[67 ProTouring Catalina]

this maybe a dumb questions guys, i figured someone in here would know. will a 4.75 stroke crank fit in a stock block ? i'm thinking i want the biggest ci motor i can stuff in my Catalina, no big cam or anything, 6X heads and a nice smooth,smooth idling hyd.roller. what do you think?

[Steve Barcak]

It can be done. I built a '59 389 with an even larger stroke at 4.800". However, I also raised the deck 2" to run a much longer rod and built an extremely heavy duty 6 bolt main lower end where all 5 mains were tied together. It is quite elaborate compared to anything else I have seen in the Pontiac world. this was done before aftermarket blocks were available.
When you increase the stroke, you do lower your torque curve but, also reduce your rod to stroke ratio. This adds side load on the block and lowers rpm level.
If you wanted to build a street motor with this much 'arm' as they call it, you could. If the power level was modest and the rpm was kept down, you would probably be OK. Building this type of motor is known as a 'hand grenade' because of the stress level placed on it. Definately make sure the block in in good shape.
However, I think you could be plenty happywith a 4.25 or 4.5" arm and a slighly better r/s ratio. You will also have to consider your chamber size on your heads.

[BruceWilkie]

At 4.15 bore thats 513 ci and a bunch of stress on both the lower end and the side loading on the bores. Might be better to go bigger bore. Aftermarket block but you could possibly swing a deal on 8 sleeves to 4.25 bore. Combine with 4.25 stroke it gets you about 482+ inches and alot less stress. But cost could be as high as just gettin the aftermarket block and building a 505 -535 combo. There are few factory blocks as strong or stronger than the 59 389. Most are rare. 59 block not so rare but needs a bell and starter mount adapter. I made my own a bunch cheaper and better(well maybe not as pretty) than kits available. Kits raise cost to near halfway to better modern aftermarket block. I got $25 bucks in material in my adapter but a bunch of labor and precision is critical. Mine can serve as a midplate as well. 59 blocks are shorter from thrust bearing to bell flange(flange recessed deeper) than 61 up blocks. 55-60 (61-64) use differing bell mount starters. My adapter is fitted to a rescued original 58 starter.(thanks Mark L) I could easier use a RobMc for a Toranado/Eldorado as it has same 3.25" register and bolt spread and infinite solenoid location. But I wont need much starter on low compression turbo motor.

Personally for what you want a forged 4.25" stroker kit for a 67-74 400 block should fit the bill quite nicely or cheap out with the cast crank version.(I wouldnt) No reason you couldnt put your Cat at low 12's or better and still be a great driver.(though a head upgrade would help ease that task)

Sorry I just cant get excited about 4.75 stroke especially in any stock block.

[rad400]

I am not an expert but as other have stated you can make plenty of power with a 4.25" stroker kit. I would save the 4.75 crank for the aftermarket blocks. I would hate to see a broken block over this. Good luck to you.

[ho428]

My question about these huge stroker cranks is at what length is the return deminished performance wise?
Other than valve shrouding using larger valves the whole down side of the 326 and 350 was the B/S ratio, with the bore being so small vs stroke, but it was still around 1:1. Using 2.5 and 2.75 strokes in a 4.25 bore or less block the B/S ratio is considerably worse than the 350, much less the 326.
You're also doing the same thing percentage wise of valve size vs the B/S ratio in a 4.25 or smaller bore and a 4.5 or larger stroke.

Are we breaking a threshold of piston speed with the longer stroke that overcomes this?

Inquiring minds just want to know since I've never built such a large engine or understood the theory on these long stroke undersquare engines.

[Steve Barcak]

Originally Posted by ho428

My question about these huge stroker cranks is at what length is the return deminished performance wise?
Other than valve shrouding using larger valves the whole down side of the 326 and 350 was the B/S ratio, with the bore being so small vs stroke, but it was still around 1:1. Using 2.5 and 2.75 strokes in a 4.25 bore or less block the B/S ratio is considerably worse than the 350, much less the 326.
You're also doing the same thing percentage wise of valve size vs the B/S ratio in a 4.25 or smaller bore and a 4.5 or larger stroke.

Are we breaking a threshold of piston speed with the longer stroke that overcomes this?

Inquiring minds just want to know since I've never built such a large engine or understood the theory on these long stroke undersquare engines.

I am not sure there is a limit. Everytime Pontiac increase the stroke, it made more torque and, generally, there is no replacement for displacement ( other than supercharging or N2O ). You just move the power curve down the rpm scale depending on many other factors- rod length ( limited by deck height ), head and induction capacity, etc.
It is the same way with other engines too. or they would not make giant engines with monster strokes.

