CAMSHAFTS

[steveww]

Too true. An old friend of mine and BRDC member, ex Le Mans racer etc... i.e. professional racing driver always says "tune the nut behind the wheel first"

[paros]

So following all the advice and also in terms of FIA regs for head, I left the valve size as Sprint but ended up with some new cams - slightly unplanned but a long story!
Cams are supposedly DA19 with shorter duration, although Kent who ground the cams say they are basically a Vegantune 450 cam.
The cam lift is .442" and the valve timing 45/75/75/45 - as measured by myself.
Does anyone have experience of these cams please?
Big flat spot with the 45's at low revs but they will potter around at 1800 in top if you are careful with the throttle. The infamous power hole at 4 to 5000 is very apparant but it comes on cam at 5200. Have a cr of 12.3 to 1 and no pinking even at low revs so a bit more advance will be tried. Idles fairly happily at 1200 rpm.
Oh for a dyno on our Greek island!!!!!!!!!
Richard

[rgh0]

Richard

How much the higher lift will help with standard valves is hard to say. You need a ferry trip to the nearest dyno I think to get the carbs and ignition timing sorted and to see what the power curve looks like first.

300 degrees and .442 lift should be a good competiton cam with the potential for around 170 hp in a 1600 at 7500 to 8000 rpm and 12:1 compression. However with the standard inlet big valves it will be contrained a little as you will not be getting the full benefit of the breathing capability of the cam. If the regulations say you have to stay with the standard valve size then a lot of time on a flow bench to get the maximum out of the rest of the head porting is the going to be needed to ensure you have the best possible breathing at the high valve lifts you now have.

Rohan

[type26owner]

My BRM Phase III has the small valves with similiar profiled cams and goes like stink. Have a tweaked Tecalemit-Jackson mechanical fuel injection fitted and it runs like a sewing machine at all rpms but comes up onto and pulls hard on the cams at 6000.

You likely have too big of air correctors fitted which is causing the flat spot. Trust me!

[paros]

Thanks all of you - I am persevering!
I have fitted a 45 pump jet to see if I can mask the gasp, however I think it is a distribution problem. The cams are very rapid opeing and the sudden lack of vacuum in the manifold may result in wet fuel lining the inlet ports and no fuel getting to the plug to be lit.
Next step if pump jet does not do the trick, is to see if a hotter thermostat will help warm the manifold [ water temp now is low at about 140 F ]. I will actually park the car when out for 5 minutes to let some heat soak into the manifold and then se if there is a difference.
Then Keith I would try a smaller air but this means soldering and drilling, current main is 145 and air is 150. So may try a 150 main.
Some more running in this week and then go to 7000 and I can see better what it is doing in the rev range I am interested in.
Richard

[type26owner]

Richard,
I've written numerous times about the air corrector delaying the flow of fuel from the main circuit and being the cause of the dreaded flatspot. To check if you have this problem I've devised a test. Search the archives here for the procedure. You'll know for sure after doing the WOT test. Don't delude yourself by assuming it's reversion or large valve overlap cause it isn't true.

The air corrector should be sized 0.1mm up from the threshold where it causes a stumble when going around hard lefthanders. My upcoming technical article will explain why this is the best compromise.

Find someone that has jets you can borrow. An Air/Fuel Meter would be a great thing for you to consider purchasing. Tuning the old way by guessing literally sucks.

[Dag-Henning]

- sorry guys for the rather skinny arguments above..... - pushed the wrong button....

DH

[rgh0]

My BRM Phase III has the small valves with similiar profiled cams and goes like stink.

Keith

My understanding of the Twin Cam head is the standard big valve inlet at 1.56 inch diameter works OK up to a maximum lift of about .41 inch. A 1.625 inch inlets work up to about .44 to .45 and 1.7 inlets are needed if trying the really high lift cams around .47 to .50. In all cases you need to match the rest of the porting as well as the valve diameter to the cam lift.

