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Roller Rockers and head flow

I need some input regarding the use of roller rockers on a race prepared TR6. If you are running a cam with .500 lift and 300 degrees duration, does the use of a higher ration roller rocker (1.65) really add that much more power? This is assuming that the head is well ported and you are running the largest intake/exhaust valves that fit in the head. I was just made aware of some engine testing recently that indicated that there was only a 2 h.p. gain by using roller rockers with the above modifications. It also indicated that the increased ratio caused more wear on the cam, tappets, etc. The logic as presented seems to be that you can only draw so much flow through the head before you get diminishing returns. Any thoughts on this?

Allen Washatko


The primary reasons for using roller rockers are to decrease side thrust on the valve stems, reduce friction in the valve train (pin bearing in the rocker and a roller) and reduce reciprocating weight. The only way that translates into higher horsepower is to take advantage of the opportunities these factors enable. A lot of speed equipment is sold to people with no plan to take advantage of the factors the equipment was invented to improve. If your cam won't benefit from a higher rocker ratio then you won't gain anything from it. If you're not prepared to experiment with shorter intake valve guides to improve flow then the reduced side thrust won't give more horsepower, just better guide wear--which none of us care about.

You can get a lot of horsepower from roller rockers, but not by bolting them on.

Bill Babcock


Bill,

That being said, the dyno tests involved a fully developed race engine run with stock rockers and again with roller rockers. The difference was 2 h.p. - maybe a 1-2% difference overall. Hardly close to a 10% gain as some promise as a bolt on improvement (I would assume that this figure is for a stock situation). It is clear that the pin bearing and roller will reduce friction but the increased leverage will also add more work at the tappet end. As in everything, there is a limit to improvement. As machines become more refined, the next increment of improvement usually becomes smaller and more difficult to obtain. My point in asking the original question was whether or not roller rockers really added more horsepower due to a longer valve duration assuming that all the other bases were covered - friction aside. I would assume that there is a limit to just how much air/fuel mixture you can suck through a Triumph head as fully developed as it may be. The dyno test would seem to suggest that.

Allen


Allen,

Bill B.is correct. The big advantage to roller rockers is reduction of valve stem/valve guide wear. High ratio rockers for your car - 1.65 - greatly increase valve train strain and wear. Even the fastest TR6 in current SCCA Production form is only runing 1.55. We sell a goodly number of roller rockers and do not recomend or sell the 1.65.for your application. Jack Drew's comment about filming roller rocker tips certainly has merit. But, at what enginespeed. The engines in our cars, othe than the small Spit engines, don't see 8500 - 9000 rpm.

Ted schumacher


I have yet to trial any roller rockers but it is planned for the in-progress rebuild of our race GT6 engine. I can however offer some real-world comments on head flow and related matters that might clear up some mis-conceptions in this thread.

Owing an engine shop means I have a bit of a chance to experiment. We have made various mods on a single 2500 head (casting # 219021) and flow tested each one of them. At the simplest end, it was dead stock, followed by various versions of most folk call a "port & polish". The final end of the head was fully downdrafted, with the head chopped away and a 35mm pipe pressed in from the top corner of the head downwards at about 60 degrees. A bit of port putty and some die grinder work, made quite a nice job of the merge. In all cases we had the stock valves and factory width seats. The guides were progressively shortened as the porting got more radical. The stock head kept flowing to 0.500" and rapidly flattened The port work increased the flow by about 20% up to 0.300" lift where the results started to come together. At 0.450" lift, the fully downdrafted port flowed EXACTLY the same as the stock port.

This tells me the stock valves/seats/chamber etc are the limiting factor in max flow, not the ports or lift. I now have some Titanium race valves with 7mm stems to trial. I'll reduce the seat width and further modify the chamber and see if I can get any better results.

There have been a few comments in this thread about roller rockers and cam lift. Because these engines are so restricted by the valve & seat, then the only way to improve them is by having very fast lifting cams and therebye having the vale open for the longest possible duration. The rocker arrangement has little to do with this. The biggest influences are the diameter of the cam follower and the mass being moved. The stock lifter can be enlarged (considerable cost as machining is time consuming) and very small changes make a lot of difference. Because of this, just reading published cam data is often a waste of time. Any reputable cam grinder will advise how fast the cam acceleration ramp can be for any given lifter diameter. Anything that can be done to lighten the valve train is a good thing.

In relation to the prior comments about dyno results being used to prove/disprove the worth of roller rockers, I have to cast some doubts over making definitive statement on measurements of 1-2%. Only the very best temperature and humidity controlled dyno cells can achieve such repeatability. We operate both engine and chassis dynos, and generally ignore results unless they show more than 3% change, once corrections are made for ambient temperature and humidity. For example, when testing a 2500 race engine, a change in engine water or oil temperature of about 10degress, can make a 3-5% difference in max power. Some other engines . like the well known 4AG Toyota, will show much larger swings from temperature changes. Because of this, comparisons have to be made under as near to identical circumstances as possible. Peak power readings can rarely achieve what you need to know, so you have to run acceleration tests and load cycles to make valid comparisons. I'd be happy to swap real results with anyone else who might have the same. I'd really like to hear from folk with cam data and anyone else who has experimented with larger valves and lifters.

