The weak link, whats weak in the 968 engine? rev to 10k
 

I’m currently trying to identify the weak or heavy internal parts in a 968 3.0L. the reason is to build an engine that can rev quite high, a target of 10k rpm’s.

after brain storming a bit I have thought of the following and currently I am leaning toward the following.

Removal of balancers. rotational variable.
Solid lifters - removing possible slap.
Low lift cam.- or looking at an adaptation of the Japanese vtec, using 2 different cam profiles, and mechanical release from the high lift to the low lift profile.
-- titanium solid lifters, titanium valves, and matching springs -- from 928 performance site
--Arrow Titanium Rods, JE pistons, and a lightened crankshaft with oil passage work--- from Rennlist threads
Aluminum flywheel. -added-
Strongest available rod pin / can easily be custom made.

I’m debating on removing the connecting chain and replacing it with a cam gear for the belt, similar to powerhaus, which will remove the variocam ability of the engine, but should significantly increase reliability and revability.

What are your ideas, as to weak links in the 968 engine, all input is appreciated.

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there is another big modification im contemplating. rotating the head 180'. this will eliminate the need for long exhaust piping and allow the exhaust to shove gasses into the turbo with about 1 foot of pipe. It allows for a much bigger turbo to be placed on the pedestal. Not to mention the full use of the head lights. Also it should allow me to contain engine bay temperatures by allowing the exhaust gasses to quickly escape the engine bay.


The difficult part would be the fabrication of an intake manifold for the small confined space of the passenger side. There is plenty of room below the engine head, but not above it. Removing the intake portions of the head and cutting them down or rotating them 180' should fix this problem. Ill knows more when i get a hold of a front clip.

You'll need to lighten the flywheel also to spin it up that far.

removing the balance shafts brings in all sorts of harmonics issues you will have to figure out - fourth order stuff is going to be a mess - you'll need to do a lot of work on the crank, pistons and rods to get that balance right

piston speed is a consideration - not sure the 968 piston size can go that fast

belt slap and stretch is going to be an issue - i know it is an issue at only 7800, so i can only imagine what it would be like at 10k

oil cavitation is something you will probably have to deal with too - likely a dry sump will be needed

crank bearings are known to get hammered after lightening things up (again, a harmonics issue), so that can be a real problem - you will probably have to replace the girdle with something more substantial

good luck and let us know how it comes out

yes i was thinking about the harmonics. and the dampeners.

I could get rid of the belts and replace them with a chain similar to the ford mustand DOHC engine. (still looking thoe)

hondas are consistantly revving up past 9k using belts. they also have alot les displacement / moving mass being only 1.6L.

thoes engines do posess a decent stroke, humm..

i haven't done the math yet to see what the limits are - i am contemplating an overbore project, and will need to do that though

I think boring out a 2.5 and sleeving it to it's max and using custom pistons is the best way to go. The 3.0 is a bored, stroked 2.5 if I am not mistaken. If you can get 2.8-2.9L with just boring your chances of hitting a higher piston speed are much better.

I already have a 968 long block.

has a company had luck with sleving a 2.5L block? fromj my understanding on one has ben able to truely do it right.

http://www.circleperformance.com/cnc_block_prep.htm

Might want to see what those people have to offer.

Seems awfully optimistic
 

mfloren,

For a 10,000 rpm red line you are running a mean piston speed of:

3.46" * 2 * 10000 rpm /12 = 5770 fpm

This is well in excess of accepted practice for a race engine with any reasonable durability. If you are building a drag engine meant to last a very short period of time then this may be reasonable. I am not familiar with drag racing engines.

Accepted maximum mean piston speeds for racing engines (e.g. F1, IndyCar, Champ Car, NASCAR, etc) are closer to 5000 fpm. This would equate to approximately 8600 rpm if I have done the math correctly.

I don't know what the bottom end of a 968 is composed of, but at this speed you are going to require all of the requisite components for a race engine rotating assembly. For example, forged crankshaft, rods, and pistons, all well balanced. Even with this work done, I have seen fatiguing of the bearing surfaces at lower speeds in racing engines.

