BHP difference before and after engine rebuild question

[Tommy Z]

Happy Holidays All! I am curious about horsepower before and after an engine rebuild.

My stock 79' SC is still running well but burns and leaks a little oil. I know at some point in the future I will be looking to rebuild the engine. I know from the factory the US Spec SC 79' had about 186 HP. I am curious if anyone has done any dino testing on an engine like mine (146K miles on the clock) to see how much HP was lost over the years .

Also is it safe to assume that after a rebuild you might be able to expect the engine to produce close to that 186 HP it has when it left the factory? Let's assume the rebuild would include new or machined P & C's, fresh cam and the replacement of anything that is noticed to be worn. I realize this is all pretty ambiguous but I'm just wondering how big a difference the rebuild will make in performance.

Photo is just because it fun to post photos of your car!

[RSTarga]

A proper rebuild should give you original HP AT A MINIMUM, with minor mods more.

[Tommy Z]

Thanks RS, any guesses on how much HP can be lost over 45 years and 150,000 miles? Wondering how mine would test now?

[Matt Monson]

186hp minus 20% for the elevation where we live. But, yes, you can expect it to make at least rated horsepower.

[Tommy Z]

I’m still really curious to know if anyone can venture a guess on how much horsepower might be lost after 145k miles and 44 years.

[lateapex911]

It's a very trick question. Loss of power could be very little, or significant. It depends.
Depends on what? Wear.

If you have the capability, do two basic tests: compression and leakdown.
(I like to do hot and cold, just because, while I'm doing it, might as well)

Compression should be pretty even across the cyclinders. If uneven, then you know something is wrong.

On to the leakdown test. This is a test where you inject compressed air into the cylinder at TDC, and the leakdown test gauge will indicate the percentage of air leaking from the cylinder.

AND, you can determine HOW it's leaking by listening.
If you hear the air leaking into the intake (take the filter off, open the throttle), then the intake valves are not seating. If you hear it in the exhaust, well, you know what the issue is there, LOL And if you hear it in the crankcase (You can feel the pressure if you cover the breather tube with your palm) then it's the rings.

I don't know the "proper" numbers for the compression psi for your engine...real deal engine builders here can help there....but if it's like 100, uh oh. I'd expect 150 and really 175+ is more like it, but it depends on the CR of the engine, and other things. And if it varies cylinder to cylinder by more than 5-10, something is up.

A fresh engine will have leakdown well under 5%, 2 is a happy number! Again, even for all cylinders.

IF all the numbers are great, 175 psi across every cylinder and 3% leakdown across the board, well, would you still do a rebuild?

Maybe, because bearings are likely getting loose, or the seals are dried up, or the cams have worn a bit and valve lift is under spec.

Your question about more or rebuilt power?
I have worked on SCCA (racing org) rules committees and we expect that a well built "stock" motor meeting all the specs will always make more power than stock, (unless it's pre SAE testing) but the improvement is all over the place. I will say that we classed Porsches at higher lower weights, knowing that the expected hp bump wasn't going to materialize as it used to in other cars, because Porsches are typically built better.

So the rebuild won't "clean up" a shoddy build like used to be the case for say Chevy LOL

Still, when I did my motor, I spent extra time doing things like balancing the con rods, paying super close attention to tolerances, and internal case mods, matching weights of all the rods and pistons...little things that are time consuming...(And expensive if a shop is doing them),

It's not worth a lot, single digits probably, but in racing 7HP more than the guy in the same model car is huge. And a better balanced engine runs nicer.

After that you can do mods. If your cams are worn, and you are buying a new set anyway....maybe a new grind?
It all works together though, so the plan needs to take lots into consideration.
The 3.0 is (i think) a popular engine for a bit of hot rodding. And they seem to be very stout and reliable.

Sorry not to venture a guess on the orignial question, but come back with the compression and leak down numbers and I bet we'll start taking guesses

[Tommy Z]

Thanks Jake ! what a really well written answer. I appreciate you’re taking the time. Very helpful.

[Bill Douglas]

Don't be disappointed if it feels like less HP. The new build will be as tight as a button for sometime. More friction etc. So you can't really expect to see anything until it's fully run in and you can really wind it up.

