I Need to Make My Engine a Little More Streetable. 365hp 3.6

[GG Allin]

I have a fantastic engine right now but it's just too much for the street and I feel that the top end could use a refreshing. I'm also running straight 100 octane at $9 per gallon, that's getting old.

Compression and leak down were done last Spring, or around 3000 miles ago:

Leak (1) 40% (6) 15% (2) 15% (4) 10% (3) 22% (5) 15%

Comp. PSI (1) 210 (6) 230 (2) 215 (4) 220 (3) 215 (5) 230

To the best of my knowledge my motor is as follows:

3.2 Case
100mm Bore, 11.5 to 1 Compression
76.4 Stroke (964 Crank)
3.2 Heads
Web Cams 120/104
Twin Plug (964 Dist)
50mm PMO's


The plan is to swap out the cams and lower the compression ratio. I already have new cams on hand. I don't know the spec on them. They were sourced through William Knight who knows this engine better than me. The drivability with the carbs is what drove the decision to change them.

My options regarding pistons are: have my current pistons shaved down or source new.

Looking at JE pistons off the shelf, there are these two options. The first one is likely what I have. Question is, would the second one be a straight forward swap? And would 10.5 be low enough for 93 octane?

Engine Size : 3.6
Bore Size: 100.000
Stroke: 76.400
Rod length: 127.000
Compression Distance : 31.800
Compression Ratio: 11.5:1
Dome volume: 40.8
Oversize: STD
Wrist Pins: 905-2500-15-52C
Ringset: JG1006-3937

https://lnengineering.com/products/porsche-je-pistons/je-pistons-353306.html


Engine Size : 3.2
Bore Size: 100.000
Stroke: 74.400
Rod length: 127.000
Compression Distance : 32.8
Compression Ratio: 10.5:1
Dome volume: 36.5
Oversize: 5.00
Wrist Pins: 905-2500-15-52C
Ringset: JG1006-3937

https://lnengineering.com/products/porsche-je-pistons/je-pistons-353290.html

[stownsen914]

Keep in mind that compression is dependent on stroke. If you buy pistons intended for a 3.2, the compression will be higher than listed with the longer 76.4 mm stroke.

Note also that the compression distances are different for the two pistons that you pasted above. You'll need to pick one that fits your application. This will be important to get the piston to head clearance where you want it (1 mm is a lot for that).

[mikedsilva]

What's compression ratio of a stock 964; I think it's a claimed 11.3:1... they seem to run OK on pump unleaded fuel? Do you really need to lower compression with a new set of pistons?

Sounds like you want to sacrifice peak hp for a gain in low end torque. Obviously William will know what to do, but I would think a smaller cam would be the way to go.
If you already have the cams, I'd imagine they have been specced to work with an approximate certain compression ratio?

With regard to leak down, where is the majority of the leak; valves?

[stownsen914]

Sounds like he's ditched the Motronic and has PMOs and stand-alone ignition. Unless that ignition is is knock-sensing (and adjusting timing to compensate) capable, then yeah I'd say 11.5 might be a bit much for pump gas.

[GG Allin]

Originally Posted by mikedsilva

What's compression ratio of a stock 964; I think it's a claimed 11.3:1... they seem to run OK on pump unleaded fuel? Do you really need to lower compression with a new set of pistons?

Sounds like you want to sacrifice peak hp for a gain in low end torque. Obviously William will know what to do, but I would think a smaller cam would be the way to go.
If you already have the cams, I'd imagine they have been specced to work with an approximate certain compression ratio?

With regard to leak down, where is the majority of the leak; valves?

No certain yet. I'm thinking rings on cylinder one to start with. I get smoke on start up from the right tail pipe which would be coming from the left bank.

Originally Posted by stownsen914

Sounds like he's ditched the Motronic and has PMOs and stand-alone ignition. Unless that ignition is is knock-sensing (and adjusting timing to compensate) capable, then yeah I'd say 11.5 might be a bit much for pump gas.

Yeah, there is no engine management currently. Diluted 100 Octane wasn't enough. I was running 100/93 75%/25% briefly last summer and heard a little knocking under load. Since then I've run straight 100 and it's been fine.

[GG Allin]

Thinking maybe just doing it right and going with an older Motec & ITB's.

