How easy it is to cause confusion with cam numbers?

How easy it is to cause confusion with cam numbers?

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Discussion

Stan Weiss

Original Poster:

260 posts

147 months

Monday 10th August 2015
quotequote all
I see people post I am running a cam with these specs. Or someone asks the question what cam should I run and gets different specs from a number of people. How is one to know which cam is correct. This post is not going to answer that question but confuse the issue somemore.

These are all flat tappet solid lifter cams: Which one is right for my engine.

cam a - lobe lift 0.425 - 0.012 lash
246.5 @ 0.050
217.5 @ 0.100
172.0 @ 0.200

cam b - lobe lift 0.429 - 0.018 lash
249.0 @ 0.050
220.0 @ 0.100
173.5 @ 0.200

cam c - lobe lift 0.433 - 0.024 lash
252.0 @ 0.050
222.0 @ 0.100
175.5 @ 0.200

cam d - lobe lift 0.437 - 0.030 lash
255.0 @ 0.050
224.0 @ 0.100
177.0 @ 0.200

While the physical lobe on the shaft is not the same. The actual design lobe minus the lash ramp is identical. If you take each of the lobe lift time 1.5:1 rocker arm ratio you get gross lift. If you subtract their lash from their gross lift all 4 have the same net / valve lift and your 0.050, 0.100 and 0.200 at the valve will be the same. My point begin that looking at numbers from the physical lobe does not tell the whole store. So duration at lifter raise does not always tell the story.

So when someone posts up numbers you better look a little deeper than just the raw numbers

Stan

stevieturbo

17,229 posts

246 months

Monday 10th August 2015
quotequote all
Cam selection is always confusing !

Mr Vizard said in the yellow book to pick the cam you want, then buy the next stage down, which makes perfect sense lol.

I dont know a lot on the subject, but it's always too easy to think bigger is better, but certainly for anything that sees the road that is rarely the case.

227bhp

10,203 posts

127 months

Monday 10th August 2015
quotequote all
None of it makes any sense to me, we've been metric since 1971....

Stan Weiss

Original Poster:

260 posts

147 months

Monday 10th August 2015
quotequote all
227bhp said:
None of it makes any sense to me, we've been metric since 1971....
Does metric numbers help you any? smile

cam a - lobe lift 10.795 - 0.304 lash
246.5 @ 1.27
217.5 @ 2.54
172.0 @ 5.08

cam b - lobe lift 10.8955 - 0.4572 lash
249.0 @ 1.27
220.0 @ 2.54
173.5 @ 5.08

cam c - lobe lift 10.9982 - 0.6096 lash
252.0 @ 1.27
222.0 @ 2.54
175.5 @ 5.08

cam d - lobe lift 11.0998 - 0.762 lash
255.0 @ 1.27
224.0 @ 2.54
177.0 @ 5.08

Stan

Pumaracing

2,089 posts

206 months

Tuesday 11th August 2015
quotequote all
Your point is well taken but a little artificial. Real cams would not have such wildly differing lash values or clearance ramps so the raw duration numbers at say 0.050" lobe lift will be a reasonable indication of performance. However I agree that only the net lift profile at the valve is an ideal indicator and that's how I've always compared cams from my own measurements.

227bhp

10,203 posts

127 months

Tuesday 11th August 2015
quotequote all
Stan Weiss said:
227bhp said:
None of it makes any sense to me, we've been metric since 1971....
Does metric numbers help you any? smile

Stan
They do, it was like finding my glasses and putting them on smile Wouldn't you spec a cam (to some degree) to what your ports flowed on a flowbench?
Like many reading this I was brought up on a diet of double OHC, direct acting tappets, turbos etc so the topic is a little irrelevant, difficult, nonetheless interesting, but I only have questions - and even then most of those are how you would spec a cam for a turbo engine (because that's where we've been and that's where we continue to go), which parts of the cam spec you would increase and why.
I have lately pondered on which one you would increase and why, what would be the differing effects (treating inlet and exhaust separately) lift or duration and what effects it would have: What if you increased a 255/8.5 inlet to a 255/11? Or then to a 272/9? what would it do to the power output and where the engine produced its power, also effect on drivability. I guess really you'd go for a 272/11. Then what about the ex cam?
I think there has been a big change in usable ex cam specs over the years due to the increases in efficiency in turbos (being much better flowing, lighter wheels, earlier spool etc) over the years, but not everyone has caught onto it.
A massive subject I think and lots of whys n wherefores.

