Please Explain Benefits of Stroking VS Destroking

If you could get them both close to the 375 cid limit in cl*** then I would say big bore short stroke. For one thing the large bore opens up the area for valves and transfer p***age over a smaller bore. But I don't see how you could get a 375 inch Flathead Cad destroker. 325 might be interesting.

 

326= destroked

375 = stroked

 

Me too. Gets tougher to suss out when they both end up in the same cl*** and the short stroke motor is smaller. Next question, what sort of flow numbers are we looking at here? Whats the valve size? Rod length? Would you be running a longer rod in the short stroke version, or a taller (heavier) piston? Also respective rod journal sizes?

sorry, see you gave the rod length earlier. In light of the rod length, and the fact that the smaller motor is going to have a heavier piston, I am thinking the bigger motor would be the way to go.

 

A short stoke motor may require a high dome to obtain a high compression ratio which comes with its own set of problems.

Just one minor edit, and I agree completely.

 

If the stroking/destroking will come from welding/grinding the stock crank I think the most durable combo will be a stock crank, magnafluxed, with full radiuses and shot peened

 

Please hunt up a copy of Ed Staffel's great book, "How to Build Max Performance Chevy Rat Motors" because on page 51, Ed has a table that illustrate's how stroke changes effect a BBC engine with no other changes.

Ed went from a 4.50" b x 3.76" s = 478" engine up to a 4.50" b x 4.50" s = 573" engine in five different strokes!

The real deal here after studying the table is that we can produce more T and Hp if each increment of stroke increase is also made along with up-sizing the topend and cam spec's.

Otherwise, Hp generally stay's very close to the same at a given rpm level, but T goes up!

pdq67

 

When I first started this thread, last August, I was trying to decide which way to go. Valves will be somewhere around 2.02, and 1.72, depending on bench flow numbers. I took an engine down to Joe Abbin last week to get flow bench numbers, and I'm waiting on the results.

If destroked, the rod length would be greater to keep the same compression, with a light, forged, coated, blower piston.

Either crank would be a custom billet crank bt Crower, with larger than stock mains 2.75"( stock 2.459'), and a smaller journal diameter-2.1" (stock 2.46").

Cam is a monster cam in either case!

So, what's the right move?

 

Not old engines but I figure this is still fine.........

Please hunt up a copy of Ed Staffel's great book, "How to Build Max Performance Chevy Rat Motors" because on page 51, Ed has a table that illustrate's how stroke changes effect a BBC engine with no other changes.

Ed went from a 4.50" b x 3.76" s = 478" engine up to a 4.50" b x 4.50" s = 573" engine in five different strokes!

The real deal here after studying the table is that we can produce more T and Hp if each increment of stroke increase is also made along with up-sizing the topend and cam spec's.

Otherwise, Hp generally stay's very close to the same at a given rpm level, but T goes up as generally rpm drops!

pdq67

 

Ok, now its back to the drawing board. I am at work right now (yea, I get away with murder around here) I am off tomorrow, I will crunch some numbers and do some syms, meditate, check the alignment of the planets, then give you a totally un-informed WAG. One more question, well, two actually, what is the projected mph, how tall are the tires, and what are the available ring & pinion ratios? Believe it or not, I am kind of leaning towards the smaller engine now. I will have to look at peak piston acceleration on both, see just how bad the stroker is going to be. Thats a LONG rod...

 

And this is bonneville right?

 

160-180 mph (dreaming, of course!), with 28" tires. 29"tires, if the engine will still pull enough.

Champ quick change rear, so gears are no problem down to about 2.4 or so.

8.75' rods on the 373.5" engine ( stroke 4.75"), 9.1875 rods on the 326"9 stroke 4.0625")

Third option is 8.75" rods, with a milder 4.625 stroke.

Bonneville, so a 3 mile run!

So, pick the one that the alignment of the planets dictates!

 

maybe I'l dis-member a chicken and read its entrails...

 

...and I want a cam that sounds the part, but goes slow- I've outgrown going fast!

 

hey, wait a minute...Your not a cop are you???

 

Lol! No, but I like donuts...