A slightly worn engine isn't likely to lose power. If you really do want more power you could look at higher compression pistons, different cams and SSIs. Now is the time to do it

[garment]

My top end last year (taking my 3.0 to a 3.2SS) yielded an immediate increase in power. But I noticed even more power as I put on more miles. Maybe 2,000 miles in and the butt dyno reports an excellent outcome.

[brianlay]

Why not just take it to a dyno shop and measure before and after?
When I lived in Denver I used Finish Line IIRC

[Tommy Z]

Brian, I may do that. Really the point of the thread was to try and get a feel for if I will notice a substantial improvement following the rebuild. Chances are I will make some upgrades while were in there. It is an expensive project for sure. I enjoy performing all manor of repairs and maintenance on my SC but I know I do not have the technical experience to do a rebuild myself. It's just too big of a risk.

[brianlay]

Many will say it takes a gain of 10% to feel the difference

[Jeff Higgins]

Engines simply do not lose horsepower over the years. Not unless there is actual damage causing that loss. Normal wear and tear does not lead to a loss of power. If the rings are shot and it has no compression, if a lobe or several are wiped flat on the camshaft, then you will see a loss of power. If it "runs well" (as you state) and has no such clear mechanical issues, it has not lost power.

If properly cared for, 146k is about halfway (or a bit more) of the service life of the 3.0 liter motor. They commonly exceed 200k miles by a good deal, many of them reaching 300k before a rebuild is required. Yes, they have issues with broken head studs, and that would be an expensive fix, but it does not require a full rebuild. Yes, they do leak oil from a number of places, but those repairs are well documented here and elsewhere, and likewise do not require a full rebuild. As a matter of fact, most of those repairs can be affected with the engine still in the car (the notable exception being the "triangle of death", to which your CIS effectively blocks access).

So, no, you will not notice any improved performance after a rebuild. That is not how this works if the engine is in good running order prior to the rebuild. Performance upgrades are another matter entirely but, again, a stock rebuild will gain you nothing.

So, at this point, simply repair the oil leaks and keep having fun driving your car. You have a wonderfully robust example of the 911 (some say the 3.0 is the most robust overall) that has many, many happy miles left in it. Just enjoy it. And, for now, save a ton of money that wouldn't buy you a thing.

[targa72e]

Hi Tommy,
This doesn't directly answer your question but will give you a idea of what you can expect with a stock CIS and a better exhaust. The dyno runs below are of my 78 SC track car from several years ago. Engine had 100K plus miles at the time and HP is at the wheels on DynoJet chassis dyno.
The Red line is where I started. Mods included SSI heat exchangers with two in one out early muffler and stock CIS settings.

Blue line is same car with Dyno tuning. Ignition timing was set up on the dyno by advancing the distributor until no more power was found. Muffler was changed to two in two out sport muffler that showed to be almost as good as straight pipes on this car. Cam timing was set to the most advanced spec used by SC and Carreras which was shown to make the most power across the board. CIS was tweaked and blue printed so that each cylinder got the same fuel and mixture was set richer than stock, again tweaked on the dyno.

Green line is after full rebuild and change from 8.5 to 1 to later 9.3 to 1 pistons. Having compared several other similarly built SC's at the time they all looked just like the green line, so that is about the max with CIS and stock cams.



All the Dyno runs were done at Dyno pro in North Denver.

john

[lateapex911]

Originally Posted by Jeff Higgins

Engines simply do not lose horsepower over the years. Not unless there is actual damage causing that loss. Normal wear and tear does not lead to a loss of power. If the rings are shot and it has no compression, if a lobe or several are wiped flat on the camshaft, then you will see a loss of power. If it "runs well" (as you state) and has no such clear mechanical issues, it has not lost power.

If properly cared for, 146k is about halfway (or a bit more) of the service life of the 3.0 liter motor. They commonly exceed 200k miles by a good deal, many of them reaching 300k before a rebuild is required. Yes, they have issues with broken head studs, and that would be an expensive fix, but it does not require a full rebuild. Yes, they do leak oil from a number of places, but those repairs are well documented here and elsewhere, and likewise do not require a full rebuild. As a matter of fact, most of those repairs can be affected with the engine still in the car (the notable exception being the "triangle of death", to which your CIS effectively blocks access).

