7.4:1 compression ration, opinions please?

[M K]

Opions please?

To low? Nice and safe etc I'm new to all this


Seen some cosworth racing pistons on 7.4:1 ratio with valve cut outs and that have done dyno time only for good money & wondered if they would be ok for my engine

200 10 studded block
Shot peened rods
Arp rod bolts
Fully worked crank, knife edged, L&B etc
High torque cams (not 100% sure on spec at this moment as info not with me)
Gtx3071r turbo
2wd manifold with tial 44mm external gate
Hart inlet with 4x4 throttle body

Power guesstimate for the above spec also please, I'd like to see a genuine 500bhp

Any info appreciated

Thanks
Marc

[mallard man]

tooo


low.

[Mike Rainbird]

480bhp with that set up .

[B13 JUS]

480bhp with that set up .

Should make more than that with the correct cams.

[Mike Rainbird]

Should make more than that with the correct cams.

Not on that compression ratio. Personally, I'd go for the GT3076, it will do 540bhp with no loss in response (both the GT3071 and 3076 use the same turbine housing / wheels).

[B13 JUS]

Not on that compression ratio. Personally, I'd go for the GT3076, it will do 540bhp with no loss in response (both the GT3071 and 3076 use the same turbine housing / wheels).

Its a gtx3071 Marc will be using.
I also think mark shead mapped a car on the gt3071 with 7.2-1 and it made over 500bhp. I'm sure his screen name was l8 ecu?

[Mike Rainbird]

All the GTX turbos surge their tits off......

[B13 JUS]

Here it is

http://m.passionford.com/forum/showthread.php?t=370883

[B13 JUS]

All the GTX turbos surge their tits off......

Even with the correct compressor housing (60 A/R)? I seen some gt30's with 70 A/R compressor housing from the gt30 which could be the cause the surge.
I don't think mark has had a problem with them.

[Mike Rainbird]

Here it is

http://m.passionford.com/forum/showthread.php?t=370883

They're running 2.5-2.6 bar to achieve that - I personally wouldn't run any turbo at more than 2.2 bar - hence the 480bhp figure I quoted. Running the turbo this hard is putting it way outside it's compressor map, which leads to large temperature increases, which means that cooling can be an issue (especially on track). Brief squirts on the road will be okay (which is what Doug's car is used for), but the GT3071 turbo is too small for track use (which is what MK's car looks like it is built for) and at high rpm will have back-pressure issues.

[M K]

That compression ratio is not what I will run it's just what the pistons I see for sale were machined for,

There is a $20 difference between the gtx3071r & the gtx3076r so if the 76 is better for my needs il go for that,

Will a gtx3076r with .60ar and 82 rear surge ??? I don't want that at all

Thanks
Marc

[cosmagedon]

I had a engine with low 7 comp ratio and it was fucking horrible. Never again. Standard comp all the way

[Mike Rainbird]

In tests Harvey has done, the GTX30 series turbos surge on a Cosworth (basically they have improved the compressor wheel, but without the spending the time to match this to the turbine wheel, so they are mis-matched like the GT35 is). This means that the turbo is generating more air flow than the engine can consume at low rpm (so it surges). I would just go for the normal GT3076R based on the conversations I have had with him.

But at the end of the day, it's your engine and you have to make the choice.

[SCS j1mbo]

The gtx3076 surges quite bad in the mid range, this is a high comp engine with big cams, but we did squeeze 600bhp from it.

The gtx3071 isn't quite as bad in terms of surge its just a flicker down low but if it didn't it would be a good turbo as it pulls low down, I think 540/550 would be done no problem.

IMO the best road gt turbo is the 3071 it pulls full boost around 3500rpm.

The gt3076 on track is a good turbo, but not in high revving and high boost situations as the back pressure becomes very high. No matter what exhaust housing as the exhaust wheel is the issue. 32 psi at 7500-7800 rpm and its happy giving a genuine 540bhp on a very good engine

[Loader]

Trusty old T4 with a rb core then Jimbo ?