Been off here a few days for school and reorganization of ideas. Good news is that I passed both of my fast track courses, so I have one class between me and graduation in May
Adam728 -
Quote:
Am I reading that right, a total of 50 degrees of intake valve opening?
You will severely limit lift this way, as there's simply not enough time to open and close the valve very far. I would be very surprised if even 0.10" valve lift could be accomplished in that duration. Trying to push it higher is going to be a fairly violent lift/stop/close cycle, the ramp angles and valve accelerations will simply be too much.
Because of the short duration and tiny lift you'll get very, very, very poor cylinder filling. A huge compression ratio won't matter, as there won't be much mixture in the cylinder to compress. If I remember from back-in-the-day the BMW system that utilizes valve lift and duration for throttling runs a minimum of 90-100 degrees duration and 0.05-0.06" lift on the intake valve. That's for idling.
I hate to be all negative, but you are chasing a path of 35 year old technology that didn't prove itself to be worth keeping around. You're looking to do a huge tear up in terms of machining, custom cam, changing efi systems, etc, to have something that will give less power and possibly no better mpg than a more "standard" build. You yourself admit to knowing little about engines, yet seem bent on designing one. Camshaft events are the heart of an engine. A few degrees change in overlap can make an engine idle smoothly or misfire and stall. There's a LOT going on, and I would argue (strongly) it's best to follow a more well known path, at least until a very indepth understanding is developed.
If I were to do a mpg build on my truck (92 C2500, TBI 350, 5200-5300 lbs on scale) it would consist of the following. Goals would be good usable torque for towing, and decent mileage.
Leave it a 350. No going to a 305 and the bore shrouding that goes with it, nor going 383 and the cost/slight mpg decease that comes with that.
TBI heads - they've proven to be great for fuel efficiency and torque. For a mild build they don't give up much on Vortecs until 4000 rpm or so. With hotter cams the Vortecs start out performing the TBI's at a lower rpm.
TPI efi system. LOOOOOONG runners, port injection. Known to make huge torque numbers, great fuel efficiency, then choking the engine to death under 5000 rpm. I think this system should have come on trucks.
Then there's the game of cam timing, compression ratio, and quench. I'd setup for around 0.035-0.040" quench, and figure out what it needs to get there (deck milled, heads, head gasket thickness), while also keeping an eye on final compression ratio (which is based heavily on cam choice) and deciding where material needs to come or go to get to that compression ratio while holding that quench number (head, piston dish).
Camshaft - short duration, as much lift as can be mustered, decently wide LSA. Stock cam is under 200° duration, which is great for torque, but signs off early (mid 4000's), and everyone wants to ditch them for more power. I'd rather see something still under 200°, but with more lift than the stocker. Won't get much, but a little could help.
Long tube headers with small primaries, 1-1/2 to 1-5/8" max. Single 3" from there out.
Then it's all up to good dyno tuning. MBT sweeps, running as much closed loop as possible (or even OL lean), etc. Keep the truck down low, geared tall, and don't expect it to get Honda-like numbers. You should end up with something that has more power than stock (mostly torque), and improves mpg a fair amount.
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I appreciate the brute honesty! Seriously, I was getting frustrated trying to figure out how to make this junk work. I've been going back through and designing this motor from the ground up, planning on building a motor similar to what you have listed.
- 350 block, bored .030 over (355)
- ported and polished 193 TBI heads. Polish the combustion chamber and increase the cc's to 66-67 for all. This will reduce the chances of one cylinder being knock prone. Also, exhaust will be ported and polished. Z28 springs, screw in studs, 1.5 rockers int./1.5 or 1.6 rocker exht.
- Flat top pistons with 4 valve reliefs, +6cc tops
- Stock crank and stock rods
- 0.039 squish/quench
- 08-525-8 Comp Cams Tri-Power Extreme roller cam, 116* LSA, 194/206 @ 0.050" duration, .470/.464 lift, 800-5000 powerband
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=1550&sb=2"]http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=1550&sb=2
- Static Compression Ratio of 10.04
- Dynamic Compression Ratio of 8.25
- Possibly TPI EFI
Found some sets around here for less than $500. If not, TBI EFI.
- 1.5" primary headers, ceramic coated, with 2.5" collectors into 2.5" duals. Y those together into a 2.5-3" single out the back with a good flowing muffler.
I'm thinking that I will take this project slowly and take my time with this build so that it will be the best possible when the motor is bolted in.
As far as exhaust piping is concerned, would a single 2.5" or single 3" be better? Pros & Cons??