Question about cylinder heads to the engine guru's

[William C.]

Re: Question about cylinder heads to the engine guru's

First, I applaud you using your Vette and having fun racing it.
Are you allowed to block off the heat riser passage in the head. I use furnace cement to close off this area and improve problems with the 3-5 heat buildup. I do road racing so this may not be a problem for drag racing.
Second and this ones for Duke i think, the question of using wide ratio vs CR transmission is one shaped by the peak torque point isn't it? If your shift point is 7000 RPM then you would want a gear split that brought you back to the peak torque point whether that a WR or CR box. This is a real question for me since I striped the teeth off my third gear at Monterey last month and I have to figure out what split is best for my engine.
Third there is a web site that has all the chevy heads numbers and what the maximum flow is out of them. It may not be representative since you aren't allowed to port your heads.
Are you allowed to relieve them? This is a BIG deal on stock heads. Jerry

Actually the question regarding close vs wide ratio in a drag car has an added component, and that is how much tire/Traction are you dealing with at launch. In My Dragracing days, we had both close ratio 2.20 low boxes and 2,54 (close ratio (special gearset with evenly spaced drops between gears) and the high $$ trans did not help much as we were running a bigblock on a 10 inch tire limit class. How much tire you are running as well as overall weight distribution are big contributors to the "right" answer to the trans.

[Duke W.]

Re: Question about cylinder heads to the engine guru's

Second and this ones for Duke i think, the question of using wide ratio vs CR transmission is one shaped by the peak torque point isn't it? If your shift point is 7000 RPM then you would want a gear split that brought you back to the peak torque point whether that a WR or CR box. This is a real question for me since I striped the teeth off my third gear at Monterey last month and I have to figure out what split is best for my engine.
Jerry

The optimum shift point is governed by the shape of the power curve and gear spacing. The various OE close ratio transmissions have gear spreads ranging from 1.34 for 1-2 to 1.28 for 2-3 and 3-4, so lets call it 1.3.

Optimum shift RPM is where the power picks up at the same point in the next gear. Say power peaks at 6500 and rolls off a little to 7000. If you shift at 7000, next gear revs are about 5500, and if the power is the same at 5500 as 7000, then you are maximizing the average power delivered through the gear. SHP engines with massaged heads, manifolds, and the OE exhaust systems always show a very flat power curve in the upper 1500 of the useable rev range, so shifting at 7000 is typically ideal. In this case, with no head massaging allowed, power will probably fall off a little faster beyond the peak.

If power at 5500 is less than 7000 increase the shift point to hit the sweet spot. If 7200 is the same as 5700, then that's the sweet spot. If power is more at 5500 than 7000, shift a little earlier. All it takes is a power curve from chassis dyno test, gear spacing value, and a straight edge to determine the optimum shift point.

What I don't like about the WR trans... whether you are shifing at WOT/redline or half-throttle/3000, the car falls on it's face when you shift into fourth because the spread is 1.46 to 1.51 depending on the year.

F1 cars have gear spacing of about 1.1. Shift at 18000 and revs pick up at about 16K and power deliver through the gears is nearly constant and the average is nearly equal to the engine's maxium output.

Like Clup said the biggest problem in this application will likely be initial traction. It will require an easy launch, but that will save the drivetrain. Once it hooks up get the engine up to 5500 ASAP, and keep it in the 5500-7000 range for the rest of the run, which maximizes average power delivery from hookup to the lights.

I don't think any head mods are allowed other than milling, so chamber relieving is probably not legal. It's not clear, but multiangle valve seats are probably not legal since the heads must be "stock".

NOTE TO TIM: Since you'll be running higher than OE valve spring forces, convert the rocker studs to screw-in, but you don't need guide plates as long as the pushrod holes in the heads have not been molested.

Duke

[Jerry G.]

Re: Question about cylinder heads to the engine guru's

When you say power curve , are you talking about Horse power or torque? From reading Carol Smith racing engineering manuals I believe i'm trying to get the gear split to get me back to peak torque from shift point. What do you think? Jerry

[Tim S.]

Re: Question about cylinder heads to the engine guru's

When you say power curve , are you talking about Horse power or torque? From reading Carol Smith racing engineering manuals I believe i'm trying to get the gear split to get me back to peak torque from shift point. What do you think? Jerry

The impression I was under, you would like the engine to pick up at or around peak torque after a gear change to help push RPM's along into peak horsepower. That is where my engine builder is going to be a key component.