If you have an exhaust port which flows well up to 9mm of lift, but not much more after that would there be any point in fitting a cam which lifted to 10mm? What would be the effect? No gain? A loss? I'm assuming you would keep the lift at 9 and increase the duration...

silly

PeterBurgess

775 posts

145 months

Tuesday 11th August 2015
quotequote all
Hi Stan

This may well illustrate your message. We prepared a head and supplied an MD310 cam for a 1328 A series race engine in a Moggy Minor. The engine was a beginners full race one. CR13:1 running a 45 side draft Weber. When the car came in for a rolling road session it was crap and hard to get the fuelling correct. The daft git had insisted his engine builder fit 1.5:1 rockers instead of the standard 1.3:1. We told him what we thought and he went off and put standard rockers on....what a difference. The graphs show 1.5:1 rockers vs 1.3:1 rockers. Fuelling and timing optimised for both sessions. The higher red dotted line(torque) and solid red line (power) shows the 1.3 ratio outperformed the 1.5 ratio everywhere (blips at end how I stop recording). The engine made around 126 bhp with the 1.3 rockers. In a nutshell the 1.5 rockers ruined an excellent race cam by increasing the overlap to such an extent the engine became inefficient!!!! Same cam just different effect of rockers as Stan shows above. Real world stuff backing up theory.



Peter

Pumaracing

2,089 posts

206 months

Wednesday 12th August 2015
quotequote all
What were the valve clearances set to with the std and the high ratio rockers?

Stan Weiss

Original Poster:

260 posts

147 months

Wednesday 12th August 2015
quotequote all
Pumaracing said:
What were the valve clearances set to with the std and the high ratio rockers?
Dave,
That question bring up an interesting point. The lash point is set by the lobes lash ramp design. What that means is no matter what rocker arm ratio is used the lash at the valve should be adjusted based on that ratio. This means if calculated correctly the seat-to-seat duration will always be the same no matter what ratio rocker arm is used.

Stan

PeterBurgess

775 posts

145 months

Wednesday 12th August 2015
quotequote all
Tappets set 16 thou " with standard 1.3:1 rockers and 19 thou" with the 1.5:1 rockers. 19 was optimal with engine even worse with tighter clearances as one would expect and rattly with 22 thou clearances so much so that valve tip damage would have occurred.
Roller 1.5:1 so very cautious setting clearances with feeler gauges in from side of roller not front which would make the clearance way too tight. 22degrees max advance 1.3:1 rockers and 21 degrees max advance with the 1.5:1 rockers for optimised power delivery.

I attach another power graph of a midget The lower bhp and torque was a 1380 12:1 CR with 1.5 Rockers and BP320 Piper cam and Weber. Higher bhp and torque same engine 13:1 and Piper cam selected to have way less overlap so it was picked to compliment the 1.5 rockers. This time we have sorted the mismatched rocker/cam selection by changing the cam to suit the rockers. Fair play to Piper it works really well. Interesting seeing the similarities in the two graphs showing wrong cam/rocker selection. The BP320 cammed engine won the Cockshoot championship in 2007 with that power curve, new cam much better and much wider power spread and more of it too!




Peter

Pumaracing

2,089 posts

206 months

Wednesday 12th August 2015
quotequote all
Stan Weiss said:
Dave,
That question bring up an interesting point. The lash point is set by the lobes lash ramp design. What that means is no matter what rocker arm ratio is used the lash at the valve should be adjusted based on that ratio. This means if calculated correctly the seat-to-seat duration will always be the same no matter what ratio rocker arm is used.

Stan
The seat to seat duration yes. The effective (some decent flow is now taking place) duration at say 1mm valve lift changes of course.

But not by as much as one might think. The actual stock rocker ratio of A series rockers is about 1.24. The 1.5 rockers are therefore about 20% higher.

However this only increases the effective duration at 1mm net valve lift by about 4 degrees, PROVIDED the valve clearances are adjusted up from the nominal 16 thou on the stock rockers to 19/20 thou on the high lift ones. If this is not done then the effective extra duration at 1mm lift increases to 6 degrees and also risks the running clearance disappearing when the engine is hot leading to all sorts of nasty happenings. Burnt out valves etc and the cylinder not sealing properly.

So just getting the valve clearances wrong can account for 50% of the undesirable effects of fitting the high lift rockers in the first place.

To fully compensate for the high lift rockers the lobe separation angle should be increased by 2 degrees and the valve clearances increased by 20%. Not doing this increases the effective cam profile by about 1 point so from a rally cam to a race one or a race one to a drag one.


PeterBurgess

775 posts

145 months

Wednesday 12th August 2015
quotequote all
I attach a link to a youtube video of the 1380 race Midget. The practice was wet so the slick tyred race Midgets were at the back of the grid, great fun to watch them carve past the modern MGs smile Our car gets to fron of Midgets then pace car comes out, then as race gets underway we get a stone through the screen frown Engine and transmission sounds brill with the vol cranked up!

https://www.youtube.com/watch?v=VJ_wtcQ7SaM

Peter

Pumaracing

2,089 posts

206 months

Wednesday 12th August 2015
quotequote all
There's a lot more going on with those two power graphs than is properly explainable. The change in engine capacity is minimal but the supposed change in torque at 3500 rpm is from nearly 120 ft lbs in the latter to less than 70 ft lbs from the best curve in the first one with similar cams, carb, CR and rocker ratio?

