How much HP does a aluminum block really lose HP wise vs. an iron block?

Did you mean gain?
Weight is everything when it comes to putting HP to work

I understand that. I've read in the past that an aluminum block doesn't seal as well as an iron block. Just wondering if anyone has done a real world test of identical engines with only the difference being the block material?

I believe at one point Best Machine took the internals and top end out of a Megablock and put them on/in an aluminum block.

They should be able to answer that question.

Has Pete been on Moparts lately? It's just a general discussion. An aluminum block has to be 150lbs+ lighter than a MEGA block?

I would guess that the difference might be just 10-15 hp if the variables (ring fit) are kept in check. There might also by dyno results that show less.

For a bracket race type BB Mopar build, whatever real difference there might be in power wouldn’t be enough to turn me away from running the 150lb lighter block.

Years ago Bob Glidden claimed the iron Boss 429 block made considerable more HP then the comparable aluminum block. They had to run a minimum weight and needed the weight on the front so weight wasn’t a factor. Aluminum doesn’t always mean lighter, a friend had a big Chief 600+ engine (aluminum) and replaced with a 900+ billit and gained 200 pounds.

The Crankshaft alone probably weighed 50lbs more... 900ci is a MASSIVE engine. No stock bore spacing, or head design there...

THIS! Whatever the power difference MAY be, it won't be enough to negate the weight loss advantage. The KB blocks are about 100 lbs lighter than a stock block and 150-175+ lbs lighter than a World block. Not to mention the aluminum block can be repaired pretty easily if you grenade it.

Actually the componites where lighter, titanium rods, rockers, Windberg lightened crank. He smoked a rod bearing from too little oil and ended up costing about 7 grand for a crank and one rod, pricy. That engine was around $85,000 new 1700 hp on motor alone, runs 4.30-4.40 in the eight, ex pro mod Stratus

My mega block is 275lbs. An Indy block is about 140lbs. Stock iron block maybe 230-240lbs, World iron 300lbs. Depending on your cars weight it would take roughly 30-50hp to make up that difference. My bet is the higher the power level the greater the difference in power output. Then depends on who's block you are using. Be it Iron or aluminum.
Doug

I have an Alum block. There Is no way I could put another 100 on the front of my car. However, I was told at BES that the iron block hands down way better. This is a power adder engine and they said the bores do not stay round. But its very hard to justify another 100+ on the front of an already heavy for the class car.
And yes youll be hard pressed to use the extra HP to pick up the ET from that extra weight. Its a loss - loss if that makes any sense...LOL

I can't give you exact numbers but I'm bless to have access to some guys that have tested both aluminum and iron blocks in heads up in a limited tire class with both aluminum blocks and iron blocks filled and unfilled. They ended up going quicker and faster with the filled iron block after a lot of testing and racing both combinations
What type of blocks do NHRA pro stock use today
This debate will be ongoing until further notice
I have a Koleno high nickel block in my shop and a new KB aluminum Wedge block on its way, I want to build a wedge B1-MC headed motor for running in NHRA Top Sportsman class in my 1965 Dart, I'll build the Koleno block first

Well, I have been lucky enough to be the victim of such a test......We struggled to get good oil pressure and cylinder sealing with our Indy alum block. I blame the bad machine work for most of our problems. Cylinders were .008-.011 out of round and we later found out, the machining around the #5 cam bearing was hemorrhaging oil like crazy and is probably why that engine threw a rod. Its pretty tough to hone a block with that much out of round cylinders. It beat the hell out of the hone, not sure how good we ended up getting it, but it was bad right from the start, along with the badly clearanced thrust face and other issues....Fastest the car went with the aluminum engine was 6.31....

A year and a half later, I have the same parts plus a new rod and a new piston in an Iron block, and the car has gone a spinning 6.25 in the 1/8th....Everything was the same, the oil pan, the oiling system, cam, head gasket thickness, carb, car.....No other changes except the loss of at least two rebuilds and lots of $$$$$$....

So the car gains probably 200lbs and still went quicker...with the mega block.

I am still really leery of buying another block from anyone to be honest. If you can't trust a shop to use a dial gauge....there is no point in buying it. Unfortunately, like most, mine was prepaid before I got it. Totally sucks on my end, but what else can I do...

As a side note with the aluminum block architecture...These things are sleeved, and then bored and honed to final size....At the standard 4.5 bore, the sleeves were either really thin or the aluminum wasn't machined correctly before the sleeve was put in, because the wear that we saw on the cross hatch in the bores was substantial....It didn't help that the honing could not take fix the out of roundness and taper the bores in the Indy block had...With a big stroke, the piston skirt is really short and it tends to shingle in there if there is any place for it to go....

If I were to do it over again, I would rather have a thicker sleeve and more material behind it, than a bigger bore....or just avoid the whole mess all together and go cast like I did....

Did you buy your block finished machined? 4.625 stroke? Was it ever on a dyno?

BTW, this would be for a 800-850hp NA bracket engine, not power adder. Something that only turns 6500-7000.

Get the block from Best , they know how to hone them, Or BES or other reputable shop. It is not like honing an iron block. They move around a lot when warm.
Then, if bracket racing, you going to need to figure out how to run it cool, 100* if possible so....I dont want to start that temperature debate , but i was told "run it as close to 100 as possible"

I built a couple of 900hp 572’s using the World aluminum blocks in 2011/2012.
Both have been freshened since with no issues.

I freshened the first one at 250 passes and the bottom end looked perfect.
The only problem with the motor at all was one of the Jesel intake rockers had worn through one of the snap rings from side loading you get with the offset rocker/angled pushrod arrangement.
That motor had 572-13’s.

The other one uses 440-1’s, and it didn’t have that snap ring issue for whatever reason.

The 440-1 build has been freshened a couple of times........ no issues.

Both of those motors made the power I was expecting, and the cars ran/run the numbers they should.

I’ve used one Indy block. It’s a mild bracket race 542(4.440 x 4.375).
I had them do the machine work to the block.
I measured everything, and it was all good.
I did nothing to that block other than clean it and put the motor together.

It’s been together and running strong since 2011.