[frbformula]

An aftermarket turbo engine is out of the reach for many people. But you still want to make big power with a stock block. What is the best way to make say 800hp with a stock block and give it the best chance to survive? Big cubes, low boost, low rpm? Small cubes, higher boost, slightly higher rpm?

[Tom Vaught]

Put a good crankshaft in the block and keep the RPM down in the 5000-5500 range.

Even though it was a Vortech Supercharger deal, (more HP lost due to driving the supercharger) Luhn Performance made 862 HP at only 5300 rpm with a 462 cid 2 bolt (studs) block and factory caps (with a 300 cfm E-Head) several years ago on Steve Morris' dyno. carb/blow-thru deal.

A Turbo would use less hp at the crank so maybe 900 HP at 5300 rpm.

Tom V.

[Junkyard Dog]

Originally Posted by Tom Vaught

a good crankshaft in the block and keep the RPM down in the 5000-5500 range.

Like the 4.5 incher another forum member put in his 400 block (the Triumph TR6)?

[Tom Vaught]

When the piston stops at Top Dead Center and Bottom Dead Center at high rpm it is very hard on the Connecting rods, the piston pins, the pistons, and the bearings.

More Boost and less RPM using makes for a long surviving engine.

With Boost, even higher boost, the forces on the pistons, Rods, etc are only a small percentage higher vs Increased RPM where the forces are much higher.

This link describes a Worse Case Formula 1 engine situation:

18,000 rpm, the best parts money can buy, small engine, small bores and strokes.

http://www.pureluckdesign.com/ferrari/cdcolumn/

At 18,000 rpm, which is the redline of a '99 Fl engine, each piston in the engine accelerates from a dead stop at one end of the cylinder to about 100 mph in less than one-thousandth of a second and then comes to a complete stop again less than one-thousandth of a second later. This start/stop cycle is repeated 600 times per second!

This is where the Ferrari Fl engine's short stroke and longish connecting rod come in. At 18,000 rpm, this combination of its 45.6mm stroke and 110mm connecting rod accelerates the piston at a peak rate of nearly 10,000 g. With the 14.9 ounce combined weight of the piston, rings, and small end of the connecting rod, such acceleration requires more than 9000 pounds of force, which is trying to rip the con rod in two or tear out the piston pin.

That's brutal enough, mechanically speaking, but if the V-10 engine had been designed with equal bore and stroke dimensions of 72.5mm and had a 130mm con rod, like the proportions in a Mustang GT V-8, then the peak piston acceleration at 18,000 rpm would be almost 17,000g accelerating the piston to a peak speed of about 160 mph.

17,000 g forces with more "Mustang Production Engine" type forces in the example vs maybe 15% higher forces over Naturally Aspirated Pontiac with a typical lower rpm boosted engine.

Connecting rods have to be stout enough to handle all the horsepower the engine can make, and be strong enough to withstand the tension forces that try to pull the rod apart when the piston hits top dead center on the exhaust stroke. If a rod is going to break, more often than not it will fail at TDC on the exhaust stroke than at any other point in its travels. RPM kills engine, decent boost and lower rpm will allow an engine (if tuned correctly) to live a long time.

Tom V.

[frbformula]

Tom,

Do you think that a 4.500 stroke crank even at low rpm 5500 and less puts to much side load on the block? Would the 4.250 crank be better suited for longevity?

[Junkyard Dog]

Originally Posted by frbformula

Tom,

Do you think that a 4.500 stroke crank even at low rpm 5500 and less puts to much side load on the block? Would the 4.250 crank be better suited for longevity?

Ditto here. Same question for a turbo engine.
The following crankshaft strokes are readily available off the shelf for a 455 turbo engine:

4.5
4.25
4.21
4.00

It sounds like you would favor the 4.5, but I do not want to speak for you.

[BruceWilkie]

IMO especially with a stock block, conservative tune-up and excellent harmonic control (along with strong reasonably light parts) are key to longevity. My view point is that a turbo'd motor can make a bunch of low to midrange torque and strong top end power without the need for big strokes or rpm much above 6000 - 6500 rpm. Therefore 3.75-4.0 inch stroke is more than enough.

Along with that, keep compression conservative, cylinder heads cool, timing conservative, and a/f on rich side and as cool as you can. (You will still make more HP than you ever could NA even at much higher rpm.)

[taff2]

If I was using a 455 block ,I'd go with the 4.00" crank, a set of good longer 6.8" rods (to lessen even more the thrust loads on the block) and some decent forged pistons with the top ring land set at least .250" down from the crown.

[Tom Vaught]

Originally Posted by BruceWilkie

IMO especially with a stock block, conservative tune-up and excellent harmonic control (along with strong reasonably light parts) are key to longevity. My view point is that a turbo'd motor can make a bunch of low to midrange torque and strong top end power without the need for big strokes or rpm much above 6000 - 6500 rpm. Therefore 3.75-4.0 inch stroke is more than enough.