[Jeff Alton]

Your carbs can make great power, and driveability. You have a great dyno if it says you make more than 100hp/liter off that combo....

What is it you want to accomplish exactly?? Power, torque curve, drivability etc.....

[GG Allin]

Originally Posted by Jeff Alton

Your carbs can make great power, and driveability. You have a great dyno if it says you make more than 100hp/liter off that combo....

What is it you want to accomplish exactly?? Power, torque curve, drivability etc.....

I want to get away from 100 octane gas without losing horsepower. Or at least not losing a ton of horsepower.

[GG Allin]

...I also need to do something about the compression and leak down numbers, full top end I'm thinking. That's where the idea of replacing or modifying the pistons came from. Ya' know, while you're in there.

I guess the question now is, if I go Motec, is 11.5 to 1 still to high for 93 octane?

[LeftCoastErik]

If you go to any decent EFI system, you can run 11:1 on pump gas all day. Just be sure to get it properly tuned. Depending on how deep you want to go, you can even use knock sensors. Nothing wrong with MoTec, but other options like Megasquirt and ECU Masters, AEM, etc offer all the functionality you will need for considerably less money. Talk to Al at X-Faktory and can kit you up. May as well go to Coil On Plug also

[Neil Harvey]

Originally Posted by GG Allin

I have a fantastic engine right now but it's just too much for the street and I feel that the top end could use a refreshing. I'm also running straight 100 octane at $9 per gallon, that's getting old.

Compression and leak down were done last Spring, or around 3000 miles ago:

Leak (1) 40% (6) 15% (2) 15% (4) 10% (3) 22% (5) 15%

Comp. PSI (1) 210 (6) 230 (2) 215 (4) 220 (3) 215 (5) 230

To the best of my knowledge my motor is as follows:

3.2 Case
100mm Bore, 11.5 to 1 Compression
76.4 Stroke (964 Crank)
3.2 Heads
Web Cams 120/104
Twin Plug (964 Dist)
50mm PMO's


The plan is to swap out the cams and lower the compression ratio. I already have new cams on hand. I don't know the spec on them. They were sourced through William Knight who knows this engine better than me. The drivability with the carbs is what drove the decision to change them.

My options regarding pistons are: have my current pistons shaved down or source new.

Looking at JE pistons off the shelf, there are these two options. The first one is likely what I have. Question is, would the second one be a straight forward swap? And would 10.5 be low enough for 93 octane?

Engine Size : 3.6
Bore Size: 100.000
Stroke: 76.400
Rod length: 127.000
Compression Distance : 31.800
Compression Ratio: 11.5:1
Dome volume: 40.8
Oversize: STD
Wrist Pins: 905-2500-15-52C
Ringset: JG1006-3937

https://lnengineering.com/products/porsche-je-pistons/je-pistons-353306.html


Engine Size : 3.2
Bore Size: 100.000
Stroke: 74.400
Rod length: 127.000
Compression Distance : 32.8
Compression Ratio: 10.5:1
Dome volume: 36.5
Oversize: 5.00
Wrist Pins: 905-2500-15-52C
Ringset: JG1006-3937

https://lnengineering.com/products/porsche-je-pistons/je-pistons-353290.html

I don't normally reply to post like this. Kinda out of our ball park. But what struck me was your request for information, when you must have discussed the cam choice with who I understand is a pro engine builder.

Why not get his advice instead of listening to all here that will always give a host of opinions and choices.

[GG Allin]

Originally Posted by Neil Harvey

I don't normally reply to post like this. Kinda out of our ball park. But what struck me was your request for information, when you must have discussed the cam choice with who I understand is a pro engine builder.

Why not get his advice instead of listening to all here that will always give a host of opinions and choices.

I like William's advice, I just don't want to harass him every time I have a question. He's incredibly generous with his time and don't want to take advantage of that.

It's also my understanding that the cam choice does not limit me to carb's.

Also, when I get to taking this engine apart, I'll need a thread like this along with the search function to get answers to the more nuanced issues.

[MoreGAS]

Realize you have a couple issues; one your leakdown results are pretty poor, where was the leakage going? 40 and 22% is very high, the rest not great. That likely needs to be addressed. You could dyno it also to see if the power has fallen off? Is your breather tank full of oil from blowby?

2nd for a 3.5 L you have a fairly moderate cam, so the power band should be very nice for a streeter hotrod.

