Destroking - Facts or fiction

Jerryl

Tall Chinese Guy
#1
Cabin fever discussion.

Let's say you have engine combo X.
You destroke, maintain CR, same cam/rpm/heads/valves/boost/fuel. You adjust the tune . . . .
What's your opinion or experience on the performance results?
 

~JM~

Wrinkled Member
#2
Can't provide that answer.

I was fascinated with a beautiful billet crankshaft that I saw at a machine shop many years ago for a BBC. The engine was for a land speed class that had a CI limitation intended for small blocks. This guy ran a de-stroked big block for the better breathing heads.
 

Mike T

Well-Known Member
#3
Jerry I have pondered the same thing and came to the conclusion that they all are fast......just different approaches.
 
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Mike T

Well-Known Member
#4
Another way I think of it is if one guy has great success with a 3.75 stroke and another may great success with less than 3.0 so why wouldn't our stock 3.4 (which falls right in the middle of both) be just fine or maybe even ideal?
 
#5
I ran a Datsun 240Z many years ago, and I built a destroked 2.4L engine for it. I used the 2.8L block, bored .060 over, and the original 2.4L crank, with the 1970 small chamber head. I had larger valves put in it, and bought a really big cam.

That engine was so much fun. I had the rev limiter set at 7500 because that's where the cam stopped making power, but it would spin to 9K if I let it. And the noise? It was heavenly. A high compression straight six revving to the moon makes the hair on your neck stand up.

I still miss that car.
 

turbo89

Well-Known Member
#6
Cabin fever discussion.

Let's say you have engine combo X.
You destroke, maintain CR, same cam/rpm/heads/valves/boost/fuel. You adjust the tune . . . .
What's your opinion or experience on the performance results?
I would say nobody that is destroking would be leaving the same cam/rpm/heads/valves/boost/fuel ... nobody is just gonna destroke and adjust the tune .. that be silly

but to answer your question I don't run a 3.8 anymore and don't believe a STROKER will take a car down the road FASTER as an absolute.
Actually the opposite is true as a destroked 3.8 optimized will be faster / quicker .

A Few people have proven this LONG ago with the Buick .. and sadly not much has changed in 30 years and the results are still pretty much the same
 

Nick Micale

Tech Advisor
Staff member
#8
Cabin fever discussion.

Let's say you have engine combo X........................
Your question is too general?

Big displacement V-8's may see a change or not, and a rather small V-6 may respond differently?

Other variables would also come into play such a RPM limits, turbo, supercharger and other power adders.

A 210 cu. in. Buick V-6 with two 67mm turbos developed 1375 HP at over 9000 RPM on Kenny's dyno!
 

mikestertwo

Well-Known Member
#9
I can give you my experience with Oldsmobile V8's in particular the 400 cu.in. motors. Oldsmobile made 3 different 400 cu. in. motors in the day. They made a 4.00 bore X 4.00 stroke 400 cu.in., A 3.87 bore X 4.25 stroke 400 cu.in, and a 4.351 bore X 3.385 stroke 403 cu. in. motor. Overall performance meaning best low end torque and high rpm horsepower the 4.00 X 4.00 combination worked the best. You can't use the same cam profiles for the different combinations because of the different piston speeds associated with each engine. The big bore short stroke engine (403) was the worst performer but the most efficient (gas mileage) due to it's low piston speed. It had poor low end torque but would rev higher. The small bore big stroke 400 cu. in. motor ( 3.87 bore X 4.25 stroke) made good low end torque but was more rpm limited. The 4.00 bore X 4.00 stroke 400 cu. in. motor was the best compromise. It made good low end torque and still made good high rpm horsepower. Bottom line is it all depends on how the engine is going to be used. Application always determines what the best combination is.
 
#10
I can give you my experience with Oldsmobile V8's in particular the 400 cu.in. motors. Oldsmobile made 3 different 400 cu. in. motors in the day. They made a 4.00 bore X 4.00 stroke 400 cu.in., A 3.87 bore X 4.25 stroke 400 cu.in, and a 4.351 bore X 3.385 stroke 403 cu. in. motor. Overall performance meaning best low end torque and high rpm horsepower the 4.00 X 4.00 combination worked the best. You can't use the same cam profiles for the different combinations because of the different piston speeds associated with each engine. The big bore short stroke engine (403) was the worst performer but the most efficient (gas mileage) due to it's low piston speed. It had poor low end torque but would rev higher. The small bore big stroke 400 cu. in. motor ( 3.87 bore X 4.25 stroke) made good low end torque but was more rpm limited. The 4.00 bore X 4.00 stroke 400 cu. in. motor was the best compromise. It made good low end torque and still made good high rpm horsepower. Bottom line is it all depends on how the engine is going to be used. Application always determines what the best combination is.
The 400s were big blocks and had better heads. The early 400 with the shorter stroke was the best performance wise. The 403 suffered from crap small block heads, and they were crappy small block heads at that, low compression, and if it wanted to rev, it couldnt, because it had the worst bottom end. But lets be honest, these are Olds motors, none of them are really good performers.
 