[estep455]

i have a friend that runs a 4.75 stroke crank and is .100 over on a 400 block. he runs it in his dragster and i wont get in the other lane against him! shifts it at 6k and runs 5.0s (1/8th mile) been 4.90s. cant get him to buy an aftermarket block and he has a gift certificate for a grand off! i still say my 535 with standard port heads will whip his and he has wide ports on it.

[Jim Zeek]

I ran a 4.500 in a stock 400 block.

[ho428]

Using 2.5 and 2.75 strokes in a 4.25 bore or less block the B/S ratio is considerably worse than the 350, much less the 326.

Sorry, I meant 4.5 and 4.75

I can see it moving the rpm and pwr range down, I just have to think the percentage gain would start to decrease as the stroke increases.
Seems like a 440 (4" stk)or 480 (4.25) would make more power per cu in than any other size due to it being pretty sq.

[KEN CROCIE]

Let's say we have a 4.75 stroke and the min. comp. height you would run is 1". The bore length on a pont. is 6.5 " So-the pin is 1" down in the hole and the crank pulls it down another 4.75". 5.5"(where the pin is )- 4.75" = 0.75". which means 0.24" of the pin is BELOW the bore . Can you say "eat your Pistons"? Run the 4.5" crank.

[67 ProTouring Catalina]

my 67 is pretty heavy, i just thought it'd be cool to have a really big motor in it, i want to put my Tripower onit and be able to say it has a 500+ motor in it, i would have o part in big cam or racing or anything, the car just looks huge to me so i figured why not put a huge motor in it

[rad400]

Than do a 505 IAII block.

[mgarblik]

If you decide to go with an aftermarket block like the IAII, I would throw my vote in for a 541. The 4.375 bore and the 4.5" stroke is super sweet and many different combinations have been built for years now. I often felt that if I still had my 66 2+2 convertible, at 4270 Lbs., this is THE engine I would build for it. It was a blast with the 421 HO, but oh what another 120 CI would have done for it! 541's are easy and with the low cost Chinese cranks are not too expensive. A 600+ HP engine that would idle smoothly at 850 RPM,s and tear your neck off your shoulders with all that torque sounds like the ticket to me.

[67 ProTouring Catalina]

would iron heads even work with this kind of ci? what compression could i get?

[PONTIAC DUDE]

Originally Posted by mgarblik

If you decide to go with an aftermarket block like the IAII, I would throw my vote in for a 541. The 4.375 bore and the 4.5" stroke is super sweet and many different combinations have been built for years now. I often felt that if I still had my 66 2+2 convertible, at 4270 Lbs., this is THE engine I would build for it. It was a blast with the 421 HO, but oh what another 120 CI would have done for it! 541's are easy and with the low cost Chinese cranks are not too expensive. A 600+ HP engine that would idle smoothly at 850 RPM,s and tear your neck off your shoulders with all that torque sounds like the ticket to me.

X2, but keep the bore at 4.350 w/4.5 stroke and then would be 535 cu.in. Just a little better gasket seal & room for next bore.
Just got a call yesterday from my buddy Paul and he finally got to take his car out. 67 droptop GTO with a 535 and my little 230/236 Butcher cam. Say's it idles like a champ and will tear the hides off his tires. LOL. about 550 hp but plenty of room to cam upgrade later if he wants to.

66 bonne is running one of my stock block 455's (4.185 bore) with a 4.5 crank.
65 Catalina and running 10.70's on pump gas and my custom hyd roller. So far.

[Junkyard Dog]

Originally Posted by PONTIAC DUDE

66 bonne is running one of my stock block 455's (4.185 bore) with a 4.5 crank.
65 Catalina and running 10.70's on pump gas and my custom hyd roller. So far.

Tell us more about this. I thought that it was not good to put a 4.5 crank in a stock block. You have two of them, one running in the tens?

[PONTIAC DUDE]

Originally Posted by Junkyard Dog

Tell us more about this. I thought that it was not good to put a 4.5 crank in a stock block. You have two of them, one running in the tens?

Yes. as his combo is a tank and needs torque thought it was worth a try. But you need heads to flow some fuel and air. Otherwise it's a waste of time.


455 block with factory 4 bolt caps and bottom filled. 5800/6000 rpm max.

To me a 4.75 stroke in a stock block is a waste of time and money that can be best spent elsewhere.