So, no, you will not notice any improved performance after a rebuild. That is not how this works if the engine is in good running order prior to the rebuild. Performance upgrades are another matter entirely but, again, a stock rebuild will gain you nothing.

So, at this point, simply repair the oil leaks and keep having fun driving your car. You have a wonderfully robust example of the 911 (some say the 3.0 is the most robust overall) that has many, many happy miles left in it. Just enjoy it. And, for now, save a ton of money that wouldn't buy you a thing.

So, his OP stated he was burning oil, if so that suggests that he's either got worn rings, or valve guides. If the valves are seating well, yes, he's fine. Worn rings will cause loss of compression and some power.

Is it noticeable?
Hard to say, but probably not. (One because it's probably slight and two, because it's been slow in happening.

Glad to read your comments about the 3.0 and it's longevity, I'm far from an expert but I was going to mention that the 3.0s are among Porsches stoutest engines.

[Matt Monson]

It’s a 3l. I’ve got $10 that says 2-3 broken head stud, in addition to the other mentioned issues.

Harvey at Boostcreep in Longmont is a good source for a dyno.

[RSTarga]

At higher altitude you can go with higher compression pistons to recoup some of the power. A better chip with a bit more advance better exhaust system etc.

[Jeff Higgins]

Originally Posted by lateapex911

So, his OP stated he was burning oil, if so that suggests that he's either got worn rings, or valve guides. If the valves are seating well, yes, he's fine. Worn rings will cause loss of compression and some power.

Yes, these things burn oil. They leak oil. What is considered "excessive"? Believe it or not, original owners' manuals told us that using up to a quart every 400 miles was perfectly acceptable. Now, I know that sounds outrageous to owners of newer cars, or even different cars (water cooled examples of other makes) of that era, but these things simply use a lot of oil. Enough to be alarming if one is new to this game, or unaware of this issue, or for whatever reason.

3.0's are not known to go through valve guides (unlike 3.2's) and, even if it has worn guides, they would not affect power output. Worn rings, yes, but several have already covered how to check for those. While not likely on a 3.0 that has been cared for, it's so cheap and easy to check that it's worth doing, just for peace of mind.

Originally Posted by lateapex911

Is it noticeable?
Hard to say, but probably not. (One because it's probably slight and two, because it's been slow in happening.

I've seen engines with bad rings that smoke a good deal that have suffered no apparent decrease in performance. The smoke gets to be a real issue long before performance suffers, at least from what I've seen.

Originally Posted by lateapex911

Glad to read your comments about the 3.0 and it's longevity, I'm far from an expert but I was going to mention that the 3.0s are among Porsches stoutest engines.

They do enjoy that reputation. As mentioned earlier, and reiterated by Matt above, their only real Achille's heel is those darn dilivar head studs. Even then, it took 20 years for that problem to begin to manifest itself. But, yeah, if they have not been replaced, I'm with Matt - I bet a few are broken. Even that is easy enough to check, just pull the lower valve overs.

The long and the short of it is that nothing I'm reading here says it's time for a rebuild. If the hope is to realize more power from an engine that is "running well", well... that isn't in the cards.

[lateapex911]

Totally agree with you on the last point, and it's not a race engine built to a class ruleset, so a couple hp isn't anything to worry about, much less spend the cubic dollars a 911 costs to rebuild, if it passes the basic tests well.

.

[Tommy Z]

Matt , thanks for your input. I’ve had the lower valve covers off a number of times, even within the last 800 miles or so. Fortunately no broken head studs. In regards to smoke, I only really notice smoke from the pipe on start up after it’s sat for a week. If I start it again the next day it’s usually pretty much smoke free.

Jeff,I’ve never heard that about potentially burning a quart every 400 miles being acceptable. That’s actually about the rate it burns now so maybe I’m not as bad off as I think!

Many thanks for the responses.