This does not say you cant make a good engine with the smaller inlets just that you will be limited to some degree in ultimate performance if you dont match inlet valve size and porting to valve lift to ensure the increase in lift actually delivers higher port flow.


Rohan
Rohan

[Dag-Henning]

- Rohan - doesn't the rule of thumb say that lifts more than 25% of valve-dia. does not improve flow...... ??

Dag

[rgh0]

Dag

Yes I have seen that rule of thumb quoted elsewhere and the lift versus diameter ratios for the twin cam I listed follow that rule approximately.
My understanding is the rule only applies when the rest of the port is not the flow limit but the valve curtain area is. If the rest of the port limits flow then lifts lower than 25% will still generate maximum flow.

Lots of other similar rules of thumb in the books "How to build, modify and power tune cylinder heads" and "How to choose camshafts" both published by Speedpro which give a good starting point but need fine tuing for application in a specific engine.

The basis of what I quoted is a mix of general information from the cylinder head modification and camshaft selection books, a range of flow bench data I have seen and the type of optimum specifications for various engines developed by people who knew what they were doing over the years for the various lift / valve sizes combinations (Hart, Bean, Cosworth, McCoy, QED)

Rohan

[type26owner]

Rohan,
Totally agree with that stuff but my point is you're not giving up all that much performance. The extra power to be gained is only in the neighborhood of like another 10%. If you're really competing to win then that's a huge factor but if you're vintage racing for the pure enjoyment of the sport then it's not.

We have stayed true to what was period correct for a 1966 BRM Phase III and that was the small valves.

[paros]

Yes I have also seen the arguments from many sources about the lift to diameter ratio. Start with rules are there to be broken!

The guy who produces race winning twin cam engines with Sprint sized valves in England has .49 of lift. Thus perhaps the efficiency is poor, but if the rules state you must use the Sprint sized valve, then power does improve but NOT to the optimum as with bigger valves.
One point, I followed Keiths comments on air correctors and viscosity and density - intriguing. The 1.1 won't be right over a big range as there are too many other variables and anyway area must be the deiding factor rather than diameter - plus laminar flow over bigger jets etc. But absolutely superb ideas.
The find I made, was that my 45's with 145 mains, and I with Wbers air correctors stamped 150 are actually only 145!! Now they have been drilled to 155 and the mixture looks better - and when I am in England next week I will buy 160 size AND will chaeck them again. I mention this as if air corrector diameter is as critical as Keith's ideas then this discrepancy could throw some wobblies into the testing!
Richard

[type26owner]

Richard,
Just to provide some perspective, when making jetting changes even large ones the results are not all that big. Rather they are subtle but if you're paying attention it's definitely noticable on the whole marco scale. Without an AFM it's hard to tell at times whether or not it's an improvement. This is why the old seat of the pants method has failed and no one has identified the true relationships as reproducible patterns. Even John Passini missed it.

[rgh0]

Richard

I never argue with race winners. The saying "The bullsh*t stops when the starting flags drops" is very very true.

And maybe pushing a standard sprint diameter valve to .49 lift gives you some benefit if you push all the other factors of porting, valve stem diameter, valve seat location and width, valve shape, seat shape etc in your favour also.

My feeling is that you could probably get the same performance with all the other elements and around .45 lift and a lot less stress on the valve gear meaning less frequent top end rebuilds. But one of my aims is always maximum performance at minimum engine stress as I cant afford the time or cost of frequent rebuilds and my race regulations allow engines internals to be free so I dont have to compromise on some of the things you FIA guys have to live with to achieve power and reliability at the same time.

But then if he's winning races who am I to argue. The differences in engine performance in any case are all relatively small as you observe and overwhelmed by driver skill, car weight, handling, tyres..........

Rohan

[steveww]

A number of successful racing drivers and mechanics I have talked to say that "torque wins races" and of course "to finish first, first you must finish". Which boils down to chasing every last horse power from an engine is not the best way to go. The difference is power/torque output between 0.45 and 0.49 lift is very small but as Rohan says the stress on the valve train is considerably more. I think a balenced approach across all areas of performance is the best way to win races.