bottom end



how much hp can the crank and caps handle in a 250 with arp bolts ???

thanks 8)
Some Engine Experts should check me out, but here's my prediction model!:-

Rods on US engines are often cast, not forged, so an unshock- loaded 5300 rpm is about it before they want to set adrift. That's around 290 hp with a brilliant head. If you use forged rods and ARP Chevy Big block-style bolts, and something to control splashing oil windage in the sump, you can go up to 5900 rpm, maybee more. In a normaly aspirated car, this is 325-345 hp(net) where there is a great cyinder head flow, and three IDA 44 Weber down drafts hanging off a cut up log or SP/ME/2v head.

As a turbo, you can go to 450 hp on lots of boost 16 lbs+, 360 hp on 9 pounds with a good head and even a stock 155 hp gross (130 hp net, pre 1970 250 1V) could go to 220 hp easily on about 9 lb's. Turbos and superchargers with low compression and good detonation control show less critical loads on an engines bottom end than a noramally apsirated engine. A 250 turbo doing 290hp will be doing less revs than a non- turbo 250 doing 290 hp, no doubt about it, even if the 290 hp turbo has a log head, and the 290 carby non-turbo a 2V head.

Bad cylinder head flow is made-up for when you apply more pressure. More pressure, more flow, less load on your bottom end!

(I don't believe I said that!)
EXECUTE wrote....
Rods on US engines are often cast, not forged.....

Correct me if I'm wrong, I thought rods on 200 sixes were forged from the factory :unsure: Would a performance rebuild (resizing, maganafluxing, polished beams etc) make the handle more?????

Rods on '78 through '80 250's are cast - D8 part number. Earlier 250's used "C9" forged rods.

Never seen a cast 200 rod, but I haven't had a close look at that BB 200 in the machine shop either. What am I going to do with that thing...
I have a 76 so I have the good ones :D

so my dream of a 300 hp 6,000 rpm 250 can be real ? :eek:
"Which rods on which engine?" is the question.

General rule is that when you have to start milling rods widths to suit the crank bearing sizes, you've lost the plot. So I guess we are talking rods which are straight swaps, not offset grinds or whatever. Unless its a cast rod, prep it and use the rods that's there. It's usual to place good rod bolts in the stock forged items, and you'll have no problems.

In Oz, you can use the stock forged 3.3/4.1 rods on Aussie 200/250 engines, these are 6.275 or 5.885" for the real tall Aussie block. In America, you're fairly stuck for choice of good stock rods, but there are custom rods available in the US that are based on the DFX Indy car engines, which are around 5 to 5.25 inches which could be made to work in a 200 US six. They are a little taller than the stock 4.78" items, so they are not a straight swop. I think Jet made them out of titanium. Generaly, the 289/302 W V8 rods are way to tall for the 170/200 blocks, but can be made to fit the 600 thou taller 188/221 blocks.

Some of the South Americans use 5.37 inch 188 rods in there 221's, and use custom shallow deck pistons.
Curious as to where you developed you rpm limits, Xecute.

IIRC, an average of 4000 fpm average piston speed is a 'good' limit for a quality street type piston ring set (I think I got this from a Gordon Jennings publication from the early '70's). I have seem to recall that I've looked at a couple of more modern racing engines and have seen piston speeds of up to 4500 average fpm.

Now coincidently, 24,000/stroke will give you the rpm that an engine hits 4000 fpm.

I'm a 200 inch guy, so I'm trying to recall what a 250 inch stroke is- maybe about 3.9"- so I'm looking at just under 6200 rpm.

Polish the beams amd put good bearings in it
RPM limits are based on what racers have used without breaking things. The Aussie 250 development engineer is on record for saying the crank and rods will cut it at 7500 rpm, but that the rod bolts are plasticine.

5900 RPM is the shift point for that silver turbo six Fairmont that drops 460 rear wheel hp without dropping his 250 X-flow engine. (see Full Boost, an Aussie drag racing site for pumped Ricers and turbo-nuts).

On the latests Falcons with DOHC , the rev limit is still below 6000 rpm, and the rods are a similar design to the 250, even though the length is different. (6" rather than 5.885, and a really good swap if you want your US 250 to hack a few more revs up high..but you'll have to shave the pistons almost 58 thou to get the same CR). So it would seem below 6000 rpm is prudent.

Normal 250's run into crank movement causing oil splash at 5300 rpm, and the rocker gear is often only up to this rpm level before needing an upgrade. The feet per minute with the 3.91 stroke 250 should mean a little 200 can rec 25% harder with a simliar type of forged rod.
Thanks Xecute-

The thing I was looking for was why the 300 rpm- it looks like windage is responsible for a big percenrage of the problem.

Theoretical maximum speed for a 200 (looking at average psitone speed as the controlling criteria) would be about 7700 rpm- but I would anticipate the same windage and valve train problems long before that point

The thing I was looking for was why the 300 rpm- it looks like windage is responsible for a big percenrage of the problem.


should read

The thing I was looking for was why the 5300 rpm- it looks like windage is responsible for a big percenrage of the problem.

I belive that Isky or Melling, or someone makes some for our motors i will make some calls and tell you. aa guy in my club got some from nevada motor works and got some high performance ones. however be prepared to pay like 75.00ea pritty pricey but are garenteed to hadle up to 7500 Rpm.