I hope you will fill us in on your intended use. I am quite curious.

thanks for doing the math - i was frankly too lazy - lol - almost exactly what i figured in my head

8500 is the highest i have ever heard anybody getting to on a 968 block, and with extensive work to get there

any higher than that and i think i would be wearing a flak jacket and concrete underwear

umm...does the 968 have the dual cam pad tensioners like the S or S2?

or did variocam eliminate that weak spot?

it has dual pads on the chain - one on top, one underneath - the variocam pushes outward, extending the pads, and thereby rotates one cam relative to the other to alter the timing

Brainstorming here….

If I’m not mistaken Indy goes the route of big bore short stroke and up until 1995ish, big boost to bring their small displacement cars up over 1000hp.

what is the max bore you can get to with a stock block. reason I’m asking is because using a 2.5L crank, (3.11 * 2 * 10000) / 12 which is 5183. much closer to the 5000fpm mark.

Now if the bore could be increased to 110mm the displacement would be 3L with a shorter stroke which would also bring the piston inertia down significantly inertia

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Using a 3.0L crank at a 3.47 stroke
Rod length of 5.726
Pistons weighing 589.5g
At 10,000 rpm the Inertia Force is 6,434.90 pounds.
with a Stroke/Rod Ratio is 0.30 .

Effect of Rod/Stroke on TDC and BDC inertia force:
Your Upward Inertia Force at TDC is 8,395.94 pounds.
Your Downward Inertia Force at BDC is 4,473.86 pounds.

Now for a 110mm bore with a 951 crankshaft.

Using a 2.5L crank at a 3.11 stroke
Rod length of 5.9
Pistons weighing 620g
At 10,000 rpm the Inertia Force is 6,035.80 pounds.
Your Stroke/Rod Ratio is 0.26 .

Effect of Rod/Stroke on TDC and BDC inertia force:
Your Upward Inertia Force at TDC is 7,626.59 pounds.
Your Downward Inertia Force at BDC is 4,445.01 pounds.

What is the maximum inertia for the best aftermarket rods? I would think that tensile strength is more important the piston speed.

with the removal of the internal chain, the force that is applied by the single belt cam would be lessened. using two cams will be less of a strain on the timming belt then one.

also using a full titanium valve train, with springs that are less stiff will bring the load on the belt down further.

Anything over 6,000 RPM even on the 2.5L engines and you're typically floating valves. This is going to be a huge challenge to overcome on a 16V engine.

Nah, that can be done using carbon fiber valves and pneumatic valve operation.

If you do it right, you don't even need camshafts!!!

:eek::eek::eek::eek::eek::eek:

with a lightned lifter that is of solid design, and titanium valves / anything else that can be lighter it should not be that big of a problem.

from my understanding the 16V head can rev higher and experiences float later in the rpm band due to the lower weight of each valve assembly. further lightening of this should promote a higher rpm band with low float.

the 968 lifters are good for a bit over 8k - i expected problems too, but have been assured by someone who races theirs that they have not seen any issues of float at 8200 - i regularly take my bone stock engine to 7800, and have never seen a hint of a problem (kids, don't try this at home)

I also take my stock block 951 up to 7000 with no problems.


Does anyone know the weight of a stock piston in the 968? Il like to do some math on fps and inertia.

with stock 968 pistons.

Using a 3.0L crank at a 3.47 stroke
Rod length of 5.726
Pistons weighing 735g
At 10,000 rpm the Your Inertia Force is 7,977.18 pounds.
Your Stroke/Rod Ratio is 0.30 .

Effect of Rod/Stroke on TDC and BDC inertia force:
Your Upward Inertia Force at TDC is 10,394.29 pounds.
Your Downward Inertia Force at BDC is 5,560.06 pounds.


The same engine at 8000rpm

Your Inertia Force is 5,105.39 pounds.
Your Stroke/Rod Ratio is 0.30 .

Effect of Rod/Stroke on TDC and BDC inertia force:
Your Upward Inertia Force at TDC is 6,652.35 pounds.
Your Downward Inertia Force at BDC is 3,558.44 pounds.

Now strictly looking at the pulling force of the pistons.

if you notice the inertia of the piston with a stock piston at 8,000rpm, 6,652.35 pounds, is 20% less then a lightened piston at 10,000, 8,395.94. this should support the thought that the stock crank, with enough oil passage preparation and a strong oil pump, should not have any problems.

any thoughts??

First thing is, Why do you want to make it rev to 10k???? IS this race only??? What kinda gears are you gonna run in the tranny????

well several reasons .

