Four-valve heads offer tremendous valve flow area, which is why these engines make such gr
Logan Farnsworth; Kaysville, UT: I am a member of the University of Utah Formula SAE team, a collegiate engineering competition in which the teams build a small, open-wheel race car using an engine no bigger than 600 cc. The motor we are using is a 600cc, dual-overhead-camshaft engine from a Honda CBR600 F3. Because of the rules, we are required to use a 20mm restrictor following the throttle, which effectively reduces the efficiency of the engine at higher rpm, which is where the engine makes most of its power. I was wondering if it would be feasible to adjust the cam timing on just the intake camshaft to reduce the overlap, as I know having a smaller overlap moves the torque curve down the rpm range. The F3 engine makes peak horsepower at about 12,000 rpm, and we are hoping to have peak horsepower occur at about 10,000 rpm because that is about where the restrictor should limit the air intake if we did our math correctly. I have already done some research, and I know there is an adjustable cam sprocket available for these motors. The stock cam specs are: intake open 15 BTDC and close 35 ABDC; exhaust open 38 BBDC close 7 ATDC. I also know that the next motor in the same family, the CBR600 F4, has the cam specs of intake open 22 BTDC close 43 ABDC; exhaust: open 38 BBDC close 7 ATDC. My thought was to adjust the intake cam on the F3 motor to close at the same time as the F4 so the specs become intake open 7 BTDC close 43 ABDC. Would this have a negative effect on the performance of the motor in the lower rpm range or would it just effectively move the peak power down?
Any insight would be greatly appreciated, as all my previous engine experience is with small-block Chevys and a few Pontiac motors, so dual-overhead cams are something very new to me. Also, for more info on the team or competition, here is the team website: UtesMotorsports.com.
Jeff Smith: You are correct that the intake closing point is the most important of the four valve events. The intake duration, and therefore the intake closing point, determines where peak torque will occur. The later the intake closing, the higher rpm at which peak torque is achieved. Peak torque then establishes the beginning point of the power curve, which is defined as the rpm band between peak torque and peak horsepower. I researched this engine and found the stock power rated at 105 hp at 12,000 rpm and 48.7 lb-ft of torque at 10,500 rpm with 12.0:1 compression. This makes the stock powerband 1,500 rpm wide. With a restrictor plate, you said you calculated that peak power will occur at 10,000 rpm. That means if you maintain the same powerband, peak torque should be somewhere around 8,500 rpm. So this is where it would be preferable to have the intake closing point establish peak torque. If the stock cam closes the intake at 35 degrees after TDC, we need to close the intake sooner to establish a lower torque peak by at least 8 degrees and maybe even earlier.
Because the engine is inlet restricted, it seems to me it would be beneficial to build more torque at a lower rpm to help the car come off the corner (assuming tire spin is not a problem). Overlap is the period during which exhaust closing and intake opening points cross over. We have to be careful here because we have lots of valve area, so too much overlap can kill cylinder pressure. Is there an octane and consequently a compression ratio limit? One way to build cylinder pressure in the lower rpm range is to close the intake valve sooner. If you can't add more compression, you can alter the point at which maximum effective pressure occurs with cam timing. I've listed your intake lobe specs again (remember, you have to add 180 degrees to intake opening and closing to get total duration) so we can evaluate what' s going on here:
F3 intake open 15 BTDC; close 35 ABDC = 230 degrees
F4 intake open 22 BTDC; close 43 ABDC = 245 degrees
This intake lobe opens 7 degrees sooner and closes 8 degrees later.
Your proposed F3 open 7 BTDC; close 43 ABDC = 230 degrees
This intake lobe closes later, which improves high-rpm power, which is not your goal.
My proposed F3 intake: open 15 BTDC; close 28 ABDC = 223 degrees
This closes the intake valve earlier, moving the torque peak lower.
The above change shortens the total intake duration and closes the intake valve sooner. Both are in an attempt to lower the peak engine rpm power point to 10,000 rpm. You may also have to experiment with overlap to help the midrange torque. That may require widening the lobe-separation angle slightly, but the only way to know for sure is to test the engine, since there are several other variables that also affect the torque curve. Also, the width of the powerband is another essential consideration because if you widen the powerband, the car will be much easier to drive and will require fewer gear changes. Every shift is an interruption of the application of power. A peaky, narrow powerband requires more gear changes. That's why I think an approach that widens the powerband without sacrificing average power will be the most successful. More area under the power curve will make the car quicker off the corners and easier to drive, reduce the number of shifts, and lower the lap times. Before you go this far, it might be worthwhile to data-log a lap and then establish a hysteresis curve that will reveal the rpm band where the engine spends the majority of its time. Obviously, this is affected by track length, gear ratio, tire size, and drive technique. But all of this will point you in a direction. The teams that concentrate on peak power and build a high-
powered but peaky engine will suffer—especially in short, tight autocross courses that dominate the Formula SAE competition.