If i bore and stroke my L81 to a 383 stroker does anyone have any idea how much power gain that will acheive alone? i dont expect alot but it should deffinently come up from 190.... ( i will be doing cam, rockers,and heads) but im just curious about the gain of boring and stroking alone.

 

I would expect it to be proportional to the cubic inch increase. In other words the higher the ci the higher the hp. Exactly how much, I have no real numbers. Going from a 350 to 383 is roughly 10%. I would think a 10% increase is a figure I would use.

 

yeah thats what pops said thanks,, people laugh when i say im shooting for 400hp( anywhere in upper 300's also) i think it can be done with the l81 dont you? it will be a whole different motor so the l81 part dosent matter. i will be doing everything else under the sun to it i think 375 is a reasonable figure.

 

As a rule, your cam determines HP.

Now, for such a small change (350 to 383) you will not see a large HP jump unless you change the cam. Given an identical cam, a 383 will have about the same HP as a 350 but at a lower RPM. It will also have more torque. HP = Torque x RPM /5252

Now what a larger engine will do is allow you to run a larger cam with an acceptable idle. A cam with ~220 degrees at .050 lift is pretty good for a 350 but a 383 can run 230-240 at .050. So, you will get more torque and more HP.

Remember, when stroking the L81, you are working with a 2 bolt main. Use quality ARP bolts or studs and keep the RPM below 6000 in order to survive. With a 6000 redline, I would probably stay with about a cam that specs about 230 @.050 lift for the best street manner.

Now, a larger engine with more cam is flowing more air. So, you need good heads or you will choke the motor. The difference can be upwards of 50 HP.

FYI, here is my setup. L48 block, .030 over for 355 cubic inches, Edelbrock RPM heads, Crane HR276 cam (214/222 @.050, .488/.509 lift), Edelbrock 2101 Performer, recurved dist, rejetted Q-jet and Hooker sidepipes. Desktop Dyno says that I have 400+ ft-lbs of torque starting at about 2000 rpm and peak at about 400 HP at 5000 rpm. I do know that I have no traction and it pulls like a raped ape with a nice smooth idle and great street manners.

Last bit of motherhood, HP is for bragging rights, torque is what gets you moving.

 

the l81 may not have much hp but its torque is way up there compared to the hp, i think the torque is 275 or 285? thats what i feel when i drive it i agree completly thanks

 

Just a guess, but if you'd simply bore and stroke your L81, and rebuild it with the same heads, cam, carb, etc., it'd probably be a real stump-puller off the line, but nothing great at the higher RPM:
the 'long-arm' 400" crank will make great bottom-end torque, which'll get your '81 off the line, as SteveG75 so aptly put it.

If the stock cam only turns 'so-many' RPM in a 350 motor, it'd probably turn even-fewer RPMs in a bigger motor, correct?

SteveG75:
what does your Vette run the 1/4-mile?

 

No idea Glensgage, never been to the track. I do know that if I am just tooling along at 1500 rpm in first and floor it, the rear end just starts going sideways and my speedo reads alot faster than the car is going. Some of that may be due to the low 3.27 first in my Tremec.

 

Ignoring the unit conversions and efficiencies for the sake of discussion, the HP is the product of torque and RPM. Torque x RPM = HP.

If you increase the bore and stroke with all else being equal, you will get more torque everywhere and since bigger torque x RPM = bigger HP! You have more torque now, so the HP is now bigger.

If you want even more HP with the new setup, you can change the cam to move the peak torque to a higher RPM. Cams basicaly move the torque peak around the RPM band with all else being equal. Since this new cam will have your peak torque moved up to a higher RPM, you get more HP. Bigger torque from the bore and stroke x higher RPM where the torque peak exists = even more HP. Simple math really.

It's all about the stickers after that.

-Mark.