4.1 Production Block Reliability Test!

Nick Micale

Tech Advisor
Staff member
After 10+ plus years of racing my TA alum block, I felt with over 1000 sub-10 sec. passes I should do a complete freshen, but did not want to not have the race car down for long so I built a "clone" using a 4.1 production block to the same 800+ RWHP specs.

Having built many 4.1 street and strip production block engines I wanted to also prove, or dis-prove, how reliable it would be?

The race car is ~3500#'s and the turbo is a 70mm BB feeding well-ported TA alum heads, forged internals w/2 steel center caps. We also converted the car from 116 octane to e-85 with the production block.

It was tuned on a chassis dyno and had over 50 high 9 sec passes.

Friday night I was testing for an event on Saturday and the first pass was at 16 psi and all was good with a 10.3 @130 MPH. The second pass with the same settings it left the line like the first but was shocked when it hit the rev limiter at 7500 and of course slowed down, but picked up speed when shifted into third?

The data log gave an indication of a converter issue, but actually the sprague failed and this has happened in years past but not at WOT? In a TH 400 this will give you another "neutral" so the RPM will immediately spike.

Car drove on the trailer but was not happy. The block did its best to hold all together, but the crank is bent, SFI balancer trashed, 2 rods twisted and the threaded holes for no. 3 main cap are cracked.

In all my years of street and strip performance cars, this is my worst disaster of all time.

Over the past year, I have built about a dozen 4.1 builds, street and strip, 4 of them I used personally, and the worst issue has been a couple head gaskets.

In none of the 4.1 engines have we seen have issues with the block, head surface, oiling or lifter valley.

Compared to the 109 block, or the early 3.8 blocks, we just do not buy the comments of how weak or problematic these 4.1 blocks are?

Any production V-6 block should survive 6-700 HP when built properly, but this is not a guarantee, especially when something extraneous like I just experienced happens.

I just installed a fresh 4.1 so it will be running this weekend [with a new, improved sprague in the 400!] so I can continue my path to find out just how hard we can push a production block to determine a reasonable and reliable HP level! ;)
Thanks for the info Nick. I know there are probably a lot of us considering that route for a future build. Watching, and listening to others experiences.
Interesting info on the 4.1 production block. Surprised you aren't using the TR6 with it's rev limiter. Might have saved the engine.
One day when the stars all align and all is right in my world I'd love a setup as sick as yours...10.30 @ 16lbs holy crap! Sorry to hear your misfortune can't wait to read when it lives again!
were you planning on using all tbe 880rwhp? i woild think you would needa girdle. love these tests. jopefully it goes well
This just provided the motivation I need to finally purchase a 4.1 motor to build. Thanks Nick!

Posted from the TurboBuick.Com mobile app
Thanks for the info Nick. Best of luck on the weekend.:) If it makes you feel any better at least you can race this time of year. Cold and wet up here:rolleyes: Take care and enjoy
Are you saying that we do not have the option of eliminating the sprag when we run a turbo 400?

I cannot answer your question as I depend on the tranny experts to keep it together. :)

The previous 34 element HD sprague and drum I replaced a few years ago cost just over $300. My friend at Hughes said a new sprague/drum has since been developed which is twice as strong as the 34 element, and it is more than twice as expensive.

I went with the new unit as I never want to experience this issue again!

This is how we learn and improve the breed by taking parts to their limit and beyond! ;)
I had a beautiful 4.1 that was murdered by a Walbro F-269 pump that quit at the top of the 1/4.
I have another one going together but its slow going.
Thanks for sharing that Nick ,great info... curious what the desired shift RPM would have been? Cant wait to hear specks on the new(old) 4.1 T/A block your buildng.
Thanks for sharing that Nick ,great info... curious what the desired shift RPM would have been? Cant wait to hear specks on the new(old) 4.1 T/A block your buildng.

The specs are pretty much the same.

I had a 4.1 block machined so we installed the cam, heads and other good stuff from the broken engine and installed it Thursday, the tranny went in yesterday, so it will be running today or tomorrow. :)

Can you post pictures of the Sprag failure? I've had this happen too. Pushed the snap ring off and rolled over the elements. I have a trick to keep the stock type 34 element snap ring in place now (TIG Brazing). I'd love to see the new Hughes pieces too.
I just got my car back this weekend after having my 4.1 rebuilt. It's now 274ci with M&A heads and a 7168. It's on a conservative tune at 19psi on pump gas and dual alky, 533whp. I'm sure with a few more psi 700+whp isn't out of the question. Any damage to the block on that pass Nick? Still haven't heard a magic power number for block failure.
Last edited:
nick the guys with high horsepower stuff in my area running the 400 don't run the sprags as they were breaking too frequent.the trans lives a lot longer without the sprag.not sure on the new piece though that might change things.