1. No one has done it yet. I have a thing for being unique / the first on something.

2. At 10,000 the mechanical top speed of the 951 is 264mph with the stock sized tire. My goal is to have a 951 that is capable of 200mph+ speeds with some room to spare. At 200mph the engine speed is around 7600rpm with the stock gears which pushes the limit of a stock 968, not to mention the power necessary for this feat.

3. I want more RPM, since Ill most likely only drive this car to 8500rpm, I want a substantial margin of error and the ability to know that the occasional over rev to 9kish will go un harmed. When in the corners and exiting, 2nd and 3rd gears are at a wierd RPM either pushing the limit of the RPM or out side of the powerband.

4. Im rebuilding / modifying this engine completely from the ground up. If it only costs a % more to gain more RPM I feel that it would be a prudent course of action.

Im acually looking at building a 600hp engine with a long flat TQ curve and feel that increasing RPM's will help this feat.

A really fun daily driver. :)

well, i have been working on this very problem for the last 4 years - top speed capability was something i wanted to pursue, just out of sick curiousity - i knew about the high rear lift coefficient issues, as well as the less than stellar drag coefficient of .34 - this meant an uphill battle to begin with, but it still seemed like fun - i've played around with this on other cars, and found pretty much all of the potholes - so far, i've only been able to make one car go 200mph and stay on the ground - it is a huge feat, and there are only a handful of cars capable of it - i have done the math on it, and determined that there were enough obstacles that barred this from reality in a street car

currently, the aerodynamic top speed of this car is about 175, even with the Turbo S splitter, hood valance, Turbo S rear wing, and the underbody work they did - it took 350hp to get there, and used different gearing

to reach 200mph in this car, you can count on having to do a LOT more aerodynamic work - it will require serious and complete belly pan work (i almost have this worked out now), venturi ducting (got a lot of that done too), deletion of all body gaps, including the headlights (done a lot of this already as well), and then the daunting task of the addition of about 200lbs of downforce, which unfortunately always means more drag to overcome - the wind tunnel testing of this is going to be very expensive - it would be EXTREMELY dangerous and foolish to start monkeying around with 200 mph without it - if you have ever driven 200mph, you would know that there are a lot of things going on there that don't happen at 180, and a world of difference between it and 160 - lift at high speed is a very serious thing, and not to be approached lightly (pardon the pun)

achieving more speed also requires a geometric increase in power - while the little quickie programs will tell you that it only takes about 550hp to get there, in real life, it will require about 700hp to hit 200mph in this car, once you have resolved the aerodynamic problems

side note - the rpm at 200 mph will be 8235 with standard gearing and tire size - 7600 only gets you 185

while i commend the idea, and wish you the best of luck, you have a huge project in front of you, but i am interested in the outcome - be careful

I found several threads on rennlist where 3.0L engines have redines up to 10k.

from what i gather they are using arrow rods and JE pistons on a 104mm bore with a 3.47" stroke.

please link those - i have spoken to most of the successful 968 engine builders, and they say the max is short of 9k, and those engines last about 20 hours, and usually bend cranks in the process - also, they don't put out nearly the power you are looking for - those engines are only putting out about 330 hp

i'm not saying it can't be done, though there are some real physics to overcome, but it is going to be a VERY expensive project, normally aspirated, it is likely to be somewhere around 30k for the engine by the time you get one running - interesting for certain, but not practical or simple

it would be a whole lot easier to turbocharge an engine, and then re-gear the transaxxle

let me find it,

http://forums.rennlist.com/rennforums/showthread.php?t=335676

a few people in this thread are talking about big bore, 104mm, engines. one in particular states a 10k rev limiter. hum

nice thread thou. I cant believe he did all that work on the block ony to place a stock 8V head back on it.

i read the thread

that is a 2.7 engine - 104 bore, but the same stroke of the 2.5

also, it said the rods could withstand 10k (though the working limit was 8k) - it did not say the engine revved 10k

as somebody pointed out earlier in this thread the max piston speed is well exceeded at 10k

what has not yet been discussed with any basis is math is the change in harmonics - this is a huge issue - lightening of the internals has resulted in a lot of problems with this - the damage has been things like smashed bearings, and bent crankshafts - somebody needs to do a lot of math to figure out what the change in mass and high revs are going to do to those harmonics

in 30 years of doing this, i have never heard of an inline 4 of this size revving anywhere near 10k - i've seen a few 2.5 liter engines doing 9k, but that's about it - it's is a long way to a 3 liter doing 10k - if it ever does get there, it sure won't do it for long

as i said, it is not the easy way to get there for sure, and even if you do, normally aspirated, the power output will be too low to use the revs in top gear

as fun as it might be to try, i think this one is a wild goose chase (or at least a grey goose chase - lol)

LOL. I love your input.