Along with that, keep compression conservative, cylinder heads cool, timing conservative, and a/f on rich side and as cool as you can. (You will still make more HP than you ever could NA even at much higher rpm.)

x2

I spoke with Travis Quillen a while back about his Turbo Engine (with the Butlers).
A 482 CID engine. 4.25" bore and 4.25" stroke =482 cid.

He said if he was to do another engine, (for him and the Butlers) it would probably have a 4.00" crank in the block with around a 4.310" bore. Only 15 cubic inches less displacement vs the 482 cid engine but the ability to keep the piston skirt more in the block and put a larger pin in the piston. Like a 1.030" diameter Hemi pin. The forces on the pin would be basically the same (under expansion stroke) but the pin would be stronger.

Tom V.

[JSPONT]

Originally Posted by Tom Vaught

x2

I spoke with Travis Quillen a while back about his Turbo Engine (with the Butlers).
A 482 CID engine. 4.25" bore and 4.25" stroke =482 cid.

He said if he was to do another engine, (for him and the Butlers) it would probably have a 4.00" crank in the block with around a 4.310" bore. Only 15 cubic inches less displacement vs the 482 cid engine but the ability to keep the piston skirt more in the block and put a larger pin in the piston. Like a 1.030" diameter Hemi pin. The forces on the pin would be basically the same (under expansion stroke) but the pin would be stronger.

Tom V.

Very true but for the street and the amount of boost for a street car that you can get to hook you can build a long living off the shelf parts engine that will last for years at 15 psi on pump gas. Make about 1000 h.p. on the street. That is more than enough.
I am not talking b.s. street cars like on tv that are pro mods no interior all cage on the street, I am talking about full interior,radio + cup holder cars.
Just about and good forged combo with the right cam will give you a bad to the bone street car.

[frbformula]

So for a 400 block you guys think its better to run a 4.00 crank and rais the rpm some (keep under 6000)?

[Tom Vaught]

Try to find a 1967 2 bolt main block, (not drilled for 4 bolt mains), put a set of splayed caps on the block, a 4.0" Good forged crank, and a decent set of heads with a Boosting Device and you will have great durability and lots of HP.

Tom V.

[bendutro]

RPM isn't the killer some here are making it. Unless you have a data showing the peak piston speeds, total reciprocating mass and acceleration forces vs material UTS graphs, you're guessing.

Yes forged internals are strongly encouraged, yes a solid tune is critical and yes the cam attributes that make a solid turbo engine do not encourage overlap and low LSAs but just saying "high RPM is bad" doesn't really tell the whole story.

[BruceWilkie]

On a factory block, especially with increased stroke, rpm is a big contibutor to stress related failures. More so than just hp/tq alone. An aftermarket block sees same stresses but being stronger it simply handles it better.

RPM is indeed a factor! Stroke is a factor! At any given rpm the peak piston speeds of the various strokes increase dramatically with stroke increase.

FWIW at 8000 rpm a 3.75" stroke with 6.625" rod will see peak piston speed of 8164.07 feet per minute at 75.252 deg ATDC with an avg piston speed of 5000 FPM

4" stroke 6.625" rod 8000 rpm, the peak piston speed = 8753.52 FPM at 74.45 deg atdc with avg piston speed of 5333.33 FPM
Same stroke but 6.8" rod the peak piston speed decreases slightly to 8734.57 fpm and occurs at 74.777 deg ATDC the average piston speed remains at 5333.33 FPM

A 4.25" stroke with 6.8" rod at 8000 rpm sees peak piston speed of 9328.95 FPM at 74.011 deg ATDC with piston speed average of 5666.66 FPM

At 4.5" stroke with 6.7" rod at 8000 rpm = 9947.28 FPM at 73.078 deg ATDC with avg piston speed of 6000 FPM.
Slowing RPM to 6565 puts peak piston speed at 8162.99 FPM, which is less than 2 fpm slower than the 3.75" stroke fpm at 8000 rpm.

Add this factor too...A piston designed for boosted applications is not going to be as light as one intended for na apps. Also a 4.5" stroke doesnt allow good pin/ring placement. Even the 4.25" stroke doesnt allow best pin placement and the skirt still hangs a bit farther than desirable out of the bore at bdc.

[frbformula]

Tom,

I have a block i plan on using for a turbi build. Its a 69 already hase 4 bolt splayed caps and a mega lifter bore brace. I want to use my heads from my N/A engine. They are edelbrock round ports and flow about 345cfm. Im trying to pin down my rotating assembly so i can start gathering parts. My first purchase will be a good crank so i need to figure out what stroke i will use. I would like to make north of 900 for a street car that will make maby 7 1/4 mile passes a year.

