The great (and unending) rpm myth for 4.3's

[Sparky2263]

When perusing the S-boards, I often come across these common statements;

a) 4.3's won't breath past 4500

b) 4.3's won't breath past 5000

c) 4.3's won't breath past 5500

d) 4.3's won't breath past (pick number)

Let's take a look at a typical 4.3 TBI engine, pre 96. The cylinder is a 4" bore with 3.48" stroke. Intake valve is 1.92" and exhaust valve is 1.5". Compression is approx. 9-1.

Now, let's take a look at a typical 5.7 (350) TBI engine, pre 96. The cylinder is a 4" bore with 3.48" stroke. Intake valve is 1.92" and exhaust valve is 1.5". Compression is approx. 9-1.

Interesting. I was able to copy and paste half of my 2nd paragraph. Now, if the stock racers can twist their factory heads and intakes on their late 80's to mid 90's 350's to 6500-7500 rpm's, what do they know (or are doing) that we're not? Good question. With basically the same crappy heads and intakes as their bigger brother, we should be able to do the same. Or at least come close, right? After all, we're spinning less mass.

Here's where you'll start seeing flow numbers and cc charts tossed around. People will swear by these numbers as if they are the do-all, end-all. Not me. I'm just gonna give my practical (and somewhat considerable) experience.

More rpm's make more power. If you can twist it harder and keep it pulling, you will go faster. Dyno chart on my 4.3 shows HP falling off after 5000. But if I shift at 5000 my ET will slow considerably. Why? 2 distinct reasons. Number two you'll never see addressed.

Reason one. Regardless of the HP falling off, it's still a considerable HP number. i.e., it's still pulling hard. If I shift early, I'll catch the torque curve wrong and the vehicle will slow. By holding the rpm's longer I'm still at a good HP number and when I shift, I'll catch the meat of the torque curve resulting in a much faster ET.

Reason two. The 700R4/4L60 dilemma. First gear is 3.06 and second gear is 1.63. That is the most horrible thing a person could do to trans. ratios. It borders on the idiotic. The ratio jump is 1.43 compared to every other trans' 1-2 ratio jump of about 1. When shifting ours from first to second, the rpm drop is considerable. It is VERY advantageous to hold 1st gear 300-500 rpm's more than second gear. Guy's with 3 speeds don't have this problem. As the cereal commercial says "Try it, you'll like it".

So, how do I get to these rpm numbers without my truck falling on it's face? It must have free flowing exhaust. Full length headers with true duals is as free as you can get (aside from open headers). Anything else (besides open headers) will reduce the rpm range, period. Shorty headers, 2 into 1 will both reduce the rpm range considerably. Will the above set-up lose some bottom end torque? Yeah, below 2000 rpms. Above 2500 it's a walk-away. Not a bad trade and you should have it geared (3.73 or 4.10) or a stall converter to make the loss inconsequential because the gain is fabulous.

Intake. If you can get it out, you need to get it in. Cowl induction or forced air from the grill/frontal area are the best bet. Scoops are way overrated and must be set up and designed properly to do the job. The top of the hood is just not a very high pressure area. Why do you think the pros have 'em sticking so far up in the air? That's where forward facing scoops work. Otherwise you're pretty much wasting time. The base of the windshield and the front of the truck are the best places.

Valvetrain. This is the biggie. Z-28 springs work good to about 6000. They're cheap and everybody has 'em. Valve rotators must be removed, good retainers must be used and roller rockers are the preferred way but not necessary. Valvetrain MUST be fully adjustable.

The above 3 mods will enable ANY stock 4.3 to turn 5500 and result in a quicker ET. Will it NEED to go 5500? Only track time will tell. But here's the kicker. Now that you've done the above, you can stick a decent cam in it. Now we're talking 5500-6000 rpm's and making some serious noise.

Oh yeah. How do the stock guys with these crappy heads and intakes turn 7000 rpms? Open headers, VERY expensive cams and extremely expensive engine prep. But, we can make 80-85% of what those guys do. It's the last 15-20% that gets real expensive ;).

Well, there it is. No fancy charts or figures. Just tried and true experience.

[KRUZER1]

wow, great write up.

[rlith]

Very good write up...

