I was curios as to why a stroker could handle a bigger cam and make a wild cam in a 350 act more streetable in a 383 or 396?

My thoughts were:

1: More cubes = more stroke

2: Strokers usually have lower rod ratio

Whats the answer?

Thanks,
-Stu

 

has to do 100% with volume of the engine. doesnt matter whether its done with increased stroke or bore.

because the motor is drawing more air in, vacuum will be higher, and the effective airfow velocity will be higher. more air in the same time = more velocity.

 

then why can a small engine such as a 302 have a similar cam (duration/lift wise) as a BBC 502. Wouldnt duration make no difference on the size of the engine, but rather the bore/stroke/rod ratio?

Just simply more displacement doesnt make sense to me. Maybe i am just ignorant. meh..

 

rod stroke ratio has nothing to do with it. Well, I don't mean to say nothing. There might be some small differences because of piston speed changes or something. But I really doubt there would be much of a difference at all with the same cam in a 4.155"bore/3.48" stroke "377", a 3.875"bore/4.000" stroke "377", and a 4.000" bore/3.750" stoke "377" (assuming the heads and compression are the same.)

 

BBC will have much larger ports and valves, so the "same" cam will not do the "same" thing at all.

like Jordon said, it's a volume to restriction ratio. a larger engine will generate the same volume of air at a lower RPM, which is why a 383 will work with a cam that's too hot for a 350. same ****, different engine speed.

that's a pretty simplified explanation, but it's not a bad one. you can dig deeper and calculate the difference between big pulses 2500 times per minute or small ones at 3500 with a constant flow/displacement rate, but that's beyond my grasp of physics just yet.

the point is to look at cubic inches times RPM, not just cubic inches.

 

I'm surprised nobody has mentioned head flow dynamics. Normally big cube motors
have head work to reflect mid to upper RPM sweet spots. That being said,
the valve lift and effective duration at the max head flow points will require
cam grinds that make smaller displacement motors bleed off pressure at equal
0.050" lift durations.

Why wouldn't stroke have a big role in the valve timing? If there is more dwell
at TDC and BDC, the overlap period, intake & exhaust opening/closing should be
tweaked to optimise the pulse timing of the intake and exhaust charge.

How about centerlines as well? If the piston acceleration is greater mid flight,
wouldn't the valve peak occur slightly later?

 

One aspect is explained relatively simply. If you look at vacuum at idle, a 500ci motor will be moving a lot more air at idle than a 350. So, even if the intake restriction is less, a larger motor can produce the same or better vacuum.

Rich

 

Yes its mostly volume but stroke makes a difference aswell, the longer stroke will hang around TDC for a much longer period of time then a short stroke engine, so things like overlap of the cam are very dependant on duration of piston at TDC, the cam & crank will still keep turning the same amount but now when the cam is 5% open on the intake lobe say the big stroke engine piston is still hanging around TDC while the short stroke is already moving air. Hence why this could change where the engine makes its power & how it behaves, with a short stroke your piston might have been 75% down the bore while the valve is at its optimum lift & flow, now you change the stroke & your valve might already closing etc... You also have to consider that although the long stroke is late on starting the race down the bore from TDC it will accelerate down the bore much faster then a short stroke setup.

Matt.

 

I thought it was more rod length that caused the piston to "dwell" around TDC. With a longer stroke doesn't the crank pin actually move farther for each degree of rotation?

So with a longer stroke, one usually needs to use a shorter rod than could be used with a shorter stroke, and "dwell" at TDC should be less with the stock stroke. Even with equal length rods this would appear to be the case.

Perhaps I'm missing something.

 

I think you know your not missing something! Of course the longer stroke engine with the lower rod ratio will linger longer around BDC and less around TDC all else equal.

 

Take the same cam, put it in a 350, then a 383 (same cylinder heads on each engine, same intake, same compression, etc). The 350 will probably peak close to 1000rpm higher and it'll make ~3" less vacuum than the 383 at the same idle rpm. So, the displacement has a good deal to do with it, just as Jordon said.

Granted, low lift (seat work) can also make an engine "cammy" or not, probably has something to do with valve seat-area flow at low lift. R/S ratio could be used to accentuate... ie, higher acceleration for poorer low-lift-flow and/or a lazy port.

-Mindgame

 

one point that hasn't been mentioned yet is that a bigger motor just makes more torque at lower rpms. the torque loss experienced with a larger cam, while still present, won't make as much of a difference because there is an abundance of torque because of the extra inches. this in my opinion makes a bigger motor more "streetable" than a smaller one with the same large cam.