3.5 heads on 3.9 bottom end , twin turbo

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Twinturbotr7
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3.5 heads on 3.9 bottom end , twin turbo

Post by Twinturbotr7 »

Hi all Im building a twin turbo injection rover v8, ive got a good 3.9 9.35cr block and Pistons that I know is ok and I've had it pressure tested , I have a pair of 3.5 sd1 heads that I built for another project they have had new wasted stem valves, seats recut ,a bit light porting and tidying up behind the valve seats etc , the stems modified to fit the new style valve guides ' new springs and retainers and steel rockers from v8 tuner , I want to use these heads on the 3.9 bottom end, I know the heads are a bigger cc so This should help to reduce the compression , will these heads be ok or am I going to loose power or run into other problems , I'm on a budget so don't want to buy some more heads and start doing the porting etc agin as I have a unused pair ready to fit ,let me know what you lot think
Thanks nick



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Post by unstable load »

There's no reason why they won't work with a boosted installation, but isn't 9.35CR a tad on the high side for your plans?
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Post by Twinturbotr7 »

unstable load wrote:There's no reason why they won't work with a boosted installation, but isn't 9.35CR a tad on the high side for your plans?

I'm hoping the sd1 heads will lower the compression even more ,here's my thinking , standard 3.9 engine has a 9.35cr with the standard 28cc combustion chamber heads and composite head gaskets , I'm using the same Pistons but my sd1 heads are 36cc so even with a steel head gasket the compression will be less than 9.35cr and if I use a composite gasket it will be even lower , does anyone know what the compression might be now with steel gasket and with a composite gasket ???

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Re: 3.5 heads on 3.9 bottom end , twin turbo

Post by russell_ram »

Twinturbotr7 wrote: I know the heads are a bigger cc so
3.5 and 3.9 heads are exactly the same and are interchangeable. The compression ration differences in RV8s come from differences in piston bowl volumes between high comp and low comp and for the different engine capacities. The only change in head bowl volume comes on the change over to 10 bolt, where the bowl is smaller to account for the thicker head gasket.

You'll probably want Low comp pistons to run much boost on a turbo motor?
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Post by russell_ram »

28cc plus composite gasket has the same total volume as 36cc plus tin gasket. You cant easily use 36cc plus composite to increase the total 'head' volume because your inlet manifold won't fit very well. - it won't be 'wide' enough.
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Twinturbotr7
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Post by Twinturbotr7 »

russell_ram wrote:28cc plus composite gasket has the same total volume as 36cc plus tin gasket. You cant easily use 36cc plus composite to increase the total 'head' volume because your inlet manifold won't fit very well. - it won't be 'wide' enough.
So has a 3.9 engine running 9.35cr that came out the factory with the later 28cc heads and a composite gasket got different Pistons to a 3.9 engine running 9.35cr with the earlier 36cc (3.9) heads ???

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Post by russell_ram »

No, if they both small pin crank then they are completely identical with 23cc (?) piston volume.

28cc head plus thick (8cc) gasket is the same total as 32cc plus thin tin (4cc) gasket. Piston volume (plus dead volume at TDC) and swept volume are the same for both - hence both then end up as 9.35:1

(Piston are physically a different component if the later motor you are referring to is a large pin crank, x-bolted 3.9(4.0L) unit although they too have same bowl cc as 'old' 3.9L engines)).
Last edited by russell_ram on Tue Jul 26, 2016 1:42 pm, edited 1 time in total.
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Post by Twinturbotr7 »

russell_ram wrote:Yes, if they both small pin crank then they are completely identical with 23cc (?) piston volume.

28cc head plus thick (8cc) gasket is the same total as 32cc plus thin tin (4cc) gasket. Piston volume (plus dead volume at TDC) and swept volume are the same for both - hence both then end up as 9.35:1

(Piston are physically a different component if the later motor you are referring to is a large pin crank, x-bolted 3.9(4.0L) unit although they too have same bowl cc as 'old' 3.9L engines)).

Maybee I'm missing something here but hears my thinking

If as you say small pin crank 9.35:1cr 3.9 engines have 23cc piston bowls?

So if I use my small pin crank with my 23cc 9.35:1 Pistons and my sd1 36cc heads+ 4cc tin gaskets this adds up to 63cc

Or if I use the same crank with the same 23cc 9.35:1 Pistons With the later 28 cc heads +4cc gasket = 55cc

So why won't my sd1 heads in this configuration lower my compresion ratio
The total cc is less ? Or am I being stupid

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Post by russell_ram »

So, your second combination has a total compressed volume LESS than the standard 3.9 9.35:1 set up (your first example).

