Hi there,

i purchased a '87 L98 shortblock out of a GTA.
From what i can tell everything looked fine when i tore it apart to the shortblock, but i know the owner could never get it to run right, which i think was due to a fuel/spark problem or something. engine has about 100kmls on it and you can still see the crosshatch, but didn't dive in further to look at the bottom end.

anyways, i'm starting this motor as a build for my '89 Formula:

• The car is a '89 Firebird Formula, stock 305 TBI, has hooker 2055 shorties and 3" from collector back with a highflow Catco in between.
• Has a 3.27 9bolt right now and 700R4, but looking to put the 350 in with T56.
• DynamicEFI EBL Flash ECU, gonna add PFI mod to this.

Parts kicking around:

• AFR 195cc
• TPIS MiniRam
• LT1 T56

What do you think?

I don't have a specific power goal in mind, i want to get as much as i can with just a refresh of the shortblock, so i'll be using the bottom end in its stock form. Gonna disassemble it first to get a picture of its condition...

So, how much power / rpm can a L98 bottom end really take?
Any input is welcome!

Thanks
ownor

 

Short blocks get killed by RPMs. A stock short block cannot withstand more than about 6000 RPM ver many times, and not more than about 5500 RPM on a regular basis, without a MUCH higher risk of an explosion. Since torque is a function of CID and compression (a 350 will produce peak torque of around 425 - 450 ft-lbs on pump gas, at whatever RPM the induction system provides the most complete cylinder fill), and HP is a function of torque and RPM, your HP "survival" level is going to be determined by how many RPM you're willing to risk. Most likely that will be somewhere around 400 HP, depending on how close to disaster you're willing to run it.

What heads?

 

Well the l98 is a two bolt main block. I've never been a real believer that you need a 4 bolt main. Are u gonna spin 6000+ rpms? Are u planing on having 500+ hp and ft-lbs.? If not your probably gonna be just fine. and besides, have you looked at those bearing caps... There huge.

As long as youve got it torn down this far, why not have the shortblock remachined? You'll squeeze out 5 more ci, have new bearings and pistons, and fresh head surfaces
A fresh shortblock makes a very nice platform to build off of.

 

sofakingdom, you made some good points here. I was also supposing the rpms derived from the short block limits would be the limiting factor here for the whole build. What would be needed for a better bottom end? ARP rod bolts, maybe main studs? Anything to keep costs down is welcome

Heads will be AFR 195cc, i got them used with little milage but they are like 3 years old. They have some kind of competition port job on them by AFR but they are not what they would refer to as the Eliminators now, so prolly somewhere between the street and competition version. I cc'd them and they came out at 67cc chambers, 197 intake runners and 66 exhaust side. Will check valve sizes in a couple of days.

To TNT_Z28, yes the L98 is a 2 bolt mains block, but then again i don't think having more bolts in the mains is more than a nice-to-have feature in applications below 500 or 600 hp. From what i gathered, the 2 bolt main is also said to have a stronger casting, although that might just be true for 400 blocks, but installing splayed 4 bolt main caps into a 2 bolt mains block will far outlast a stock 4 bolt afaik.

I don't plan 6k+ rpm at this time. Just what the bottom end with a refresh can take. If i don't find anything wrong, i'm gonna use the stock crank, rods and pistons. As mentioned i want to keep costs down on internals if possible. Also no boost will be involved just in case you were wondering.

Keep the good input coming guys Thanks so far

 

The Mini Ram is a high rev, high horsepower manifold and goes against the lesson that sofakingdom just taught. You'd be better off with long runner TPI on your stock bottom end.

 

well said, that was the point i was trying to make...

IMO, its never a bad idea to have some machining done to a block with some miles on it, especially when youve got it torn down.

On the other hand, you said you can still see crosshatches from the hone, and if the bearings look fine, you should be good to go.

100k kilometers=about 60k miles right?

 

I would not put that manifold and those heads on a stock short block, unless you have another (and another and another and another .... like the guys that run the motor claimer classes, the sponsorship TO HAVE is a junkyard...). You will pop it. All it will take is one slightly too enthusiastic test drive.

The extra 2 bolts in the main caps are totally ineffective against the stresses of high RPMs. Not worth paying even one extra sime for, or spending even one extra minute looking for. In that situation, the block is completely irrelevant. You need better rods and a forged crank.

 

Thanks again for all the input.

I'm well aware that the 195 & MiniRam combo would be better off with more rpm or more cid (or both) lol. I just want to throw something together for now that works with what i have. I'm not too concerned by the torque loss the MiniRam might give in my application. I get what you are saying though.
So i guess i'll be getting ARP rod bolts installed at least, and maybe ARP main bolts/studs and an align bore/hone done (along with new bearings and prolly rings). Please let me know what you think about a bottom end refresh of this kind for an NA application of about max. 6krpm and 400hp..

p.s.: yup 100 kilometers would be about 60 miles, but where did i talk about km's?

 

By the time you buy and install those rod bolts, and have the rods reconditioned, you will probably have spent MORE on a bunch of stock rods, than just buying better ones.

But if you're just going to "throw it together" regardless, then ... it doesn't matter how likely it is to survive; you're going to do it, and whatever happens, happens.

I don't think anyone said anything about any "torque loss" from a MiniRam. Shouldn't be a problem (or a solution). What is being said is, the MiniRam is a high-RPM, high-HP type of piece; the heads you plan to use are high-RPM, high-HP parts; and if a motor is to be properly built to take advantage of those parts, and not just simply "consume" them from sitting on a shelf somewhere, then a stock bottom end is inadequate for the RPM that will result.

As said, you're evidently going to do this, no matter what we say. It will "work", but will have a high risk of a very short life and spectacular death.

Align-honing won't fix the basic problem. Main bolts or studs aren't the weak point in your proposed plan, so that won't change anything. New rings & bearings are a sine qua non of rebuilding a motor, no help there either. Nothing about any of that will keep the crank from breaking due to RPM-induced internal harmonics and stress. You are proposing to spend as much money on stock rods, as what MUCH better aftermarket ones would cost. All in all, not a sound plan IMO; very risky.

To keep costs down, spend your money wisely. Identify the WEAK points and ADDRESS THEM. Avoid spending money on stuff that will not provide tangible benefits (main studs), machine work that is not necessary (align-boring and/or honing if the main bores are straight and true already), excessive spending on attempting to "upgrade" something that you can spend THE SAME money on and make a bigger improvement (stock rods), and overstressing stock parts that will be at serious risk of total catastrophic failure (stock crank). Each of those items is an unwise use of you limited resource (money): it either accomplishes nothing, costs more than alternatives without delivering the same benefits, or fails to address the MOST LIKELY failure modes.

 

The Mini Ram will level off the torque curve, trading the peaky mid range torque of the TPI for a wide flat torque curve that will carry the engine to 7,000rpm with the right cam(ZZX). This is too much power at too high revs for a cast crank and stock rods. The ARP studs and rod bolts are a good idea but will only help on a rotating assembly designed to handle torque at 7,000rpm.

To assure you of any lasting success, I can only tell you to find a good machinist and follow their recommendation on choice of parts. You may save $$ by using a rotating assy kit from someone like Summit or Jegs. My machinist usually provides parts for less than even the discount houses.