Jumpy ride at low revs

[Higgins56]

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/[email protected]"

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.

 

[73pony]

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.

 

[Don C]

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-camshaft-selection?pid=278395#pid278395

 

[Hemikiller]

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.

 

[Higgins56]

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-camshaft-selection?pid=278395#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....

 

[73pony]

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.

 

[Don C]

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.

 

[Higgins56]

OK... you got me !

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.

 

[73pony]

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. best of both worlds. I also do what Hemi Killer mentioned in marking lot and get it moving and then just let it roll.

 

[Don C]

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?

 

[73pony]

Tire size was in first post 275/60/15 or 28" tall also having negative effect on this.

 

[Don C]

Yes, that certainly didn't help his final effective ratio.

 

[donkost]

It's certainly a interesting question. My '71 Mach 1 429 CJ-R 4 speed (close ratio) has a bit of an aggressive aftermarket cam that does not prefer the lower RPM ranges. Personally I just put up with parking lot or other slow speed areas with things like coasting as suggested. That's me though, and I'm very rarely in a parking lot with that car. I take my drive and then it's back to my garage. Too many people here in the states that have no respect for parked vehicles. Maybe it's the same on the other side of the ocean. Unless you have a ton of money and can make all of these changes yourself with no labor charges, you could spend a small fortune and get back very little as far as positive results with low speed driveability. Some of them would be interesting changes to try regardless if you have the time/money. This is why the factory normally installs a mild cam. So Aunt Betty can slowly cruise around the grocery store parking lot without the car lurching all over. lol

 

[Higgins56]

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?

Hi Don,

the tires at the back are Nitto 555 275/60-15. Car has adjustable traction bars and shocks at the rear

By the way the flywheel is  Aluminium McLeod and the clutch is McLeod  twin disc RST.

it just occurred to me  that in case I change the cam to a milder one and the car stops "jumping" the speed would remain the same meaning that at a parking lot  it would  run too fast at idle ( 700 rpm) anyway without changing the rear ratio - right ?

 

[Higgins56]

It's certainly a interesting question.  My '71 Mach 1 429 CJ-R 4 speed (close ratio) has a bit of an aggressive aftermarket cam that does not prefer the lower RPM ranges.  Personally I just put up with parking lot or other slow speed areas with things like coasting as suggested.  That's me though, and I'm very rarely in a parking lot with that car.  I take my drive and then it's back to my garage.  Too many people here in the states that have no respect for parked vehicles.  Maybe it's the same on the other side of the ocean.  Unless you have a ton of money and can make all of these changes yourself with no labor charges, you could spend a small fortune and get back very little as far as positive results with low speed driveability.  Some of them would be interesting changes to try regardless if you have the time/money.  This is why the factory normally installs a mild cam.  So Aunt Betty can slowly cruise around the grocery store parking lot without the car lurching all over.  lol

Good comment ! The main concern  in my driving is standing in a queue with that car - not that much  a parking lot but the behavior is the same. This car certainly not is an everyday car -  and never meant either  - because of that but the original idea was to go for a strip and the  engine builder took it perhaps a bit too literally forgetting the  road manners when moving the  car on public roads to club meetings etc. Maybe the cheapest way would be to change the rear end ratio to 3.9-4.11 which was the  original Ford specs for  CJ cars.... What ratio do you have and what  are the revs at 60 mph in your case ? How comfortable  do you find it inside the car ? I have   2 x  3" exhaust without an H-pipe and  straight through24" silencers and you can feel the sound  - maybe a bit too much at  3000 rpm's.

 

[71coop]

Just my opinion, so take it for what it's worth - (probably not much)

I believe the best way to improve low-RPM drivability is a cam change. It can be relatively inexpensive, depending on if you change to hydraulic rollers or not, and will be a fair amount if work. It can be done in a weekend if you don't have too many plans.

My thought is that 242/[email protected] is a fairly long duration (big) cam. I would not worry about lift, my opinion is that you should run as much lift as your heads allow (within reason). I also agree with an earlier post that if you go down to a 225 to max [email protected] duration your drivability will be much better (usually the lower the better). However - Don't go so small you cause detonation with your engine combination, so you would want to research that a bit.

For example, I also have a 79 corvette with a 427 cubic inch small block, 10.25:1 compression. I run a comp Hydraulic roller with 286 degrees advertised duration, about .230@050, and around .6" lift. It idles around 800 RPM and has good low RPM drivability. It does not jump or lurch at "parking lot" speeds.

As far as rear gearing goes, my transmission is a TKO 600 paired with a 3.75"1 rear. It's a decent combination, and I would not go higher than 3.73. I have considered going back to 3:55:1 for highway cruising, but don't want to drop the cash on the part to replace the rear end. What I have is a decent setup as is.

The idle sound is a bit lumpy, but steady at 800 RPM and I don't have to fight it moving around parking lots or in traffic - I have good torque everywhere I need it.

On my mustang, I have a 351c with trick flow heads, about 10:1 compression with a 272/280 degree hydraulic roller that has around .6" lift. It idles well at 700-800 RPM and has great low-speed drivability. It has an FMX Automatic transmission. Rear is very conservative (2.73:1 and I may bump it up to something just above 3:1. I won't go above 3.55:1. If I were to go that high, would probably want an AOD or a TKO to offset highway RPMs.

Your best bet (if you choose to do a cam swap) might be to call Comp and get a recommendation on a hydraulic roller that works well with your engine.

A rear end change can help low-speed drivability, but will not really deal with low RPMs (a higher numeric ratio just gets your RPMs higher at a given speed - which may not be very enjoyable, depends on what you like). My guess is that will probably not really satisfy you. I also think your 3.55:1 ratio is a decent one for the street.

 

[Don C]

Idling at 700 RPM shouldn't be too fast around a parking lot, half as fast as your going now, and no jumping. Download the spreadsheet in this thread and enter your tire size, rear end ratio, transmission ratio, and RPMs and you'll get your speed (in mph).

https://www.7173mustangs.com/thread-tire-sizes--11566?pid=115406#pid115406

 

[73pony]

With 4.11 gears, 28" tall tires, 2.78 first gear at 700 rpm you would be going 5 mph.

 

[TommyK]

I think the aluminum flywheel may be part of the problem.