Engine Acceleration and Cam Timing

[Bazman]

If you wanted a motor that rev'ed quickly to opening the throttle, besides friction reduction and lighter reciprocating mass - does cam timing have a big impact?

e.g. tighter LSA's sharper compared to wider? or is the actual LSA less important than the valve timing events - and the LSA will be what it is?

Assuming a given LSA - will retarding or advancing the intake (thereby spreading the lobes) significantly affect engine acceleration?

My example is an LS3 I want to rebuild and I want it to be lively to throttle inputs (assume power will be what it needs to be - we're talking engine response not power). Limitations are pump gas (93) and sticking with the stock LS3 intake manifold (for now), so it's never going to match the pinnacle of responsiveness achieved by Sprint car methanol ITB injected motors.... but EFI LS engines are by nature lazy compared to older school decent built small blocks that actually run carbs.... and much tighter lobes.... is there a connection?

Even my mild old Mopar 440 on OEM rods and crank could rev from idle to 7k in under a second on a single carb (Predator variable venturi). Never heard an LS that could do that in person - though I'm sure there are plenty that could.

Engine acceleration may not win races but they sure feel more alive to play with

[peejay]

I am interested in this as well.

One of Toyota's design criteria for the 4A-GE engine (1600cc engine in first generation MR2, rear drive Corolla GTS) was that it had to be able to accelerate from idle to redline in under a second. The 4A-GE has a cult following due to its lively feel.

[MadBill]

For a manual trans, flywheel/clutch inertia is a much larger factor

[groberts101]

Besides a shorter stroke and lighter inertial advantages Bill speaks of the big difference in rev response comes from retuning the drive by wire tip in response. Most tuners will adjust this in conjunction with higher ignition advance and makes for a much touchier accelerator pedal. OEM's have the ability to make an engine far more responsive but it comes down to a safety factor for the average driver who may nor be able to control the sudden surge of power. Especially on road debri/sandy, wet or snowy conditions. Same goes for deceleration fuel cutoff(dfco) settings as well. Cut fuel too quickly all at once coming off the throttle and compression braking can get a lot of overzealous drivers into trouble real quick.

But to answer your specific question.. yes advanced cam position can create higher cylinder pressures earlier in the rpm curve which has the ability to enable slightly faster rev's when compared to another ICL position which is retarded.

[Bazman]

Good post, thank you.

I was not referring to PEDAL sensitivity but ENGINE RESPONSE given the same degree of throttle body opening in a given moment. I agree with you, I would not alter the throttle sensitivity for the reasons mentioned, the graduated pedal of drive by wire allows me to use 663rwhp in the wet from top of 3rd gear.

I want an engine that will react fast if I really prod it, but is graduated by pedal movement so I can control the torque in traction limited situations. That way I have a responsive engine or a very controllable one - simply by the speed and amount of pedal used.

Lighter flywheels are great for race only, but no good when they reach the point of stalling around carparks and in traffic jams because the engine hunts or stumbles slightly and there is not enough enertia to get over the hump. It's the one downside of a bigger cam, the mild one I have allows 4000lb car to idle and release clutch at no throttle and it will happily drive off at idle on any flat surface. I have to accept going to a cam with a little overlap will required more clutch slip and throttle interference to keep it going smoothly.

Not sure yet how much overlap I can tolerate at low speeds, probably as much as my tuner can make civilised and still snap from 3k to 7k. I'd like to match stock LS3 torque by 2500rpm, and be ahead by 3k. Cams I've looked at have ranged from 8 to 13 degrees overlap. I doubt my turbos will like 30 degrees + but who knows. The old 440 Mopar loved a 30/70; 70/30 cam and that had a heap of overlap yet was streetable enough... cough.

[Warp Speed]

Whatever cam timing that produces the most power in a given rpm range, will accelerate the quickest! Lol

[CGT]

Whatever cam timing that produces the most power in a given rpm range, will accelerate the quickest! Lol

Thanks for closing the door to the "it doesn't make any more power but revs faster" crowd...in the rain...around corners....with burr finish etc.

[427dart]

I will go with the lightest clutch assembly you can find. I've had engines on my run stand where of course you only have the flywheel on and the difference is night and day as far as throttle response without and with the clutch.
Without the clutch it feels and sounds like a more radical engine!

[CamKing]

If you're talking about acceleration off idle, a earlier intake closing will usually make the engine more efficient at lower RPM's, as long as the fuel can handle the added cylinder pressure.

[Bazman]

Good info, thanks. I know it doesn't make the car faster, power is power, but a crisper engine is a lot more fun to drive when that is a factor, as opposed to just comparing timeslips.

A friend took me out in his 430hp small block Ford 302. It makes no where near the power my LS3 makes (about 780 at crank), but OMG does that thing get going fast. It's almost violent in the way it reacts to sudden WOT. It is built by a nationally acclaimed race engine builder to endurance racing specs, and on pump gas it is hard to imagine a motor any more responsive.

That's what got me wondering as LS engines, while powerful, are lazier in delivery.... a lot of that is intake manifold design and big single TB but I have also believed part was due to them using wider LSA's that most builders insist on.... (likely because they have to tune them). The few that run tight LSA's say they work real well.

How much overlap is too much is the question lol

[Monza Harry]

I will go with the lightest clutch assembly you can find. I've had engines on my run stand where of course you only have the flywheel on and the difference is night and day as far as throttle response without and with the clutch.
Without the clutch it feels and sounds like a more radical engine!

Back in the late seventies my family were racing a midget race car with a 0.030" over Chevy II with no flywheel [standard fare for midgets then, maybe still I'm way out of the loop] and just the ~2.5# in and out box drive flange, that thing would scare you with the throttle response, lift the arm about 3/32" and you were "Red-Lined" before you know what happened, weight here is exponentially important. Harry

[cjperformance]

Whatever cam timing that produces the most power in a given rpm range, will accelerate the quickest! Lol

Classic! And true!!
In general for a given rpm range used Earlier intake closing/more cyl pressure and later exhaust opening/more use of cyl pressure after ignition give more response before peak.
But that doesnt necessarily mean just close up the lsa as the increased overlap may not help response in your desired rpm range.
If you want responsive street fun(relativeto your combo), lean toward a smaller , (and in most cases a single patterrn, or close to if going shelf cam shopping), cam that that 'just' covers your real world max rpm, maximise compression within reason, dont go for big overlap, pick an intake that will just be big enough, pick a carb that will just be big enough, etc. It may not be the biggest hp beast and may not like to rom much past peak but will be a lot of fun getting there.