I have a 1975 L48 which I assume is a two bolt main. I have put on 64 cc double hump heads, comp cams 262 h with .460, .467 lift, roller tip rockers, headers, edelbrock intake and holley 650 double pumper. It is time to do the bottom end. I would like to able to bring this motor to 5500 or 6000 rpm and am presently reluctant to do it. Will installation of a forged crank and new main bolts increase the rpm capacity of a two bolt main. I do not want to purchase aftermarket caps. I am unsure of the exact reason a four bolt main is stronger than a two bolt. Is it because of increased bolt capacity or the rigidity of the cap.

 

I have a friend with a 75 with almost the exact same setup. He has been drag racing it and autocrossing it for the past 2 years with no major problems.
He takes it to 5800 rpm when he runs it at the track and often when we are just running around town.
By the way he runs mid 13s with 308 gears and automatic.

 

5500-6000RPMs on an L48 is nothing! You could rev that right now if you wanted to. Your rev limit is determined largely by your rod/stroke ratio. Your 350 has a healthy 1.64 rod/stroke ratio. It's not so much the mains that you need to worry about so much as the rod bolts. When revving an engine to higher RPMs the rod bolts are usually the first to bite the dust....breaking a crank should be the last thing on your mind.

To be honest with you I wouldn't think twice about revving your L48 to 6000RPMs it could easily accomplish this. 6500RPMs is still do-able in an L48 (I would), but if you don't have an engine that will pull past 6000RPMs (which you don't) there's not much sense in revving much higher than 6000RPMs. I hope this helps.

 

I had a 2 bolt small jounal 327 that would see 7000 rpm a lot. It did have a forged crank but he bottom end was rock solid.

 

Sorry Bence, but this is bad advice. Please do not take this as a flame.

There are a number of reasons that his L48 should not be run over the factory assigned redline, but (perhaps) the most compelling is the fact that it has *cast pistons*.

I don't think they would be happy (for long) at 6500 . . .

*Properly* built, a two-bolt block can deal with a fairly high redline. My (former L48) has seen 7000 many, many times without any problems. It saw 7300 once and I pumped up every one of the damned lifters (yeah, hydraulics). After changing them out, I settled on 7000 as the practical redline.


[Modified by 45ACP, 6:13 PM 3/20/2002]

 

In some defence of Bence's statement. I had a '69 Camero several years ago. It was a stock 350-300hp, with a 327 "151" cam, modified Q-jet, headers.
cast pistons, cast crank.

This car saw 7000+ rpm, many, many times. Maybe I was just lucky. Never had any engine problems. Well, never had any drive line problems.
M20 Muncie with 3:73 12 bolt rear. One tough Camero.
Barry

 

I'm not at all surprised that there are cast pistons out there than can/did handle 7000+, but having handled OEM cast pistons from that era (mid-70's) out of my own L48 . . . I don't think they are up to the task. I never even considered replacing them with oem units. I got on the phone and ordered up some forged TRW's. Waaay stronger pieces!

FWIW, the figure that I have always heard for cast pistons (in SBC's) is 300hp max.


[Modified by 45ACP, 6:55 PM 3/20/2002]

 

45ACP:
No offense taken :blueangel:. The truth is I wouldn't think twice about winding the L48 out to 6000RPMs...6500RPMs is pushing it but is still do-able(but with the added risk). My point was a 350 has a fairly good rod/stroke ratio and could easily do 6000RPMs. Your comment about the pistons is valid but 6000RPMs is hardly anything to lose sleep over. Just my opinion.

*EDIT* I would also like to throw in that if I did rev the L48 to 6000RPMs it wouldn't stay there for long....keeping it at 6000RPMs for a prolonged period of time is just asking for trouble.


[Modified by bence13_33, 6:19 PM 3/20/2002]

 

im sorry but i disagree ..i build smallblock motors for circle track guys all year and use 2 bolt blocks which we turn 6800 rpm out of them some times 7200 with missed shifts..so they do handle it figure we run sustained rpms for laps upon laps..and ive only had one motor failure in 8 years...and it wasnt the block it was the crank..i use a L-48 block in mine 2 bolt ive turned it to 6500 and 7000 a few times..so im a firm beleiver 2 bolt blocks can take it.

 

The block is not a problem. I gave some friends a 68 2 bolt block which they put studs in. If I told you how hard they turned it in a super comp car you would call me a liar. The two ton cast pistons are a problem but 6000 is not going to hurt them.

