• 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Jumpy ride at low revs
#1
Symptoms: Jumpy ride at low revs ( less than 1500 rpm)

Engine is tuned  to give ca 600-650 hp  and the cam is  a Mechanical roller Comp Cams Ex. Energy 242/248@0.050"
Toploader 4 sp. has wide ratio: 1st: 2.78, 2nd 1.93, 3rd 1.36 and 4th 1.00
Rear end is 1:3.55
Tires are Nitto 275/60 with  15 " wheels
Question:  is it possible to have  good  off the line acceleration  and decent  motorway revs  with  this set up with minor changes...
Currently  the problem is that low speed driving  is  jumpy as  the car won't  drive nicely  with low revs in the city / parking place.
Is this problem caused by the cam or the rear end ratio or both ?

The  1:3.55 rear end is a compromise  to give  decent  motorway  revs and lower  decibels....also the Toploader was  close ratio before but changed to wide ratio to give a shorter 1st gear.
  Reply
#2
What are you running for a clutch? What is lift on the cam? Often times the jumpyness at low RPMs is due to very high lift cam and an aggressive clutch. I have the same issue and find I have to keep the revs a little higher a lower speeds or feather the clutch a little more to avoid this. What is your idle set at? With higher lift cams your typically see less torque and power down low which coupled with an aggressive clutch will cause the jumping.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason


[Image: 082-hot-rod-power-tour-2017-1970-1970s.jpg]
  Reply
#3
Your problem is the camshaft, that is a fairly aggressive grind, causing a very lumpy idle, good for the dragstrip, not very good for the street, unless you're willing to put up with the low RPM problems. If you want to keep the 4-speed and want better low RPM manners you need to get a camshaft that is in the 230° @ 0.050" range. That will give you good low end, off idle power, as well as a decent top end. It will still have a little lope at idle. A lower rear-end ratio will give you a higher RPM at low speeds, and better low speed manners, but lose your highway cruising. To keep that camshaft you need a lower rear end and a lower 1st gear, to keep a good highway cruise you'll then need to get an overdrive. With a 503 cubic inch engine you don't need that much cam, rely on the engine's ability to produce a lot of torque.

Unless you like the clatter and like adjusting valve clearances I would also go with hydraulic lifters. If you like the sound, get Rhoads hydraulic lifters, not as loud as solids, but you'll still hear them, plus they'll smooth out your idle a little.

The spreadsheet in this thread may help you
https://www.7173mustangs.com/thread-cams...#pid278395



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
  Reply
#4
Nature of the beast.

Guessing that it happens when you're trying to idle through a parking lot with the clutch engaged in first and the cam starts lopeing at low RPMs. Yeah, it's not fun, but you didn't build a stock engine, so it won't behave like one. I usually get the car rolling, then slip it out of gear and let it roll.

You have a large tire and what I'd consider borderline rear gears for the setup. The 2.78 first helps with the takeoff and the torque of the big block helps quite a bit, but you effectively have a 3.25 gear when corrected to a stock 26" tire.

You can try messing with idle RPMs, mixture and timing to make it a little "softer", but primarily it's the cam. You have to learn to live with it or change it out.


  Reply
#5
(08-28-2019, 09:16 AM)Don C Wrote: Your problem is the camshaft, that is a fairly aggressive grind, causing a very lumpy idle, good for the dragstrip, not very good for the street, unless you're willing to put up with the low RPM problems. If you want to keep the 4-speed and want better low RPM manners you need to get a camshaft that is in the 230° @ 0.050" range. That will give you good low end, off idle power, as well as a decent top end. It will still have a little lope at idle. A lower rear-end ratio will give you a higher RPM at low speeds, and better low speed manners, but lose your highway cruising. To keep that camshaft you need a lower rear end and a lower 1st gear, to keep a good highway cruise you'll then need to get an overdrive. With a 503 cubic inch engine you don't need that much cam, rely on the engine's ability to produce a lot of torque.