[Tim S.]

Re: Question about cylinder heads to the engine guru's

I don't think any head mods are allowed other than milling, so chamber relieving is probably not legal. It's not clear, but multiangle valve seats are probably not legal since the heads must be "stock".

NOTE TO TIM: Since you'll be running higher than OE valve spring forces, convert the rocker studs to screw-in, but you don't need guide plates as long as the pushrod holes in the heads have not been molested.

You are correct about the chamber work. I have to ask about the multi angle valve job. I understand my choice of valves is relatively open provided it is a stock head and stem size. Good call on the valve springs Duke! I am with you on the rocker studs. I can only imagine things will get scrutinized much more carefully if this gets into the 12's. If I wanted to cheat, I could always use these guys, they are local to me http://www.castheads.com/undercover_porting.php. But.............................I don't want to be known as a cheater.

[Duke W.]

Re: Question about cylinder heads to the engine guru's

Jerry, Tim - It's horsepower, which is energy per unit time that accelerates a car. The engine's mechanical energy is converted to vehicle kinetic energy, and the faster we add energy per unit time, the higher the vehicle speed at any point in time. The power curve has the steepest increasing slope up to the torque peak. Once torque starts dropping slope is reduced until torque decreases faster than revs increase. This is the power peak, and then it drops off.

If we had a 100 percent efficient CVT we would set it up so WOT causes revs to immediately increase to the power peak so maximum power is delivered regardless of vehicle speed. (The trouble with CVTs is that they are far from 100 percent efficient under maximum power input at slow speed.)

Conventional transmissions are a compromise. For a good road car we need a fairly wide total ratio spread to give good startup and initial acceleration performance, an optimal gear for top speed, and a gear for low rev cruising, but for a race car a much narrower total spread is required based on the minimum and maximum expected speeds, and low rev cruising is not an issue.

For Tim's drag racer the necessary speed range is zero to about 105 and about 40-140 for Jerry's road racer on typical track, but Laguna Seca is different than Road America, which would probably require different axle ratios for these two tracks to optimize performance.

For a road engine with a limited number of gears it's okay for the 1-2 shift to drop revs from redline to the torque peak. This takes advantage of the fattest part of the power curve, but there is still a big dropoff from maximum power on the 1-2 upshift. On typical road engines that have relatively broad torque/power curves, it's not that noticeable, but inter-gear spacing should get tighter as you go through the gears to keep average power in a higher range. As vehicle speed increases more power is required to overcome total drag and less is available to accelerate the vehicle, so you want to keep average power input to the wheels as high as possible as speed increases.

For a race engine you want, ideally, maximum power available at every speed within the range dictated by the track, and in lieu of a CVT, F1 cars do it by having seven very closely spaced gears so available power at any speed is always very close to peak power.

The CR four-speeds can be considered "racing transmissions" because of their closely spaced gears, but the total ratio spread is only 2.20. Depending on axle ratio I jokingly refer to the CR four-speed as a "five-speed with no fifth gear" of a "five speed with no first gear."

A new Z51 C7 has 1-4 total spread of 2.97 and spacing is near constant averaging 1.44, which means revs drop to about the 4600 torque peak shifting at the 6600 rev limiter. Top speed is achieved in fifth, and sixth and seventh are strictly cruise/fuel economuy gears. That's a good set up for a road car with a very wide torque and power bandwiths, but you can bet that the ALMS GT Corvettes have much closer spacing. In fact you can "hear" the close spacing during the in-car camera segments.

As I always say, race cars/engines are totally different animals than road cars/engines, but either way it's available horsepower at the wheels minus drag horsepower at any given speed that determines how fast either will acclerate, and the higher the available power, the greater the rate of acceleration, which is what wins drag races from either a standing start or from the exit of one corner to the braking point for the next corner.

Duke

[Tim S.]

Re: Question about cylinder heads to the engine guru's

Here are some more pics I have of the car.

[Tim S.]

Re: Question about cylinder heads to the engine guru's

66 L79 Pure Stock ride along http://www.youtube.com/watch?v=ZGrYZ0O0I0U