Rover V8 Turbo Compression ratio..

[neal1980]

I am building a low boost Rover V8 Turbo engine. I have had my compression measured and its coming out at 9.9:1.

I have a late cross bolted and top hatted 4.6 engine. Its on the original pistons.

Is this compression ok?? I have a megasquirt for fuel and ignition.

I have come across a place selling de-compression plates by using a set readily avilable I can get the compression ratio to 9.4:1 , 9.1:1, 9.0:1, 8.85:1, 8.6:1 or 8.3:1

Any suggestions??? Dont want to spend thousands on pistons as I plan to LS engine eventually anyway.

Thanks
Neal

[Boosted LS1]

That compressions fine. With your engine you get the solid skirt pistons and better rods as well (when compared to non cross bolted engines) so you'll be ok. Expect to use 7-10 psi with that c/r.

I love the LS but still lean towards turbocharging the rover in some instances.

ETA, I think decompression plates are nasty and they'll screw up your inlet alignment on the rover. As Eliot says, do the tune carefully as you are at the higher end of the compression scale. Don't forget that the dynamic compression can be lowered by selecting a different camshaft.

Edited by Boosted LS1 on Monday 2nd November 16:16

[eliot]

I would suggest you creep up on your tune slowly. Start around 4psi and work your way up slowly over a period of time.

[neal1980]

Thanks boosted & elliot. Saved me a few quid then. I thought it might be ok. I plan to use front mounted intercooler and possibly a snow performance water injection as well. And a quick one what spark plugs you reckon I should run?

Neal

[Boosted LS1]

NGK Projected nose types with a 7 grade/heatrange. I've forgotten the part numbers but there's info on the NGK site. It'll be a BP7ER number iirc or something like that.

[Pumaracing]

Sounds very high to me for turbocharging a 2v engine especially with a short duration cam. I would have thought you could get away with a few psi of boost at 9:1 but nearly 10:1 is seriously into normally aspirated territory. 4v engines can usually cope with about 1 CR ratio higher than 2v's in any given state of tune because they burn faster and have better swirl but I usually like to see turbo 2v engines somewhere in the 8s. High 8's for mild boost and low 8's for serious psi.

I'm not a fan of deck plates either but you can usually find some extra volume from combustion chamber machining and/or piston machining.

To drop the CR by 1 point you need about 8cc per cylinder which I would have thought should be doable quite easily.

[GreenV8S]

Pumaracing said:

High 8's for mild boost and low 8's for serious psi.

This is a discussion I'll be following with great interest. What do you call mild boost?

I've seen similar figures quoted on other forums, then contradicted by people who sounded quite experienced saying that higher CRs could be tolerated these days due to improved management systems. My own limited experience is that my 4.6 RV8 at 10.5:1 worked OK at 0.3 bar but I had to run rich and late to avoid detonation; at about 9:1 now and 0.6 bar I'm running at 12.5:1 AFR and pulling some timing off to avoid detonation but it seems quite happy at around 20 deg advance. Mine is running very hot and a properly intercooled system ought to cope cope with boost + CR far better than mine, I should think.

[Boosted LS1]

I've had very basic turbocharged 4.5's and above on over 9:1 c/r which used to be perfectly happy on a sustained 7 psi boost and flick up to 10 psi during gearshifts. With good management and WI then 9-10 psi will be fine on a rover street car where boost won't be sustained for very long.

Detonation would kill the early engines with their weak slotted pistons.

[stevieturbo]

Pumaracing said:

Sounds very high to me for turbocharging a 2v engine especially with a short duration cam. I would have thought you could get away with a few psi of boost at 9:1 but nearly 10:1 is seriously into normally aspirated territory. 4v engines can usually cope with about 1 CR ratio higher than 2v's in any given state of tune because they burn faster and have better swirl but I usually like to see turbo 2v engines somewhere in the 8s. High 8's for mild boost and low 8's for serious psi.

I'm not a fan of deck plates either but you can usually find some extra volume from combustion chamber machining and/or piston machining.

To drop the CR by 1 point you need about 8cc per cylinder which I would have thought should be doable quite easily.

Im running over 20 psi with around 9.5:1 CR, although it isnt a turbo. Its still a 2v engine though.

With proper intercooling and tuning, 10psi should be ok on his CR. With water/meth, then it will defo be fine. Although a little lower CR would add some safety.


Easiest option to lower the CR, is just to buy some OE 4.6 low compression pistons.

[eliot]

I'm running 10psi on 9.25:1 (350 chevy with ally heads). I retard from 36' down to around 24' (depending on fuel grade).

[neal1980]

Interesting views. I have the heads off the car at the moment how much cc's can you get by machining the combustion chambers??

Neal

[stevieturbo]

What heads are you using ? 4.6 or the earlier SD1 type ?

Earlier SD1 type have larger chambers to start with.

And Ive never been a fan of tampering with combustion chamber shapes

[Boosted LS1]

neal1980 said:

Interesting views. I have the heads off the car at the moment how much cc's can you get by machining the combustion chambers??

Neal

Not much. The sd1 28cc heads can go out to 34cc but that's about it. I've got a sectioned head on the bench at the moment and the head casting isn't very thick near the waterways. I may get a cnc programme done to make life easier. I'd leave the squish area alone.

I don't know if there's any more meat in the later heads but I doubt it.

[neal1980]

Boosted whats your views on running a 50/50 meth/water mix? Been reading about this today and seems like a good way of protecting the engine a bit. On ebay you can get a 205L drum on methanol for £85.00.

So many options!

[eliot]

Why not just run it on 6-8psi? I could barely tell the difference between 8-10psi and that was on a 5.7L

[neal1980]

Im only looking for around 7psi that should still give the car a kick up the arse. I want to use water injection but looking around if you add methanol gives you a nice healthy octane boost as well which cant be a bad thing.

[stevieturbo]

at 7psi and decent intercooling, you should have no need for water or methanol.

[neal1980]

stevie when you had your rover what power was that making? and what do you think i can expect from around 7psi?

[Boosted LS1]

neal1980 said:

Boosted whats your views on running a 50/50 meth/water mix? Been reading about this today and seems like a good way of protecting the engine a bit. On ebay you can get a 205L drum on methanol for £85.00.

It's something I'll probably do next along with air/air. I did use WI before and liked it but I didn't use methanol back then. I like the Snow Performance stuff though and they're very helpful people.

[stevieturbo]

neal1980 said:

stevie when you had your rover what power was that making? and what do you think i can expect from around 7psi?

Havent a clue.

I was running a bit over 15psi I think on the 4.6, but no notion about power. Never had it on a dyno.