Just curious questions here.

Block and or Crank

1. On a high compression motor - Let's say 10:4.1 - can you bore the crank 0.30 over without doing any damage?.
2. Will that increase the compression or decrease compression?.
3. What are the benefits of doing so?.
4. How Much HP is likely to be gained?.

On Euro Heads

1. Can you port the heads to let it Breathe better to match the block bore of .30.
2. If so, will different valves be needed?.
3. Anything negative in porting the heads.

I've had a few domestic V8 engines and I was thinking that the same performance enhancing measures could be used on the Porsche V8. If this possible - or is it wish-full thinking?.

A curious mind would like to know.

TIA and

 

Drew!!

Bore a crank??
I would guess you mean turn the journals to some undersize diameter... Of course it's what is done to salvage a crank damaged by a bad bearing. Usually you do this "on center"..... certainly for the main-bearing journals. You could do it "off center" for the rod-journals to get some added "stroke" but it would decrease the journal size (weakening the throw) and be of minimal amount (not worth the effort) because you'd get 1/2 of the change in diameter for change in throw (0.015" in this example).

As for compression; if you turned "on center" there would be no change. If you changed the stroke, you'd have to consider the change in piston travel. A small stroke change would have the same effect as shaving the block......probably requiring a change in piston pin placement or con-rod length. Neither directly affects the compression ratio. Pushing the piston up into the head this way would increase the compression ratio....... at the risk of piston-to-head/valve collision, not to mention altering the squish band designed into the current combustion chamber.

Porting of heads is done to decrease resistance to air flow. This is usually done by increasing the port diameter. This is a good thing until the increase results in a reduction of intake flow velocity to the point of hampering the mixture of fuel with the fast moving air as it passes into the combustion chamber. As a rule, ports are drastically hogged out in high RPM applications (like race cars) but not so much where idle quality is a concern......... like street cars. Back in the day, porting was a tricky "black art", now subject to the cruel but obective eye of the dyno!

 

Thanks for the info - Dunno what I was thinking when I typed crank. I never thought of "boring" a crank. I would just replace it.

All in all, I was just wondering about boring the block and porting the heads to increase "flow" for maximum HP in the Porsche 4.5-5.4L V8 engines. Including the Euro motors.

I know there are stroker engines but I was just curious if there was an alternate, less costly method of increasing HP.

Thanks again!

 

Drew,

All else being the same, a larger bore will give a higher CR; your example of 10.4:1 bored .030 might yield 10.5 or 10.6, not a big gain.

Increasing the CR will raise performance, but the increase will be small at that level.

Then there's gas; raising the CR on your engine could put it outside of the octane capabilities of pump gas, which would require the timing to be retarded to run properly, with the result being a net loss of HP.

I wouldn't think that the increase of displacement and CR that boring your engine .030 would require porting the heads and porting them to gain slightly more HP on the top end could take away low-end drivability.

 

Drew, just for reference:

Going .030" over = increase in bore from 3.74" to 4.04" (funny, same as a SB Chevy 350)

Your CR would be raised to 10.68:1 (actually, 10.677184).

Jim V's guess was on to 1/100; nice.

 

I know you meant bore the block, not the crank. If you really meant bore the block .30" over, no you can't. If you really meant bore it .030", yes you can.

Boring will require new pistons. The new pistons used will be what determines the compression ratio.

If you aren't doing the boring to repair currently damaged cylinders, and everything else remains the same, not much.

Again, if everything else remains the same, not much.

The heads can be ported and/or polished whether the block is bored or not.

Need, as in required, no.

It depends on what your objectives are, and whether the guy that does the porting knows what he's doing and does a good job.

 

Sounds like it's back to the huffer, Drew!!

 

Can these blocks be bored out? I thought they had a nickasil coating. That means you may have to sleeve the block or use a specially coated piston. Anyway, as suggested, boring over is intended to fix a worn bore, which you probably do not have, not as a performance enhancer.

Cylinder head work is a different story. The problem is, the 928 is not like my old Buick. I am having that engine rebuilt with larger valve heads that will be ported and polished. On that particular engine I will pick up 60-80hp. You would not see that type of gain on a 928. Porsche rarely leaves a bunch of easy to get hp on the table. Valve sizes and port flows are pretty optimal from the factory. You may gain 20hp from a $500 port job.

I think that the best way to get more power is to go with euro heads, cams and intake. That or sell the car and buy one that is faster to begin with, ie 32V or 300hp Euro.

 

I must be missing something here. The difference between 3.74 and 4.04 inches is 0.3 inches, not .03 inches.

3.74 + 0.03 = 3.77

 

They can be bored, but then need to be honed with a special process using some sort of paste. It's alusil, not nicasil that's used in the stock 928 blocks. You do not have to sleeve the block, but you do need to use the specially coated pistons.

 

Good catch, old & new. The CR I gave is correct for Drew's example of a 10.4:1 gone 30 over, however, I mistakenly used the bore and stroke for his MY.

CR should be 9.42, .030 over stock bore and stroke, with no change in chamber volume, compressed gasket thickness, or deck clearance.

Sorry and thanks.

 

As Z stated, the blocks are alusil; they are impregnated with silicon rather than the nicasil which is a coating.

They are bored out using a Sunnen or similar machine which uses stones to bore and finish-hone rather than a steel cutting bit which can be used to rough bore an iron block.

The final step with the paste; which I think is caustic rather than abrasive, is done with felt "stones", this laps away the aluminum leaving microscopic silicon particles exposed which the rings ride on.

The pistons are plated with either chrome or iron; don't know which is used for which year or application or if that is just a supplier choice.

 

Drew....just blow the engine and be done with it. A modest 5 lbs of boost is all you will need to make some pretty good power.

 

Just a quick heads up to say Thank You. I appreciate all the replies and I learned something new. That alone made it worth while.

Thanks Again.