CR is compressed volume divided by swept (+ compressed) volume. Your second example therefore has a HIGHER CR than the std 9.35:1 (your first) set up. Something over 10.6 : 1 with that configuration.


If you want to lower the CR then you need to INCREASE the total compressed volume from standard, for instance 36cc heads plus thick gasket.

But now your inlet manifold now won't fit very well because you've effectively moved the heads apart.

(Slightly doubting my 23cc piston volume now but the principal is correct)


You need something less than 9.35 for a turbo motor. LR low comp pistons will get you 8.13:1 with either head combo (36+4 or 28+8 )
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Post by stevieturbo »

russell_ram wrote:
You need something less than 9.35 for a turbo motor. LR low comp pistons will get you 8.13:1 with either head combo (36+4 or 28+8 )
Sweeping statement.
You dont need....it all depends on the build, the application, the tuning.

For a lowish boost boost build I'd have no concerns at all about 9.35:1 unless it had no intercooler and no real means of tuning it correctly.

Lower always gives a safer margin...but depending on goals really may not be needed at all.

I see no mention at all of goals, power hopes, the install, what controls etc may be in place.
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Post by Twinturbotr7 »

stevieturbo wrote:
russell_ram wrote:
You need something less than 9.35 for a turbo motor. LR low comp pistons will get you 8.13:1 with either head combo (36+4 or 28+8 )
Sweeping statement.
You dont need....it all depends on the build, the application, the tuning.

For a lowish boost boost build I'd have no concerns at all about 9.35:1 unless it had no intercooler and no real means of tuning it correctly.

Lower always gives a safer margin...but depending on goals really may not be needed at all.

I see no mention at all of goals, power hopes, the install, what controls etc may be in place.


I would like to achieve 250 to 300hp maximum but not going to get to upset if can't get that much, yes it will be inter-cooled and running emu stand alone ecu distributer less setup ,boost performance manifolds bought 2 years ago
, I've got 2 old rotomaster turbos from an old janspeed kit I bought years ago that I might use if I can get them refurbed I've also bought one of john eales new hybrid timing covers with poly v pullys
That's about it
Last edited by Twinturbotr7 on Tue Jul 26, 2016 4:29 pm, edited 1 time in total.

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Post by Twinturbotr7 »

If as you say all 9.35cr 3.9 engines have the same pistons

So an early 3.9 with 9.35cr with 14 bolt (36 cc heads )with standard factory fitted tin 4cc head gaskets and 23 cc pistons ,will have a gap above the piston including the dish of the piston of 63 cc agreed ?

But the later 3.9 9.35cr engine with the same throw crank with the same 23cc pistons ,but with 10 bolt 28 cc heads with factory fitted composite gasket 8cc have a total gap above the piston of 59cc

That's a difference of 4 cc how can both these engines be 9.35 cr

I thought if the total cc above the piston was a lower number the compression would be higher I thought thats why people skim heads or blocks to increase the compression ratio ?

Am I getting this completely wrong

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Post by stevieturbo »

Buy a pair of modern turbos.

There will be loads of used units about for little money that will perform better and not need rebuilt

Given the low goals, a couple of smallish units from a diesel would even do and spool very fast

TBH at only 300hp...I'd probably suggest a single turbo might be easier.
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Post by russell_ram »

stevieturbo wrote:
Sweeping statement.
You dont need....it all depends on the build, the application, the tuning.
Well true, but I was kind of assuming you wouldn't go to all the trouble of a twin turbo build unless you were going for some extreme power - turns out he's not . . .
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Post by russell_ram »

Twinturbotr7 wrote:If as you say all 9.35cr 3.9 engines have the same pistons

So an early 3.9 with 9.35cr with 14 bolt (36 cc heads )with standard factory fitted tin 4cc head gaskets and 23 cc pistons ,will have a gap above the piston including the dish of the piston of 63 cc agreed ?

But the later 3.9 9.35cr engine with the same throw crank with the same 23cc pistons ,but with 10 bolt 28 cc heads with factory fitted composite gasket 8cc have a total gap above the piston of 59cc

That's a difference of 4 cc how can both these engines be 9.35 cr

I thought if the total cc above the piston was a lower number the compression would be higher I thought thats why people skim heads or blocks to increase the compression ratio ?

Am I getting this completely wrong
You're using 36cc which is the number everyone quotes, my calcs used 32cc (measured by me).

Ignore the numbers and concentrate on this statement - an old 14bolt head plus tin gasket has the same volume as a 10 bolt head plus composite gasket. What you are suggesting now doesn't work.
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