8:5:1 v 9:0:1 comp ratios

[Chriscooke]

There is a slight chance I may have to have my sr rebuilt, after speaking to a few people some have suggested raising the comp ratio to 9:0:1 to aid off boost torque etc. What are the downsides to this, does it affect top end and make det much more likely?

I'm guessing any machine work to the block/head and mhg thickness will affect the ratio too?

I've currently got a standard bottom end with a cosworth 1.5mm mhg, I guess this has dropped my ratio?

I'm a total noob to this and don't really have much clue as you can tell so please forgive my ignorance.

[dan_evans]

I have gone for 86.5mm pistons 8:5:1, I had the head skimmed & also the block decked so used a 1.5mm cosworth gasket.

Depends on how much work your doing, oversizing, decking, skimming etc to know exactly the route to go down.

[Chriscooke]

Thinking standard bore, decked block head was skimmed a couple years back so will take advice on this.

Car would be running a gt2871r for a while but upgraded to a gtx hybrid or gt3076 etc in the future.

[Chriscooke]

will only know about bore once been checked I guess. Do you gain much from the over sized piston

[zeppelin101]

No reason not to run 9:1 with modern fuels imo. Increasing ethanol content makes them slightly more knock resistant than the RON rating suggests in practise.

CR is a function of head volume, piston volume, gasket thickness and swept volume. So just buying "9:1 pistons" without knowing what the dish cc isn't going to help you much in working out where you have ended up.

For instance on my V8, I've ended up with 1mm head gaskets, -1.5cc pistons (i.e. they take 1.5 cc out of the chamber volume) with a 1mm overbore and a slight increase in head volume from OEM which was then skimmed to within an inch of its life. The end result was 11.4:1 versus my target of 11.5. So you need to take everything into account to see where you have actually ended up. I had to take a flyer on the head volume when I ordered pistons because I hadn't finished machining them and hadn't had them skimmed but it all kinda worked out in the end.

If you overbored by 1.0mm but changed nothing else then CR would go from ~8.5 to ~8.65 for example.

All things being equal, a 1.5mm MHG will drop CR more than expected because they are usually larger bore diameter than the OEM gasket (which is 87mm I think? Most MHGs are at least 88mm) so that would take you from ~8.5 to ~8.2.

I can send you a calculation sheet if you wish to see what the impact of each change is.

[big_mac]

You won't notice a big increase in off boost driveability imo, the only real clue to the engine compression being different from a similar species engine is the cranking speed.
I've driven a 6:1 comp ratio 2ltr and couldn't tell any difference, my old .ca was 7:1 and nobody who drove it or was a passenger in it could tell.

If you are thinking it will give a huge change then I think it may be a bit of a letdown

[Chriscooke]

No reason not to run 9:1 with modern fuels imo. Increasing ethanol content makes them slightly more knock resistant than the RON rating suggests in practise.

CR is a function of head volume, piston volume, gasket thickness and swept volume. So just buying "9:1 pistons" without knowing what the dish cc isn't going to help you much in working out where you have ended up.

For instance on my V8, I've ended up with 1mm head gaskets, -1.5cc pistons (i.e. they take 1.5 cc out of the chamber volume) with a 1mm overbore and a slight increase in head volume from OEM which was then skimmed to within an inch of its life. The end result was 11.4:1 versus my target of 11.5. So you need to take everything into account to see where you have actually ended up. I had to take a flyer on the head volume when I ordered pistons because I hadn't finished machining them and hadn't had them skimmed but it all kinda worked out in the end.

If you overbored by 1.0mm but changed nothing else then CR would go from ~8.5 to ~8.65 for example.

All things being equal, a 1.5mm MHG will drop CR more than expected because they are usually larger bore diameter than the OEM gasket (which is 87mm I think? Most MHGs are at least 88mm) so that would take you from ~8.5 to ~8.2.

I can send you a calculation sheet if you wish to see what the impact of each change is.

If you could send it to me that would be good, not sure I'll understand it straight away but I can learn lots of good info there, thank you.