 

:cheers: :cool:

 

also we use cast pistons and cast crank ...and i have pictures of the motor if you done believe me.

 

My 010 350 block is a 4 bolt main, but I got interested why you guys started talking about cast cranks and pistons. My old engine used cast speedpro pistons and a stock cast crank (budget) and I can't tell you how many times I have overreved that motor to infinity and beyond. The motor was apart last year for a freshning and cam change and the same used crank and pistons went back in as well. They wil take quite a beating. It has atleast once seen 10,000rpm when I dropped for second and went too far and got first. Only a tenth of a second of course, but it regularly saw 7000+ on the autocross track. If you don't beleive me watch the first video in my sig. This year the same old crank and pistons are going to get the same treatment.


[Modified by Stingy74, 7:35 PM 3/20/2002]

 

Jeesh I see almost everyone has seen the light ! When I saw the topic I thought everyone would be putting down a 2 bolt main block saying it could'nt handle high revs or high horse power.

A little while ago I was talking to my friend who owns a engine shop and we started talking about the 383 I was going to build. I told him I wanted to start with a four bolt main and he said " Why, we'll use your block and keep it numbers matching."

I replied that he must want to put in four bolt caps and he said no he'd just use ARP bolts. He then took me over to a picture on the wall of his old 69 Chevelle popping a wheelie with 9.80 written on the windshield and said "That's got a 2 bolt main in it, low et of the day" :D

 

don't grade 8 bolts have like a 40,000 psi tensile strength anyways? Seems like 2 are up to the task

 

Wow, pretty opinionated string....Here's mine :D
The two bolt block will handle 6000 RPM all day with studs, forged crank and even hypertuetonic (sp?) pistons. Stock cast crank and pistons are the limiter. Rods may need to be ground and peened to (suggest arp rod bolts too).
Build strong, last long. My two bolt has a forged crank & pistons, balanced with HP rods and all ARP studs & bolts. It has seen 6000 a few times. My current rev limiter is the cam that peaks at 5500. When it goes away, I shift! :cool:
:yesnod:

 

Ultimately I think it comes down to how long you want it to last, really. People say "my motor revs to 6 grand all the time" but longevity is not usually the issue for them. Our 2 bolt budget racecar motor runs 5000 to 6000 rpms for 20 miles a race night, and it holds up, but to say it would last for 50,000 miles doing that would be rediculous. The four bolt 350 is a stronger piece than the two bolt, I dont think anybody will argue that, and if I was building a motor asking what kind of R's it could handle I would use a four bolt or a splayed 2 bolt (still 4 bolts). The two bolt truck core i just got with the intention of rebuilding had spun a main bearing. The block was essentially junk. I could have put a new cap and crank on/in it, and line honed it and had a great motor, but a new core was cheaper. Mains can fail. I think maybe if I had a four bolt, it would have spun a rod bearing and it would have been $10 for a new X rod. However, if you have a good balance, good rod bolts, maybe some main studs and nodular caps if they are in the budget, and a line bore/hone, you should be fine to rev to or past 6000 or even 6500 occasionally and your motor would still last. If longevity isn't an issue, rev away man, rev till the cam stops pullin or the valves start floating, or the bearings start spinning. :lol:

 

Your small cam is going to decrease in hp as rpm go up. So your going to reach a point where it's pointless to rev higher A well balance 2 bolt can be can be drag raced for years with good rods and crank. Hot rod mag has done articles of 450-500 hp two bolts. The block doesn't fail. It's always the other parts. Splayed caps and all the high dollar stuff is for 600+ hp. I have a 4 bolt block with studs and just a good crank and parts.

Valve train failure is is much more common

 

gkull and some of the other above posts are right...your two bolt block should be good for at least 6500-7000 RPM if set up right. I personally would not run the stock rods and pistons past 6000 RPM and would feel much more comfortable keeping it maxed at 5500. Your cam will have signed-off well before 6,000 RPM anyway, it's power range is roughly 1000-5000 RPM (it's a MILD street grind)and you'll need more CFM than your 650 will flow above 6000...go with a 750 (after youv'e up-graded to a larger cam). Deen

 

FYI, a 2-bolt block can be tapped out to receive a 4-bolt main bearing cap. When this is done, the 2 outer bolts are usually splayed outward, which makes this modification even stronger than a standard OEM 4-bolt block.