Unless you like the clatter and like adjusting valve clearances I would also go with hydraulic lifters. If you like the sound, get Rhoads hydraulic lifters, not as loud as solids, but you'll still hear them, plus they'll smooth out your idle a little.

The spreadsheet in this thread may help you
https://www.7173mustangs.com/thread-cams...#pid278395
Thanks,
 what would happen if I   change the rear end to ... say...1 :3.9 ? Any better  at low revs ? My original idea was  to go with this   present setup to a quarter mile but did not realize that it  ( the aggresive cam) will effect so much to the low speed behavior. I fully agree  with the benefits of a hydraulic cam....
  Reply
#6
A 3.90 rear would help, as would a smaller diameter tire. As mentioned the stock size in 26" diameter, the tires you have at 28" diameter. With that said maybe go a 4.11 rear and an OD trans like a TKO600. You are still likely going to have some chugging issues at very low speeds but will be a huge improvement over your current set up plus more comfortable revs on the highway.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason


[Image: 082-hot-rod-power-tour-2017-1970-1970s.jpg]
  Reply
#7
You'll need to decide if you want a dragstrip car, or a steet/strip car, or just a street mannered car. You won't like it on the highway with a 3.9:1 rear end, the RPMs will be too high, lot of noise and poor economy. It will behave a little better at 15kph, because the engine will be running at a higher RPM. I have a 3.73:1 and will be going to an overdrive transmission.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
  Reply
#8
OK... you got me ! Smile
either a new milder cam - probably a hydraulic one - or a Tremec TKO 600.

Can you roughly describe what would I loose in a quarter mile when going to a 230 @ 0.050" hydraulic cam withe otherwise the same set up.
[Image: IMAG0252.jpg]
  Reply
#9
I would go with the 4.11 and the TKO. you will likely improve 1/4 mile time and be able to drive at slower speed. Smile best of both worlds. I also do what Hemi Killer mentioned in marking lot and get it moving and then just let it roll.

73 Grandé H Code. Headman long tube headers, T-5 Transmission, 3.70 Traclok, Lowered 1" all around, Aussie 2v heads w/ 2.19 intake, 1.71 exhaust, screw in studs, full roller cam 608/612 lift 280/281 duration LSA 112, Quick Fuel 750 CFM double pumper, AirGap intake.

- Jason


[Image: 082-hot-rod-power-tour-2017-1970-1970s.jpg]
  Reply
#10
It's really hard to tell, maybe none. The gear ratio you're running now isn't what should be run with that cam, a 4.10 or lower would match it better.

What size of rear tires are you running, do you have traction bars and sub-frame connectors?



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
  Reply
Share Thread:  


Possibly Related Threads…
Thread Author Replies Views Last Post
  351C Stumbling at low RPM TheDude 21 1,296 12-19-2018, 11:53 AM
Last Post: barnett468
Wrench low oil pressure or bad sensor or something else John J 3 683 07-13-2018, 07:29 AM
Last Post: Blue Mach 429 CJ
  429 headers that dont hang too low? RWR 7 1,141 02-06-2016, 10:57 AM
Last Post: MotoArts
  High revs on start up Thin Lizzy 25 2,474 11-27-2014, 06:09 PM
Last Post: c9zx
Stop how much can h.p. I build a 351c2v up to and still have a reliable ride nightmare stang 9 1,500 09-09-2014, 01:54 PM
Last Post: Mister 4x4
  Is low compression ok? cazsper 6 1,033 11-30-2013, 09:55 PM
Last Post: cazsper
  Uh Oh! Low oil pressure jmcgill89 21 6,099 07-04-2013, 07:11 PM
Last Post: Totalled
  PerTronix / Relay issue at low RPM akherber 4 1,784 09-04-2011, 06:25 PM
Last Post: akherber



Users browsing this thread: 1 Guest(s)