1988 Chevy K5 Blazer build - An unorthodox approach

[BabyDiesel]

Alright y'all, Brain Harris just e-mailed me back... not looking too promising. Here is what I wrote -

Hello Mr. Harris. What I am attempting to do is replicate the Atkinson cycle on my engine. I am looking into using a camshaft the has around 300* @ 0.050 duration for the intake. I understand through research that this will cause issues with the MAP sensor. Is there any way to tune around this?
Also, is lean burn (15.5 - 17.5:1) possible with the right precautions and tuning?

Thank you sir.


Here is his response -

nope, you need a mass air sensor computer system to attempt that not a map based system. All of my attempts at running an air fuel ratio leaner than 15.2 resulted in significant loss in gas mileage.

Looking back at what I wrote, I should have mentioned that lean burn would be used cruise-only.

Soooo, any bright ideas boys and girls??

I have seen conflicting reviews on TBIChips and their ability to tune is worth a half-penny. I have heard reports that EagleMark over at Gearhead-EFI has fixed many of his chip problems. I will go over there and ask for their knowledge and second check Mr. Harris's answer.

[2000mc]

... Not really a work around, but start over with a MAF system. Find a donor truck with a bad bottomend, 96-2000 with a 305 would get your heads, possibly a good trans, and then take everything else needed to make it run.

[cRiPpLe_rOoStEr]

Originally Posted by BabyDiesel

I was wrong, it had to be a straight 6.

IIRC the 4.3L straight-6 had been phased out at that time, leaving only the 4.1L and the 4.8L ones. But there was a 4.3L V8, which was the first small-block V8 and reintroduced in the 70's due to the oil crisis, and this one lasted until the 90's in the Caprice.

[hondaguy72]

To address my previous reply and your response, there is also a technique of angle milling that can reduce chamber volume even further. The 5.7 vortec heads are the best low cost solution. Small Ports, excellent port velocity. With a small custom cam you will achieve great results.

[BabyDiesel]

CrippleRooster - Thanks for the info on GM's I6's! I did not know about the 265 V8 being first. I assumed it was the 350

hondaguy72 - Yes, I believe you are talking about angle milling. That will be done as much as possible. Though I am thinking about keeping the 193 heads. I already have them, and they can be ported and cleaned up to make near the power and flow of a Vortec. And they are torquier down low, which is needed with me going to the Atkinson Cycle.

2000mc - That idea is great! I'm probably not going Vortec though. However! I may have figured out a way around the intake reversion problem while keeping the MAP system.

I have the idea of opening the intake valve on the intake stroke @ ~90* ATDC. Then, close the intake valve @ ~140* ATDC/BDC on the intake stroke.

Lemme preach on it! With the piston traveling down the hole (All valves closed), I do believe the cylinder will have an increasing amount of suction within. As soon as the intake valve opens at the late angle, I am thinking the charge will be pulled in more vigorously, promoting turbulence and mixing on top of what the swirl ports will do. With the quick close, less charge is trapped, allowing me to the run high compression. Also, the piston is still traveling downward as the mix is done coming in and the cylinder is sealed off. My thinking is that this will further the mixing of fuel molecules. Then it's high compression and BOOM. Beautiful ignition!

Am I missing something in the above paragraph? Feel free to point out any ignorance. I am smart, but I know little about internal engine dynamics.

I will be configuring the exhaust valve timing in the next few days. I will dive into the the Mazda Skyactive engines to see how the do it. IIRC a major contributor their engine running the 14:1+- CR was an efficient exhaust.

Probably means headers for the K5

[cRiPpLe_rOoStEr]

Originally Posted by BabyDiesel

I will be configuring the exhaust valve timing in the next few days. I will dive into the the Mazda Skyactive engines to see how the do it. IIRC a major contributor their engine running the 14:1+- CR was an efficient exhaust.

Remember, it also has direct injection, which makes it less prone to pre-ignition in spite of the high compression ratio...

[adam728]

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.

[elhigh]

I find the whole idea fascinating but I think your best money would be spent making yours the best SBC 350 it can be, not trying to retcon it into an Atkinson-ish model.

The TBI isn't really sophisticated enough to handle the tune you want for Atkinson function. Like the other guy said, the sensors are wrong.

Your truck is an aero disaster. I think there are likely a lot of things you can do, things that aren't too gross to look at, that will clean it up a lot. You're already doing nearly 15mpg and for a vehicle that size with that engine, that's pretty good. Tune your mill to its very best, and leave it alone.

I remember reading about Hot VWs "Mileage Motor" project - virtually none of which will make a hill of beans' difference to your project - but they found a lot of mileage after giving their test vehicle a chassis tune: setting up bearings correctly, alignment, fixing brake drag, etc.

You don't have to give your truck a heart and brain transplant in order to make it perform well, I think. I suspect you're just going for the biggest, sexiest project that has the greatest possible potential - really I think the Atkinson project does have enormous potential, but starting from a TBI SBC is probably the second-hardest way to do it (first hardest being carbureted) - but skipping past the more mundane but proven techniques that might get you closer to your goal anyway.

[Big Dave]

Think about what you are trying to do.

The OEMs have engineering resources you cannot even imagine. You can bet the farm they've taken a long hard look at this and rejected it unless it uses a direct injection engine and/or hybrid drive. You run into control issues and a loss of torque that makes the vehicle a burden to drive. The OEMs rejected it for good reasons.

Another very doable and proven effective strategy (maybe long term for you) is to ditch the automatic and get a stick. That's a slam-dunk 2 MPG improvement. there are all sorts of kits available to convert these vehicles to a NP4500 five-speed manual. The manual enables you to run a lower ration final drive. Yukon makes a 3.23 R&P set for GM 12 bolts front and rear. That's a 14% reduction in engine RPM at a given road speed If you need more torque, push the pedal and yank the stick and Viola! There it is.

A NP4500 and 3.23 gears. We're talking a 4 MPG improvement and a big improvement in reliability. You keep your 4x4 for off-road driving, and don't have to do any body work.

Not cheap, but entirely doable and you can expect a 33% increase in gas mileage.

[Big Dave]

Every time i park my F350, I feel like the Captain of the USS Nimitz trying to bring her alongside without any tugs. A short but still roomy vehicle like yours might be the answer.

I don't think any K-Blazers were ever built in a 4x2. There were some 4x2 Broncos but those are as rare as honest politicians. 3" front and 6" slam kits are available for F150s so should work on a Bronco.

4x2, moderate slam, manual transmission, a Cummins QSB (4.5 liter four-banger), 3.08 gears, and some aero cleanup. You'd have a easy to park but roomy vehicle that would scare 30 MPG.

Too pricey for a college student but within the reach of an average working person.

With my aero package right (it need work right now), diesel engine, 4x2, manual transmission, 3.08 my F350 routinely gets 27 MPG in the summer without even having to bother with aggressive hypermiling. That's not an instantanous ScanGauge reading but a three tank full (1,500 mile) average in real-world mixed driving. And all that in a 7,800 lb vehicle. A 4x2 Bronco or Blazer would come in at roughly 5,000 lb. A 37% weight reduction would have to buy you some MPG improvement even in flat terrain. 30 MPG looks attainable.

Imagine. A roomy, comfortable vehicle with all the virtues of a SUV but getting (stock) Honda Fit range mileage.