Alex,
To determine compression it depends on whether you talk about static
compression or Dynamic compression.
Static Compression
i.e.: the compression based on the swept volume of the cylinder bore that
is displaced by the engine as it moves the piston. You simply take the bore
say 4" an put it into the formula for area of a cylinder lets presume the
stroke is say 3.48 (Chevy 350 sorry guys) for (4/2)squared X pi X Length of
the cylinder 3.48" for 43.7485 cubic inches or X 8 = 349.988 ci (small block
350) . or in Metric 4" = 10.16 cm and 3.48" = 8.8392 cm the formula then
gives roughly 25.8064cm X 3.14285714285 X 8.8392 = 716.9106 cc.
If we assume zero deck height no valve relief or dome and no gasket the
ratio would be ((716.9106 swept volume + 54cc Head volume)/54cc) : 1 or
13.276:1 compression.
To do your computation we would need Bore size, dome and piston valve
relief, volume and thickness of the compressed head gasket and piston to
deck height.
That gives you static compression.
Dynamic compression
What you really want is dynamic compression and get it around 195lbs of
compression for pump gas. And you can do this with a 10:1 or a 11:1 or a
12:1 or a 13:1 compression ratio depending on when you choose to close the
intake valve in relation to the position of the piston in the bore. It
depends what you plan to use the engine for but I have seen 10.5:1 motors
perform significantly better than 11.8:1 motors because the produced more
cranking cylinder pressure.
I hope that helps,
Tim Ronak
B382000680
A Canadian forced to stay indoors while it is cold out.
-----Original Message-----
From: Alex MacLaurin [mailto:Alex_MacLaurin@telus.net]
Sent: Friday, January 21, 2000 12:11 PM
To: Ronak, TP (Timothy); 'Bob Palmer'
Cc: 'Tiger News Group List'
Subject: Re: Performance Tuning the Tiger
With all this talk about compression,I have a question: if a
motor has 11:1
compession with a 54cc chamber and all variables stay the
same, how much
compression does the engine have with a 64cc chamber?
----- Original Message -----
From: Ronak, TP (Timothy) <Timothy.P.Ronak@akzo-nobel.com>
To: 'Bob Palmer' <rpalmer@ames.ucsd.edu>
Cc: 'Tiger News Group List' <tigers@autox.team.net>
Sent: Friday, January 21, 2000 10:14 AM
Subject: RE: Performance Tuning the Tiger
> Bob,
> 195 ....so I was told ... is about the optimal pressure to
avoid excessive
> heat generation on the compression stroke and not use too
much power to
> compress the next charge when running pump gas. If you
were to optimally
> tune your engine you could probably retard the cam 1
degree close the
> intakes a little later drop the cylinder pressure for an
average of 195
> (190-200 range between cylinders) and get a little more
power out and it
> might even run slightly cooler. IMHO This explains why you
do not have
> detonation. But didn't you run the car at 60 degrees
advance. I hope now
you
> are around 36-38 Total at 5000+ RPM. I hope someday I can
go for a ride.
> Regards,
> Tim Ronak
>
> -----Original Message-----
> From: Bob Palmer [mailto:rpalmer@ames.ucsd.edu]
> Sent: Friday, January 21, 2000 8:57 AM
> To: Ronak, TP (Timothy); MWood24020@aol.com;
> TSMIT@isotel.com; rdmallory@earthling.net; Ronak, Timothy
P;
> tigers@autox.team.net
> Subject: RE: Performance Tuning the Tiger
>
> Tim,
>
> After you guys taught me the proper way to do a
compression
> test (plugs
> pulled, WOT) I get 195-205 psig depending on which
cylinder
> and which day.
> I'm at sea level here, but of course that doesn't affect
> psig, just psia.
>
> Bob
>
> At 07:57 AM 1/21/00 -0600, Ronak, TP (Timothy) wrote:
> >Bob,
> >I have to agree with you about the effect removing and
> sharp spots from the
> >combustion area will have on detonation. The key to
> avoiding the excessive
> >pressures is by changing where the Intake valve closes in
> relation to the
> >piston traveling up on the compression stroke. What is
your
> cylinder
> >pressure warm with all of the plugs out ... or have you
> tested it? The
> >method I will use on y 302 will be to chose the desired
cam
> profile and then
> >increase compression (or advance the cam) until I obtain
> about 195 lbs
> >cylinder pressure cranking. My experience is similar to
> yours in that
> >compression is fine provided that the combination once
> assembled gives you a
> >usable cylinder pressure. It may be that the guy who has
to
> run race gas
> >needs to install a cam with a little more overlap or one
> that closes the
> >intake valve a little later reducing the amount of
cylinder
> pressure the
> >piston can build. I do know that back in the race car
days
> we had an engine
> >that was 14:1 (remember that car Theo .. it was blue and
> yellow) it made
> >over 220 cranking cylinder pressure and we had to run
C-14
> and methanol in a
> >15% and 85% ratio respectively or it detonated like
crazy.
> >My 2 cents too,
> >Tim Ronak
> >B382000680
> >PS: It is amazing how a well placed email gets us back on
> the performance
> >topic....YEEHaaaww!
>
> Robert L. Palmer
> UCSD, Dept. of AMES
> 619-822-1037 (o)
> 760-599-9927 (h)
> rpalmer@ucsd.edu
> rpalmer@cts.com
>
|