Maxwell

Is there any way that a street port and/or 3mm apex seals could alter the compression ratio? Please explain.

wwilliam54

no

well the apex seals could alter it alot, if they break

peejay

There is no way at all that seals or porting will alter compression ratio.

Even if the apex seals break that doesn't change the compression ratio! Static and dynamic compression however will go to **** in a hurry

edit: If you port the rotor (relieve the rotor face so the intake ports open earlier/later) then your C/R will change by a couple of points lower at the most. Note that one point is .1 of a ratio... so you might go from 9.4 to 9.2:1.

12abridgeport

I've heard that on some bridgeports if you make the exhaust too big you will loose some compression.

Liquid Anarchy

iTrader: (2)

You heard wrong. You'll have a lot of overlap, but your compression is unaltered.

88IntegraLS

If you make a port close later than stock, the uncompressed air/fuel volume becomes slightly smaller than before. Hence, the ratio of uncompressed air/fuel volume to compressed volume goes down, effectively lowering the compression ratio a tad. Don't believe me? Port your own engine plate and lay out a rotor on it before and after the job. You will notice that moving the rotor up the plate along its rotational path to where the new port (street port in my case) closes creates less volume in the upper rotor housing as the compression phase begins compared to a stock port which closes earlier.

REVHED

Come again?

peejay

Do not confuse dynamic compression with compression ratio.

Compression ratio has nothing to do with a running engine. It is just the ratio of the working chamber at its largest size, to the working chamber at its smallest size.

In other words, an engine with 9.4:1 compression ratio has a chamber that is 9.4 times larger at BDC than at TDC. This will not change from porting... hell you could port the engine until you hit coolant all around and remove all of the apex, corner, and side seals, and the compression ratio WILL NOT CHANGE. The engine will have no *dynamic* compression, and it won't run, but the ratio of chamber at BDC to chamber at TDC is still the same, and this is the ONLY thing that is meant by compression ratio.

88IntegraLS

If you count chamber volume while the rotor has not yet closed the ports then that volume is pointless because none of it will be able to begin compression until the port closes. Effective compression ratio is what gives an engine its characteristics . . . you guys were discussing this in theory?

REVHED

Ok, I understand what you're saying now but it's incorrect.

The whole concept of porting is to try and increase the volumetric efficiency of the engine. By extending the port upwards you delay the closing of the port which allows more air and fuel to be drawn in which in turn increases the VE. This means the dynamic compression will actually be higher because it's directly related to the VE. If what you're saying was true there wouldn't be much point in porting like that at all would there?

88IntegraLS

True, that is a valid point that without knowing the VE of a stock ported or street ported engine, dynamic compression could not be debated. Looks like I advocated an undebatable perspective.

I would guess that the VE of a late closing street port would increase with RPM due to increased intake velocity, Seesh, this intake stuff is a science. I am not an engineer yet so I don't have math equations to back this up.

Maxwell

To simplify things- if a copression test was performed, would there be a difference in results because of seals or ports?

1st to 3rd

I think a couple different terms are being confused.
Static compression ratio will never change (unless you change rotors).
Dynamic compression ratio - http://members.uia.net/pkelley2/DynamicCR.html
It's for piston engines, but same principals apply. This is why piston guys can get away with running 11:1 static compression with pump gas if they use a long duration cam.
I think dynamic compression ratio and dynamic pressure are being confused. The pressure is related to VE, CR, etc.

Yes, if one was to perform a compression test on a ported engine the results would be lower then a non-ported engine.

Nathan Kwok

Exactly what Peejay said. Porting won't make a difference in C/R, large apex seals will raise it *very sightly* but it is miniscule, due to the thicker apex seal taking up some space in the combustion chamber.