Which is better?
(A) Slightly advanced camshaft, and slightly retarding the timing?
-or-
(B) Slightly retarding the camshaft, and advancing the timing a little?
It seems like (A) gives you more bottom end torque but then you have to loose a little due to needing to retard the timing a bit, because advancing the camshaft increases the chance of detonation…whereas retarding the camshaft per (B) raises the torque curve higher, and puts the horsepower slightly outside of the normal driving range, but you can get away with slightly more advance which makes up for some/all/more of the lost bottom end torque.
Which way is better, (A) or (B)?
How about C: setting the camshaft timing to what the cam maker/grinder recommends, then tweaking the spark timing.
I once had a modified '81 Accord with an adjustable cam timing gear. It’s main benefit was setting the cam timing back to spec after milling the head.
The aftermarket cam maker recommended setting the cam so intake and exhaust valve lift were equal at TDC, end of exhaust stroke.
Turns out that setting was the best compromise of bottom/top torque.
What’s your measure of “performance” – HP, torque, fuel mileage, longevity, etc.? Change to a hotter cam, slap on a turbo or go with nitrous.
Twotone
Well - normal daily around town driving RPM range is roughly 2000-3300 rpm. Freeway cruise at 4000 rpm equates to about 84 mph. Acceleration seems to get flat after 5000 rpm (I know you’re thinking why drive with the engine that high - no I’m not street racing, but it’s nice to be able to pass trucks on an onramp quickly, and redline is above 7500, and I keep up with valve adjustments, etc.). Gas mileage isn’t really my concern since the car is barely over 2000 lbs and being easy on the throttle it runs fairly lean on cruise and gets over 30mpg (down to about 3 mpg when autocrossing - ha!). The distributor curve is not stock and overall the car has a really good amount of bottom end. My current setup is 4 degrees cam advance, and I retard the distributor about 3-degrees from my normal setting during the summer when it’s above 90F (June thru September).
I suppose I wouldn’t mind losing a few HP below 3000rpm to gain a few above 3500…just curious if someone else has played with their settings a bit.
I no longer autocross (maybe once every two years), and I do use the car as a daily driver, but I have another car for comfort…this one is the fun one. I’m also currently accumulating parts for the “funner” engine, and have yet to fine tune the DCR I want to end up with and camshaft selection vs. head cc’s, and the answer to my question here may help me decide the final engine build, too.
Current upgrades installed are:
Lightened Flywheel
Higher flow exhaust
Dual Carbs
Stock build engine (although 2.0L instead of 1.6L)
Larger Radiator
Oil Cooler (thermostatically controlled)
O2 Sensor w/ A/F ratio gauge (for tuning)
Recurved distributor (more initial, same total, vac adv. retained)
electronic ignition
We here at CarTalk like questions about lose drainplugs,I don’t think messing with my camshaft timing is even on my list. Now I do like to overclock my 955BE got any hot memory timings you can pass on?
This is old foggie ville here no one does that stuff here.Maybe Elly
Oldschool, did you ever figure out why a clutch might slip more in a higher ger than a lower?? And I never mess with engine valve timing!!
Oldschool - I understand the nature of the questions this site typically sees, and it’s helped me out in the past. I’ve always tried to follow my rule…answer at least one or two questions before posting a question to help keep the forum active. I thought with the experience some have here, it would be a question worth posting.
I strongly recommend not trading off your bottom end torque for slightly more high end horsepower. Most people will go faster with a torquey engine.
Here’s why.
Tom and Ray are both late for church and they both live exactly two miles away from church. The first mile goes through a neighborhood with a 15 mph speed limit and the second mile is on a highway with a 60 mph speed limit. The first mile at 15 mph takes four minutes and the second mile takes one minute for a total trip time of 5 minutes.
Tom obeys the 15 mph neighborhood speed limit but when he hits the highway goes 120 mph which makes the second mile take only 30 seconds and gets to church in 4.5 minutes.
Ray goes 20 mph through the neighborhood but obeys the 60 mph speed limit. At 20 mph, it takes 3 minutes to cover the first mile and his trip only takes 4 minutes. The guy who only goes 5 mph over the speed limit in the neighborhood gets there 30 seconds before the guy who doubles the speed limit on the highway. Even if he went the speed of light on the highway, his trip would have taken the four minutes needed to cover the first mile at 15 mph. Once you understand the above, it sort of looks silly that everyone goes down the freeway like they are on their way to a fire but nobody runs to their cars and fills gas as fast as possible.
The secret to low lap times isn’t going as fast as possible on the straights but never going slow in the corners. A torquey engine makes it easy to accelerate out of the turns and that is why the seasoned racers will tell you “horsepower sells engines but torque wins races.”
A good track school and a professional driving coach will do most people a lot more good than spending tons of money on the car.
Yes, I understand the whole concept of an engine being able to put out 1000 HP from 7800-8200 RPM, but less than 50 everywhere else…and that you’ve got to look at the values under the ENTIRE engine speed range, in other words, the area under the curve of the range your driving in.