[Maximus]

I could not have said it better.I have always said that these engines even if they are falling off in power are still pulling higher in the rpm range.At least when ya shift your back down at a higher tq level than you are at before you shift.I have seen my TBI go to 5500 and shift out and pull stronger than at 5000.Don't believe me then all you guys with our whimpy TBI's as some say just try it and then tell me the same thing.But timing is everything ans shifting at the right are before it does fall on it's face is crucial.Oh and gears gears gears boys.I have never thought 3.42's were good enough for our tranny.At the least 3.73's should be used for optimal power.I noticed a real significant difference when I swapped to 3.73's.It allowed me to unrestrict our very restrictive exhaust.I am running a catless 2 1/2 exhaust in to a Magnaflow muffler and it sounds great and pulls great.

[jdhogg]

this is a great article,same we do on motorcycles.makin em breath better is probably the single most important upgrade there is

[jdhogg]

maximus,so 21/2 in exhaust werks fine?wow i thought it wood be too big.my headers are 15/8 in did ya get rid of thr o2 sensor too?

[Joker2040]

What about fuel delivery at high rpm's? Can the little injectors hold up under 6500 RPM's?

[Sparky2263]

What about fuel delivery at high rpm's? Can the little injectors hold up under 6500 RPM's?

Probably not. That's getting up there for factory injectors. The pulsewidth required would pull a lot of juice from a factory driver. But, there's ways around that also. Rat knows more about that than I do.

Now, in a drag race, they'd probably do okay. But sustained rpm's, I just don't know.

[Sparky2263]

maximus,so 21/2 in exhaust werks fine?wow i thought it wood be too big.my headers are 15/8 in did ya get rid of thr o2 sensor too?

I run 2 1/2 on mine out the sides. 1 bend and it's gone.

[Sparky2263]

I got to thinking about the injector question. Isn't the LS1 redline 6500? What is different about those injectors?

Here's the other thought. What if you ramped up the voltage to the injectors to say, 16-18 volts? That would reduce operating amperage 10-20% possibly providing that last bit of margin needed? Make it a bit easier on the driver and injector. Less heat produced also.

Random thoughts...........

[rat]

I don't think i would be worried about the drivers on the ecm for the injectors. Wouldn't raising the voltage increase the current as well? I know that is how it is in a normal dc circuit, v=ir, resistance is fixed, voltage is what the battery is, and current(i) is dependent on voltage. Raise the voltage, keep resistance the same, and current must go up to keep the formula ballanced. Of course this is more of a wave. I don't know how the inductance comes into play with all of this either. My thinking would be physical limitations of the injectors, with the popets could be a problem, and that is the main difference between the '96 up spider setup, and "normal injectors".

Another thing related to fuel injector size, and rpms is the higher you rev a motor the less time you have to inject the fuel. So if you were building a high rpm engine you might want to go with an injector size larger than what the formula would indicate. Or viseversa, you might be able to get away with a smaller injector if you plan on keeping the rpms down, and only ocasionaly romping on it at the track. Programing could also come in to play there, if you are comfortable keeping them on for more than 80% duty.

Another problem with reving these motors is in the ecm. The vortec ecm seems to have a 5800rpm rev limit, weather it is a bug in the tuning software, or if the processor just runs out of processing power. I am not sure if the 2001 up computers have this limit or not, i doubt they do as they are the ls1 style computer. I don't think there would be a bennifit of reving it higher than that, unless the engine was built for the rpm's in terms of camshaft specs, and valve springs.

[Sparky2263]

I see what you're saying. I like the idea of going larger on the injectors to make sure the job gets done.

Realistically, if I was heading over 6000 rpm's, I'd probably go carbed. Then I'd know I could dump enough fuel.

I'm putting the Comp 270AH-R14 in mine. That should take the CPI all the way to 6000 and maybe even 6200. The CPI seems to be able to handle running at 80-90 psi w/o any problems so I'm thinking I won't have any fueling issues. Only thing seems to be the stock fuel pump ends at around 300 hp so for me to continue my massive dry shot is gonna take something else.

