Lookin for a review of my 3.4 specs?

[rdane]

One of the things I just got clarified this morning was my new engine's compression and how I got there. I had bought 9.8:1 Max Moritz pistons for a 3.0 to a 3.2 swap thinking I was getting the 3.2 to 3.4 set. In the excitement of building a 3.4 I somehow ignored the fact that with the longer throw of the 74.4, 3.2 Carrera, crank I was going to up the compression a good deal from the get go with the smaller crank's set. My compression ended up being 10.67:1 measured and will require twin plugs.

Happens when an amatuer gets involved. Good lesson, as it could have been worse.

V1 = 561.2cc cylinder volume at BDC (which gets you a true 3366CC instead of the reality of a 3.4)

V3 = 58.0cc cylinder head volume at TDC

Compression ratio (actual measure, not calculated)

V1 + V3 divided by V3 = 10.67

exh valve clearence is 3.375mm
int valve clearence is 2.75mm
cam timing 1.9-2.2mm

I am told the recommended deck height is 0.9mm - 1.2mm. Or a flat 1mm for factory spec.

Deck height on my engine as measured with a 1.0mm
shim was 0.99mm.

Wayne says a min of 1.5 for intake and 2.mm on exhaust and 1.30mm on deck height on a stock engine.

Should I mill the pistons down and start over? Or are these specs going to be OK with twin plugs?

Best guess anyone? TIA

[Shuie]

This is a really interesting engine configuration. The compression ratio really shocked the hell out of me. Anybody else built one of these that has any input? Why the huge CR jump?

Grady? Wayne? ChrisBennet?

[Wayne 962]

???

Moot point, right? Considering that the piston pin diameters of the 3.2 rods are different than those of the 3.0 - the pistons just won't work...

What engine do you have now?

-Wayne

[rdane]

Wayne I would have thought the same thing. But no, actually the piston pin dia is 23mm. Which is why I thought i had the correct 3.2/3.4 Mahle parts @ 9.8:1. Pauter rods are correct for the P&Cs and 74.4 crank.

[Grady Clay]

rdane,

I assume you have been test assembling the engine and you have good confidence in your numbers.
You are right, that CR is at the extreme for an air cooled street engine. If you keep that CR, you will certainly want twin plugs. The second plug recess will lower the CR ever-so-slightly. Those pistons are already designed to be as light as is structurally prudent. I don’t have one of those pistons in my hand but it is very difficult to change (reduce) the compression ratio unless you carefully machine a controlled surface. That means taking a controlled cut with a lathe or mill of the same amount on each piston. The critical issues are; not to reduce the strength of the piston, have all six be the same CR, and have then be all the same weight. If they all weigh the same now and afterwards there is a difference among pistons, then that is an indication something went wrong.
Do not try and reduce CR by adding to the piston-to-head clearance. Your 0.99 mm sounds just right.

What is your intended use?
What is your access to high octane fuel?
What emission requirements where you live, do you have to use the catalytic converter?
What other choices in pistons do you have available (cost)?
What are the other specs for the engine – cam, induction, exhaust, etc?
How do you plan to keep this cool?

You might consider just adding twin plugs and assemble it the way it is. Do the brake-in with high octane fuel and then run some tests with lower octane (pump) fuel. With twin plugs it is easy to use a bore scope and light to inspect the combustion chamber for signs of detonation and flame pattern. Proctologists don’t have anything on Porsche enthusiasts.
If you don’t have a problem with the possibility of pulling the top end off again, this might be a good solution.

This should be a great street engine.

The similar engines we did were 100 x 70.4 = 3317, and 100 x 74.4 = 3506, 10.3:1 on 3.0SC with MFI or carbs.

OK, WHO KNOWS ABOUT DETONATION SENSORS? Someone start a new thread.

Best,
Grady

[Shuie]

Thanks Grady! Im always interested in the different possible engine configurations and I've never been able to find much info on the 3.4s.

Im curious as to how a set of 9.8:1 Mahle P&Cs built to be used with a 3.2 Carrera crank and rods ended up at over 10.5:1. Is the cylinder head volume that much different between a 3.2 Carrera and a SC?

Anybody out there started out with a 3.2 Carrera motor and went to 3.4 using these 98mm Mahles? Was there a consistent compression jump over the rated 9.8:1?

[rdane]

Grady thanks very much for stepping up and giving me some useful feed back. I'll go through the list and aswer your comments.

