Is this Pump Gas-able?

[AWDBOB]

Hey R3V-

I recently acquired this freshly built 2.8L M20, and it seems pretty aggressive. The bottom end consists of an M52b28 crank, 135mm Carrillo rods, and custom JE pistons, that appear to be quite high compression.

The head is worked over and has Ferra valves, a 304 cam, etc.

I want to run it on pump gas, but it looks like this will be a no go on the current setup.

Cranking compression was 210psi.

What's the path of least resistance to running this thing on the street?

Untitled by Bobbie Morrone, on Flickr

[2mAn]

I think you can get a thick MLS gasket?

[AWDBOB]

I think you can get a thick MLS gasket?

That was my plan- I need to find a way to get a reading on dome volume (clay mold and fill to measure CC’s) to get the proper sized HG.

I’m really just hoping that digger comes in here and gives me a magical answer that makes me feel warm and fuzzy inside.

Obviously ethanol and ITBs would be ideal, but not exactly practical.

The cranking compression seems moderate, which I assume is due to the bigger cam. Cranking compression on Linda’s 3.1L is 240psi LOL

[Tinkerer007]

What is the static compression on Linda's 3.1L?

I have a used stroker that was advertised as 10:1 but cranking compression is ~235 PSI (W/ a 274 cam).
It was sort of running on pump gas if I didn't push too hard...

I am hoping to measure CC with a burette at TDC and BDC. (My engine is still assembled.)

[digger]

What did it run on before with the PO? any issues?

The thing is cranking CR tells you only so much. 210psi is not a deal breaker for pump swill, though a 14:1 static engine with big cam that cranks the same as a 10:1 static and small cam will be more knock prone as when the thing comes on cam the bigger cam engine makes more VE and cylinder pressure (or at least it should as that's the purpose of a big cam). Generally it does this only at mid range and topend so at low rpm it can be better.

What are the deets on the pistons as far as bore/cc/cr that the PO told you?
Those pistons look awfully a lot like the 14.5:1 2.8L from Victoria that runs on e85 and Webers

I'd be tempted to run them with 1mm squish, colder plugs and see what ignition timing it will take. I've heard from several sources who have tested that setting with a proper squish circa 1mm is more likely to cut down on likelihood of knock than using a thicker gasket with sub optimal squish in the 1.5 to 2mm range.
There are several ways to cut down the CR if you want to go down this route

• A thicker gasket but as above this can be worse unless you go quite a bit thicker and then this hinders burn characteristics.
• Then there is removing some meat from the combustion chamber (this may have already been done with the bigger valves perhaps you could cc the chambers)
• You could machine a shallow dish in pistons in situ but will be a pita without some fixture that makes them all the same.

[AWDBOB]

What is the static compression on Linda's 3.1L?

I have a used stroker that was advertised as 10:1 but cranking compression is ~235 PSI (W/ a 274 cam).
It was sort of running on pump gas if I didn't push too hard...

I am hoping to measure CC with a burette at TDC and BDC. (My engine is still assembled.)

Linda is 10.5:1 with a 284/280 Billet IE cam.

What did it run on before with the PO? any issues?

The thing is cranking CR tells you only so much. 210psi is not a deal breaker for pump swill, though a 14:1 static engine with big cam that cranks the same as a 10:1 static and small cam will be more knock prone as when the thing comes on cam the bigger cam engine makes more VE and cylinder pressure (or at least it should as that's the purpose of a big cam). Generally it does this only at mid range and topend so at low rpm it can be better.

What are the deets on the pistons as far as bore/cc/cr that the PO told you?
Those pistons look awfully a lot like the 14.5:1 2.8L from Victoria that runs on e85 and Webers

I'd be tempted to run them with 1mm squish, colder plugs and see what ignition timing it will take. I've heard from several sources who have tested that setting with a proper squish circa 1mm is more likely to cut down on likelihood of knock than using a thicker gasket with sub optimal squish in the 1.5 to 2mm range.
There are several ways to cut down the CR if you want to go down this route

• A thicker gasket but as above this can be worse unless you go quite a bit thicker and then this hinders burn characteristics.
• Then there is removing some meat from the combustion chamber (this may have already been done with the bigger valves perhaps you could cc the chambers)
• You could machine a shallow dish in pistons in situ but will be a pita without some fixture that makes them all the same.

Yep, that's what I figured I'd find re: cranking compression - I expected Linda to be lower with the cam I used, but the small chamber head and weird piston design didn't help, I'm assuming.

With mid range and top end increasing in VE/cyl pressure, my worry was that weird mid-range knock that seems to plague many stroker M20s would be present.

I finally got ahold of the builder and ding ding, they're 14:1, and they already took material out of the chamber to get it all set up properly. 84ish mm bore, and he didn't have the CC info I was hoping for.

He ran it on 93 - I had a plug from the motor and it didn't show signs of detonation, but I don't think he drove it- just broke it in and then lost interest.

Also, that is fantastic info via 1mm squish. I need to measure it up, and then shoot for that number.

[AWDBOB]

Well I'll be damned..... I did some digging and that Victoria/Weber 2.8L is 14:1, and I think these are the exact pistons that they used in this motor. See below for a shot of their JE 14:1 piston.

Picture085_zps5d36ad21 by Bobbie Morrone, on Flickr

[digger]

you should probably cc the chambers, pistons and work out the actual static CR for point of reference but id run it assuming you have a standalone with good control over everything

[varg]

you should probably cc the chambers, pistons and work out the actual static CR for point of reference

This. Anything else is speculation, and since the head is off it's borderline trivial to measure everything needed to determine compression ratio.

[AWDBOB]

Agreed- I just don't have a good way to CC the pistons, or I'd already be on it. Will be CCing the chambers tomorrow.

[digger]

Ccing piston is similar to chamber.

[TobyB]

just drop the piston exactly 5cc into the bore.

And use Vaseline. No, to seal the rings, you perv!

I agree- without static numbers, you're never going to know.
WITH static numbers, you can make an educated guess.

t

[AWDBOB]

Thanks all. I have a question that is likely a dumb one, but it's holding me back.

When calculating dome volume, do I only calculate the dome area that sits above deck, or does the whole dome get calculated from where it is slightly below deck?

Saying dome this many times is not satisfying..... dome

5cc = 0.9mm down into the bore, so with TobyB's suggestion, I am assuming that the entire thing gets calculated, essentially from rings up (where it'll hold fluid).

Untitled by Bobbie Morrone, on Flickr
Untitled by Bobbie Morrone, on Flickr

[digger]

The reference for piston cc's is the top of piston from a compression height perspective (CH), i.e. this is the datum. This isnt the actual top of dome, it's the corner where the "chamfer" starts.

Use grease in the corner/edges to help it seal and prevent liquid getting into the crevice between the top ring and CH.

the piston position w.r.t deck is defined using the same datum as above, this also needs to be measured. Yours look like it about 1mm below deck. So your deck clearance is +1mm (insert actual number)

[TobyB]

Digger,
you might be saying this, but I'm not getting it.

To cc a piston in the bore, stick a dial indicator on it.
Lower it some arbitrary amount. (I said 5mm, but it can be anything that lets you put a flat cover over)
Then put your head cc cover plate over, add fluid until it's full, then
do the math to figure out how much of the arbitrary cylinder volume the piston crown is filling.

So let's say if you lower the piston 10 mm, and then get a volume of 40 cc,
your piston crown volume is ((8,4/2)^2 *pi ) = 55.4 - 40 or 15.4 cc.
Which you subtract from the chamber volume to get the tdc volume.

clear as mud?

t
worked it out on a napkin