[Tom Vaught]

Originally Posted by bendutro

RPM isn't the killer some here are making it. Unless you have a data showing the peak piston speeds, total reciprocating mass and acceleration forces vs material UTS graphs, you're guessing.

Yes forged internals are strongly encouraged, yes a solid tune is critical and yes the cam attributes that make a solid turbo engine do not encourage overlap and low LSAs but just saying "high RPM is bad" doesn't really tell the whole story.

So tell me who you are:

I personally have been playing with Pontiac Traditional Engines for 50 Years.
Know the good parts of the engines and the not so good parts.

I personally have 38 years of experience working for the Ford Motor Company as a Turbocharger programs Senior Engineer. Been helping the Butlers with their low 6 second Turbocharged car (and other cars) for 20+ years.

I have run THOUSANDS of Hours on Research Dyno cells and have a pretty good idea what causes engines to fail.

Also have real world knowledge AND data showing the Pontiac peak piston speeds, total Total reciprocating mass and acceleration forces vs Different materials. Pretty basic really.

You buy a Moldex pontiac Billet crank made out of Timkin ball Bearing steel and it will live a long time (The Butlers ran one for years and years at 50 psi of boost). You buy a cast chinese crank and you are on your own immediately. Same deal with most of the Chinese Forged cranks. There are a couple of decent Forged Chinese cranks out there though, SCAT stuff comes to mind.

Same APPLIES TO THE CHINESE CONNECTING RODS. BUYER BEWARE.

The Physics does not go away. RPM and crappy material makes for broken parts.

Lower rpm and maybe you can get away with less money and poorer quality material connecting rods. Pontiac had cast rods that lived forever at 5500 rpm.

just saying.

Tom Vaught

[Elarson]

Disclaimer: I'm not a turbo expert but we've done our fair share of torture testing Pontiac parts. A few general comments:

In general, I'd lean toward smaller cubes. Smaller bore leaves stronger, stiffer bores and more head gasket sealing surface between cylinders. Shorter stroke leaves the piston skirts farther up in the bores and reduces acceleration loads on the rods and crank. Let the turbo make the power for you.

As far as strength of your short block, there are conflicting parameters that have to be considered.

The connecting rods will be most highly stressed at the top of the exhaust stroke; where negative acceleration is the greatest and there are no combustion forces pushing on the piston to help slow things down. RPM is your major killer here and stroke is a big factor too.

On the other hand, your block is stressed highest at peak cylinder pressure (usually about 15 degrees ATDC), where the combustion pressure is trying to push the head up off the deck and trying to push the crank out of the bottom of the block. Cylinder pressure is your enemy here and is directly proportional to torque output.

Horsepower = torque x rpm/5252. What that tells you is that for a given horsepower, if your rpm is lower, the torque has to be higher (to make the same power) and the cylinder pressure has to be higher.....creating higher loads trying to tear your block apart.

So now you have to make a judgment call on what your weak link is. If you had an entirely stock short block assembly, the cast rods are your weak link and you would pick lower rpm/higher cylinder pressure. On the other hand, if you have a stock block but really great connecting rods, you may choose to run higher rpm to reduce torque, reduce cylinder pressure and reduce the block loads.

No black and white answers...but understanding the forces will help you make a good judgment call.

Eric

[Elarson]

And if your tuneup puts you into detonation, none of the rest of the decisions matter...you're going to bust stuff.

Eric

[v869tr6]

I probably have 20,000 miles on my 4.5" stroke 0.035" over 1970 400 block.
Have lifted the heads 2 different times but that has been at 25 plus psi.

I never did hurt my old cast rod, cast crank 0.030" over 455 with boost and I had it at up to 18 PSI many times.

I way overheated 5 qts of Moble 1 5-30 so I have 8 qts of 20-50w oil now and two oil coolers. One large one for all the oil and a second one with a fan that cools the turbo supply oil.

When I first built my 488 I melted pistons twice, when the front power valve got blown shut. Didn't know that would happen when the boost got up to around 20 psi.

I think the chassis twisting around has killed many of the blocks over the years, last year I was looking at a LS turbo Powered Trans Am with a big hole in the radiator, had what looked like plenty of clearance, but I guess the 2 foot wheelie had other ideas.
I run a mid plate and the top part of my front suspension is tied together with a 1.625" piece of DOM tubing. Plus I have the two bars that come from the firewall.

If your car really starts to haul, make sure the oil pickup stays in the oil.

Put a camera in the car to watch the gauges as you won't be looking at them when your sideways, LOL

[frbformula]

V869tr6

Can you elaborate more on your combo? Crank rods pistons heads ect?