*I assume you have been test assembling the engine and you have good confidence in your numbers.

Yes, double checked.

*The second plug recess will lower the CR ever-so-slightly.

I have already done the twin plug milling so the 10.67 is with twin plugs in the head. I had planned on 9.8:1 and was going to add electromotive later in the year.

*The critical issues are; not to reduce the strength of the piston, have all six be the same CR, and have then be all the same weight. If they all weigh the same now and afterwards there is a difference among pistons, then that is an indication something went wrong.

Gotcha...my first thought was to cut the piston tops.

*Do not try and reduce CR by adding to the piston-to-head clearance. Your 0.99 mm sounds just right.

thanks, one less worry.

*What is your intended use?

Street use and an occasional track/DE day. The original intent was to max out CIS and my case while on a budget.

*What is your access to high octane fuel?

I have 25 gallons of Trick 114 leaded race gas sitting in cans now and more easily available, I as planning on a mix of 3:1 of unleaded 92 and 114 for a 97 octane blend for the moment on a single plug.

*What emission requirements where you live, do you have to use the catalytic converter?

Zero thankfully, car is past the last emission test so I am good to go as is. It is a '79 so no cat or O2 sensor.

*What other choices in pistons do you have available (cost)?

These are new Mahle P&Cs so not a lot of options at the moment although I am going to talk with my supplier as these were suppose to be a 3.2/3.4 set @ 9.8. with 23mm wrist pins.

*What are the other specs for the engine – cam, induction, exhaust, etc?

Cam is a web cam 20/21, still running CIS intentionally and SSIs witha 2in/2 out Danske

*How do you plan to keep this cool?

B&B front cooler is already in. Good enough that I had a hard time warming up the old 3.0 to check the oil.

*You might consider just adding twin plugs and assemble it the way it is. Do the brake-in with high octane fuel and then run some tests with lower octane (pump) fuel.

Do you think this engine as is will run daily on 92 octane unleaded after break in? Or is 10.67 going to require a race gas blend even with twin plugs?

*This should be a great street engine.

Man I hope so, I would hate to be throwing money away here!

*The similar engines we did were 100 x 70.4 = 3317, and 100 x 74.4 = 3506, 10.3:1 on 3.0SC with MFI or carbs.

These sound like really fun engine. Any dyno info on these?

Thanks again Grady!

Dane

[Grady Clay]

rdane

At 10.7:1 I don’t think just 93 octane pump fuel will ever be safe enough to insure no detonation. Blending some leaded race fuel with premium unleaded is a relatively inexpensive and easy way to go and might be suitable. A little TEL goes a long way. For track use you absolutely must use 114 race fuel and really good cooling.

Be very careful storing your race fuel. It has a shelf life, is photo reactive, must be in a well sealed steel container, and kept cool particularly when transferring. Refrigerated at 38F is about right. For my son’s shifter karts I’ve been buying VP 112 octane in the sealed factory drum. Any that has been left out, I dump in our old 460cid van.

Does your proposed ignition system have the ability to switch curves. That would allow you to back off the ignition advance somewhat if/when you are concerned about the fuel or engine temperature.

What are the individual cylinder’s CR? I’m interested in the mean, standard deviation, and max-min.

Cooling is a big issue and also affects the octane requirements. Does your cooler system have a bypass thermostat like the factory system and a pressure relief safety valve? How much cooler do you have up front (how many, how big, and where)?
The engine fan is critical also. What fan do you plan to use and how fast to turn it? Here in Colorado we never use anything less than the ’76 -> 930 fan setup. This cools the most important parts – heads and cylinders. If they get too hot, detonation becomes even more of a problem.

Since you are using CIS you have a mild (no overlap) CIS cam. This exacerbates the detonation problem in the 1-4000 RPM range. If you use MFI or EFI and “S” and wilder cams that reduces the low RPM detonation problem but shifts it to 4500-7000 RPM. Most detonation issues are most pronounced from just below torque peak to horsepower peak. Remember, most detonation you can’t hear.

Is your second sparkplug 14 mm or 12 mm?

Tell me about your connecting rods.

As a philosophical note, keep as much as possible stock, un-machined, so you can have easy replacement parts. A “little bit” machined off everything makes the engine difficult to repair in the future.
Another admonition is to maintain your own photocopy workshop-parts-custom machining scrapbook/manual for your car. Include photos, invoices, diagrams, measurements, copies of relevant parts of Wayne’s and BA’s books, all your research, etc.

