Can cam support gear change?

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plumkrazee70

plumkrazee70

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70 Dart 340 rebuilt, compression coming in right at 9.8:1. With the lunati 703 cam. Tire height is 25.7" (245/45/17) Currently have 4.10 gears with Hemi OD 4 speed gears. Ratios are:

1st: 2.66
2nd: 1.59
3rd: 1:0
4th: .80

I'd like to have more drivability on the hwy. Was thinking about 3.23, but not sure if my cam will like that high of gear.

Thoughts?

Screenshot_20230411-064903.png
 
So do you have it put together and running it with this cam and 4.10 gear combo right now and it works well, or this is yet to be tested?

In general you have more flexibility with rear gears in a stick car because you control the motor rpm at gear changes, but I really don't believe there is a perfect max performance, good fuel economy to be had. It's always a compromise, but for me at this point in life I question how many miles a year that I'm actually going to drive it. It's not like I'm commuting to work every day on the freeway in my hobby car, so I lean toward the bigger performance gears and enjoy the car more.
 
Outsder said:
So do you have it put together and running it with this cam and 4.10 gear combo right now and it works well, or this is yet to be tested?

In general you have more flexibility with rear gears in a stick car because you control the motor rpm at gear changes, but I really don't believe there is a perfect max performance, good fuel economy to be had. It's always a compromise, but for me at this point in life I question how many miles a year that I'm actually going to drive it. It's not like I'm commuting to work every day on the freeway in my hobby car, so I lean toward the bigger performance gears and enjoy the car more.
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In the car and running. It performs fine (I don't have anything to compare it to) It is at 3k rpm at 65.

however at this stage in my life, I just want a nice cruiser that I can drive everyday or close to it.

When I started to build the car (going on 10 years ago) Plans were different. Lol.
 
Just the opposite view I have; I figure my remaining time is short, so I want to do it one more time while I can. LOL

It's not a huge cam you have, but you don't have a lot of compression and probably don't have a lot of pop below 2k rpm which I would consider good mannered street driving rpm. You might want to go to a smaller cam when you change the gears to make it a better, more useful power combination for street cruising.
 
Outsder said:
Just the opposite view I have; I figure my remaining time is short, so I want to do it one more time while I can. LOL

It's not a huge cam you have, but you don't have a lot of compression and probably don't have a lot of pop below 2k rpm which I would consider good mannered street driving rpm. You might want to go to a smaller cam when you change the gears to make it a better, more useful power combination for street cruising.
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Yea, I was afraid of that. Maybe I'll just leave it as is.
 
I have a 230@50 480 lift in my car with 3:21 gears and a tire like yours, and mine runs good. If you go to a 3:55 gear, I bet you will like it.
 
Your cam isn’t that big. [email protected] can take the 3.23 gear just fine, especially with a 4 speed. I wouldn’t do it in a heavy car but an abody will be ok. Is it a perfect match? Nope. But it isn’t terrible either.
 
plumkrazee70 said:
70 Dart 340 rebuilt, compression coming in right at 9.8:1. With the lunati 703 cam. Tire height is 25.7" (245/45/17) Currently have 4.10 gears with Hemi OD 4 speed gears. Ratios are:

1st: 2.66
2nd: 1.59
3rd: 1:0
4th: .80

I'd like to have more drivability on the hwy. Was thinking about 3.23, but not sure if my cam will like that high of gear.

Thoughts?
Click to expand...

You need an online calculator to figure out the rest.
You need to be honest with your cruising speed.

3.23 X .80 = 2.56 final drive ratio.

With that math done, add in tire size and speed you want to travel at on the calculator. This will give you the RPM. For automatic transmission guys, converter slip not included but it maybe an option on the calculator. The known slip percentage of your converter is what?
 
rumblefish360 said:
You need an online calculator to figure out the rest.
You need to be honest with your cruising speed.

3.23 X .80 = 2.56 final drive ratio.

With that math done, add in tire size and speed you want to travel at on the calculator. This will give you the RPM. For automatic transmission guys, converter slip not included but it maybe an option on the calculator. The known slip percentage of your converter is what?
Click to expand...
Exactly! DeJa Vous from a thread yesterday.
 
3.23 with a 0.8 OD and a 26.5" tire would put you at 2130rpm at 65, 2290rpm at 70.

How's the car handle hills right now at that rev range (1900-2300)? If it can accelerate decently starting from 1700-2000 rpm up a steep hill with mulitple passengers in 4th gear, then dropping to 3.23's might not be so bad.

3.55 would be even less bad, and still get you under 3k at 65 (~2350). 3.91's would be around 2600 @65, and at 3k you'd be at 75mph. Personally I'd pick 3.91's.