750H.P.V6

Old confused member
#11
I'd say the output will be less if everything other than the stroke stays the same. A shorter stroke crank will make less torque. The point at which the engine makes peak torque is a function of the camshaft profile and probably won't change much. That being said if you make less peak torque at the same RPM you will make less H.P. If you were to change the cam and increase the max RPM of the engine and or run more boost the results will be different.

Neal
 

mikestertwo

Well-Known Member
#12
The 400s were big blocks and had better heads. The early 400 with the shorter stroke was the best performance wise. The 403 suffered from crap small block heads, and they were crappy small block heads at that, low compression, and if it wanted to rev, it couldnt, because it had the worst bottom end. But lets be honest, these are Olds motors, none of them are really good performers.
The OP's post was about bore stroke combinations not who's engine is better. And by the way the 403 we built had big block heads (yes they do fit with a little work) with a custom made intake.
 

Ttype6

Well-Known Member
#13
Cabin fever discussion.

Let's say you have engine combo X.
You destroke, maintain CR, same cam/rpm/heads/valves/boost/fuel. You adjust the tune . . . .
What's your opinion or experience on the performance results?
Since the displacement will be smaller,it will produce less power at all rpm. On a street car,it will lose power where you want it the most,down low in the rpm. To make the same power,you will need to run more boost then rev it higher. You don't destroke to make more power,you do it because you're forced to by running in a class that limits displacement. You want to have the biggest bore possible to unshroud the valves so you give up some stroke. The fact that the destroked engine can rev high is nothing to brag about. It had better rev higher. It needs to because it is smaller.
 

Ernie GN

Just floor it
#16
Cabin fever discussion.

Let's say you have engine combo X.
You destroke, maintain CR, same cam/rpm/heads/valves/boost/fuel. You adjust the tune . . . .
What's your opinion or experience on the performance results?

Let's say you have engine combo X.
You destroke, maintain CR, same cam/rpm/heads/valves/boost/fuel. You adjust the tune
Less power
You need to rpm the heck out of if and turn boost up.
It's just math
Not Magic
 

Mike T

Well-Known Member
#17
Was hoping this thread went a little further in detail with build recipes of "destroked" motors that are making big power. IE actual bore and stroke, cam, heads, rpm.

A friend of mine who has built a ton of Buick race engine (big cubic in strokers) made a statement that has stuck with me. Regarding RPM it was his opinion that higher RPM was "less" destructive than loading an engine "diesel" style. Lots of way to interpret that simple statement.
 
#18
Was hoping this thread went a little further in detail with build recipes of "destroked" motors that are making big power. IE actual bore and stroke, cam, heads, rpm.

A friend of mine who has built a ton of Buick race engine (big cubic in strokers) made a statement that has stuck with me. Regarding RPM it was his opinion that higher RPM was "less" destructive than loading an engine "diesel" style. Lots of way to interpret that simple statement.
It's hard to go into detail with so many variables.

Destroking reduces piston speed over the default. That's the benefit. How you utilize that benefit (lighter rods, higher RPM, etc) is completely up to the builder and what the application wants or can do.
 

Mike T

Well-Known Member
#19
Yes, was aware of the benefits of reduced piston speed. Curious how much destroke and RPM others are saying is the ticket. I'm assuming Jerry started this thread based on another recent thread where destroking was said to be the way to go. Like Jerry I'm looking for potential benefits beyond what conventional thinking tells us.
 

Jerryl

Tall Chinese Guy
#20
Elementary knowledge is;
Higher RPM typically increases mass flow, everything else staying constant.
A shorter stroke will decrease piston speed, allowing higher RPM operating conditions, if the combo is setup to spin higher.

So let’s say you have “Out of the Box champion ported iron heads”. De-stroke the engine, and the cam is kept the same.
There is 0.XXXX seconds to fill the cylinder, and it fills at 80% (Random number.) The same fill time is available because engine is run at same RPM, but the displacement volume is reduced.

Will the VE increase?
If VE increases, will the usable RPM increase?