Can I raise compression on my US 3.2?

[jsoderbe]

Hi,

I am working on my US 3.2 Carrera engine. I discovered it was a US low comp engine, and I really dont want to replace the pistons.

My question is what I can do to bump the compression and keep the pistons? Can I fly cut the heads? Mill of the cylinder mating surface?



Thanks,

Johan

[Raceboy]

Flycutting heads and/or milling cylinder mating surfaces will create problems with chain housing fitting.

[jsoderbe]

Could I shave that one too?

How much is needed bump the compression 0,5-0,8?


Johan


Johan

[Steve@Rennsport]

Originally Posted by jsoderbe

Could I shave that one too?

How much is needed bump the compression 0,5-0,8?


Johan


Johan

Johan,

You really cannot cut the chain boxes either since the chain tensioners run out of travel and you'll be unable to keep them tight.

The way to raise CR is by using different pistons since you cannot machine the heads sufficiently to make any real gains without chain problems.

[911pcars]

Originally Posted by Raceboy

Flycutting heads and/or milling cylinder mating surfaces will create problems with chain housing fitting.

I'm curious as well about the potential compression bump.

In regard to chain housing (box) machining. Yes. Any machining to the cylinders and cylinder heads, will change the distance between the cams and the intermediate shaft and how the cams are centered in each chain box opening. Recall the oil seal between the cam and cam box depends on the large o-ring being centered within the opening. The solution is to remove material from the cam box mounting surface to restore this relative position.

On some rebuilds, all stack components may have been machined for one reason or another, including the cylinder mounting surfaces on the crankcase - the alert builder will discover the timing chains may be too long because the cams are now closer to the engine centerline. This is evidenced by the chain tensioner extending further, perhaps beyond a reasonable working range. The timing chain, esp. on the driver side, may be uncomfortably close to or even contact the chain box wall

I experienced this situation on my old 2.7 build and took B. Anderson's suggestion to source a couple of oversize idler arm sprockets to take up excess chain slack and return the above components to their normal, at-rest position. One might be motivated to simply remove a link from the chain, but that wasn't recommended by the engine builders I spoke with.

FWIW,
Sherwood

[Raceboy]

Just an idea but perhaps watercutting a spacer from aluminium sheet between heads and cam tower could help compensating the lost material?

I know that the best way is to change the pistons but hey, the spacer even is not a combustion sealing surface and I've seen even more weird things done on engines

Am I missing something obvious?

EDIT: Nevermind, the rocker distance from the valve would be affected of course. Guess this is a show-stopper?

[safe]

Another way to raise compression, probably not cheaper than new pistons.



Old school with a twist

[911pcars]

Originally Posted by safe

Another way to raise compression, probably not cheaper than new pistons.



Old school with a twist

Are twin plugs necessary for this combustion chamber shape? Looks fairly compact. Will this work with factory pistons?

S

[AlfonsoR]

Not a stopper. If you really want to experiment and like to revel in the frustration of leaks, engines that "just don't seem to run right", then go right ahead and proceed.

Otherwise, if you like to keep it boring with an engine that always runs, then just follow the recommended repair practices.

Seriously speaking, if you are looking for low budget performance improvements, an incremental bump in CR is not going to get you much. Instead, reduce your car's weight. You can piecemeal it, and will also improve handling and braking.

[safe]

Originally Posted by 911pcars

Are twin plugs necessary for this combustion chamber shape? Looks fairly compact. Will this work with factory pistons?

S

I think Henry found then in a core engine as a single plug.

[KTL]

Also if you fly cut the heads you're inviting valve clearance issues. Seems you would be chasing your tail a bit because you cut the heads to lessen combustion chamber volume but then you cut some valve reliefs in your pistons to gain clearance. Those valve reliefs diminish the gain you got by reducing the combustion chamber volume.

[911pcars]

Originally Posted by AlfonsoR

Not a stopper. If you really want to experiment and like to revel in the frustration of leaks, engines that "just don't seem to run right", then go right ahead and proceed.