If you're at all concerned about how the engine will perform at lower revs, I'd make a smaller change in gear. Even the 3.91's will make a substantial impact to your cruise rpm. I know my car at 2600 is pleasant, but at 3k it's starting to rattle the ears and by 3500 it's annoying for any length of time (I have 25.5" tall tires, 3.91's and no OD, so 70mph is 3600rpm, 65 is 3350). I've got a 408 with a bunch more cam (295 adv) and it will pull from 2k, but it's much happier if I keep it over 2200. The smaller cam I used to run (284 adv) was fine down to about 1800rpm.
 
Overdrives have spoiled us .
Commercial engines used to get started, moved to the throttle stop, and buzz at 3 - 4k all day, generators, boats, compressors .
 
rumblefish360 said:
You need an online calculator to figure out the rest.
You need to be honest with your cruising speed.

3.23 X .80 = 2.56 final drive ratio.

With that math done, add in tire size and speed you want to travel at on the calculator. This will give you the RPM. For automatic transmission guys, converter slip not included but it maybe an option on the calculator. The known slip percentage of your converter is what?
Click to expand...

Oh trust me ,I've played around with the calculators. 3.23 seemed like the best.

70 mph is 2364 (3.23)

70 mph 2599 (3.55)

I like the rom of 3.23 but not sure if my cam will be in happy at that rpm.
 
Phreakish said:
3.23 with a 0.8 OD and a 26.5" tire would put you at 2130rpm at 65, 2290rpm at 70.

How's the car handle hills right now at that rev range (1900-2300)? If it can accelerate decently starting from 1700-2000 rpm up a steep hill with mulitple passengers in 4th gear, then dropping to 3.23's might not be so bad.

3.55 would be even less bad, and still get you under 3k at 65 (~2350). 3.91's would be around 2600 @65, and at 3k you'd be at 75mph. Personally I'd pick 3.91's.

If you're at all concerned about how the engine will perform at lower revs, I'd make a smaller change in gear. Even the 3.91's will make a substantial impact to your cruise rpm. I know my car at 2600 is pleasant, but at 3k it's starting to rattle the ears and by 3500 it's annoying for any length of time (I have 25.5" tall tires, 3.91's and no OD, so 70mph is 3600rpm, 65 is 3350). I've got a 408 with a bunch more cam (295 adv) and it will pull from 2k, but it's much happier if I keep it over 2200. The smaller cam I used to run (284 adv) was fine down to about 1800rpm.
Click to expand...

I have a 25.7" tire. So your calculations are a tad off. The thing accelerates like crazy now. I'm out of first gear in a hurry, figured that was due to the 4.10s. I'll play around with it and see in the lower RPM range.

3.55 - 2599
3.23 - 2364
 
plumkrazee70 said:
I have a 25.7" tire. So your calculations are a tad off. The thing accelerates like crazy now. I'm out of first gear in a hurry, figured that was due to the 4.10s. I'll play around with it and see in the lower RPM range.

3.55 - 2599
3.23 - 2364
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D'oh, I missed that you put it in your OP and just shot for the size I plan to use in the future.
Difference should only be about 3%, or 93rpm off at 3k, 62 rpm off at 2k. Personally, I'd still choose 3.91's, but that's just me.

I'm sure it accelerates like a rocket right now. My question was more what does it do under load in high gear at around 2k rpm. If it bucks, surges, or is otherwise a bit lazy that low in the revs (in high gear) then that should give you some indication of what the car will be like if you try to cruise it there at higher speeds. If it can pull from 2k in 4th on a steep hill, then you should have no issues cruising at 2k on the highway. I call this type of a thing a 'sanity check' since it's easy to get lost in pages of theory.

I suspect your cam isn't too big to run the 3.23's, but pulling a hill from 2k in 4th would help confirm it.
 
Phreakish said:
D'oh, I missed that you put it in your OP and just shot for the size I plan to use in the future.
Difference should only be about 3%, or 93rpm off at 3k, 62 rpm off at 2k. Personally, I'd still choose 3.91's, but that's just me.

I'm sure it accelerates like a rocket right now. My question was more what does it do under load in high gear at around 2k rpm. If it bucks, surges, or is otherwise a bit lazy that low in the revs (in high gear) then that should give you some indication of what the car will be like if you try to cruise it there at higher speeds. If it can pull from 2k in 4th on a steep hill, then you should have no issues cruising at 2k on the highway. I call this type of a thing a 'sanity check' since it's easy to get lost in pages of theory.

I suspect your cam isn't too big to run the 3.23's, but pulling a hill from 2k in 4th would help confirm it.
Click to expand...
That's a smart way of testing it.
 
Phreakish said:
D'oh, I missed that you put it in your OP and just shot for the size I plan to use in the future.
Difference should only be about 3%, or 93rpm off at 3k, 62 rpm off at 2k. Personally, I'd still choose 3.91's, but that's just me.