Not expecting it to make a massive difference but if it's more efficient then I can't see it being a bad thing. Will it not help me reach similar power to I am currently with less boost?

[big_mac]

If you could send it to me that would be good, not sure I'll understand it straight away but I can learn lots of good info there, thank you.

Not expecting it to make a massive difference but if it's more efficient then I can't see it being a bad thing. Will it not help me reach similar power to I am currently with less boost?

It has the potential to allow you to run less boost but so will a bigger turbo or a better flowing exhaust/manifold or a better intake/intercooler setup.

Are you planing on getting to a certain power level and staying there or would you want to push the limits

[Chriscooke]

Not looking to push the limits as such but thinking what will give me the best potential long term.

Eventually I'd like to swap the turbo to a garret gtx 2871 or 3076 with an apw manifold most likely. Still only running 740cc nismo injectors and a Walbro so imagine these will be a limiting factor too.

[dukey]

Do you not want to look at thing that give you better torque through a greater range rather than power if you are interested in 1/4 mile figures?

I know they are a product of one another but area under the curve is better than peak power in this environment no?

[dukey]

http://dsportmag.com/the-tech/boost-...ession-ratios/

Nice bit of the basics here and little table at the end on page 3 for what is expected.

I imagine you will have to decide how far you want to go with the build really first and base it on power/torque output Vs cost and maintenance.

[zeppelin101]

Half a ratio won't lead to a huge change in boost requirement unless you fiddle with cam timing as well.

I'm in the middle of putting content together for a tech blog that goes into the science behind all this stuff since a lot of the verbatim floating around is wrong or misinformed.

Would you consider water injection? As close to the chamber as possible to get the most benefit. If you have the capability for switchable maps you could hook this up to a fluid level sensor which will then automatically drop the boost level when the tank is getting low. Then you can run some proper CR and start to realise some of the benefits of doing so.

It's not just off-boost torque you're interested in since that is a function of the valve lift along with CR, but when driving around un-boosted it will give you better fuel economy if it's tuned correctly.

[Chriscooke]

Not looking at water injection. I'm only looking at the 450-500bhp range but with an engine that's capable of more in the long term so don't think it's needed plus I don't really understand it if I'm honest. The car is running 260 pro cams currently too which I understand aren't well matched to my currant 2871r?

The car is a road car but one that I'll be using on the odd drag day

[green_rs13]

the americans seem to have good results with bolting turbo's to stock NA SR20 which have 9:5:1 compression ratio. but in the end they all seem to die of ringland failure though, but that's probably due to greed and/or DET.

ive not long rebuilt mine with CP 86.5mm 9:1 pistons in.ive not changed anything else(map/tubo/injectors etc..)yet.
it does seem more perkier than before but that maybe because the old engine was tired. old compression results were about 130 now its a solid 150 on all.
only thing ive noticed is that before it was set to run 15psi of boost but now it tries to pull 18psi.

[BLAKTOOTH]

I'm only looking at the 450-500bhp range but with an engine that's capable of more in the long term...

That's a lot for a little 2.0-2.2L engine to make. You can get there but it requires a dirty great big blower that doesn't hit full boost until 4500rpm. Personally I'd much rather stick to a smaller turbo, have full boost by 3k rpm and have a much nicer car to drive. Changing diff ratios has quite an impact as does loosing weight from the car. I've driven lesser powered cars that accelerate as quickly as mine due to better gear ratios and/or being lighter. I'd be speaking to whoever would be building and tuning the engine to find out about CR. Or just stick a VR38 in and be done with it

[green_rs13]

TBH I would look ino the borg warner turbos. I was quoted something like £900 new on a twin scroll 7163. drifworks have just put a 7064??? on thir S15 and made 550hp @ protuner.


I would never go more than 4.08 diff ratio on a road car, I have a 4.3 in mine and it was a joke on the road, I was changing through the box like lightspeed, it needed about 10 more gears in the box. you would be in 5th and it would be revving like it was in 3rd.

[Chriscooke]

I'm running an auto diff with the standard 5 speed box. Not sure how long that'll last mind.

Don't know anything about twinscroll setups. Seems I have more research to do.