Change Rear End Gear Ratio

[mark69sportsroof 31]

Ok let me add a few more items to clarify my engine.  The cylinder compression tested at 145 psi.  Pro-Comp 4340 forged I-beam rods, Mahle 4032 forged pistons, hiperf rod bearings, hiperf hd cam bearings and hyd lifters.   The block was magnaflux, shot blast cleaned, sonic map block cylinder walls and blue printed.  I have the Quickfuel "Hot Rod" 750 CFM carb with mechanical secondaries.  The carb comes with 72 jets in front bowl and 82 in rear bowl.  When it was dyno we had to change the front jets to 64 and the rear to 72 jets.  The reason for changing the jets was to correct to air/fuel ratio.  It was running way to rich.  The was 2 guys at the dyno.  One who just runs the dyno test and the second did all the adjustments.  He also is a engine builder and was a NASCAR crew chief for 20 years.

They were concerned about the 64 jets in the front bowl being to low and would cause pinging.  Now remember I'm not even a journeyman mechanic so most of this is over my head.  I took it for a drive and it ran strong and responsive.  When I got back to the dyno both guys were anxious to see how it ran.  I told them nothing and handed them the keys.  They took it out and came back dumbfounded that it ran great and no pinging.  They tried to get it to ping but couldn't.  Later that week I my engine builder and the mechanic that does work on it were also very suspicious of the 64 jets.  Both took it out for a drive and drove it hard, high RPM's and low speed in high  tranny gears and couldn't get it to ping.  I asked them what would they change and both said absolutely nothing.  Both are now believers.    

The altitude and thinner air really does affect the building, tuning and performance of cars at 5000 - 10,000 feet.  I drive it over 3 mountain passes that are of 11,000  feet.

Sounds like I will just leave the rear gears alone and just drive it.  Thanks Mark 

 

 

   

[1969_Mach1 333]

Someone with more engine building experience than me will eventually chime in (like Barnett).  I'm no expert, but 145 psi is kind of low for an 11:1 CR motor.  I imagine it's because of a long duration and long valve overlap cam.  Either that, or somebodies math was off when selecting parts.  Yes, a number 64 main jet sounds small.  Especially for that large of a motor.  Strange why such a small main jet is needed.  The cylinder pressure might be contributing to why it is not pinging.  Like I mentioned somebody with more experience should eventually chime in.

[barnett468 418]

3 hours ago, mark69sportsroof said:

5000 - 10,000 feet.  I drive it over 3 mountain passes that are of 11,000  feet.

Sounds like I will just leave the rear gears alone and just drive it.  Thanks Mark   

ok there is part of the answer as to why it only has 145 compression which is exactly why it doesn't ping. the other reason it has only 145 compression is because the cam has an advertised duration of 316 degrees, which is in fact about the highest duration i have ever seen in a cam.

i would ask them if they also set the timing "curve" on the dyno to get optimum performance.

 

if it was me i would consider the cam below. it will also increase your compression a little. add 3.43 or 3.50 gears and it will have a lot more low end pulling power.

http://www.lunatipower.com/Product.aspx?id=8612&gid=289

• Advertised Duration (Int/Exh): 282/290
• Duration @ .050 (Int/Exh): 231/239
• Gross Valve Lift (Int/Exh): .611/.611
• LSA/ICL: 110/106
• Valve Lash (Int/Exh): Hyd/Hyd
• RPM Range: 2600-6600
• Includes: Cam & Lifters (#72440-16)

 

 

 

 

 .

 

[barnett468 418]

I forgot to mention that air density will also affect compression and therefore power etc, and the higher the elevation, the more the air density can change throughout the year, so if you did a compression test in the summer when it was 90 degrees out, the compression would be lower than if you did it in the winter when there is snow on the ground and it is 28 degrees outside.

At the very least, If you don't drive long distances at high speed, installing the next numerical higher set of gears will make a noticeable change in power feel and acceleration but it won't be a dramatic change, and gears are less involved to change than the cam but if it was me i would still yank that cam out as it really is not the best one for your app imo. Also, it has very little lift for a big cube engine and more lift will give you more torque if all other aspects of the cam remain the same which isn't really possible but it can be reasonably close.