Terry O'Beirne


Terry, thanks for sharing this with us. I found the same thing with my flow bench work on both Spitfires and TR6's.

Thanks for telling us about the straight port experiment - you saved me all the work of trying this next.

One thing that I found interesting is the flow I measure on both 4 and 6 bangers with the valve totally removed did not increase as much as I expected, and like you, could only conclude that the diameter of the seat was pretty limiting. I have found that the 3-angle valve seats act as advertised and are the biggest single improvement that one can make.

Jack Drews


Years ago a sharp machinist in Long Beach, CA did what he called a spherical seat on the intake of my Matchless G80CS desert sled (single cylinder 500cc motorcycle) Gave it a noticeable power boost with no apparent change in the power band. I always figure an improvement you can feel with the seat of your pants is at least 10%. The "spherical seat" was just a curved seat--no angles--that was lapped by hand to seat the valve.

Bill


Rocker ratio has everything to do with valve velocity. Lifter velocity times rocker ratio equals valve velocity. Boring the block and installing .874" Ford lifters instead of .800" stock lifters allows the cam to be ground for 13.6% more lifter velocity. Changing from stock 1.45:1 ratio to 1.65:1 ratio rockers increases the valve velocity 13.8%. Do both and you get a 29% increase in valve velocity.

Richard Good


Richard is correct. The caution is boring out the lifter bore.These are on a 4.5 degree slant so you can't run straight into the block and overbore the lifter bores.

Ted Schumacher


On our current race motor for the TR6 we bored the lifter holes for ford lifters...Ted is correct...the lifter bores are not perpendicular to the block face....What we did is used a CNC machine and wrote a program to do the job...the blocks are between 1 and 2 thousands from one to another using the locating tabs on the pan flange surface...aside from calculating the degree of offset, each hole center was found, and lastly the lifter directly in front of the cam drive needs to be moved forward about 25 thou when using the 875 lifters...it is also important to do aprox. a .900 or so cut initially in the very top of the bore to remove the webbing curve to give a flat face to start the boring process and get a clean cut and not destroy your bit..

Chip


I stand correcting......... for any given camshaft, the average valve velocity will change with a rocker ratio change. For example, if changing the rocker ratio increases valve lift to say 16mm instead of 13mm, then the average velocity increase is 16/13 or 12%. this is because the lift occurs in the same time (cam/crank moves the same amount). What I really should have said originally, is, the biggest useful influence over the rate of valve opening, is the camshaft, because flow testing suggests you need a big increase in rate of lift early in the cycle, not just averaged.

I will take some measurements of cam rates on various Triumph cams and report back. To the various folk who emailed me about lifters, I understand various sized lifters can be fitted---its just a case of boring out the 12 lifter bores (tedious job). Cam grinders will specify the minimum cam follower diameter their cam will successfuly work on. The faster the acceleration rate of the cam, the larger the lifter diameter required. If you dont get the lifter diamter correct, the cam will edge ride. This means the lower edge of the lifter will try to dig into the cam, rather than ride up and over it. I would be deligthed to ehar form anyone who has such cam data, or even what is the most agreeive cam people have found to work on stock lifters.

Terry O'Beirne


just as reducing the rocker weight isn't an overall weight reduction issue. Skip that second donut and you'd be way ahead for a lot less money. It's all about making life easier for the tappet, camshaft, and the valve so that you can either make the head flow better or rev the engine higher. So to answer the original question as succinctly as possible, you can do much, much better than 2% with roller rockers if the overall engine has been designed to have roller rockers. If not, then you won't.

A high performance racing engine designed for standard rockers probably won't benefit from roller rockers to a large degree. But a good cam designer knowing exactly what's going to be in the engine can shape a more radical profile for a roller rocker than a stock one, accelerating the valve more harshly, and holding it at maximum lift longer. Just as they can if they know you're going to have roller followers (only moreso). The head porting guy that knows you're using a roller rocker with properly and precisely set valve geometry can eliminate the valve guide from the intake port, knowing that the remaining stub is going to be enough.

As with any other flowing system, as the valve gets open further, it has less influence on flow. Its why you don't use a gate valve to throttle and why carbs flow about the same from 3/4 to full opening. It's pretty well understood that the value of the higher rocker ratio isn't really ultimate lift, it's acceleration during the early part of the opening cycle. Given that rocker ratio doesn't influence duration of the cam, or the point that it begins to open, it does have an effect on early flow since the valve is accelerating faster.

The problem with conversing with me about this is that I'm re-reading the Engine Builder's Handbook, and trying to translate all the great V8 knowledge in that book to puny tractor motors. A frustrating practice that makes it impossible for me to look at any individual part without considering the entire system.

Bill

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