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Stroking an engine..
#1
I've always had stroker motors since they are pretty much the same cost when it comes to a rebuild and you end up with a "bigger" engine.
My question pertains to "what does stroking an engine do in relation to camshaft profile and idle quality?"
Example- if I have a 4v 351C with a "lopey" idle, and I don't change anything except the displacement to a 408c, would it only add torque and lower the power band? Or, would it also "tame" that cam some also? Does stroking..(hehe) also add horsepower or mostly just torque?
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#2
I recently did a short block swap in my 91 Mustang reusing all of the aftermarket top end parts and F303 cam. This was only going from a stock 302 to a D.S.S. 331 and it noticeably ate the cam in terms of lope and "sound".
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#3
Like everything in an engine there are compromises. It really boils down to what you want from the engine. A longer stroke will naturally add more torque output as the bore to stroke ratio moves toward "oversquare" (like a diesel, stroke larger than bore). The cam selection will need to change to maximize output with the extra inches. Think of it this way given a cam maximised with a certain lift and duration for a given cubic inch displacement will move X amount of air. If you just add to the stroke without changing the cam you lose volumetric efficency by not being able to fill the larger cylinder volume as well. It is rare for an engine to fully fill the cylinder to atmospheric pressure on the intake stroke which would be 100% volumetric efficiency. Most naturally aspirated engines are at 75 to 80% VE.

It is really more complicated than just cam design. Port configuration and valve size would be your biggest gain in raising VE in a stroked application.

I hope that makes sense.

Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

[Image: 1_12_09_14_10_15_11.png]
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#4
Sort of. I reason I was asking is, let's say I had a little more can than I intended to get in a 351. If I were to stroke the engine 40-50 cubic inches, would that tame the cam and idle "characteristics"? Also, if a given "recipe" (displacement, compression, cam) produces a certain HP/TQ, would that same increase in displacement increase the HP also, or just the TQ.
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#5
The longer stroke will increase torque with all things being equal. I do not see an appreciable increase in horsepower by just lengthening the stroke. It seems like alot of expense to cover up a cam choice. Wouldn't it be more cost effective to change the cam?

Jeff T.

Low buck, touring style, '73 Convertible "rolling restoration", 351c, 2v heads with a shave and a haircut, Performer intake, Holley 650(ish), roller rockers, screw in studs, guideplates, stainless valves, Duraspark / Motorsports MSD, T-5 conversion. 1-1/8" front, 3/4" rear swaybars KYB shocks and some home brewed subframe connectors. Future plans; JGC steering box, Cobra brakes and... paint, interior, etc.

When I die I want to die like grandpa, peacefully in my sleep... not screaming, like his passenger.

[Image: 1_12_09_14_10_15_11.png]
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#6
Adding 50+ cubic inches will tame the same cam a bit as well as lowering the RPM at which peak HP occurs. How much depends on the cam and the displacement increase.
Torque and horsepower are linked together. Given a constant RPM, if torque rises so does horsepower.

(Torque x RPM)/5252* = Horsepower

Ex. (375 x 4000)/5252* = 285.6 HP

(425 x 4000)/5252* = 323.6 HP

* 5252 RPM is the point where torque and power curves cross.
Chuck
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#7
droptop73;106960 Wrote:The longer stroke will increase torque with all things being equal. I do not see an appreciable increase in horsepower by just lengthening the stroke. It seems like alot of expense to cover up a cam choice. Wouldn't it be more cost effective to change the cam?

It certainly would. That isn't what I did, but just an expmple of what I was asking. I found a horsepower chart in another forum pertaining to the 460 bigblock. But it only shows horsepower and RPM levels. So, if I found a "horsepower" rating that I wanted to reach but wanted slightly milder idle characteristics as well as lower the max redline, would a stroker do the trick?
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#8
cazsper;106985 Wrote:
droptop73;106960 Wrote:The longer stroke will increase torque with all things being equal. I do not see an appreciable increase in horsepower by just lengthening the stroke. It seems like alot of expense to cover up a cam choice. Wouldn't it be more cost effective to change the cam?

It certainly would. That isn't what I did, but just an expmple of what I was asking. I found a horsepower chart in another forum pertaining to the 460 bigblock. But it only shows horsepower and RPM levels. So, if I found a "horsepower" rating that I wanted to reach but wanted slightly milder idle characteristics as well as lower the max redline, would a stroker do the trick?
I hesitate to even attempt to answer this question. The field is full of land mines. If you want more power but milder manners, more displacement is an obvious solution, especially if it can be done with little to no increase in engine weight and, at moderate cost, i.e. a stroker motor. However, The choices are not infinite and each decision comes with consequences. For example, a true pump gas, naturally aspirated, 351C based stroker motor of 408 CID that has 17 in. hg. vacuum at 900 RPM, and make 650 HP is NOT going to happen. Basing an engine build on fixed HP or TQ number is not something the average person should do, especially for a street car. Goals should be based on what you REALLY want, can afford, have the skill to execute, and can live with the real outcome. Be realistic with your goals, match all the parts for the entire car, and you will be happy with the outcome. On the flip side, a pump gas 408C that makes 17 in hg. at 900 RPM that makes 430-450 HP is not out of the question. Not matter what you do quality machine work and technical support is critical to success. Good luck with the build. Let us know what you decide to do. Chuck
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#9
Thanks Chuck.
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#10
c9zx;106966 Wrote:Adding 50+ cubic inches will tame the same cam a bit as well as lowering the RPM at which peak HP occurs. How much depends on the cam and the displacement increase.
Torque and horsepower are linked together. Given a constant RPM, if torque rises so does horsepower.

(Torque x RPM)/5252* = Horsepower

Ex. (375 x 4000)/5252* = 285.6 HP

(425 x 4000)/5252* = 323.6 HP

* 5252 RPM is the point where torque and power curves cross.
Chuck
Good formula to know
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