 

Ok, the wife is working, and I am in charge of my two year old daughter today, so I am going to have to chip away at this in bits and pieces. I started with peak piston acceleration. I know winston cup motors are seeing around 6000 g's peak, and they do it for 500 miles, so that seems like a reasonable upper limit.Pretty sure I am gonna get a BUNCH of fuss from guys who havent done the math, but here goes.
The 4.75 stroke motor will be at 5913 g's @ 8300 rpm. The 4.625 stroke motor will be at 5864 g's @ 8400 rpm, and the little motor will be at 5967 g's @(brace yourself) 9200 rpm. Long rods REALLY help reduce the peak piston acceleration, thats why these numbers are WAY more relevant than overall piston speed.Speed doesnt kill, its the sudden stop at the end. Most guys depend on piston speed rather than peak piston acceleration because the math to calculate acceleration is complex.
Bottom line,I dont see the bottom end strength as being the limiting factor in this engines ability to rpm, its going to be a question of pumping enough air through the motor. I will follow up on that this afternoon when my daughter takes a nap, and see if I can establish the max. RPM that this thing will actually be capable of making power to. What are you using for a blower, and what is the size/number of throttle plates? Carb or injectors?
Once I found out the car had a quick-change my thinking changed back to the big motor. If you were using some kind of p***. car rear, you would probably be limited to a max. 2.70ish rear gear, and I am betting the engine wont flow enough air to feed the big engine at the required rpm. With the quick-change, Running out of rpm is no longer an issue, you can just stick a taller gear in.

 

The other issue of course is the valvetrain, and there you are on your own, I have no valve in block experience to draw on. I ***ume you have probably talked to PackardV8? I know he lurks here and Speedtalk, and also has Bonneville experience, I believe in the same cl***. I dont have Bonneville experience, just approaching this strictly from an engine standpoint.

 

Very cool George!

The engine has a 6-71 blower, with an intercooler. It's topped with an Enderle birdcatcher, and mechanical injection. I'll find out the throttle ****erfly size for you.

As for the valve train, I believe we'll be ok. Solid adjustable lifters, ***anium valves, etc.

New billet cam core, ground to .525 lift, and .280 duration. Engine will be ported and polished, and probably relieved- we'll know exactly when the numbers come from Joe Abbin.

Engine will be about 8:1, with 12-14lbs boost.

If we can pull 4th gear at 5500 rpm, I'll be thrilled! If it will stay together at 6000 rpm, that's icing, icing, and more icing on the cake!

 

I believe that you would do well to let the flow numbers dictate the displacement necessary. Once you have the numbers in hand and are confident of them, ask on Speedtalk, look at Wallace Racing formulas, etc. There is no sense in having a larger engine if the ports won't feed it. Especially in a flathead.
JMO
Jim

 

Well there is some guy on here that has some Bonneville experience in XOB/Alt and has some connection to a Packard V8. But no experience with Cad flathead V8s. I do know that the 6:71 makes flow numbers somewhat less compelling.

 

Okay, so if I can only get a 3.75 inch bore out of my '28 Chevy block, the optimal stroke is 3.375 (stock, of course is 4 inch)?

Yeehaw.

 

The A/MP Vette that we still have is 403ci its a 454 bb chevy .060 over i have a 348 chevy crank in it with spacers in the mains and a custom gear drive .
On the dyno it made 867 hp @10,500 rpm and the sound is like some thing you never heard in your life.
BB Chevy 10,500 rpm 5 speed = One hell of a ride

ROCKY

 

Hmmm, who would that be Rich?

The flow numbers really are for comparison, hp estimation- and for a possible N/A build.

I feel your pain. My stoke, stock, is 129% of bore!

That sounds like a wicked ride!

 

Can you either give me the check height the 280 was taken at, or the duration #'s a .050? Its quite possible the 280 IS @ .050. Like I said, I have no flathead experience, and I havent had time to do any syms yet, but my gut reaction is filling the cylinder at high rpm is going to be your biggest challenge, even with the blower. I would think you would be looking at a pretty wide lda, probably 114-116. Maybe Rich can chime in here, I would think the higher rpm you can make power at the better. I am still waiting for the little dictator to go to sleep.

Unless somebody REALLY messes up, or the valvetrain wont do it, it should stay together WAY past 6000 rpm, I THINK the issue is going to be filling the cylinder at high rpm.

 

George, I gave you bad info- here is the correct info: 275 dur. @.050. I'm trying to find the specs, but can't find the file. I believe LSA was 110 or 112

 

I havent forgotten about this, just ran out of weekend. can you also provide me with the port hieght/width? The more I have, the less GIGO.

 

George, that's kind of tough at the moment! I'm at work 1600 miles from home until the 11th.

 

GOD GAWD MAN! THIS IS IMPORTANT! Call your wife, mom, whoever, tell them to grab some verniers at GET OUT TO THE SHOP!