There are a lot of factors that increase operating temperatures:

Overbore amount
Compression
Engine size (as in cubic inches)
Forced Induction
Camshaft?

Just how much hotter does a LT engine run if a .030 383ci overbore is increased to a .050 overbore making 387ci?

Being cars run hotter with more compression, wouldn’t a longer duration cam actually reduce daily driving temps because it makes less static compression in the rpm ranged used in daily driving?

Guess the bottom line is I’m thinking of a bigger bore for better breathing unfortunately knowing it will increase operating temperatures. Wondering if more duration in a cam (which I plan on) will offset the heat somehow.

 

Internal combustion engines are heat engines. Some of it makes power, a bunch goes out the tailpipe and some of it is transferred into the coolant and then to the air via the radiator. The amount lost thru the coolant is proportional to the amount being produced, so if "daily driving" isn't all at wide open throttle (WOT), there won't be much more heat going into the coolant, because the hp produced only equals that required. For example, cruising at 60 may require 35 hp or so, whether it comes from a 387 or a 383 or any other engine size.

Even at WOT there's only about a 1% difference (387/383=1.010) in hp and therefore heat, and a cooling system which can hold the WOT output won't notice 1% more.

I'm assuming you don't have the 383 yet, because just buying new pistons .020 bigger (but otherwise identical) unless the engine needs a rebuild, is a waste of money. If you are starting from scratch, catalog .050 pistons are probably harder to find than .030 over, and therefore more costly.

Assuming you already don't have the "optimum" cam, increasing duration could increase WOT power, even if Dynamic Compression Ratio (DRC) was lower. Remember pumping more air at a given rpm is what makes more power, not a small fraction of a point of compression either static or dynamic. There is a good chance the slightly higher CR and bigger cam might make power more efficiently, and have almost no additonal heat rejected into the coolant.

Botom line: don't worry about the extra 1% and choose a cam based on the operating range you plan to use at WOT, and how it acts during your daily driving at part throttle.

If you live in a sunny climate, a black car with the 383 will run considerably hotter than a white car with the 387.

PS: Didn't you pitch for Detroit from '63 to '70?

 

Actually I do have a 383 and I'm not happy with the balance job or the clearances that a "name" shop did for me. Not only do you feel the vibration over 3000 rpm, but the oil pressure sucks indicating way too much clearance. I using 60 weight oil and when the oil temp get to 210-215, the idling oil pressure is only 20 psi.

Needs to be redone no matter what. If they screwed up the clearances and balance job the first time around, I don’t have a whole lot of faith in anything else they did. Figure it probably will need a new set of pistons anyway so why not bump it up a bit? “While I'm at it”, basically upgrade the whole engine with new cam and new heads. Only $ ya know and really not that much.

Realizing fully that 4ci extra even with better breathing from a larger bore won't set the world on fire, but hey if there are no downsides……… why not?

What concerns me is the potential of extra heat from going .050 over. I know Jordon Musser has problems with the engine running hotter and cracked a block when he did a .060 and I’m trying to split the difference. Also frankly wondering if more camshaft duration made any difference in running temps.

Did I pitch for the Detroit Tigers? Why no……. thank you. I did stay at a Holliday Inn though.

 

.060 over is probably stretching the block's integrity. It only takes a little core shift to get one thin wall and a cracked block. IMO, that's where the cracking and subsequent overheating problem comes from, not the extra cubes as you suggested.

If you need to overbore to freshen the engine, go from .030 to .040 over...or start with another unbored block which nobody has screwed up.

Good luck with your next 383/387.

PS: Glad to hear your're not "that" DMc.

 

Thanks for the get backs buys.

I'll probably stick my tail between my legs and just do the .040 over on the block I've got with new cam n heads. Afraid I've got too many "dead" blocks from "name" shops running around as it is.

Still curious.........I have heard in the past that larger bores do breath better. Any truth or is this just another wives tale? Sure, I'd bet going from 4" to 4.25 would make a difference, but .020 more? .050 more?

Thanks again guys!!

 

A few things to consider.

A bigger bore will flow better then a smaller bore, but most significantly, if the bigger bore allows you to unshroud a valve that would otherwise be shrouded. This is not that big a concern with most SBC heads on a 4” or larger bore.

Cooling with large overbores is not a issue because of the extra displacement or power, but instead with the fact that you’re removing highly heat conductive metal that also acts as a heat sink which better absorbs and distributes the heat from combustion, prevents hot spots…. The end result is that it’s easier to control and keep engine temps even with thicker cylinder walls. Unfortunately, I’m don’t have (have never seen) any data on how big an issue this really is. FWIW I don’t think I’d worry about this in a drag or street/strip car, but I would in a road race car.

 

Denny-
I would recommend sticking some .040 pistons in there..

 

.040 would be the next step up from .030, but I know people who had to go .060 n have no heat problems at all, I am at .030 over and its fine too.

Are you sure the vibrations over 3000rpm are the balance job? My IROC wasnt balanced n at 5000rpm its still running smooth, unless u really ****up a motors balance, the vibration is usually in the drivetrain. What I originally thought to be a motor balance issue turned out to be a cracked flex plate.

 

My 383 is going to be bored .020 over. (ok, so a 382) Why go more than you need to? Because if you go .060 over, and something breaks requiring you to bore it again and you have no material left in the block, you'll have to either junk it or sleeve it. I rather just leave room for error. 4ci isn't going to be noticable, sure it adds hp, but in the long run, it's like a K&N filter, do you really feel that 20hp gain?

 

Just to bust *****, actually its a 380.77..... at least that's what mine is .

 

 

Posted this is because I also have never heard anything objective regarding increase in temps from larger overbore’s. Bits of hearsay saying it does increase temps, but nothing you can put your finger on..

Like everyone on this list, love that little extra hp stuff.

Am I sure the vibration is the balance? About 99.99 percent sure. Nothing to do with speed……. Everything to do with rpm.

The engine was originally specked for internal balance with a neutral balance flywheel. It came externally balanced instead. After sending out the original flywheel and having weights put back on and then purchasing a new flywheel, it still has vibration over 3,000 rpm.

This particular shop admittedly jobs out it balance business and also builds a spec engine. They used a different brand of pistons on mine (probably used because the ones I got are more expensive) and there is a possibility that this may be the issue. The also admitted using flexplates as a reference for balance, but I can't imagine this having much effect. Sloppy work or pistons in my opinion.

Thanks guys!!

 

I get my balancing done by CRE, I give em my pistons, rods, bearings, crank, flex plate, balancer, crank gear and piston rings, then for $285 I get back a completely balanced assembly that takes into account every part of my rotating assembly. I wouldnt trust it any other way.