Otherwise, if you like to keep it boring with an engine that always runs, then just follow the recommended repair practices.

Seriously speaking, if you are looking for low budget performance improvements, an incremental bump in CR is not going to get you much. Instead, reduce your car's weight. You can piecemeal it, and will also improve handling and braking.

Not so. Incremental bumps in CR increases both volumetric and thermal efficiency. The results include increased fuel mileage and more power, not a lot but incrementally on the plus side. What else can you do while you're in there that accomplishes both? Nada. If the head is to be machined for a new sealing surface, it takes little effort to increase the cut as long as that amount doesn't compromise engine longevity or create significant compatibility or tuning issues. This really isn't in the same category as, for example, incrementally increasing boost pressure from 19 to 20 psi.

And there are those who prefer to play it close to the vest. That's okay too. I have an old Corolla sedan in the driveway, so I understand.

At the end of the day, if increasing CR isn't feasible due to mechanical interference or other tuning issues, that's fine too. Just want to be aware of my options.

Sherwood

[911pcars]

Originally Posted by KTL

Also if you fly cut the heads you're inviting valve clearance issues. Seems you would be chasing your tail a bit because you cut the heads to lessen combustion chamber volume but then you cut some valve reliefs in your pistons to gain clearance. Those valve reliefs diminish the gain you got by reducing the combustion chamber volume.

Was wondering how close that invitation might be. How far can a cylinder head sealing surface be cut w/o getting too close to piston-to-valve interference and/or cutting the piston crowns for valve clearance, +.010"?, +.020"? +.030"? I understand factory pistons don't have all that much material to remove anyway, so that's moot.

I'm aware of the general valve clearance issue. Is the current status on these engines typically borderline, and if so, by how much? Surely this isn't the first time someone asked the question or considered it. In the overall scheme of putting together an engine, this is hardly pushing the proverbial envelope.

Sherwood

[AlfonsoR]

Originally Posted by 911pcars

Not so. Incremental bumps in CR increases both volumetric and thermal efficiency. The results include increased fuel mileage....

Sure, they do. On these engines, when done through a piston change, increasing CR should be part of several changes that could be done to increase the engines efficiency.

But when done through milling heads or cylinders and chain boxes, those small gains in CR are not worth it. I would argue that the difference in HP will be minimal and where the rubber meets the road, the car will not be significantly different.

Why not Install a 964 cam or similar and a chip instead?

[911pcars]

Originally Posted by AlfonsoR

Sure, they do. On these engines, when done through a piston change, increasing CR should be part of several changes that could be done to increase the engines efficiency.

But when done through milling heads or cylinders and chain boxes, those small gains in CR are not worth it. I would argue that the difference in HP will be minimal and where the rubber meets the road, the car will not be significantly different.

Why not Install a 964 cam or similar and a chip instead?

I agree. However, all modifications are incremental. Not all machining operations provide noticeable power. Some mods produce almost nil performance. However, most add up to a greater whole. e.g. Subtract 1 lb. from the curb weight of your car. Now repeat that incremental weight reduction 50X.......

Off the top of my head, here are some popular yet incremental "rubber meets the road" engine mods, all with a cost/hp ratio benefit:
- Moon bottom of cylinders, est. $100. Power increase =
- Boat tail crankcase webs, est. $300. Power increase =
- Polish crankcase webs, est. $400. Power increase =
- Polish ports, $500. Power increase =
- Extrude-Hone intake manifolds, $600. Power increase =
- Twin-plugs on a low-compression engine, $350. Power increase =
- Twin-plug ignition system, est. $600-2000. Power increase =
- Various strategies to increase pre-filter intake volume, including Oil-gauze filter elements, abbreviated/shaped air filter housings, various "cold air" intake accessories. $0-$300. Power increase =

For some reason, many folks have a "You get what you pay for" mentality to explain and/or justify a positive or negative purchasing decision. However, that mantra doesn't really apply in all cases. It all depends. Thus, not sure why a question about slicing off a few thousandths more on a pre-paid cut is discounted. Some might think it worthwhile if instead that operation costs $500 (see above price list).

