but when a t-76 dynoes something like 500rwhp and 680rwtq (on stock ci)...wouldn't a long stroke just accentuate that even more? almost like diesel truck numbers or something.

and lets say one could only choose between a stroker or all bore, for a turbo application what would be best? (neglecting block strength or durability)

I know lower rpms wil be easier on the parts, but alot of those outlaw mustangs also are known to rev bit high and they use the 101mm+ turbos. also alot don't have "big" ci.

one mroe question, woudl twins be better for a 427?

 

A large bore would be better hands down for a turbo motor.

When you start looking at the physics of it, you realize a supercharged motor doesn't care about stroke when looking at peak power potential, it only cares about the total volume when the piston is at TDC. More volume = more charge compressed to the limit without detonation. This is why supercharged motors lower compression, because it increases the total volume of the air/fuel mixture. Stroke doesn't add any volume here, although it can help in other areas (like at low RPM before a turbo has spooled up). Turbos usually make awesome torque, so this isn't really a problem in most cases.

Hope this helped somewhat.

I'd go big bore if it was me

 

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There's a big difference between changing your rpm points a few hundred rpm, and saying "diesel truck". Yeah, the car will make more power theoretically - at the low and high end (low end as it spools up sooner, high end as the compressor is a little more effecient) - but since when is more power, anywhere, a bad thing? If it's more than you can use get a boost controller and turn it down for lower rpm's. The point is to have it available




I would probably go for the larger bore since it would decrease valve shrouding. That said that isn't a realistic situation - you are going to be buying a new crank, pistons, rods, etc. - so it's hardly any extra cost to go to a larger stroke as well.




They are also in a racing class that restricts/gives weight breaks for different cubic inch limits. If you are in a class like that then yeah, max out to the limit, then just turn the rpm you need to spool up the turbo - what you have to remember is with a turbo it's about your exhaust massflow - whether you get it from displacement, rpm (or both) is irrelevant - you just need to get the turbine spinning to flow the air.



A larger bore has virtually no effect on the compressed volume at TDC - especially on a ls1 where the piston pops out of the bore. But if we talk a 4.1 vs. 3.9" bore, and say a 0.040 quench, we are looking at (4.1/2)^2*pi vs (3.9/2)^2*pi difference, or 0.528 in^3 vs. 0.478 in^3, or 8.626cc's vs 7.84cc's. So if we figure a 60cc head and 20cc dish, we are talking 88.626cc's vs. 87.84cc's, or given the same displacement (bore vs. stroke) at say 6.000L, that gives us 8.46:1 vs. 8.53:1 compression ratio. Obviously not a big difference *at all* in the TDC volume - heck, the heads probably vary cylinder to cylinder by more than 1cc, which is about all the difference we are talking about here.

And a supercharged motor *does* care about the stroke. A greater stroke generates a greater depression (longer travel/faster movement), which mean the motor "sucks" in more air. If we assume on a turbo our boost pressure is fixed (via wastegate) that means at the same pressure we are flowing more air - hence more power. Same with a bore - you have a greater piston area, so you generate a greater depression. Either way it boils down to displacement.

Sure you can make as much power with a smaller motor, but it will be at a higher psi. If you are boost pressure limited the bigger motor will make more power. If you are CFM limited then the bigger motor will still make more power as it won't work the compressor as hard, so it will be more effecient/take less power to turn. And it will do so at a lower psi.





As I pointed out a bigger bore makes virtually no difference in compression vs. a bigger stroke. Plus you can set the compression ratio to whatever you want with piston/head volume. You just need to maintain enough compression height on the piston (which is the first thing I mentioned initially).
Turbos make "awesome torque", but that's at WOT, after they spool up. A bigger motor will help them spool up faster, which equals more power, at least early in the powerband. If you don't want it run a boost controller. The point is that it is an option. Also if you are driving the car around at all you will *not* be in boost all the time - and then your motor will be just a low compression NA motor - a dog. With more displacement it will be much more fun to drive at part throttle. Finally, moving the same cfm of air into a larger motor results in a lower boost pressure and a more effecient system.

 

Your going to want more than 350cid if your going to get that thing to spool. You'll be sitting at the line so long you'll cook the tranny. My buddie Mike Keenan is sponsored to Turbo People. He runs a 106mm on a 400 incher. Takes a while to spool up but when it does, the thing is a rocket. 3600lb car, and runs 8.0's @181mph. It's a 99 Silver Saleen. He runs in Wild Street.

 

It should spool fine if I leave off a transbrake and have a convertor that keeps it in the rpm powerband through the 1/4 mile, no?

 

No, That thing will come on past the 60' mark and you'll just blow the tire away. Way too much power coming on all at once. I guess the only good thing about that is you'll have a insane MPH. Too much lag on a turbo that big to just leave off the transbrake. Look at it like this. You'll leave the line with lets say 300rwhp. Then 60 feet out the turbo comes on in less than a second and there's another 5 to 600 hp at the tire. You'll be skating have way down the track. It's cool to see and hear, but makes for one hell of a ride. If your going to destroke it, I wouldn't go bigger than a 91mm. That's enough turbo to run low 8's depending on race weight.

Neal

 

Here is an example of what Neal is talking about
http://home.comcast.net/~jsbird98/keenan.mov
its 5 megs but worth the wait, same mustang that is being discussed above.
J

 

Thanks Jeremy. the video say's it all. of course that thing makes 1800hp but you guy's get the point. Big turbo=Big lag. I'd still take it though.