Standard rod/piston limits with boost

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sowen
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Standard rod/piston limits with boost

Post by sowen »

I've been away from V8's for a while now, but I've got the itch again to build something silly and V8 powered 8)

I have a 3.9 with ARP main, head and rod studs/bolts which I want to use with a turbo or supercharger mounted on it. I can suss out the sizing/fueling/ignition stuff as I've quite successfully built my Rover P6 with a turbo'd M16 four cylinder engine running twin blow through SU's, running about 180bhp at 7psi so I'm quite confident in my abilities there.

Question is, what can the standard Rover rods and pistons handle before they start to fail, I've searched but not really found anything conclusive?

Is there any difference in strength/reliability between the 3.5/3.9 pistons and rods against 4.0/4.6 pistons and rods? I know a crossbolt block would be more beneficial to what I want to do, but I've got a 3.9 and may as well blow that up first :lol:


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Post by Rossco »

The answer depends on if you're asking how much bhp a standard setup can take before failing, or at what bhp standard rods and pistons should considered as being a cause for concern.

Standard rods start getting marginal around 350bhp although as always there will be someone out there running a zillion bhp on a standard setup.

Pistons can probably take a little more depending on how aggressive your settings are and how well built the engine is, i.e. piston clearances etc

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Post by sowen »

Thanks for that, it's more of an open question really, and I expect an open answer! I understand that there are so many factors involved, really just want to see if there's a ballpark safe max at whatever example combination there is.

350bhp on a relatively stock bottom end would be quite impressive, what capacity, boost and other factors would be making that kind of power?
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Post by Rossco »

On a 3.9/4.0 you'd need x-bolted block, decent heads especially on the exhaust side, headers, and a good cam. All buttoned up with ARP etc.

Standard ignition isn't going to live with this.

Boost would have to be above 10psi and progressive rather than suddenly coming on at 3500rpm

Easier to do it on a 4.6

sowen
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Post by sowen »

I have a 3.9 with ARP head and bottom studs, and am about to fit some ARP rod bolts. If I was to keep boost below about 7psi I guess it would be getting close to an accepted upper limit of reliability, but if I was to swap in a crossbolt block 4.0/4.6 I should in theory be able to squeeze a little more out of it, possibly to the magic 10psi mark and be somewhere near the upper limits of reliability then?
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Post by Coops »

running 4.0HC pistons on 4.6 rods in my 4.6 and running 14psi of boost,
BUT fuel and ignition are controlled via an aftermarket ecu,
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Post by sowen »

Thanks for that, plan is to get it running with the stuff I currently have, which includes a Megajolt set-up off my old Land Rover V8 which will sort the ignition side of things, then decide whether to stick with carbs or go efi. I have a few superchargers and turbo's to pick and choose between, and just sourcing a car now.

It's just that I want a ballpark figure to set it to when it's running, then see what happens, run as is or evolve it further :twisted:
1972 Rover 2000TC M16 turbo
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Post by DaviesDJ »

Think furling and ignition are critical in preserving the stock parts, take away all hot spots, cooler plugs, conservative advance etc.
Spent so much on trial and error!

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Re: Standard rod/piston limits with boost

Post by stevieturbo »

sowen wrote:I've been away from V8's for a while now, but I've got the itch again to build something silly and V8 powered 8)

I have a 3.9 with ARP main, head and rod studs/bolts which I want to use with a turbo or supercharger mounted on it. I can suss out the sizing/fueling/ignition stuff as I've quite successfully built my Rover P6 with a turbo'd M16 four cylinder engine running twin blow through SU's, running about 180bhp at 7psi so I'm quite confident in my abilities there.

Question is, what can the standard Rover rods and pistons handle before they start to fail, I've searched but not really found anything conclusive?

Is there any difference in strength/reliability between the 3.5/3.9 pistons and rods against 4.0/4.6 pistons and rods? I know a crossbolt block would be more beneficial to what I want to do, but I've got a 3.9 and may as well blow that up first :lol:
The first engine I used was a 3.9 with standard rods, non cross bolted etc etc
It used ARP studs and composite gaskets, and the original Omega pistons machined to lower the CR.

Even at around a very estimated 350-400hp it seemed to be very reliable. When it did go bang it was my own fault lol.

My main concern would be to use decent pistons. Standard ones have a huge slot behind the oil ring ? they are not suitable at all.

I wouldnt be concerned about the rods etc unless you're really going to push quite hard ( relatively speaking )
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Re: Standard rod/piston limits with boost

Post by sowen »

DaviesDJ wrote:Think furling and ignition are critical in preserving the stock parts, take away all hot spots, cooler plugs, conservative advance etc.
My turbo'd Rover P6 seems to be quite reliable so far, that's running twin SU's and 10:1 high compression. Megajolt and AFR gauge made it so much easier to set up and get running nicely :D
stevieturbo wrote: The first engine I used was a 3.9 with standard rods, non cross bolted etc etc
It used ARP studs and composite gaskets, and the original Omega pistons machined to lower the CR.

Even at around a very estimated 350-400hp it seemed to be very reliable. When it did go bang it was my own fault lol.

My main concern would be to use decent pistons. Standard ones have a huge slot behind the oil ring ? they are not suitable at all.

I wouldnt be concerned about the rods etc unless you're really going to push quite hard ( relatively speaking )
That's good to hear, I'm not after crazy high figures, just a fun little project using stuff I've got lying around, and if it works really well, all the better!
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Post by Eliot »

sowen wrote:I have a 3.9 with ARP head and bottom studs, and am about to fit some ARP rod bolts. If I was to keep boost below about 7psi I guess it would be getting close to an accepted upper limit of reliability, but if I was to swap in a crossbolt block 4.0/4.6 I should in theory be able to squeeze a little more out of it, possibly to the magic 10psi mark and be somewhere near the upper limits of reliability then?
7lb should be ok on the std engine - as long as you are looking after the fueling and ignition and keep the inlet temperatures sensible.
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Post by sowen »

Eliot wrote: 7lb should be ok on the std engine - as long as you are looking after the fueling and ignition and keep the inlet temperatures sensible.
I've learnt a lot turbo-ing my P6 with a 4 cylinder lump, that is running an AFR gauge, and has an air temp gauge on the plenum going into the carbs, and I've got a second to put on the air intake under the filter. I did a few motorway runs with my laptop fiddling with the ignition to get it running nicely, and after a few thousand miles I removed the head and there was absolutely no signs of damage to the pistons or combustion chambers from the setting up period. Not bad for running about 7-10psi on 10:1CR :D
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Post by Wotland »

Generally it is not power who kill rods but RPM.

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Post by sowen »

Wotland wrote:Generally it is not power who kill rods but RPM.
That's one of my thinkings, I have an idea of setting the rev limit to 5k, with a softcut on the Megajolt. Not particularly high but easier to live with than sat beside the road waiting for the recovery truck to come :lol:
1972 Rover 2000TC M16 turbo
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Post by thewedgeshop.com »

I am currently getting 275 HP to the wheels and 305 TQ running a stock 3.5 single turbo setup.

Running 10 psi and often spin it up to 6500rpm.

Specs below

http://www.thewedgeshop.com/clints-coupe.html
www.thewedgeshop.com
Rover V8 Performance
Single Turbo Rover V8
http://goo.gl/aDa3L

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