I have never calculated for harmonics, and do not even know where to begin.

this car will be turbocharged. ;)

I did think about using a 3.11 stroke crank in this engine with the 104mm bore i believe that the fps was closer to 5000.

It is ok if 10,000 is out of reach, it is only a goal. I could easily say i would not be disapointed with a 8k engine that can accidentaily rev up to 9k since most of the time the engine will be driven in the 3K to 6K mark.

Is it possible to use what a lot of the v8 guys are, a harmonic dampener, on the front of the crankshaft instead of the balance shafts. the thought here is that the location of the passenger ballance shaft would be a great place for a dry-sump pump.

no worries - it's all fun stuff, and at the end of the day, there is a cold newcastle to make it all better

8k is very do-able - 9k is an attainable goal, but not streetable - any engine that could hit 9k would not run well at all below about 4k

there is talk of a front dampener attempt, but so far nobody has successfully done it - the dual mass flywheel seems to be a problem there

i would contact pete at rs barn - he knows pretty much everything there is to know about these engines, their realistic rev limits, what it takes to get there, and those who have tried to exceed them and the issues they have had

just a note on the top speed thing - you are better off with a 951 than a 968 - the drag coefficient is a touch better, and it would be a LOT easier to get the power you are looking for

You should move over to vw type one and try to get 9K rpm out of a 2.2 liter :) you won't be destroying expensive hard to get parts, and you can build one or two at a time for the cost of those P car P's n C's etc..

Or lower speed and larger displacement with a BIG turbo on that 968. If you are going for top speed, switch up the final drive, use a taller rear tire, increase charge cooling/watermeth injection, decrease boost retard to match egt and knock readings with increased charge cooling, also higher flowing more efficient IC, and turbo and turbo inlet/outlet/ also free flowing exhaust after the turbo, bigger injectors, stand alone EFI/SPARK control hard hitting coil per plug, ceramic coatings, graphite coatings, mabe even nickasil and j/e p's,

propper heatsinks on your computer/injector drivers, maybe even watercooled transistors.. Big fuel pump/s, aditional stuff. and maybe an ejector seat/parachute....

Turbine engines can spin in excess of 50K rpm and push your car well over 400 mph like a brick through water. I wouldn't want to be inside it at anything over 150 without corner weighted air-tunel studies though...

I've been in excess of 124 mph on flat ground in a type one vw that's older than me :) a large flat windshield and large frontal area compared to any 944... no seatbelts, no second chances..... the engine would spin up to 7900 rpm in the lower gears but lay flat around 5400 5500 rpm in top gear with all the wind resistance... Fuelling and air delivery was sorted with dual ida 48's and big fuel lines and pump.. A type one chassis at 132mph with only 2 liter's N/A displacement pushing it, it's freaky, turning input takes about half a mile to do safely without upsetting the car.. The 944 under belly is scary looking even with the plastic trim and inner fender work on the s2 and turbo, but top speeds should be pretty fun without all the frontal area and long flat sides for big side winds to push on...... It looks like alot of air can get trapped under the belly too... I would put considerable time into making it look like a leggo brick upside down, except the distruptors should be domed about the size of those street bumps.. the rear air exit and rear tire inner valance should be as straight and as flat as possible inline with the belly pan, or tapering UP towards the top deck of the vehicle slightly you can create inner rear ducts to evacuate air pressure from under the rear of the car, vent it through the wheel well exit air above the rear tires at an upward angle to help make a venturi effect on the rear undercariage, but be carefull not to create a negative pressure space under the rear end as that may create a potential for the front to pressurize and lift up like a pickel fork boat.. use fender flares for wider tires and put a big old wing from the back rear deck area up to just a couple inches within the roofline. a large front air-dam with ducts to the brakes, radiator and intercooler... don't neglect to cool off your injectors with air blown in, or fan/blower.. you will need exceptional amount of "TORQUE" at upper rpm, this means air and fuel flow must match mixtures and volume etc.. do the math for the fuel system and let the airpump try to keep up, if it can't add a bigger turbo.... Don't worry about the horsepower numbers..