence and you can only swap timing by also swapping the cam. But you can't just randomly pick another firing order, you are limited by the degree spacing between the cylinders and which cylinder fires next in the sequence, or which one has just fired and the one your trying to fire now. Much like the 302 and 351W having different firing orders because they also have different cam profiles to allow this. You can only use the 302 firing order on a 302 if it has a cam ground for that firing order, and the same is true for the 351W and all other engines. But more importantly, the crank journal also has to be in the correct location to match the valve timing.