[Hammer]

there's more to it than just the rpm limit on the 02+

the maps installed on the ecm are using the last readings from the pcm 200rpm+/- from the redline.

delphi didn't program the maps to go beyond redline i.e. fuel cut-off which IS the rev limit.

you can change the redline or remove it... but it is extremely difficult to upload new maps.

it's far easier just to get an after market ecm with everything you need and an editor program like ls1edit/tuner cat etc and work from that instead of hacking the delphi pcm

it will run beyond the limit once you hack the limit/fuel cut off, but you may pay the altimate price for it unless you have the equipment installed to shut it down during detonation etc outside of the pcm controls.

[Hammer]

an inline fuel pump is far more effecient than at pusher pump, especially considering the volume and or psi they run at. i installed a 110psi pump 12years ago on my 84 converted to 86camaro mpfi setup and it had so much pressure we were more concerned with vapor lock, etc.

you can run past 80% on the injectors but in my mind.... i figure i better be ready to buy a new block if i'm going to do that, so why not just fabricate an intake instead of rebuilding the engine?

[Sparky2263]

Good points.....

[Hammer]

this has been suggested before by other people but half mapping is relatively reliable.

i'd still try to hack closed loop and upload new maps before trying to skate by with other less reliable measures such a choke on the return fuel line... just as an example.

running rich doesn't always give you more power. optimizing the rich/lean and the timing as many of us already know provides far more power than dumping raw fuel out the exhaust and building up huge amounts of carbon in the cylinder/pistons/valves etc which leads to pinging, detonation, etc etc.

do the tune right is my suggestion. if you want to run high rpm, destroke it make it a big bore lightweight balanced application with free flowing low lift long duration cam and purchase aftermarket sensors computers and tuners to do this correctly... or install a carb and tune it daily.

[Sparky2263]

A good carb doesn't need to be touched. Does it offer the same perfect mixture under all driving conditions? Well, they wouldn't have done away with 'em if it did.

But to say "daily adjustment" does carburetors a great injustice. They definitely have their uses in a broad range of applications.

[jdhogg]

will 454 injectors work?

[Maximus]

Yes 454 injectors will work but you HAVE to have the engine with the need for that amount of gas.A TBI is similar in design to a CARB.If you get the throttle body bores out to 2 inches or more to flow the needed CFM then you can build a engine that will still be pulling in the upper RPM.I am helping a guy at work build a power house TBI 4.3 that will be dynoed to show if it is still making power past 5000/5500 RPM.I know this engine like the back of my hand.I'm helping him,well no I am basically building it for him at his garage and we are gonna put it in at my garage.He doesn't wanna go port injection as some of us TBI guys don't want to.I don't want to.You just can't beat the durability and simplicity of them.the TBI's major flaw is CFM flow.Even the the 454 TBI isn't as much as you would think at only 670 CFM.The engine is bored 30 over,running forged flat top pistons,Comp 270HR cam and new lifters,Comp double roller timing chain and 2 piece aluminum timing cover.The heads are 99 vortecs.The exhaust side has been enlarged slightly and smoothed out,and the exhaust valves have been cut to 1.60.Compression ratio will be 10.30:1 with the vortec heads.He's using a Edelbrock intake for the heads with a Holley TBI adapter plate.Instead of using just a 454 TBI he had the guys at Xtremefi to rebuild it and install a AFR and bore it out to 50mm to flow 750cfm.He has both 350 injectors and 454 injectors to try out both.There are other little details but that's about it.It's going in to a 91 S10 with a T5 and 3.73 gears.We are waiting on the intake to come in and a custom burn chip to finish it up and get it ready to drop in.I am really anxious to see just how much power it does make and how high in the RPM's it continues to make it.Once he gets it dynoed I'm gonna post the results even if it doesn't make power past 5000 rpm.You might ask why someone would put that kind of money in to a 4.3 TBI.I asked him why and he just said "why not".I didn't go any further lol.

[jdhogg]

i gotta normal electronic tach in my mg,but it goes up to 7k all the time.and dont cut out or anything,it backfires once in a while but thats it

[Rob_0126]

So with a CPI engine, would running bigger injectors help an all around stock motor?

See, If I got a CPI truck, I might want to get an engine rebuild, then put some beefier parts in. I wouldn't mind it having good power up to 6k.

