Short Stroke 2.8

[Henry Schmidt]

Originally Posted by svandamme

Henry, 280HP with milder street cams?
define milder? DC60? DC40?

DC 62 with a 106 lobe center might do the trick.

[svandamme]

Just scored a set of 3.3 turbo heads heads in the UK
port size was 38 mm , i'll have to open that up a bit, but with 49mm in, and 41 something out, in the valves department, i guess it will do , right?
They already came with twin plug and mfi injector work done, so that helps too

[rswannabe]

If you are building a 2.8SS then those ports might be OK as they are. I beleive Henry recommends 38mm intake ports for a 2.8SS to keep the port velocities fast enough. I have 3.2 heads on my 2.8 and I know Henry thinks the stock 3.2 ports are too large at a little over 40mm (I forget the exact size). Granted my motor runs great at all RPM's, but I've always wondered what it would be like with slightly smaller ports.

[svandamme]

Originally Posted by rswannabe

SC HEADS PORTED TO 40 MM IN 38 MM EX.

already got the MFI stacks bored to 40mm (bought them this way), so i guess it will have to be 40, which is what Henry described initially
won't have the lip for pulse control, and it will have more high end then low end optimisation, but i have to get to 40 or i'll have a lip in the wrong direction.
3.2 ports are 41 someting mm

[rswannabe]

Stijn,

You are correct. It has been a while since I went through this thread and I was misremembering the sizes. Sound slike you got the right heads to start working with. Have fun!

[svandamme]

haven't decided yet, but i might end up with 97mm jugs... that would make it a 2.9 short stroke.
Not sure if that makes much difference for revability, i suppose the pistons would be a lil bit heavier...
But then i would not need to make the same revs to get the same output.which makes it easier on the valve train and gains a bit of torque while i'm in there.

[PFM]

Henry,

You mention using the "solid" chain tensioners in this 2.8 motor build as apposed to hydraulic chain tensioners as they never fail. I am building my first flat 6 a 3 ltr for track use high lift cam etc. Would you recommend the solid chain tensioners here and why? The motor will not see street usage and will be removed and refresher once per year.

Also what is the part number for these solid chain tensioners.

Stay tuned,

PFM

[svandamme]

not sure there is a part number for those, my 73S had solid tensioners,
it was just a machined aluminium block, with a screw to tighten things up.



And although the tensioners can't fail, they won't adjust when the chain stretches...
it was very slack when i opened up my 73S...

[PFM]

svandamme,

Thanks for the input and photo, I do not think the adjustment would be a big deal, it is a track car after all. I believe I have a set of those in the garage but thought of them as a tool to check cam timing with not to run in the motor. I like simple in a track motor, less oil lines simple parts less prone to fail etc. My only thought is there is zero "shock" absorption for the chains with these installed.

Stay tuned,

PFM

[Walt Fricke]

PFM - you hit the nail on the head, sort of. The solid tensioners only set a "correct" tension for one temperature. Colder than that and the chain will be too tight, hotter too loose.

Opinions seem somewhat divided here for race motors. But for advice based on the early 911 motors, where the stock trensioners failed more often, I'd be careful about accepting it.

Me, I run either Carrera tensioners (never failed me absent my having messed one up removing it) or the immediate pre-Carrera tensioners (narrow mount to go with the gear wheel carriers which have the wider bushings where they pivot) with collars. And at some point I will do the modification to my Carrera tensioners which limits how much they can collapse.

I have a set (milled to work with the wide gear carrier) of these mechanicals in the tool box as replacements at the track should the non-pressure feds fail. Problem with pressure feds failing is that you can't replace with something else without a spare set of chain box covers - ones without the holes for the pressure system.

Gluing a plastic cap into the hole only works for 40 minutes or so - ask me how I won our Club's Exxon Valdez award once in the best forgotten past.

Walt

[PFM]

Walt,

Again good info, I understand the chains would be "loose" until the motor comes up to temperature but to be honest I think stomping on the throttle of a cold motor is a bad idea anyway. But you gotta admit this is a very simple and bulletproof part. The oil fed tensioners have the advantage of not needing to adjust them, handle the cold motor better etc. but there are many more parts and pieces added to the mix to gain those advantages.

I am still on the fence.

