Posted by alloy_625 on April 21, 1998 at 20:33:59:
I think my previous question rolled off into oblivion, so I was wondering about the practicalities of getting "mirrored" pistons and low compression height ring packages. JE and others don't seem interested in going through the trouble to program the proper contour for a set of 4 pistons so I was wondering if there's anything special that needs to be considered, or if a copy of Surfcam, a VMC, and a casting of the combustion chamber is sufficient. Is there a rule of thumb for the minimum thickness in the center?
Also, is there anyone you can recommend that has a proven track record with the design of low compression height ring packages for forced induction applications? People still seem to want to move the package down...
Thanks
I've simply gotten to the point where I tell my piston guys that it's
normally aspirated, and there's no arguments. On a blown application with
a relatively small bore, about .200" is as thin as I'd go, and Idon't think
you'll have any problems.
If you can orient a straight piece of wire across the chamber centerline
for proper bore axis orientation, any two part rtv compound should be enough
for a machinist to use to modify your "slugs". Although there are better
pistons on the market, Aries has been good when it comes to custom "small"
orders. You might give them a call, as my guys also require quantities
or you wait forever.........T.O.O................
Posted by alloy_625 on April 30, 1998 at 19:27:02:
I was wondering about chamber design, specifically the design of the pistons as you once said that piston dome should be thought of as the bottom of the chamber.
Is it sufficient to duplicate the squish areas on the piston or (for a low compression application that would otherwise be dished) should the piston dome be formed so that the air gets squished into a football like shape when it's at TDC?
Also, what do your Popular Mechanics :) derived stagger cams accomplish? Do they help accelerate the intial intake charge by momentarily reducing cross section and help develop swirl or am I way off base?
In general, what can the average enthusiast, without access to your programs, do to achieve relatively high boost pressures on pump gas without detonation? Is there any trickle down from your "soft head" work...
Posted by T.O.O. on April 30, 1998 at 21:23:05:
In Reply to: T.O.O. - Misc Questions posted
by body on April 30, 1998 at 19:27:02:
Creating the "Soft Head" is the trick. Assuming you're intent on lowering
static compression for turbo or what ever, I would say that maintaining
the quench area is absolutely critical if you're attempting to eliminate
detonation. In many tests we ran in conjunction with both the ETHYL CORP.
and LIVERMORE FABS., quench was commonly called artificial octane.
The problem you'll end up with if you drop the CR and attempt to "mirror"
the chamber with the piston is one of depth...depth of combustion shape.
A shallow chamber with shallow valve angles helps this situation regarding
depth and surface area to volume ratio. Spark plug location is another
major player in this area of creating a detonation resistant chamber. As
considerable ionization occurs prior to actual ignition, placing the plug
in that "sweet spot" is crucial, and to become really efficient twin plugs
are a necessity, even on small bore engines. Now, there are alternate forms
of ignition which will ultimately replace spark plugs, and you'll be reading
about this particular method shortly.
Swirl.....I'm ready to take my lumps again.
The ability to induce the proper swirl frequency and depth of rotation
is paramount to maintaining a layered homogenous mixture which will provide
a lengthened primary burn followed by a rapid secondary burn, all of which
will yield considerable resistance to detonation, greater over all combustion
efficiency and fewer bad guys coming out the exhaust. The over all time
of burn is so short and complete that the spark advance may be reduced
to the extent that you're not doing as much "negative work", and the exhaust
gas temperatures generally are in the 800 degree area, which means that
the heat of the burn provided considerable better thermal efficiency, and
something we and our competition noticed early on was the sound of the
exhaust....it was almost a "whisper" rather than what you'd normally hear
from an un muffled race engine.
I do indeed use staggered intake valve opening events to allow the
flow from the first valve to "influence" the flow of the secondary valve...and
it works very well. I have used sequential inlet valve opening with 4 and
5 valve engines for over 15 years, and it still works.
I cast some small block Chevy heads back in the mid 80's, and although
I did rotate the deck to lessen the 23 degree valve angle, and reduce chamber
volume. The plug position was optimised, and of course the ports were adequate,
and the inlet ports were properly biased to promote swirl. The pistons
were "unique" in shape...all I'll say is they had no dish, except two .120"
valve reliefs. They were certainly of the domed variety. Those small blocks
were 358 cid. engines with 1.75-1 rod length to stroke ratio, very short
cam timing...235 degrees @ .050", and the intake manifolds were some of
my Edelbrock "specials" with Murray Jenson prepared Holley 830 cfm carbs.