[Sparky2263]

CPI is one injector and 6 poppet valves. Don't know about getting it larger. I just up the fuel pressure for a corresponding increase in fuel delivery. The guys at Lindertech would be able to answer on whether or not the CPI could be made larger.

From my extensive testing, the limiting factor has been the fuel pump, not the injector unit. The pump is pretty much at it's limit at 300-325 RWHP. The injector unit, I'd say would peak at about 350-375 RWHP at 90-95 psi fuel pressure. That's a rough guess.

[Rob_0126]

Well, 300 rwh would be enough power to move an s-10 mighty fast. :)

Getting it to hook though would be a diff. story.

[Sparky2263]

Yup, it'd be in the 12's. But you're right, you gotta get it to hook.

[biglouie_underpressure]

Well, 300 rwh would be enough power to move an s-10 mighty fast. :)

Getting it to hook though would be a diff. story.
hooking is easy in a blazer or regular cab s10. i see your in GA, come out to dougles!! on that track i have two buddies with blazers that drag the rear bumper and a friend with a reg cab 87 bigblock on 300shoot runing mid 5's the blazers both have stock suspension with caltracs!!!!!

[Rob_0126]

hmph. Wish I had the time. I still have to get me a truck. What would some decent heads and intake do for a regular tbi 4.3?

[T Man]

Edelbrock makes an intake, and the heads........the cpi heads with a little valve work are pretty good castings.

[redman20]

i have a 2.8 that i can hit 5500 to 9000 rpm and it fine but it has true duals and at 6000 to 7000 rpm's it screamin but whats the worst that could happen

[jdhogg]

great 2 be back guys,well my 4.3 will now run a little past 6k.i aint tried anything higher yet.these little twisters will breath great if ya let em

[nrlanni]

I'm sure as many of you know, GM simply shortened the classic V-8 350 CID (5.7L) block and made a great 4.3L V-6. It no exageration to say that Chevy's 350 CID is the most popular engine of all time. More raciing and high performance builds are based on it than any other engine known to man. It follows that the same great features of the chevy 5.7L would apply to the 4.3 (sans 2 cylinders).

It may be a slightly limited performer (although mine's great) in stock street trim, but that can be said of virtually all factory production. I marveled at my Lincoln Navigators' 300 hp from 5.4L and then I realized that still well less than 1 HP per CID, something that Detroit can do in it's sleep. 1.25 HP per CID in a daily driver street engine is very obtainable with good camming, mild head/valve work and decent compression.

Only people who lack knowledge about building engines would say "you can't" do something with any motor, or that it performs poorly.

[Mikz86TA]

Diggin up threads, huh newbie. hehehehe You are right and you basically touched on what Sparky was talking about. Being the 4.3 and SBCs are nearly identical, they should have the same intended use and possibilities. Given its a smaller cubic inch motor and not capable of the same possible torque and HP numbers. But looking back my '86 orig had a 305 vin H motor. Which was a whopping 150HP 240TQ. Durable, but when you started to squeeze out more HP that fell off a bit and you still come up short of 300 ponies unless a power adder or stroking was involved. Drawbacks in the 305 lies mainly in the smaller cylinder resulting in a smaller combustion chamber....58cc heads compared to a 350s availible 64cc plus designs. The 4.3 sharing the 350s platform means those arent the drawbacks. IMO, a well thought out build with high strength componets and using the SBCs power building knowledge, the 4.3 can acheive an easy 300-320HP at the flywheel without a sweat. I hope to one day find a block and have the time/money to do a build. I wait 4 now until..... 1: I have the time and funds 2: I gain more 4.3 knowledge and 3: I do it right the 1st time with others' builds as inspiration.

[nrlanni]

:) True I'm new to this forum but have been wrenching on cars since I was 13 - I'm almost 51 (August, 1956), so do the math.

I've owned several makes of domestic cars, including Chevy, Ford, Mopar, and Rambler/AMC - my favorite. I've had a few non-domestic makes, too.

I'm always amazed when people get stuck on one brand or another, or say you can't do something or that one brand is better than another, so the 4.3L Myths touch a cord. I have no brand loyalty but find that various years and models of just about every manufacturer has produced superior products. The S-10 Blazer is one of those enduring products.