Do you have access to a dyno?

We dynoed most every engine we built and all of the hotrods. Around 500 engines, mostly 911s. All that info was lost in about ’86. I remember the 3.5s pushed almost 300 hp at 6500 or so. We did very few as twin plugs back then because of the cost. Luckily today there are lots of relatively reasonable choices.


I assume you have done the appropriate suspension mods.
What is the transmission? LSD?
Where are you going for DE events?

You can just e-mail those details but I would like to keep this Engine Forum discussion going so everyone can learn from your success.

Best,
Grady
gradyclay@hotmail.com

[rdane]

Grady I really apprreciate the input. The info has been short from my builder, supplier and here for what ever reasons. You are getting to the stuff I had to know. Thanks again up front.

*At 10.7:1 I don’t think just 93 octane pump fuel will ever be safe enough to insure no detonation.

OK that is bad...if I can't run 92 unleaded from the pump I need to change the engine. Race gas is fun but this is a street car.

Going back to the drawing board on that one.

* For track use you absolutely must use 114 race fuel and really good cooling.

Did not know that.....again not the engine I intended to have built.

*Be very careful storing your race fuel.

Another heads up...

*Does your proposed ignition system have the ability to switch curves. That would allow you to back off the ignition advance somewhat if/when you are concerned about the fuel or engine temperature.

Don't know..a lot of that...I was thinking the newest version of Electromotive. But at least now I can ask the right question.

>What are the individual cylinder’s CR? I’m interested in the mean, standard deviation, and max-min.

My wrench put the lower together and one cylinder to get measurements. Nothing has been done past that because of my concerns on the 10:67 compression. I am beginning to realize now why he wouldn't guarantee the engine.

>Cooling is a big issue and also affects the octane requirements. Does your cooler system have a bypass thermostat like the factory system and a pressure relief safety valve? How much cooler do you have up front (how many, how big, and where)?

Just the one B&B in the front valance. No fan. No pressure relief value that I know of. It is B&B 911 radiator style cooler, 19-row I think, direct from Pelican in the $700.00 range for the part IIRC.

*The engine fan is critical also. What fan do you plan to use and how fast to turn it?

Using the stock '79 SC fan at the moment and something I had asked about.

>Since you are using CIS you have a mild (no overlap) CIS cam. This exacerbates the detonation problem in the 1-4000 RPM range.

Sticking to CIS

>Is your second sparkplug 14 mm or 12 mm?

Normal size....14mm I assume.

>Tell me about your connecting rods.

I used Pauter billet rods for a 3.2 crank

*Do you have access to a dyno?

Yes the plan was to tune with the dyno once I was up and running.

>I assume you have done the appropriate suspension mods.

Yes the car is fairly well sorted out.
More info than you need here:
I had no clue.......long

>What is the transmission? LSD?

915, short box, 2 through 5 with a LSD @ 80%
and all the WEVO parts.

>Where are you going for DE events?

At the moment Seattle and Porland but I expect to do less of them instead of more.

At the moment the bottom of the engine is together. That means I would like to stick with the 3.2 crank and rods for the bigger 3.4.
The actual 3.2 to 3.4 pistons are not available which is what started this circus.

I could start all over with my old crank and brand new rods and these pistons and get 9.8:1 compression and a 3.2....not happy about that idea.

I could mill off all the piston tops, which seems the best idea, and cheapest, back to a a 9.8:1 compression or lower.

My build demands are 1st a street engine with CIS and 20/21 cams. I'd like to have the full range of torgue that is available from a 74.4 crank. Twin plug is OK as the mill work is already done. But the car MUST be able to run on pump 92 octane unleaded, safely. How far must I drop the compression?

If you have any other ideas on how to set the engine up with those parameters I would love to hear them.

[Grady Clay]

Rdane,

Don’t get feeling this is a circus. These are all normal issues when building a good “mix & match” 911 engine. Some of the fun is in the chase. For those who aren’t willing, Porsche makes great stock replacement parts that just bolt on. For your effort you will be rewarded with an outstanding engine and many years of fun.

I encourage you to find a suitable alternative to cutting your pistons. Baring a replacement, the critical issue is the structural integrity of the piston crown. Also you cannot remove metal in the deck/squish area. There is a limit as to how much you can safely lower the CR. I don’t know where that is. It may not be possible to get it down to 9.8:1 or so. Do some measuring and some math.

