Race 2.4 engine valve size increase

neilca · 10-09-2010, 11:20 AM

I am in the process of building a 2.4 liter engine (85mm X 70mm) for my 914. This will be a full race engine capable of 8000 RPM. I am starting to think about the heads and have been reading a lot about 8mm valve stems. The stock Chrysler hemi's from the 60's used these small stems on some very large valves and it seemed like a good idea for the Porsche.

I have not read much about increasing the valve size from the 46/40 stock configuration. Is this done on these engines? How large can you go? Can you use the valves from the larger engines (49/41)?

Thanks,
neilca

Walt Fricke · 10-10-2010, 07:11 AM

Neil

I think you can use smaller stemmed valves with no special problems not normal to race motors.

But I think there is little, if any, room for larger valve heads in an 85mm bore (not the stock 84mm the 2.4s used?). I once had a 2.3 (85 x 66) motor, and there wasn't much spare room. When Porsche built the 2.8, even at 92mm, it had to change the valve guide angle in order to go to 49mm intake valves.

Though there are shops out there who can make custom heads for you, and no doubt could get you an increase in valve size (along with a decrease in wallet thickness).

I think you can usually pick up one millimeter more by deepening the seats, though. At some cost iin valve longevity.

Walt

neilca · 10-11-2010, 03:00 AM

Thanks Walt,

I really try to keep the wallet from thinning. I wanted to be sure I wasn't leaving some extra breathing room on the table. The 2.4 valves are already much larger than my 2.0.

MBruns · 10-11-2010, 04:53 AM

We have done 47.0 and 47.5 in. and left the ex at 40mm, the bore size is a concern but the cam selection and lobe seperation is also critical for valve to valve at overlap
Mike Bruns JBRacing.com

Steve@Rennsport · 10-11-2010, 08:53 AM

Adding to Mike's comments,.....

We have done oversize intake valves as well and used a custom intake valve with a smaller stem to help pick up airflow. Naturally, one needs the appropriate guides to do this.

For 8K power, I'd be focusing on headwork to make power at those RPM's. We do some CNC flow augmentation that works very nicely.

Its a combination of the whole package that gets the results,....

kenikh · 10-11-2010, 06:19 PM

You have to go a lot further with a motor to be valve limited...you will be port limited long before being valve limited in conventional motor configurations.

Make sure you reef down that flywheel to the 6 bolt 70.4mm crank beyond factory spec, too. They like to "go frisbee" at factory torque spec.

Walt Fricke · 10-11-2010, 09:30 PM

Seconding Kenick: you need to apply red loctite to each flywheel bolt. And torque to 150 lbs/ft. Install all bolts and tighten some. Remove one at a time, add just a little loctite (you don't want any on the sealing face, nor any loose, so to speak, in the dead pocket below the end of the bolt), and retighten. Then apply the big torque in the pattern of your choice.

If you don't do this, 8,000 rpm will loosen those bolts. Even tack welding them (!!!) won't stop them from backing out at stock torque. Porsche sort of solved this in the 2.8 RSR motors by replacing the bolts after every race, and were amazed to find that some US guys had come up with a cure.

Took me two bad experiences before Bruce Anderson explained how to stop this from happening. Nary an issue in the 15 or so years since running the 70.4 6 bolt crank.

Walt

kenikh · 10-11-2010, 09:42 PM

Closest I can figure is that the looser spec keeps just enough wiggle room in the tension of the fasteners that the violent direction changes, coupled with the vibrations of the 70.4mm crank stretches the bolts. When they go slack enough that there is enough room to shock load the fasteners, they shed the flywheel like a winter coat.

There are also some aerospace fasteners out there that are even better at the job than the factory units. I think Tadd found a set.

cameron.arnott · 10-11-2010, 09:57 PM

Here's an example of Herr Weiner's flow augmentation.....currently working a treat in my 2.3 twinplug engine

neilca · 10-12-2010, 03:02 AM

Hopefully I won't have a problem with the flywheel. I run a twin disc 7.25 clutch with a flywheel of my own design. It is very light compared to the stock set up. I will try to post a picture later.

I have planned on poting the heads, my 2.0 has 34mm ports intake and exhaust. While we are on that subject what do you suggest for size. Sorry, I do not have the money to pay someone to port my heads.

neilca · 10-12-2010, 03:45 AM

Here it is

Steve@Rennsport · 10-12-2010, 07:52 AM

Quote:

Originally Posted by neilca

I have planned on porting the heads, my 2.0 has 34mm ports intake and exhaust. While we are on that subject what do you suggest for size.

Hi Neil,

Its not about size,....its about shape and all in the correct places. If you want power, this is the right place to allocate resources.

kenikh · 10-12-2010, 08:14 AM

Big hole not mean big power. Ugh.

Walt Fricke · 10-12-2010, 10:17 AM

The flywheel bolts on the 6 bolt 70.4 don't break. They back out (because tension has been released due to something - 4th order harmonic?). This announces itself with an inability to get full clutch release.

No doubt if you kept soldiering on something would break. I never noticed the issue on the track, so my shifts were matching revs close enough. But in the paddock it was obvious - can't get into a gear from neutral with the engine running. So maybe in a long stint racing things would come apart.

At a minimum you get metal to metal transfer from the flywheel to the crank end, and vice versa. Can be really ugly (ruiins both parts), or if lucky you can clean up the end of the crank, and only junk the flywheel.

At least that was my two experiences.

I have no idea if that flywheel you have, and that pressure plate, would behave differently or not. My two flywheels were lightened RSR aftermarket clones, and I was running, as I recall, the 8 lb alumiinum housing pressure plate (now running the RSR at 4 lbs). But the books all say it was a pesky 4th order harmonic, whose node was right at the flywheel/crank mating surface, which was the culprit.

And these issues arose with the 2.8s, whose peak HP was at 8,000, so you know they were spun higher to make the upshifts more efficient. My 2.7 obviously was close enough (though not in power!) to have the same issues.

But I think all this surprised the factory, because I don't recall the usual authors saying the earlier race motors had this issue. For sure the 66mm cranks didn't. Perhaps this was a problem with the long stroke 2.5s?

Anyway, just torque to 150 with stock bolts and you'll be fine. I used to worry about this, and held my breath when I got up above 125 or so, and started breathing again when the wrench clicked. But I've done this dozens of times, and never had a bolt break. You want a really good (Assenmacher or the equal) 12 point socket tool because the hole in the bolt is so shallow.

None of this seems to be an issue if you shift at 7,000 or so.

Walt