And A series engines don't produce 93 ft lbs of torque per litre. If they hit high 70s that's doing bloody well.

PeterBurgess

775 posts

145 months

Wednesday 12th August 2015
quotequote all
I never said the engines were the same and had the same spec heads did I? The 1328 is a beginners race engine and the 1380 is our dogs Wotsits engine. I used the two engines to show the effects of high lift rockers with unsuitable cams. The 1328 was improved by taking off the high lift rockers and the 1380 was improved by fitting a cam suited to high lift rockers. 13:1 as opposed to 12:1 makes a dramatic difference with A series engines but they don't last as long between builds and need a more seasoned driver to run them safely. The beginners engine at 13:1 is softer so safe for a novice which the driver is.

Peter

Stan Weiss

Original Poster:

260 posts

147 months

Wednesday 12th August 2015
quotequote all
Pumaracing said:
The seat to seat duration yes. The effective (some decent flow is now taking place) duration at say 1mm valve lift changes of course.

But not by as much as one might think. The actual stock rocker ratio of A series rockers is about 1.24. The 1.5 rockers are therefore about 20% higher.

However this only increases the effective duration at 1mm net valve lift by about 4 degrees, PROVIDED the valve clearances are adjusted up from the nominal 16 thou on the stock rockers to 19/20 thou on the high lift ones. If this is not done then the effective extra duration at 1mm lift increases to 6 degrees and also risks the running clearance disappearing when the engine is hot leading to all sorts of nasty happenings. Burnt out valves etc and the cylinder not sealing properly.

So just getting the valve clearances wrong can account for 50% of the undesirable effects of fitting the high lift rockers in the first place.

To fully compensate for the high lift rockers the lobe separation angle should be increased by 2 degrees and the valve clearances increased by 20%. Not doing this increases the effective cam profile by about 1 point so from a rally cam to a race one or a race one to a drag one.
Dave,
Would you have any camshaft profile lift data that I can examine, even if it is from the stock cam.

Stan

PeterBurgess

775 posts

145 months

Wednesday 12th August 2015
quotequote all


Hi Stan

If you mess with A series figures there are two standard ratios, the early forged cooper and midget rockers and the pressed rockers are 1.3:1 measured and the later sintered less reliable ones are 1.28:1 measured. We know this because we ended up with just too much valve lift running an STR930 in a Metro race series. We had to fit later less reliable rockers to get the lift down to legal numbers. My thanks to Mike Garton who worked on drawing up the STR930 cam for the miglia minis for guiding me with these differences in rocker ratios. Sadly not all that is in books is spot on and a lot is hard earned from hands on experience.

At the end of the day the proof of the pudding is in strip times, track times and race work. As the red Midget shows in the clip from youtube it overtook all the other slick tyred race Midgets and the slick tyred MGB. It closed up to being behind a 180 and a 190 Z whatever they are modern MGs with blueprinted engines so they should be around the 200 and 210 bhp mark one would hope. From that I would assume we have a lot of power smile

Peter

Pumaracing

2,089 posts

206 months

Wednesday 12th August 2015
quotequote all
PeterBurgess said:
It closed up to being behind a 180 and a 190 Z whatever they are modern MGs with blueprinted engines so they should be around the 200 and 210 bhp mark one would hope. From that I would assume we have a lot of power smile

Peter
Well obviously your engine must also have 200 to 210 bhp then or even more to be faster than these other cars.

PeterBurgess

775 posts

145 months

Wednesday 12th August 2015
quotequote all
I guess you must have been out of the racing game for too long and resting on your laurels Dave. I would assume we have a distinct advantage in terms of power to weight ratio/C of G and handling including better tyres?

Peter

Pumaracing

2,089 posts

206 months

Wednesday 12th August 2015
quotequote all
PeterBurgess said:
I guess you must have been out of the racing game for too long and resting on your laurels Dave. I would assume we have a distinct advantage in terms of power to weight ratio/C of G and handling including better tyres?

Peter
Ah, I see now. So by eulogising the performance of your car against another of different type, weight, engine design, tyres and driver you find it possible to both..

A) Infer "From that I would assume we have a lot of power" and also
B) Believe that it is not possible to infer your engine power because lots of other more important factors come into play.

I believe we call that "cognitive dissonance". The ability to hold two conflicting or even mutually exclusive ideas to both be true at the same time.

Pumaracing

2,089 posts

206 months

Wednesday 12th August 2015
quotequote all
Stan Weiss said:
Dave,
Would you have any camshaft profile lift data that I can examine, even if it is from the stock cam.

Stan
Do you mean specifically from an A series engine?