Don’t be afraid of completely disassembling the engine many times to get it right. The only significant use-once-only parts are rod bolts & nuts.

Since you already have twin plugs machined, use them. This will allow a few tenths CR or a few octane numbers. At 9.8:1 CR and 92 octane you will have a more than reasonable safety margin and a better running engine.

What are “heat sheets”?

Go back to the OE engine & trans mounts.

What are the actual 2-5 gear ratios?

Next time you corner balance, I like no cross bias. A little heavy on one side is preferable to any diagonal stress.

Get rid of the cam oil restrictor, get a larger oil pump. On the old race engines I have the 935 cam oil setup – both center oiling and spray bar. I now would use that on any air cooled street engine with an even larger oil pump. I have even considered putting small oil coolers in the cam oil feed lines but decided I wanted the larger flow.

Tell me more about your clutch assembly.

Let’s talk about lightweight.

I just read your original great post: I had no clue.......long
I read most of that at the time you posted, I just didn’t connect it with our current correspondence. I lurked on Pelican for way more than a year prior to my first post.

An outrageous suggestion:
Get a second engine and transmission. Build it so it is a direct interchange. You clearly have found a good mechanic who doesn’t leave things loose. He can do a swap in a few hours, double checking everything. Besides, you get an oil change.
Your imagination can define the differences between the engines and transmissions. One for driving across country (say to the Parade) with no worries about fuel octane, heavy original clutch, no muffler noise, no buzzing along at 5000 RPM, no LSD while parking, etc. On the other hand, you can have your wildest DE, auto-X, street hotrod. Another advantage is that when one engine is in for service/upgrade, the 911 still rocks.

Best,
Grady
gradyclay@hotmail.com

[dd74]

Dane: Grady makes a good suggestion, which in my years of modified cars I too have considered. A second engine is in a way, insurance. Your 3.4 sounds as if it needs a bit more massaging. I don't like piston cutting because it's fiddling with a design and structure. Weakness has to be the outcome - if any - of cutting pistons to lower C/R.

Will you twin plug it? Will twin plugging suffice the 10.7 C/R? I completely concur with you about your demands for a "streetable" car on pump gas: I'd hate to have a daily driver that would require any sort of gasoline blend or race fuel. It's one reason why I've considered 9.8:1 pistons only in passing. In CA. of course, we have 91 octance, and I don't even know how long we will even have that (91 octane) before it's taken away.

Anyway, I've seen 3.0 and 3.2 engines around fairly cheap. A backup might not be a bad idea.

[Shuie]

One thing that really bites about this that it looks like its going to difficult to come up with a different engine configuration with the current components. Everything Dane has seems like it was made to work together. The 3.4 is cetainly not one of the most common configurations Ive heard of, but Dane's motor is not the first 3.4 Ive read about here either. How do the rods, crank, and Mahles rated at 9.8:1 end up at over 10.5:1? What happened?

Im not saying he is considering this but, its not like Dane can even sell off the 74.4mm crank and do a 98mm 3.2 at this point is it? IIRC, those P&Cs will not work with the SC crank and rods. Will the Pauters work on the 70mm crank? If they do work, is the CR closer to the rated 9.8:1 or does it go to ~10.5:1 also?

[dd74]

I'd sell off the pistons before I sell off the crank. JE can build pistons to suit compression. But if I were to go this route, I'd order pistons with, say, a 9.3:1 compression in lieu of the compression actually being higher once the engine is assembled as is Dane's case now.

[Shuie]

Good point dd. Those cranks are tough to find. The bottom end of the motor is probably buttoned up anyway by now. Just trying to think of the options.

[Grady Clay]

rdane,

Since everyone is offering suggestions, let me reinforce mine. Find a stock ‘79 SC engine and trans. Rebuild as original but with later tensioners and twin plug. This way the fuel lines, oil pipes, wiring, ignition, clutch linkage, etc. all fit your car. EZ swap. AND, it will run on pump fuel.

Most Pelicans would give their first born for an aluminum case 3366 cc, 10.7:1 twin plug. Use that as your “RennSport” engine.

Keep both engines in identical configuration so there isn’t any part swapping between engines or rearrangements in the car when switching engine-transmission assemblies.

Besides, this allows the engine assembly process to continue. Boy, Dane, it sure is easy for us to spend your money.

Best,
Grady

[rdane]

btt