Those engines had "over" 16-1 static CR, and dynamic compression was so
high we had to use custom starters run off 24 volts.
They were installed in some Camaros and two pick-up trucks. They all
ran 91 octane unleaded pump gas. They never detonated, the mileage was
37 (combined) for the cars and 25 for the trucks. The Camaro's had Turbo
400 automatic transmissions, and from off idle you'd swear that there was
at least a 454 under the hood...the throttle response was almost too quick.
Those "loaded" cars all ran 12's with ease. The trucks had pulling ability
that no body imagined, and were a dream to drive, especially compared to
their street counterparts.
So, yes. If something as crude as a small block Chevy can be that efficient,
some of the more "modern" chambered heads can certainly do the same and
considerably better.
Admittedly, for people who have simply followed the rules governing
pressure and detonation, trying the other way is risky business, and if
you fail to pay attention to each part of the puzzle, you'll have trouble.
Many of our customers do not want to know the numbers, which is fine because
we'd never bother measuring static CR if people didn't ask. CR. has nothing
to do with the ability to burn effectively, the preparation and delivery
of the mixture, and the shape of the combustion space and all the cam timing,
intake design, header config., etc. mean everything. We're not done on
this subject by any way of thinking, so be prepared to listen to some really
crazy stuff in the future.....after all being crazy is what racers pay
me for, and I'm too old to change...........................T.O.O. ...................
Posted by alloy_625 on June 15, 1998 at 22:54:02:
T.O.O., I'm trying to package a long rod into a short block and I've
got some questions/concerns about designing a small compression height
piston. I'm shooting for 24 to 25mm, and JE's recommended ring package
works out to 15.5mm plus the
piston pin radius plus the material between the oil ring and the piston
pin.
1) Is it really bad to sneak the piston pin under the oil ring so there
is a portion of missing land underneath?
2) The stock pin is 22mm (for an 86mm piston). What are the concerns/tradeoffs
involved in reducing the pin diameter to 20mm or smaller?
3) What else can I do to squish the ring package (currently 0.25"/.060"/.090"/.149"/??/pin
given the turbocharged nature (~20 psi) of the project without sacrificing
reliability?
Thanks a bunch
Posted by alloy_625 on June 19, 1998 at 22:23:37:
I'm hoping you missed the question the first time and didn't think it too mundane, but here goes. In the quest for a small compression height piston (25mm), where can you skimp? The stock piston pin is 22mm, and JE's recommended ring pack (turbo engine) is 15.5mm from top to the bottom of the oil ring. I'm willing to make the pin smaller since the rod is custom, but I don't know what the tradeoffs are. Also, you mentioned to someone else to keep compression at 9:1 or higher. Is this honda specific or a general guideline? Is this in an effort to improve quench by minimizing dishing or is there something else going on.
Thanks
Posted by alloy_625 on June 19, 1998 at 22:23:37:
I'm hoping you missed the question the first time and didn't think it too mundane, but here goes. In the quest for a small compression height piston (25mm), where can you skimp? The stock piston pin is 22mm, and JE's recommended ring pack (turbo engine) is 15.5mm from top to the bottom of the oil ring. I'm willing to make the pin smaller since the rod is custom, but I don't know what the tradeoffs are. Also, you mentioned to someone else to keep compression at 9:1 or higher. Is this honda specific or a general guideline? Is this in an effort to improve quench by minimizing dishing or is there something else going on.
Thanks
Posted by T.O.O. on June 21, 1998 at 04:03:05:
In Reply to: T.O.O. piston design and compression
posted by body on June 19, 1998 at 22:23:37:
For a turbo application the 15.5 mm distance from the top of the piston
to the bottom of the oil ring is a good package. As far as pin diameter
I think an 18mm would do. I'd make sure that it's not too short, and I'd
also use some "good" tool steel.
On the compression ratio, we use much higher numbers than 9-1, but
your assumption that increasing the quench area is dead on.
As far as the 25mm. piston compression height, I've got one here in
my hand that I've been measuring for your pin and ring pack information.
The distance from the pin bore to the bottom of the oil ring groove is
only .030", but there's nothing risky at all here.
.....................................T.O.O. ......................................