Porting Timing for Torque

[Glastron1987]

Been reading a few things on porting and can't say I really understand it that well. From what I can tell it seems narrower carbs and more compression will add to low end torque. Also, it seems lower exhaust porting is probably good for low end torque.

Anyone know which 2.5 engine is ported to favor low end torque? Namely, would it be the 135, 150, 175, 200, XR6, EFI, Optimax, ProXS, etc?

[FORBESAUTO]

In a nut shell on port timing, important factors are port duration on both intake and exhaust ports, and blowdown degrees ( amount of degrees from the point when exhaust port begins opening until the transfers/ intake open.) Both duration and blowdown degrees affect the rpm range where max power/ torque is achieved. Lower durations/ shorter ports make power at lower rpm while longer durations/ higher ports are needed to make power at higher rpms. Even more crucial on what rpm max power is achieved, or even if any power is made dependent on transfer duration is blowdown. Higher degrees of blowdown duration is needed to make power at higher rpms to allow proper scavanging/charging of cylinder at quicker piston speeds but at lower rpms will cause a loss of charge mixture out the exhaust killing low end power. Both durations and blowdown have to work hand in hand for an engine to be efficient at particular rpm.

[keefallan]

As to carbs, smaller carbs do help lower torque as well. When I was messing with carbs on my 150 crossflow, there was a significant difference in holeshot with 1-1/4 carbs versus the larger 5/16 or 3/8 carbs. But it also ran outta steam up top too with the smaller carbs.

[Glastron1987]

In a nut shell on port timing, important factors are port duration on both intake and exhaust ports, and blowdown degrees ( amount of degrees from the point when exhaust port begins opening until the transfers/ intake open.) Both duration and blowdown degrees affect the rpm range where max power/ torque is achieved. Lower durations/ shorter ports make power at lower rpm while longer durations/ higher ports are needed to make power at higher rpms. Even more crucial on what rpm max power is achieved, or even if any power is made dependent on transfer duration is blowdown. Higher degrees of blowdown duration is needed to make power at higher rpms to allow proper scavanging/charging of cylinder at quicker piston speeds but at lower rpms will cause a loss of charge mixture out the exhaust killing low end power. Both durations and blowdown have to work hand in hand for an engine to be efficient at particular rpm.

Excellent explanation. So if I understand that correctly an engine with lower port duration and lower blowdown degrees would be the one I am asking about. Of the V6's, which ones come stock towards that end of the scale?

[FORBESAUTO]

In the 2.5 family the 150’s make good low rpm torque, EFI versions better and opti makes the best. The higher hp models like the 200 make more torque but at little higher rpm. The 150’s make great working motors because of this.

[mach351]

Xr6

small chest, lowest port timing, longest tuner on the planet, and tiny venturis on the carbs....while still at 2.5L of displacement. They run good.

[racer]

Another item not mentioned is the angles on the ports. The top roof angle on the intake ports also has an effect on where power is made.

[Glastron1987]

So being a skier and barefooter it sounds like a 150 carb, XR6, 150 EFI, or 150 Opti would probably be preferred over a 200 really?

[FORBESAUTO]

So being a skier and barefooter it sounds like a 150 carb, XR6, 150 EFI, or 150 Opti would probably be preferred over a 200 really?

Kinda depends, the 200 will make more torque just doesn’t have as much right off idle and getting up. I’d guess somewhere around the 3000 rpm range things change and the 200 will begin making more power. So depends on propping and set up and what’s the most crucial rpm for ya which will work best for ya.

[FORBESAUTO]

Personally a 3.0 opti would be the ticket, if it’s an option.

[Glastron1987]

Kinda depends, the 200 will make more torque just doesn’t have as much right off idle and getting up. I’d guess somewhere around the 3000 rpm range things change and the 200 will begin making more power. So depends on propping and set up and what’s the most crucial rpm for ya which will work best for ya.

Got it, makes sense, thanks.

[Glastron1987]

Personally a 3.0 opti would be the ticket, if it’s an option.

Oh my, loads of torque on that one I'm sure.

[Merc 2.5]

So being a skier and barefooter it sounds like a 150 carb, XR6, 150 EFI, or 150 Opti would probably be preferred over a 200 really?

Get a stock ee 200. Lower port timing and longer exhaust runners in chest , add long tuner should have great torque,maybe keep compression to 135ish

[Glastron1987]

Get a stock ee 200. Lower port timing and longer exhaust runners in chest , add long tuner should have great torque,maybe keep compression to 135ish

What I think you are saying is that the stock 200 EE has lowered port timing and longer exhaust runners in the chest already, but probably has a short tuner and that should be replaced with the longer one for maximized torque... and no need to cut heads?

[FMP]

He said get an Etec/G2 lol