ANY significant change will have a positive impact AND a negative impact…as an example, a set of headers will improve freeway speed performance, but at the cost of low end torque (below 2500 is my guess) due to reduced exhaust velocities at low engine speeds; therefore the exhaust gas has less momentum and gas reversion back into the chamber through the unclosed exhaust valve after bottom dead center will occur to a greater extent. Top end (if the headers are well designed, and if the downstream piping is complimentary in size) will be increased to the extent that the change will be well worth it, especially if other areas are matched up well (increased carburation, etc.)
The comparison between the two cars you’re talking about only compares two cars at constant speeds, not straight acceleration or the area under the curve which it what I’m asking about…which brings me back to the original question of:
(A) Slightly advancing the camshaft (say +3 degrees) to get the intake valve to close early and improve bottom end torque, which requires timing to be reduced by a one or two degrees to prevent knocking and detonation…
-or-
(B) Slightly retarding the camshaft (say -3 degrees) to increase mid range and top end torque because now the intake valve closes later and exhaust scavenging is improved at higher speeds, which tends to be a more detonation resistant adjustment and allows the timing to be advanced a little, perhaps by one or two degrees?
Say, if doing (B) above, does the 1-2 degree timing increase come close to making up for the torque lost in the bottom end of the range with a retarded camshaft? (Obviously it would at least be somewhat less, otherwise that would be free horsepower of which there is no such thing)
You probably can’t get back the power lost from the reduced volumetric efficency with ignition advance. It’s not like the more you advance the spark, the more power you make. Rather there is a perfect ignition advance where maximum power occurs and advancing it more or retarding it lowers the power.
A pound of gas releases about 18000 BTU of heat when it burns, a BTU of heat = 778 ft.-lb. of energy, every BTU of heat that migrates to the cooling system or comes out of the exhaust is a BTU of heat that the engine failed to convert into 778 ft-lb of energy.
Overadvanced ignition = less power and high head temperature with low exhaust temperatures. The hot gasses had more time to give up its heat to the engine’s cylinder head and piston crowns before the gases were expanded to extract the energy of the hot gasses.
Underadvanced ignition = less power with high exhaust temperature and lower head temperature. The expansion already started before the fuel was completely burned and instead of converting the heat into energy, the fuels heat remained as heat in the exhaust.
You probably can’t get back the power lost from the reduced volumetric efficency with ignition advance.
You’re absolutely correct. As the cam timing is advanced, the intake valve closes sooner. As the rpms increase, you loose VE due to the early closing of the valve relative to the intake charge velocity. No amount of ignition timing can overcome this physical limitation.
Thanks for the technical answers - that’s EXACTLY the type of info I was looking for (I’m an engineer)…it actually also explains why my engine temperature and underhood temperature became more consistent between stop/start and freeway driving when I switched from the stock vacuum advance source (from ported to manifold).
I also found this info on a Madza forum which from the discussion posts and the photos looks to be a single cam setup like mine (intake and exhaust cannot be changes independently)…this one has some actual data. One guy states that his MPG went way down, although he doesn’t say what type of driving he does. I’d imagine it would go down if city driving cycling through lower RPMS, but I would also think it would be about the same on a freeway cruise, or better MPG if you’re cruising above 3500 RPM…
I recall a magazine article from way back on cutting a key offset on the timing pulley of a 2.2 litre Ford Pinto engine to allow a few degrees of cam retard for better high rpm breathing. Curiously, the gas mileage actually improved according to the article.
Just because the volumetric efficiency is low, it does not nessesarily mean that the thermodynamic efficiency is low. I believe that some Toyota VVTi engines run the intake cam at full retard during idle and at low throttle once the engine is fully warmed up. This lowers intake manifold vacuum and lowers pumping losses giving the engine a remarkably low idle fuel consumption.
You might say that these engines switch from a Miller cycle to an Otto cycle as you open the throttle.
There’s an underlying curiousity in the OP’s question that is great to see. It reminds me of my first few years learning about cars. It’s when you’re so passionate about learning something that you view your tireless energy to continuously assemble/reassemble your car as no problem at all.
Today, a person in that position like the OP, has the power of the internet to help accelerate his learning’s.
The youthful enthusiasm and tireless energy I had years ago is now long gone. But those were the years I learned the most. I learned more from my experimental failures than my successes.
Without a dynomometer for the OP’s specific car, there are too many variables to know exactly what will happen as he plays with valve and ignition timing. But the car will definitely behave differently with each experiment, and he’ll then know something he didn’t previously know.
OP: If you do pursue this, please share some of your findings. I’m sure you’ll find many here who will be interested and will be glad to help you understand some of the behavior changes you’ll run into.
both answers depend on what head you are running, and compression ratio. lets assume this is a 350 chev. i know thats not what you drive but the rules play the same. stock heads arent that efficient, and require a bit of advanced timing for the distributor, and a bit of the cam. in the top end, try advancing the distributor. you can always go back. it may give you that extra oomph you want.
if you are fine with the way its running other then above 5000, try advancing the distributor a bit, then try advnceing/retarding the cam. good luck!
You raise an interesting point. The OPs original question included the term “older cars”. I immediately associate that with non-variable cam timing. Someone with fewer years behind them might consider “older” to be more modern than I do. My assumptions were based on symetrical cam profile and static mechanical timing. In that situation, you set up the engine to one thing well and everything else is a compromise…