Rebuild Questions

[RonD922]

I ran a dry compression test on my 75 911 5R case that has built to RS specs--approx. 260HP

1-75
2-75
3-100
4-115
5-125
6-125

So I am thinking I need to take it apart and see what the problem is. Anyway what I was wondering was

1. What should I check first??

2. How detialed should I go when rebuilding( Replace Everything or just machine heads and new piston rings)??

3. Since the case is the 5R and is RS specs and a more are the any mods I should do while it is out??

4. If I had the money how could I get more HP??

[J P Stein]

Before you get too het up....
Has it been sitting for long?
Did you open the throttle all the way?

For more power:
The usual, more compression, more cam.

If you are going to crack the bottom end:
Knife edge crank
"Eyebrow" the cylinders
Radius the web supports

Do the tin cut for added cool, while you're there.

Every litter bit helps.

[RonD922]

JP,

It has been sitting for 2 years at least not being run, hell I don't even know if it has been turned over. I got the engine and replaced plugs, points, coil, Mechanical injection pump belt and static timed it. I filled it up with oil ( which it does not leak any and then turned over by hand, and gradually worked up to trying to start it. It ran for 10-15 seconds on only 1-3 cylinders and then I shut it down. I tried to see how much gas was going through the fuel lines to each cylinder but didn't get conclusive results(although it seemed to me like all cylinders were getting gas--but uneven amounts??). Also I checked the plugs to make sure all cylinders were getting spark. So my dad wanted a compression check before we went to much further and I got the bad news=$$$, which is in short supply in a college student's budget!!!

Thanks For the Help
Ron Dailey

PS what do you mean by Eyebrowing Cylinders and Knife Edging the Crank?? I have no knowledge with Engine rebuilds so please forgive me

[J P Stein]

Best advice I can give is got get a copy of Bruce Andersons book

"Porsche 911 Performance Handbook"

Read that sucker cover to cover.....twice. A good read if you're a Porsche gearhead.

The cyls/rings may be dry as a bone and skewing your results.

Pull the plugs and spin it over on the starter for at least a minute.
Some folks say...till pressure shows on the gauge, but I've never has any luck with that. Put the plugs in and crank it over some more 15/20 sec.....then light that sucker off. If pressure don't come up on the gauge within 10/15 seconds....shut it off, then worry.

If it lights and the pressure is there, check for oil leaks. Spin it up to 2000 rpm or so and run it for a while.....15 /20 mins. It's gonna smoke.....if it makes rude noises.......shut it down and worry some more.

If you get past that, do your compression test.
Remember, WOT for that test.

[Early_S_Man]

Ron,

After sitting for that long, large chunks of carbon may have come loose and be stuck under valve head of cyls #1, #2 and #3. There's no reason to get worried, yet!

Get a couple of 16 oz. cans of Berryman's B-12 Chemtool and pour 4 oz. down stacks for cylinders 1-3 while spinning the engine over, after pulling the 3-pin plug from CDI-unit. Let engine sit overnight, and repeat treatment again ... then do another compression check. Be sure throttle is wide open and all 6 spark plugs are out, and freshly-charged battery before compression test.

[ChrisBennet]

Originally posted by J P Stein
Before you get too het up....
Has it been sitting for long?
Did you open the throttle all the way?

For more power:
The usual, more compression, more cam.

If you are going to crack the bottom end:
Knife edge crank
"Eyebrow" the cylinders
Radius the web supports

Do the tin cut for added cool, while you're there.

Every litter bit helps.

Just thinking out loud...
Would knife edging the crank make any difference on a dry sump engine?
I radiused the web supports on my 3.2 but I wonder if it is worth the reduction in strength in a 5R case?
A factory RS motor puts out 210hp (DIN). If RonD922's "RS spec" motor puts out 260hp (SAE) it probably already has any mod we can think of.
-Chris

[J P Stein]

Good question, rather than just mouth what I read, it got me to thinking about it. The conclusion I came to is...probably:-)

I can't concieve of what the interior of the engine looks like at high RPM, but "stuff" has got to be flying around in there to beat the band. All that oil "probably" doesn't neatly slip by the crank and rods to be sucked out and put in its proper palce. It would seem that the atmosphere in there might be a tad thick, in which case, knife edging should be of help. I over filled my engine last change. The motor responded by quickly pumping it out into the catch can via the crankcase vent....oops.

As for boat tailing the webs, you may be right. I don't think that it was invented with the coming of the 7R case, but I've been wrong before. Larger radii on the corners shouldn't hurt, but may not be worth the time and effort.

I've been over on the 914 list debating the virtues af the type 4
vs the 911 six. Exagerated hp claims roll of me like water off a duck.

[Roland Kunz]

Hello

A 5R case is 2,4L and not the best start for a 2,7 RS.

Knifedgeing the crank will save power due the windage loses inside the engine and this power loss is relatet to the rev level.

On race engines some 8-10 HP can be gained by optimizing internal vent flow. Race engines also get a teflon coating on moving parts to make them "slipstreaming". Race pistons are very short and have sometimes only one ring made by busak & shamban. This is high $.

Grüsse

[Wayne 962]

Agreed, knife-edging is a bit extreme, and reductions in the case supports are not recommended for the mag cases. The Knife-edging makes the crank more aerodynamic so that it reduces air friction when spinning around inside of the case. Makes sense to do on a race motor when you're trying to squeak even one more HP out...

-Wayne