Just like my '65 Ford Ranchero w/ factory 289 CID 4V & AT, PS. It has the original engine and tranny. I drove it work for a couple years until I decided it deserved a full overhaul after 40 years!

[Mikz86TA]

Dang!! Your old! Hehehehehe J/K I was fortunate to have a great father and good teacher on the values of mechanics, do it right...do it yourself and pride in your work. I started helping him out when I was old enough to and havent quit since. Our family has been a GM and Ford loyalist. I prefer GM over any other, but some Fords are dear to me. I know other brands are decent and have respectable potentials, but mostly Im 100% American. Funny thing I want to add. I was searhing something about the 4.3s and came accross a Google search with 4.3 rocker arms. I checked it out and it was a Ford site.....WTF?!?! Did they finally wise up and drop a 4.3 in a Ranger hehehehehe Not hardly, its a stroker kit that makes I think a 3.8 into a 4.3. Hmmm, sounds like a decent running V6. Now if they can stroke a 3.8 to 4.3 liters and get decent HP out of it, we got a Diamond in the Rough sitting under the hood!! I cant wait to get another daily driver one day and Unleash the Four-Three!!!

[whowhat28]

jdhogg ask if u got rid of the O2 sensor. i would like to know this also and how you made it work. thanks

[bespurcell]

You can buy an o2 simulator. I bought mine from Jegs for about $46

[whowhat28]

where can i find an electric fan conversion at a great price?

[firefighter]

The usual "great price" option is either a Taurus or Volvo e-fan from the junk yard. Do a search. I got a Flex-a-lite 210 on mine with the VSC control.

[Toypar]

All i can say is he has a point. I finalized my Edelbrock Package and when i bury the tach needle and the truck is still building more power im very happy. as for the exhaust i like it quiet you dont have to make alot of noise to hit the revs plus with the stock muffler i still cant hear the radio at 6000 RPM GM put a v8 exhaust on those 4.3's that flows great.

[brianj1976]

where can i find an electric fan conversion at a great price?
Auto Zone has a universal kit I saw on saturday and plan on getting this upcoming week. It has the fan and mounting compunents in a box and a seperate switch and harness kit wiith everything you need to install without having to "redneck " it with a toggle.

[Mikz86TA]

Yeah, I got 2 of those adjustable controllers from the autoparts store on my TransAm. One for each fan. I wired up my own relays and such...because its what I do. But those cheapo adjustable 'stats have lasted since the 305 resided in the engine bay. I reused it all after the 355 swap and just replaced the stock e-fan motor. All has been well for about 8 years with the setup.

[Mikz86TA]

BTW, Welcome to The PlANET Brian!!
Introduse yourself and your ride over on the 'New Members' section.

[Apprentice #4]

I've had my buddies 4.3 5-speed Silverado wrapped well past 5500rpm. His tach reads as high as 5000 and I had it at about 5500 and it wasn't moving as a needle would at redline, so I think it was more like 6000rpm.

[nomaboy]

the myth is not that it won't rev up that high, just that it won't produce power that high in the rpm band

[firefighter]

Yeah but that is a Cam thing not a motor thing. But you know that.

[Apprentice #4]

the myth is not that it won't rev up that high, just that it won't produce power that high in the rpm band

I'd say it was making plenty of power seeing as the truck was sideways with smoke pooring off the tires.:D

[nomaboy]

thats why it is called "the great (and unending) RPM myth for 4.3's;)

[biglouie_underpressure]

does anyone know where stock valve float is lol

[bespurcell]

I would like to know this too. I bet if you called comp cams they could tell you.

[Toypar]

My Edelbrock RPM Performer Cam and other boltons breath Great past 6000 RPM's the only issue i find is the adjustable fuel pressure regulator is great for tuning top end / Rich mixture and bottom end / lean mixture but i cant have both... And as for the LS1 Injector Question the redline is 6500 but Superchips raises it to 6750 and the 35th anniversary Z28 edition has bigger injectors by the info i have seen on the ls1.com forums.

[tonylinc]

someone told me I should look into buying some valve floats. where can I get some and what do they do?