PFM

[Walt Fricke]

Here is another thing about regular tensioners: their pressure is mostly, I believe, generated by the hefty spring inside of them. The oil serves as a shock absorber, being forced out a small orfice and elsewhere when the chain applies enough pressure to push the spring down, so the natural frequency of the idler wheel is something other than that of the spring in the tensioner. Something like that.

The pressure feed serves to keep the chamber full of oil when, for some reason, the tensioner is extended beyond whatever its most recent retraction was. The earlier tensioners did this by catching oil in the cupped top of the tensioner body, and somehow getting it inside as needed.

Someone once told me that they ran their engine without the chain box covers (or maybe just one cover) just to see what was happening. Said that surprisingly little oil flew out (unlike the experience of those who have tried this with valve covers). And that it was amazing to see how much the chain jumped around.

But I have yet to try this, much less compare a regular tensioner with a solid one.

For real security, the airplane regulators required Porsche to use gear drives for its airplane motors. I suppose those would be hard to come by.

Walt

[svandamme]

anybody know anybody who flies one of those PFM3200 engined Mooney's?
Maybe they can come up with some part numbers, schematics, and price tag for those gear drives?


EDIT, never mind, only 80 PFM3200's ever built, no factory support... it would probably be easier trying to make em from scratch

[safe]

Originally Posted by Walt Fricke

Here is another thing about regular tensioners: their pressure is mostly, I believe, generated by the hefty spring inside of them. The oil serves as a shock absorber, being forced out a small orfice and elsewhere when the chain applies enough pressure to push the spring down, so the natural frequency of the idler wheel is something other than that of the spring in the tensioner. Something like that.

The pressure feed serves to keep the chamber full of oil when, for some reason, the tensioner is extended beyond whatever its most recent retraction was. The earlier tensioners did this by catching oil in the cupped top of the tensioner body, and somehow getting it inside as needed.

Someone once told me that they ran their engine without the chain box covers (or maybe just one cover) just to see what was happening. Said that surprisingly little oil flew out (unlike the experience of those who have tried this with valve covers). And that it was amazing to see how much the chain jumped around.

But I have yet to try this, much less compare a regular tensioner with a solid one.

For real security, the airplane regulators required Porsche to use gear drives for its airplane motors. I suppose those would be hard to come by.

Walt

When I rebuilt my 930 tensioners I discoverd that the spring isn't really heafty at all. When doing the rebuild you insert a steel wire trough a hole to release the check valve in the tensioner. To bleed the tensioner from any trapped air you then pump the piston a few times and you can actually do that with your thumb!
But when you remove the steel wire you need a vice to compress the piston...

These tensioners are sealed no oil gets either in or out. When they fail I guess one of the o-rings inside fails and the oil can flow "free" inside or from the inside out.

[Flieger]

956s also used the same gear drive for the valvtrain as the PFM

[svandamme]

great, now to find a breakers yard that has a 956 on the chopping block

[Flieger]

Well, there are ways... http://forums.pelicanparts.com/911-engine-rebuilding-forum/248909-rebuilding-962-motor-10.html#post2398960

[PFM]

OK now the 956 gear drive looks like the way to go but I believe they are out of my price range. Even if I whittled it up from billet .... Not enough hours in the year

Thanks for the post.

[Walt Fricke]

Yup - massive overkill. But look what those engines were built to do.

Tensioners just aren't a weak point in a race motor built by and for the likes of most of us. I had one go bad (and that was a fluke due to my bad wrenching) in 25 years of DE/racing/street amateur use. Once the idler bearing issue was addressed these should be good. And all that meant was I swapped in a spare at the track. No disaster.

How much extra money do you want to spend? Extra time fussing with adjusting to get the best compromise setting with the solid setup? Are you shooting to win a 24 hour pro race? Porsche used aluminum cam chain gears in their race motors because they expected to replace them after rather few miles/hours. Collars/pressure fed mods will insure a failure of the tensioner does not cause the chain to skip a link.

To each his own, of course.

Walt

[Henry Schmidt]

We've been using solid tensioners in race engines and miscellaneous street engines for years with 100% success.
The key is high performance chains and experience that tells us that the distance between crank and cam centers expands at a predictable rate with heat. This means that the chains tighten as the engine heats up.
The short duration between freshen ups in race engines means that there is a limited need for maintenance and zero failures in solid tensioners.