Next, your compression ratio if True is a problem, and realize that when you CC a 3.6 L Porsche says is 11.3:1 w knock sensing on both banks, it is actually 10.4;1 max in my experience. So if you CC'd your engine when built and it is really mid 11:1 you have an issue.
Hard to tune safely for that compression on shi__ty fuel. It is better to be much lower in compression, and keep and achieve a more aggressive tune with more safety margin when temps get up there, which is the primary limiting factor on these air cooled engines.

Kevin Roush
GAAS Motorsport

[Jeff Alton]

What Kevin says,
also the more aggressive the cam overlap wise, the lower your dynamic compression will be vs static. That may let you get a way with a wee bit more static compression. That cam is often used in 3.0/3.2/3.4 motor so it is a bit mild for your 3.6 depending on what else is the engine configuration.

You leak down number are interesting because your compression numbers are all pretty close. Number 1 leakdown is high, yet it makes almost as much compression as number 4. That said, you need to get the bottom of the leak down. Find out where it is leaking and make sure that gets addressed. However, if you are going to rebuild properly, that will all get taken care of at that time.

We leak down every motor build before it even gets started to get an idea that all surfaces/materials are sealing properly. When engine is on the stand, it takes about 15 min to perform. We do this prior to cam installation as a quick double check. You can check each cylinder at TDC and BDC this way. May be a waste of time to some, but it is a quick check.... for a baseline.


Cheers

[Peter M]

Originally Posted by LeftCoastErik

If you go to any decent EFI system, you can run 11:1 on pump gas all day.....

That's a pretty big call Erik!

Can you explain why you think EFI allows for more knock resistance than carbs please?

[GG Allin]

William Knight called out of the blue, he does that. As usual he's been very helpful and generous with his time. I also spoke with another Pelican member also very knowledgeable and helpful. There is a plan to move forward. First stage will be a reduction in compression and swapping of cams. In doing that I shouldn't lose a whole ton of power and it will run on 93 octane. I'm also going to do another leak down and compression test when the motor is out. Second stage will be to go Motec if I still don't like the carbs. Not that I don't like them, there are just some trade offs to live with them. A well tuned EFI system seems like something I'd like.

I'm slowly selling off my BMX collection (if anyone's looking) and trading TQQQ to fund the Motec system. Hobbies gotta' fund hobbies.

https://bmxmuseum.com/user/242280/forsale

[faapgar]

Usually when you have a leakdown number like this it is piston rings.I recommend after 8 % leakage it is time to inspect.When you have blowby past the rings you are preheating the engine oil with the byproducts of combustion.Not cool.Also when new the end gaps of the ring are0.006 in inches.Where you are now is after 10% the end gap of the ring is close to 0.040 which is toast.Fred

[GG Allin]

Originally Posted by faapgar

Usually when you have a leakdown number like this it is piston rings.I recommend after 8 % leakage it is time to inspect.When you have blowby past the rings you are preheating the engine oil with the byproducts of combustion.Not cool.Also when new the end gaps of the ring are0.006 in inches.Where you are now is after 10% the end gap of the ring is close to 0.040 which is toast.Fred

The motor is coming out soon, as soon as I get off my ass and do it. The pistons will be cut down to a target CR of ~10.25 to 1. New rings while I'm in there. Hopefully no issues found when it's apart. Will post as the engine comes apart.

[stanglife]

Somewhat related question. Everyone says over 11:1 seems high.. Ever since I can remember, OEs include a safety margin in most things for overall reliability. Bumping the compression slightly here, while ensuring good fuel/spark/timing feels pretty normal. OEs also account for a range of fuel quality - so if you can buy 93, it also seems like there may be some power left on the table with OE compression. Many suppliers offer engine kits in the 11.3:1 compression range - I had the impression that is for a street car - and their 12:5+ options are for track/race gas.

So my main question is - is it common for people to experience issues at the mid 11 range on 93? It seems like the entire hobby wants me to build a 3.8 with 11.3:1 - based on whats available

[stownsen914]

Depends heavily on what engine management you are using. Modern OE engine management systems as highly optimized for the engine package, and include knock protection which is critical at higher CRs. Once you drift from stock, you're into custom tune and all the variability in quality and results that come with it. Good engine builder and tuners know how to do it.