Is 9.3:1 too much??

[andy198712]

Hello,

Over the years I’ve got very jealous of all your turbo builds so I’ve decided this year or early next mine will get one too.

Here’s the spec

Type 1 case hoover mods
82x90.5
40x35 oval ports
1mm deck
9.3:1 compression
Webcam 86b

Just finished match morning my new speed shop manifolds with inboard injectors without throttles in them, running to a single billet throttle body.

It’ll be water/meth injected at first and charge cooled as and when the boost increases.

But can I get away with 9.3:1? We have 99ron uk fuel which I already run it on.
And it was on megasquirt 1 V2.2 extra but I’m just finishing my v3 board which will start off MS1 but will upgrade to 3 when funds allow.


Probably looking at a HX35 holset turbo.
I’ll do a build thread when I get closer but at the mo the bug is having some major works done on it, new frame head, irs conversion ect ect

Thanks

Andy

[Schweg]

I’m not sure about 9.3:1 on an air cooled engine. On other 4 cal getting to 8.5/9:1 used to be good. Sorry I can’t be of more help there.

As for an HX35 I had trouble spinning one with a 1600cc. You may have better luck with a 1900cc for sure but it may also be on boost super late in the rev range.

Best of luck!


Sent from my iPhone using Tapatalk

[Chip Birks]

Any plans for water/meth injection? 9.3 isn't crazy high. Mine is at 9.1, and was at 9.4 before that. Funky pistons have let me take boost to stupid high numbers on pump gas here. Just run a little less boost and enjoy the snappy around town cruising of a compression engine.

[Wally]

I’am at 9.1-9.2-ish also, but the major factor is dynamic CR and the cam (valve opening times) determines that.
A 86b and over 9 CR is kinda high (my cam has more duration). LC of the cam also influences valve timing. Try to get a feel for it and look especially at intake valve opening timing. The earlier it opens, the more CR you could run, all other things being equal.
To be safe, a little less would be wise though

[andy198712]

Mine is 2110cc so should spoil up....
I don’t really want insta boost anyway tbh, want to be able to drive it off boost mostly then have that bit extra when inrev it out

[andy198712]

Any plans for water/meth injection? 9.3 isn't crazy high. Mine is at 9.1, and was at 9.4 before that. Funky pistons have let me take boost to stupid high numbers on pump gas here. Just run a little less boost and enjoy the snappy around town cruising of a compression engine.

So water meth would be from the start, probably twin nozzle if I can’t work out a good place to run a single one.
Are you running SS pistons right?

I’m thinking high compression will make good power at lower boost am I correct or is it not quite like that?

Cheers

[andy198712]

I’am at 9.1-9.2-ish also, but the major factor is dynamic CR and the cam (valve opening times) determines that.
A 86b and over 9 CR is kinda high (my cam has more duration). LC of the cam also influences valve timing. Try to get a feel for it and look especially at intake valve opening timing. The earlier it opens, the more CR you could run, all other things being equal.
To be safe, a little less would be wise though

I see I could probably easily add in some copper head spacers to drop it a tad to below 9.
but still have it running well off boost
Or add to my cylinder base shims and lower it that way.
If I do that I’d probably rehone and maybe add total seal rings though. Will do a compression test soon and see how my engine is.

Cheers guys

[andy198712]

If I were to add 0.5mm shims under the cylinders it would be 8.9:1..... I guess it’s trying to find that balance of on and off boost performance.

Does the boost coming in later help?

[madmike]

not really It's all about not pinging on boost

[andy198712]

not really It's all about not pinging on boost

This is true
I wondered if it would tolerate it more as the the cam came one or no....
Looks like I’ll lower the comp to 8.9:1 with a .5mm shim, that’ll give me 1.5mm deck height so still okay.

[buguy]

I do believe the likelihood of pinging is a bit less if it comes in at a higher rpm. Only because of less load on the engine than at a low rpm. That said, I think 8.9 will still be on the upper end of safe on pump gas, but maybe if you keep the boost low it will be fine. Worst case you could always pull the heads back off in favor of a little thicker shim. Water/meth and a charge cooler will of course help.

[Wally]

For all practical purposes, compression ratio does NOT spool your turbo sooner or later!!

^^ should be a 'sticky' here in the forum

Higher CR does make the engine more 'alive' down low in the RPM range (has more power), but this is when the engine is off-boost! Spool is the same as exhaust gas flow is ~ the same.

Decreasing your CR by adding more deck, could make your engine MORE knock/ping prone!!
So what you intend to do, might not help at all or be getting worse. It hard to be very sure about the effect, but more deck equals worse fuel mixture and that makes it earlier to knock!
Personally I think you can better run your current higher CR, then removing the squish from your combustion room, even though 1,5mm is probably within the limit of loosing squish altogether. But its a toss as good squish is very important imo. Case in point: its the main reason "Squishy pistons" get away with crazy high CR's! The best advise is always the most expense. You surely know what I mean

Think of it this way: K20 Honda engines with stock 10,5-11 CR can run a lot of boost without reducing CR or different cams! The reason: excellent (the best in the industry actually) combustion in the combustion room because of excellent gas mixture.

Thats why I get away with high static CR (and a lot of other things that help): supernice billet heads with excellent shaped combustion chamber.
Be carefull to adapt single things from others peoples engine to your own for this very reason. People also ask me often how much timing I run. Talking about dangerous information! I do always want to help, but that info I don't share because I don't want to feel (even partly) responsible for their split cylinder walls...

[andy198712]

Yes I fully understand your point, and greatful for your input.
Basically “send it” but don’t be surprised when I kill it, and then open my wallet and build it better next time

I think it’ll be a good learning experience but at the same time I may look into how the k20 does it so well and see if any of that design can be ported across for us....

I’ll give it a go, it’ll either work or it won’t.... just put an offer in on a turbo on eBay (new) so we’ll see what they come back with.

[madmike]

Don't be afraid to run huge 'deck' mine is .120" and has great off boost throttle response!

[Wally]

Don't be afraid to run huge 'deck' mine is .120" and has great off boost throttle response!

Imagine how it could have been!

(although throttle respons is the least of the benefits of a tight deck )