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

[mfloren]

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.

[deekay911]

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

[flash968]

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

[mfloren]

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)

[mfloren]

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..

[flash968]

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

[Schumi]

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.

[mfloren]

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.

[Schumi]

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

Might want to see what those people have to offer.

[944Jim]

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.

[flash968]

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

[harryrick]

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

or did variocam eliminate that weak spot?

[flash968]

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

[mfloren]

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.

[mfloren]

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

[mfloren]

Originally Posted by flash968

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

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.

[Porsche-O-Phile]

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.

[944 Ecology]

Originally Posted by Porsche-O-Phile

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!!!

[mfloren]

Originally Posted by Porsche-O-Phile

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.

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.

[flash968]

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)