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turbodave
dave cox

(22 posts)

Registered:
04/30/2018 03:00PM

Main British Car:


Forged pistons - chevy 305 or ford modular 3.700? Rod options?
Posted by: turbodave
Date: December 30, 2019 09:18AM

What are your thoughts?
Will the factory 3.9 liners be reliable with the 3.736" bore of the chevy pistons?
Is the shorter 1.22" compression height of the ford compared to the 1.56 of the 305 better or worse?
What rods could be made to work with either?

If I'm going to go forged, I'm wanting an engine I can take to 7000rpm in a race engine; one used for endurance racing, so it'd need to be a robust solution...

Or I may plan a different route and see just what I can do with the most power/widest torque spread for the stock cast pistons (see other post).


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5863 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Forged pistons - chevy 305 or ford modular 3.700? Rod options?
Posted by: BlownMGB-V8
Date: December 30, 2019 10:26AM

I use custom pistons. Cost a couple hundred more but I get a no-compromise fit.

Jim


minorv8
Jukka Harkola

(190 posts)

Registered:
04/08/2009 06:50AM

Main British Car:
Morris Minor Rover V8

Re: Forged pistons - chevy 305 or ford modular 3.700? Rod options?
Posted by: minorv8
Date: December 31, 2019 03:23PM

I am building a stroker engine with 5.950" SBC rods and low C.H. pistons. Plenty strong for any Rover spec.


minorv8
Jukka Harkola

(190 posts)

Registered:
04/08/2009 06:50AM

Main British Car:
Morris Minor Rover V8

Re: Forged pistons - chevy 305 or ford modular 3.700? Rod options?
Posted by: minorv8
Date: January 02, 2020 05:50AM

I asked years ago Omega Pistons in UK if they either had or would do me a set of pistons for 4.6 Rover. They did give me an offer for minimum batch size of 12 pistons at 100 quid each. So pretty steep and I lost my interest.

If you consider other rods I would choose (in fact have chosen) SBC rods, preferably 6 inch rods. These are reasonably easy to adapt to Rover crank and cheap. And, you have a wide choice of bearings, even factory inserts will fit. When you narrow the big ends, you might think ahead and see how the inserts locate in the big end bore. Worst case: you either have to modify the insert lock tab grooves or machine the bearings. My stroker crank is a TVR 500 specific with narrow crank pins. The rod width is only 19.40 mm (around 0,76") . I had to machine the inserts narrower, too.

The rods dictate the C.H required, as well as pin design. Press fit or bushed. Chevy pin size is common. I prefer a short piston, typically less mass at higher revs. OTOH, being short it may also rock whereas longer pistons might be more stable in that respect.

My spec was supposed to be 90 mm stroke, 6.000" Crower rods and Real Steel +20 stroker pistons. These are forged Icons with 1,195 C.H. I was expecting a slight skim of piston tops. I could not do a dummy build since my block was still std size. I had to block bored and assembled the internals. OK, pistons +60 thou out of the block at TDC. To be honest that was not what I expected and could not really undestand why. Until I checked the stroke, it was 92 mm, not 90 mm as I was told when I bought the crank. I contacted Icon and asked if I can machine the tops +60 thou lower. Not recommended… I have less than 0.25" from piston ring to top. The piston might take it easily but when you think about the valve reliefs that I need on the pistons. Valve size is 1.94"/1.60" and the reliefs are very close to the ring groove. Not ideal.

So, I had to get 5.950" rods. That helped and I only had to machine some 20 thou off the pistons. The only snag is that these rods are SBC 350 items and for 2.100" crank pin. Luckilly there are 2.100 to 2.000 pin bearings available. Thick and stiff as hell, you canhardly fit them with your bare fingers. Way too wide to my spec so I had to narrow them from both sides. 16 setups on a lathe. Not cool.

To be honest I do understand why people opt for either stock engines or simple stroker packages.

Being a somewhat careless person I managed to destroy one piston ring. These are NLA from Real Steel. Turns out they are oversize rings for a V6 and not even for a V8. Apparently they made 3 V8 sets out of 4 V6 sets. Again, something to consider when you think about your pistons options.


tr8todd
Todd Kishbach

(367 posts)

Registered:
12/04/2009 07:42AM

Main British Car:


Re: Forged pistons - chevy 305 or ford modular 3.700? Rod options?
Posted by: tr8todd
Date: January 02, 2020 07:16AM

Before you dive into the bottom end, what are you doing for heads? What displacement are you planning? 7000 rpms is a lofty goal if your heads can't flow enough air. Rover heads are limited to how much air they will flow no matter what you do with the bottom end. Smaller displacement engines can run higher rpms to get to the max air flow. Larger engines can't run those higher rpms because there just isn't enough air getting into the engine. You can get away with more on street engines, but race engines are not as forgiving. Even my 4.0s would max out with rules limited semi stock heads, and the engine temps would get very high on 45 minute sprint races. On the Rover, you have to think backwards. Find out how much your heads will flow, and then figure out what's the best way to use that number on any given displacement.


BlownMGB-V8
Jim Blackwood
9406 Gunpowder Rd., Florence, KY 41042
(5863 posts)

Registered:
10/23/2007 12:59PM

Main British Car:
1971 MGB Blown,Injected,Intercooled Buick 340/AA80E/JagIRS

authors avatar
Re: Forged pistons - chevy 305 or ford modular 3.700? Rod options?
Posted by: BlownMGB-V8
Date: January 02, 2020 12:24PM

The TA heads are expensive but less than most aftermarket heads I think. The ports and valves are quite large. I bought a set of bare heads for right at $1600. By the time you add valves, springs, and rockers it can get expensive. I bought titanium valves as seconds and am cutting them down to fit. That keeps the cost down to about what steel valves would be. I also bought beehive springs, further reducing the valve weight. This should let me get the redline up with reasonable spring pressures. And of course I will be mounting a blower, simply because I want to. Many ways to fry this fish.

Jim


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