Torn........ 331 or 347??

[nucjd19]

Hey folks. Newbie here. Thanks for the excellent info. I have purchased the blueprint 347 (Carb) and TKO 600 combo for my build about 2 weeks ago. I spoke at length with the guys there about the 306 vs the 347. They really thought the 347 would be the right fit for my driving plan and overall gameplay for my build. I plan on using mine like MPTech. The price difference was about 800 dollars between the 347 and the 306. Also my wife loves here some torques when she is scooting around the back roads where I live ( she can autocross something fierce ). Anyway cool info and appreciated.

[CobraboyDR]

If we're talking "all things being equal" the 347 will have only slightly more torque "down low" as the poor rod ratio sucks a bunch of power on a 347 (see post 23 above). And it will be much harder on rings, pistons, and cylinder walls plus that extra friction is turned to heat the cooling system has to deal with. Offset pins will help to mitigate that but I didn't model that to quantify the result. The OEMs have been playing with offset bores and picked up a significant amount of power with engines constrained to short rods, but that won't help the poor short deck SBF.

I am told those bugs have been eliminated in 347 stroker kits.

[NAZ]

It's not a bug, it's geometry -- has to do with the extreme rod angle. Short rod & long stroke equals extreme rod angle which during combustion forces the piston hard into the cylinder wall.

Not the end of the world, but it is something to consider when building an engine as there are drawbacks to short rod ratios. The 302 based strokers all have the same low 8.206" deck constraints and require low compression height pistons which will limit power capability due to piston durability.

The takeaway is: there's more to power output than just adding cubic inches. Building an engine is an exercise in balancing compromises.

[GoDadGo]

My Dark Side Chevy Examples:
1. A stock 400 SBC from the 1970's had a 3.75" stroke crankshaft with crazy short 5.565" rods, that loved to break, which yielded a Rod Angle Ratio of 1.48.
2. My 383 SBC (All Dart Engine) which can be easily punched to 400 cubes shares that same 3.75" stroke; however, my rods are 6.0" long so my RAR is 1.60.
3. I went with the longest rod possible in an attempt to get close to the RAR of the 350 Chevy which has a 3.48 stroke with a 5.7" rods which has a RAR of 1.64.

How It Equates To A SBF Mini Stroker Motors:
1. A 5.4" Rod wiht a 3.00 stroke yeilds a RAR of 1.80.
2. A 5.4" Rod with a 3.25 stroke yeilds a RAR of 1.66.
3. A 5.4" Rod with a 3.40 stroke yeilds a RAR of 1.59.

Just make sure that if you go with the 347 stroker that the sucker doesn't have the stock 5.09" stock rods because that would equate to a RAR of 1.50.
By the way, a 302 Ford with the stock shorty rods has a RAR of 1.7 in case this is something you'd like to know.

NOTE:..Please be advised that I am NOT an engine builder, just a Dark Side Banker who is obsessed with numbers and engines.
............My interest is specifically limited to our domestic American V8's and some inline 6 cylinder engines of various countries of origins.
............In addition, my extreme knowledge of engines was granted to me by sleeping at more than a few Holiday Inn Express Hotels over the years.

Good Luck From The Dark Dart Side!

[Hoooper]

Keeping the wrist pin out of the oil ring is obviously one thing to watch out for. After that, I would focus more on mean piston speed than rod angle ratio. The actual angle difference between at rod with RAR at 1.54 vs 1.8 really is not all that significant, but for reliability keeping the mean piston speed within reason for the quality of parts you are using is very important.

That said, if it were me with $7k to spend on a motor and want something around 400 HP, streetable, with good reliability and fuel injection, I would pick up a 376 from a 2008+ escalade with wiring harness, computer, and stick the other $3k back in my pocket.

[rich grsc]

Ya, like that'd just drop right in for a straight up swap. Sorta 100% opposite of what Mark is looking for.

[Hoooper]

He wouldnt need to modify the footboxes and should only need to weld a new flange on his headers + get a bellhousing adapter. I guess it depends on how much he likes that extra $3k

[edwardb]

[MPTech]

Hooper, Thanks, but ah................................................ ................................NO

I'm ok with spending the $3k to put a Ford in my Cobra. You are free to put whatever makes you happy in your Cobra.

Rich's ride and NAZ's comments have me re-thinking the 331, but I think Rich's 331 may be more performance than the Blueprint 331 I was looking at. Still undecided.

[bobl]

I think you guys are way over thinking the rod ratio deal. The difference between the 331 and 347 is minimal and would never be noticed in a street engine. Sure, as Naz stated a longer rod would be better (if it would fit in the block). But the benefit of increased stroke far exceeds the losses due to increased rod ratio. Both engines can make nearly the same power all else being equal, but bringing that power down by several hundred rpm in the 347 will be noticeable. That increased torque holds true all the way down the rpm scale. For example: 2 identical builds except for the stroke, at 1500 rpm the 347 will have roughly the same torque as the 331 at 1700-1800 rpm. You could definitely feel that in normal driving. My thoughts on the longer stroke being harder on pistons… yes it would be to some extent. However it will turn less rpm during the life of the engine to generate the same power so I believe it is somewhat of a moot point. I’ve built a ton of stroker engines and never seen issues. The pin overlapping into the oil ring groove is also not a concern. In all modern pistons that I’ve seen there is a support ring that goes on before the oil ring if required.
Attached is a dyno simulation of my 347, which I tweaked parameters so it closely matches the actual dyno results. I then changed the stroke to 3.25 with no other changes.

Bob

[CobraboyDR]

I think you guys are way over thinking the rod ratio deal. The difference between the 331 and 347 is minimal and would never be noticed in a street engine. Sure, as Naz stated a longer rod would be better (if it would fit in the block). But the benefit of increased stroke far exceeds the losses due to increased rod ratio. Both engines can make nearly the same power all else being equal, but bringing that power down by several hundred rpm in the 347 will be noticeable. That increased torque holds true all the way down the rpm scale. For example: 2 identical builds except for the stroke, at 1500 rpm the 347 will have roughly the same torque as the 331 at 1700-1800 rpm. You could definitely feel that in normal driving. My thoughts on the longer stroke being harder on pistons… yes it would be to some extent. However it will turn less rpm during the life of the engine to generate the same power so I believe it is somewhat of a moot point. I’ve built a ton of stroker engines and never seen issues. The pin overlapping into the oil ring groove is also not a concern. In all modern pistons that I’ve seen there is a support ring that goes on before the oil ring if required.
Attached is a dyno simulation of my 347, which I tweaked parameters so it closely matches the actual dyno results. I then changed the stroke to 3.25 with no other changes.

Bob

Agreed.

I totally understand NAZ's perspective in a racing engine which run balls-to-the-wall constantly where squeezing out the last hp & RPM is critical to winning or (as Ricky Bobby says) "First Loser."

But a street engine is a different animal. It will rarely suffer the same physical forces for as long as a race engine. And the additional grunt down low makes the street experience enjoyable.

I'm unaware of the 347 engines so popular in street Cobra replicas having major mechanical issues because of RAR. Even the FFR preferred engine builder offers a substantial warranty on their 347.

That said, the RAR discussion is educational, and I appreciate NAZ's input. I learn a ton from specialist geeks. Thanks!

[Duster]

I went with the Blueprint 347 and the Holley Sniper with the TKO 600. Fired up instantly, has a ton of torque and plenty of grunt. Have a power curve similar to what Papa posted.

All this and a warenty!