5 Speed Gear ratio advice?
your pretty close,,,It starts real nice,,
340 +.030, 10.1 cr, flat tops zero deck, .039 quench, roller 222/226 @ .050, .550"/.548" with 1.6 rockers, Eddl RPM's closed chamber, Air Gap and FiTech,,, 21.59 MPG cruising with one dig with a Jeep GC SRT8,,,I'm satisfied but looking for more of course.!!!
Oh, I shoulda guessed roller. That'll bump your VP up and make your starter-gear bearable.
I ran a practically identical combo but on a 360 base, and no roller. That was a dynamite combo, and by far my favorite. I ran it with 3.55s and no EFI. So practically identical,lo. At least from the decks to the top of the intake.
But as to your combo, I think as a streeter the engine is pretty much optimized. I guessed that roller was a 262 advertised, and that might look like this
Static compression ratio of 10.2:1.
Effective stroke is 2.72 inches.
Your dynamic compression ratio is 8.55:1 .
Your dynamic cranking pressure is 174.25 PSI.
V/P (Volume to Pressure Index) is 148.............................................148
So the next bigger cam would drop your Dcr and VP
Static compression ratio of 10.2:1
Effective stroke is 2.66 inches.
Your dynamic compression ratio is 8.38:1 .
Your dynamic cranking pressure is 169.79 PSI.
V/P (Volume to Pressure Index) is 141............................................141
It doesn't look like much on paper, but Everything is going in the wrong direction.
Your heads and Q will accept over 9/1 Dcr and still run pumpgas. I ran 8.9Dcr/11.3Scr on 87E10 for about 8 months, then tore my 367 down for inspection. She was good to go back together. Others here have said they also run over 9/1Dcr.and over 200 psi.So your current combo is about right.
The Dropping VP is a little concerning to me. I swapped out that
[email protected] for a
[email protected], just one cam size difference, and lost 9 points of VP, and I could
really feel it. I was already running 3.55s, and no overdrive, so I had to do something. My solution .....eventually, was to get a 3.09 low box AND an overdrive, and still the 3.55s. That restored my take off power, and reduced my cruise rpm simultaneously. I found the same thing when going from the 292/108 Mopar cam, to a 270/110 Hughes cam, again of 19 points of VP, at close to the same Dcr. That is what I expected, and it was awesome. I swapped out the 4.30s that the Mopar cam liked, back to the 3.55s. which the Hughes liked.
What I learned was that 19 points of VP was worth 21% gear. And 9 points was worth 10% gear.
So knowing that,now, I can confidently say that in your combo, the 7 point change in VP is gonna demand a 7% increase in gear. That comes to 3.46 and rounds to 3.55s.
VP is just a measuring stick to help predict low rpm performance. Which, IMHO, is pretty important for a streeter.Cuz we spend most of our time in that Zone. You for instance, with 3.23s can take those, to 28mph @3000rpm. Then you shift and will be in second at 2000. So that's all lowrpm stuff. You better have some VP there to pull away cleanly.
On the other hand the next bigger cam would move your power peak up about 200rpm, and with that and the Eddies, you'll pick up about 20 horsepower...... at 5200rpm. That's a nice kick at 44mph with 3.55s. Of course the power begins increasing at maybe 3000/25mph in first, but you're not likely to begin feeling it until 4400/38mph still in first.
SO IMO, if you have to buy gears anyway, to take advantage of that bigger cam, you might as well do that first. And with the .69od you might as well optimize them for the current cam. And when you do that, man that 340 is gonna jump! So then you will forget all about more cam for a few years.
So what gears?
Well it depends on what you're gonna do with the car. I can tell you my opinion and you can take it from there.
With a VP of about 148, (remember I had a 143 that liked 4.30s), and the .69od, IMO, 3.91s would be the minimum with a 2.66 low. That will get you a starter gear of 10.4, which is 21% more TM than current. That's a lot of additional jump. I currently run a 10.97 starter with a VP of 153. It's a little more than I need, so it'll be just right for you. Your cruize rpm will still be pretty good at 65=2180. I run 2240 and am very happy with that. So now you have both ends covered. From here you can let those fabulous splits fill in the blanks.
Now you should know that those 3.91s are gonna let your 340 buzz up to the shift rpm pretty quick. What that means is she'll be putting down more average HP per unit of time than she currently is. And that means a lower zero to sixty time. And those 3.91s are gonna let you hit 60 at 5200 just about where your current combo is peaking. You could run a little more gear, but 4.10s will jump your cruize rpm up about another 5%.
To recap; slam some 3.91s in there and never look back. Probably less than $500 installed. Maybe tighten up your SureGrip while it's out.
So that's my opinion, and it's on the cheap.
OK wait, if your still not sold,Here's another argument;
Say your engine makes 400 peak ftlbs and your trans multiplies that by 2.66 and your rearend multiplies that by 3.23. And lets ignore any powerloss thru the gears for now. That totals 3437ftlbs at peak.
Well with the 3.91s it totals plus 21% or 4160. That maths out to an additional 723 ftlbs. And every single gear will show the same 21% increase.
At the torque peak say 3900rpm, this will feel like 400 x 21% x 3900 /5250 =62 more hp. That's about triple of what just one cam size will feel like, with the 3.23s.
Case closed.