BTW, I've got the 964 cams and chip covered. Comparing the specs of the 964 cams vs SC/3.2 cams, it's quite minor (est. $400), but again incremental when performed with other complimentary and incremental modifications.

Now add up all the incremental power mods described above. Add up above typical costs as well. Use those figures to justify why removing some metal from the heads is out of the question/not worth it/risky (given a yet-to-be-announced negative consequence).

Sherwood

[Raceboy]

I like Sherwoods thinking and logic, no need to just "believe" something, but actually make it clear through rationale and common sense.

If removing the material, some measuring of valve clearance is in order and of course the amount that can be shaved safely depends on many things (rpm limit = rod stretch, cam specs etc).

If sealing surface needs to be machined anyway, I would also give it a bit more to raise the CR, just the homework needs to be done.

[KTL]

The factory CIS or Motronic 95mm pistons do have a considerable amount of material around the smushed dome as I call it. I have a set of used Carrera pistons I got with some unexpected smiley faces on them due to valve contact. Couldn't see them until all the carbon buildup was removed. The smileys are on the exhaust side and are about the length across of an average adult's (mine) pinky fingernail and maybe 1mm wide/tall. In looking at the piston crown here, it would be rather simple to have a machine shop mill a slight crescent relief in that location and not compromise it's strength. There's a considerable amount of material in that area. We're not talking about a ton of material to cut. Here's an example of cutting 0.020"

Valve Pocket Machining.wmv - YouTube

With respect to cutting the heads, there's a good bit of meat in them and they can be cut down quite a bit. I've got a set of heads on my '79 that have been milled numerous times, simply to true up the mating surfaces. They're now below the minimum thickness & next time they need to be reconditioned, either shelve them or have them welded up by someone like Ollie's. That minimum thickness is based on stock components. Since I have JE pistons and higher lift cams, the pistons have considerable reliefs in them to allow for valve clearance and that's how the heads can still be used below minimum thickness

The mention of chain length is indeed quite important. My idler arms are extended quite high into the chain housing. I am "fortunate" in that I use the dreaded solid mechanical tensioners on the racecar so there is no concern about over-extending the operational range of a standard 930 self-contained tensioner or the pressure-fed Carrera tensioner. Were I to use the normal tensioner. I would certainly be looking to install a larger idler sprocket to take up the slack in the chain.

Back to the heads, when cut down quite a bit, you're nearing the valve seat of the large intake valve. My heads were also chamfered to give some additional room for the 98mm pistons (heads are '78 SC 3.0L heads, originally intended for 95mm pistons). When my engine took a crapper, the #5 rod got slightly loose, increasing stroke & the piston kissed the head ever so slightly. So my 1mm deck height (measured) helped to prevent excessive head smushing when the rod got sloppy. Still, I think my valve seat & bore is disturbed by that kiss job so that head is now junk.

It's very easy to increase or decrease your compression ratio with combustion chamber modification. For example, my JE pistons are spec'd at 10.5:1 ratio. With heads cut down to the minimum thickness, I would have expected to exceed 10.5 by a bit. However, the addition of spark plug bore #2 and the chamfer of the periphery to accommodate the 98mm pistons, I believe the meas'd/calc'd ratio was 9.9-ish if I recall correctly. Kinda disappointed but it was what it was since I chose to go with an off-the-shelf set of JE 10.5 pistons.

I guess my point is that chasing the compression ratio via the head depth is a bit of a challenge. I think it's better to fit a different set of pistons that makes a safer adjustment to the combustion chamber squish area.

I think many of the other modifications Sherwood listed are arguably miniscule hp improvements. The cylinder mooning & case web stuff is typically reserved for high rpm operation. As in, they were found to provide noticeable HP for a engine intended to run at sustained high rpms? Dunno. My understanding is those modifications typically don't do much of anything for the vast majority of us. Twin plug is a detonation-preventer more so than anything. The HP gains there are also quite small.