I'm sure it accelerates like a rocket right now. My question was more what does it do under load in high gear at around 2k rpm. If it bucks, surges, or is otherwise a bit lazy that low in the revs (in high gear) then that should give you some indication of what the car will be like if you try to cruise it there at higher speeds. If it can pull from 2k in 4th on a steep hill, then you should have no issues cruising at 2k on the highway. I call this type of a thing a 'sanity check' since it's easy to get lost in pages of theory.

I suspect your cam isn't too big to run the 3.23's, but pulling a hill from 2k in 4th would help confirm it.
Click to expand...

Thank ya sir. So test in 4th, even though it's the OD gear? Or should I test in 3rd?
 
plumkrazee70 said:
Thank ya sir. So test in 4th, even though it's the OD gear? Or should I test in 3rd?
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Yup, in 4th. It will give you the best 'simulated' feel of the power available in that range. Your 4.1 with a .8 OD is about like a 3.28 ratio - so the power available in 4th at 3k will be very close to the power of a 3.23 in 3rd after you swap.

I'd play around with a few WOT pulls in 4th from varying RPMs as a 'passing test' to see how the car feels at 2k, 2300, 2500, 2700, etc. If you find that certain ranges are a no-go, then gear accordingly.
If you can comfortably pull from 2k in 4th on a good hill then 3.23's should be no problem on flat ground at speed. If it lugs too hard for your liking, then you may need more gear than 3.23, but less than 4.10. Finding which rpm range it pulls from will help make that call.

Obviously the lower you go, the longer you'll be pulling 1st-3rd, but I bet the difference won't be too noticeable for 3.55s, and probably just starting to be notable with the 3.23's. The shift to 3rd will probably be where disappointment shows up if you go too low.
 
MoparLeo said:
Borrow a set of taller tires and try it. Easy peasy...
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Unless his rear is moved back, the '70 wheel opening gets real tight with taller tires. I moved mine back 1/2" just for that reason. If I remember right, 28" is the tallest I could fit after the move and I'd probably have to do some trimming to keep the tire from making contact under compression/launch.
 
Phreakish said:
Unless his rear is moved back, the '70 wheel opening gets real tight with taller tires. I moved mine back 1/2" just for that reason. If I remember right, 28" is the tallest I could fit after the move and I'd probably have to do some trimming to keep the tire from making contact under compression/launch.
Click to expand...
255's fit nicely in 70 wheel openings. I do like the way they look moved back though.

002.JPG
 
Phreakish said:
Yup, in 4th. It will give you the best 'simulated' feel of the power available in that range.
I'd play around with a few WOT pulls in 4th from varying RPMs as a 'passing test' to see how the car feels at 2k, 2300, 2500, 2700, etc. If you find that certain ranges are a no-go, then gear accordingly.
If you can comfortably pull from 2k in 4th on a good hill then 3.23's should be no problem on flat ground at speed. If it lugs too hard for your liking, then you may need more gear than 3.23, but less than 4.10. Finding which rpm range it pulls from will help make that call.
Obviously the lower you go, the longer you'll be pulling 1st-3rd, but I bet the difference won't be too noticeable for 3.55s, and probably just starting to be notable with the 3.23's. The shift to 3rd will probably be where disappointment shows up if you go too low.
Click to expand...
of course its going to lug in 4th with a .80 ratio@2000 rpm with 226@50 cam
 
a-bodyguy said:
of course its going to lug in 4th with a .80 ratio@2000 rpm with 226@50 cam
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That's the point - he'll get a quick idea of what RPM the thing will actually want to run at with that cam and his actual combo. From there, it will be a much more informed decision.
 
plumkrazee70 said:
Thank ya sir. So test in 4th, even though it's the OD gear? Or should I test in 3rd?
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You do the test in 3rd 1.00 how many hills are you going to be running you cant pop it out of od for abit
i ran a 340 11 to 1 243@ 50 108 lsa 727 26.99 tire 3.23s it didnt lug at 2k
i also ran the same setup 4sp no issue in 4th@2000
with a 226 cam it should be no issue with 3.23s
What you have is a glorified 727 with no verter slip with slighty better ratios
 
a-bodyguy said:
of course its going to lug in 4th with a .80 ratio@2000 rpm with 226@50 cam
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Lug? In the OD gear I’d agree. That final drive ratio of 2.56 is super low. While at 70 mph, it’s just the beginning of that cams power range. Yea yea yea it says 1800 plus, but the reality is it has to ko e a box shape through the air at speed. It’ll be OK if it’s rpm-ing better than 2100, IMO. I’m running a 226@050 Comp Hyd. roller in the wife’s 11-1, 360 engine. At low rpm, it pulls but it doesn’t really take off. Once it starts to get above 2500, it picks up good and really goes well after 3K.

Not in a great rpm at cruise is what I’d call it. A tad low, IMO.
 
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