To Stroke or Not to Stroke
On a 2.6 liter 4G64 stroker with a rod stroke ratio of 1.47 (106mm stroke & 156mm rod) I was planning to CNC Port the head, but what oversized valves would you recommend for the intake and exhaust? Also what cams....was planning on the Cosworth M3 Mivec. I plan on running a Borg Warner S300SX 91-79 twin scroll turbo set-up and revving to max 8000rpm.
Rod ratio doesn't matter. It's been played with since the 70's and we've found no gains, dyno or otherwise. The theory is there but in practice the longer dwell time just doesn't help. And Scorke, IIRC, a longer rod increases dwell at one end of the cylinder, but increases acceleration at the other. You have to remember that no amount of rod lengthening is going to change the time the crank takes to make a rotation, so any extra time spent at TDC has to be made up for elsewhere. We have a joke around the school about the 7 mile rod. A friend of the teacher who works at NASA put together a simulation in which he used a progressively longer rod to find any gains. The rod was 7 miles long before it made any appreciable difference.
The thing that should concern you is not rod ratio, but piston speed. Piston speed kills wrist pins, rods, cranks, bearings. Scorke gave the formula for it a few posts above, and it's something I addressed in my first post(#28). With your 106mm stroke(~4.17in) you have a mean piston speed of 28 m/s. Here's some other engines you might find interesting:
A 2.0l 4G63 at 8k has a piston speed of 23 m/s
The Peugeot WRC engine has a piston speed of 25 m/s
A MotoGP engine has a piston speed of 25 m/s
A Formula 1 engine has a piston speed of 25 m/s
An SB2.2 NASCAR engine has a piston speed of 26 m/s
A 2.3l 4G63 at 8k has a piston speed of 27 m/s
An NHRA Prostock motor has a piston speed of 29 m/s
An NHRA Nitro motor has a piston speed of 31 m/s
As for head porting, when someone said "port the hell out of it" they couldn't be more right. When you have 40 pounds of pressure on the port, air doesn't really care about the shape of the port, it's going to go in the chamber. We've found minimal gains through head and cam work on turbo engines, but what we've found works best is a huge port. Just, make it big. It may not show on the bench, but it will show on the dyno. Doesn't work very well for lower boost apps, like anything under 30.
Why not go with a 2.0 with higher compression say 9.5 or even 10.0? It should spool the turbo up comparable to a 2.3 and still capable of higher revs safely. Just a thought as thats what I'm currently considering as I get ready to order my pistons...
The higher the compression the greater the octane demands, the higher the risk for detonation and the lower the margin of error for tuning mistakes, especially on cars with forced induction.
Spool is not increased by compression only displacement. I can confidently say we have tested that theory in the last few weeks. Compression maximises the power for a given airflow value however, so more power without changing anything is certainly possible.
I also do not believe nitrous helps spool, it only helps off spool power which means it gets to where the turbo would normally spool faster than without.
This thread about displacement increase and spool though is certainly true and this is an accepted fact. I really wonder though what the line on percentage of increase in displacement to spool is though. For instance is a 2.2 likely to spool 10% faster? What losses come into play when you are increasing the displacement (other than pumping losses)?
Aaron
I also do not believe nitrous helps spool, it only helps off spool power which means it gets to where the turbo would normally spool faster than without.
This thread about displacement increase and spool though is certainly true and this is an accepted fact. I really wonder though what the line on percentage of increase in displacement to spool is though. For instance is a 2.2 likely to spool 10% faster? What losses come into play when you are increasing the displacement (other than pumping losses)?
Aaron
Well any time you have increased cylinder pressure you have increased exhaust pressure/energy, so doesn't that make the argument for nitrous?
Its all motor horsepower, so the engine rpms (that is revs up) that much easier and gets to the exhaust gas value where the turbine begins to spool. 10:1 compression to 11:1 didnt increase spool...at all. Its an increase in non boosted power to be sure, but I dont think the extra cylinder pressure made it spool faster.
Its all motor horsepower, so the engine rpms (that is revs up) that much easier and gets to the exhaust gas value where the turbine begins to spool. 10:1 compression to 11:1 didnt increase spool...at all. Its an increase in non boosted power to be sure, but I dont think the extra cylinder pressure made it spool faster.