 

 

 

 

[mark69sportsroof 31]

Thank you Barnett468 once again for the additional information.  So if I understand everything correctly the best option would be change the cam.  Second would be changing the rear end gears.  Thanks to all.  Mark  

[barnett468 418]

17 minutes ago, mark69sportsroof said:

Thank you Barnett468 once again for the additional information.  So if I understand everything correctly the best option would be change the cam.  Second would be changing the rear end gears.  Thanks to all.  Mark  

that's what i would do in your particular case. the engine builder probably built it how he thought you wanted it. we have no idea exactly what you both talked about so all we here can do now is make suggestions based on what you tell us which is why we ask a lot of questions. you might want numerically higher gears even if you install the cam, it's just hard to know exactly what someone wants sometimes, but irregardless, i never would have used a cam with 316 degrees of advertised duration but i'm sure your builder had a reason for doing what he did.

you might even like a cam one size smaller then the one i suggested but it would knock a lot of rpm's off the top end, plus it might raise the compression enough to make it ping some.

 

the single pattern cams tend to operate over a narrower rpm range than a dual pattern one like the one i mentioned also, so the single pattern ones have traditionally been more for drag racing.

 

 

 

 

[mark69sportsroof 31]

Hey Barnett468, could you explain to me what the "Duration Int/Exh" means and the difference from my 316 to the 231/239 you suggested.  Thanks Mark

[barnett468 418]

59 minutes ago, mark69sportsroof said:

Hey Barnett468, could you explain to me what the "Duration Int/Exh" means and the difference from my 316 to the 231/239 you suggested.  Thanks Mark

Yes but i have to wait until i get my computer back from the repair shop which should be monday.

[barnett468 418]

On 11/23/2018 at 7:10 PM, mark69sportsroof said:

Hey Barnett468, could you explain to me what the "Duration Int/Exh" means and the difference from my 316 to the 231/239 you suggested.  Thanks Mark

I am not a cam scientist but I know a reasonable amount, plus I have built many engines and done cam swaps, so I know what the cams did in those engines. You can also do a google search for cam info but you will find some info that appears to be conflicting. I have not found a site that explains cams well in simple terms and the problem is that if you change one aspect of a cam, it automatically changes another to some degree which complicates explaining them. 

In addition to the differencs I already mentioned, a short explanation that is relevant to your situation is that the advertised duration is the total time the cam keeps the valves open. The longer they are open the less compression the engine will have. In general, the longer the advertised duration and duration @ .50" lift is, the higher the overall operating range of the engine. For example, your cam with durations of 316 advertised and 244 @ .050" might have an operating range of 3500 to 6500 in a 454 cube engine and a cam with less durations like 290 advertised and 232 @ .050" might have an operating range of 3000 to 6000 rpm in the same size engine.

The farther apart the intake and exhaust lobe peaks are, which is referred to as LSA (lobe separation angle), the wider the powerband will be and the "softer" it will be, and the higher the engine will rev.

The greater the lift is, the more power it will produce up to a certain point, but this does not mean that one should use a cam that has .650" of lift in every engine. There is such a thing as too much lift for a persons intended use of the vehicle and/or for the particular heads they are using etc.

I frequently use cams with around .600" lift in 347 strokers. Your cam has around .040" less than that yet it has around 100 more cubes.

In your case, based on what it sounds to me like you want, more lift would give you more power/torque per se. Less advertised duration will increase the compression which will increase the torque and improve throttle response. Less duration @ .050" will lower the overall operating range. A wider LSA will give you a wider and softer power band but the softness will be countered some with the shorter advertised duration and increased lift.

Also, lowering the overall operating range of the engine will also make the existing diff gear ratio more useful, but as I mentioned, you may still want numerically higher gears even if you do install a different cam.

[mark69sportsroof 31]

Thanks Barnett 468.  I had no idea what it meant.  Your explanation is easy to follow.  I will follow up with my engine builder and see why he used that one.  Thanks Mark