Newbie engine questions

wha-tah-hey Wed Oct 26, 2016 4:24 pm

I've no interest in hopping up my standard engines (there are plenty of TDs, BWs, etc. out there) but being new to the intricacies of Cox engines, to get up to speed somewhat I have a few questions.
I know that many of you are tech/mech minded, involved in tach racing and other venues needing high performance from custom tinkering, so if you'll indulge me:

Auxiliary ports:
Why not on both sides of each bypass?
Why not just a wider bypass?
How about an angular widening of the existing ports? / \
Why not on either .020?

Pee Wee:
How does the later single-vent tank vent when filling?
How to adjust SPI?

Adjusting SPI:
Doesn't cylinder shimming also lower CR?
CR dealt with afterwards?

Adjusting CR:
How about lowering cylinder top and shortening head threads to raise CR for (cheaper) FAI fuel?

Inquiring (idle?) minds want to know - just for the helluvit. Popcorn

Thanks.

pkrankow Wed Oct 26, 2016 7:27 pm

Ports were done in all kinds of configurations. With the dual exhaust there will always be less than excellent configurations - there really is too much exhaust port. With double bypass, one boost per bypass the performance is consistently best. It apparently keeps more charge in the cylinder than with two boosts per bypass. Of course I am an armchair engineer on this one.

If you are modifying cylinders then it is all on you. Pay attention to port timing and make sure it gets done as best you can. If I were to start on this line of crazy I would set up my lathe to mill out a blank cylinder I have with a single exhaust port indexed to the case I intend to run it on. I would look up Schnurle porting and go from there.

If you were really into it making one-offs in BRASS then having them hard plated followed by hand lapping the chrome is relatively economical... Then again the original cylinders are "mild" steel, nothing special material wise. It really is about holding the tolerances, and the original prints are available.

The single tube tank back on the peewee and bee has a pinhole next to the fill tube to vent the tank while filling. Don't block it with your fill hose.

SPI cannot be adjusted without changing other timing in the engine, unless you are recombining parts.

The compression ratio can be adjusted by adding or removing head gasket shims. Only one is required, and thinner than stock gaskets can be found or made. Generally speaking for stock thickness gaskets 1 gasket per 10% nitromethane. 25% nitromethane will typically have 2 or 3 gaskets. Lapping a head or using a high compression head or drop in insert is a cheap way to increase compression for low nitro fuels.

These engines run fine without nitro, but are a little picky to set the needle. Nitromethane has oxygen in the molecule so less air is required to get it to burn right. Oil content can be bumped up slightly to both increase apparent compression and make the no-nitro blend easier to needle. I ran 25% castor oil in my experiments prior to ordering good fuel.

Phil

wha-tah-hey Wed Oct 26, 2016 8:35 pm

Thanks for the tech info, Phil.
I don't have the machine tools, skills or the technical bent to experiment with porting and such, but the methods and principles involved are of interest - I just wonder why some things are or aren't and want to scratch the itch prompting questions that pop into my head.

Adjusting Pee Wee SPI, deck height and CR are of definite interest because I have a few of 'em.

As for FAI fuel, I have methanol and castor for blending/adjusting mixes so I usually try it during my bench running sessions and wondered if worth trying in a reedie.

So when cylinder base shims are used to adjust SPI, how is deck height returned to or set to optimum?

I've gathered from reading that lower compression is the usual need when building a hi-po, but there's also a lot of recommendation for the Tee Dee head to raise CR in the standard engines, so I wondered what other approaches have been tried or used.

dckrsn Wed Oct 26, 2016 8:39 pm

pkrankow wrote:Ports were done in all kinds of configurations. With the dual exhaust there will always be less than excellent configurations - there really is too much exhaust port. With double bypass, one boost per bypass the performance is consistently best. It apparently keeps more charge in the cylinder than with two boosts per bypass. Of course I am an armchair engineer on this one.

If you are modifying cylinders then it is all on you. Pay attention to port timing and make sure it gets done as best you can. If I were to start on this line of crazy I would set up my lathe to mill out a blank cylinder I have with a single exhaust port indexed to the case I intend to run it on. I would look up Schnurle porting and go from there.

If you were really into it making one-offs in BRASS then having them hard plated followed by hand lapping the chrome is relatively economical... Then again the original cylinders are "mild" steel, nothing special material wise. It really is about holding the tolerances, and the original prints are available.

The single tube tank back on the peewee and bee has a pinhole next to the fill tube to vent the tank while filling. Don't block it with your fill hose.

SPI cannot be adjusted without changing other timing in the engine, unless you are recombining parts.

The compression ratio can be adjusted by adding or removing head gasket shims. Only one is required, and thinner than stock gaskets can be found or made. Generally speaking for stock thickness gaskets 1 gasket per 10% nitromethane. 25% nitromethane will typically have 2 or 3 gaskets. Lapping a head or using a high compression head or drop in insert is a cheap way to increase compression for low nitro fuels.

These engines run fine without nitro, but are a little picky to set the needle. Nitromethane has oxygen in the molecule so less air is required to get it to burn right. Oil content can be bumped up slightly to both increase apparent compression and make the no-nitro blend easier to needle. I ran 25% castor oil in my experiments prior to ordering good fuel.

Phil

Thanks Phil+, saved and printed.
Bob

bamboozler Thu Oct 27, 2016 4:13 am

I've never used FAI fuel myself but know 0% nitro can be hard starting. Has anyone tried using an Ignition Enhancer (Propylene Oxide) to help things?
Duke Fox used an Ignition Enhancer of 1% in has 5% nitro fuel and it was an excellent blend. What's nice is Propylene Oxide is legal and easy to buy for us hobbyists.

balogh Thu Oct 27, 2016 9:39 am

pkrankow wrote:Ports were done in all kinds of configurations. With the dual exhaust there will always be less than excellent configurations - there really is too much exhaust port. With double bypass, one boost per bypass the performance is consistently best. It apparently keeps more charge in the cylinder than with two boosts per bypass. Phil

Single bypass flutes on the opposite sides of the bypass ports add a spin to the fule charge that helps better mixing and more efficient combustion. Some people have the impression that the later, thick-wall TD cylinders with only a single booster allow higher rpm that the classic stepped wall and thin wall TD cylinders with dual boosters...I dunnnnooooo, I prefer the classic one made during ther heyday of COX

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