Best way to up my compression ratio

JAS · Post by **JAS** » Sun Oct 06, 2013 10:26 am

So I’ve been driving the gold beast around a bit and loving it. But its slow to come on boost and my tuner said he thought the compression seemed quite low as the engine is quite lazy. He said that I could easily make the same power on 95 octane fuel. And noted that advancing the timing didn’t really do much.
When I built the engine I ordered 8.5/1 pistons but the builder said when he put it together it was around 8.0/1 but said he bought it up a bit. My tuner seemed to think it behaves like its got around 7.0 to 7.5/1 compression.

And being the fussy prick that I am, Id like it right. I know its not going to make any more power. It would be nice to have it more responsive.

Would having the head planned give a decent amount more compression or am I better to buy some 9.0/1 pistons and have them fitted?

ProZac · Post by **ProZac** » Sun Oct 06, 2013 11:59 am

I'd do the new pistons dude. Shaving the head could introduce a few other problems, the squish area gets changed and it might become pretty detonation prone. Also, piston to valve clearances tighten up.

You'll need another tune as well?

You'd surely be able to on-sell your current pistons pretty easily too.

Komeuppance · Post by **Komeuppance** » Sun Oct 06, 2013 3:25 pm

Perhaps a better suited Turbo to match your power needs?? Would be quicker than pulling apart the block.

-Robert

ProZac · Post by **ProZac** » Sun Oct 06, 2013 4:33 pm

Doesn't matter what turbo chosen if the static compression is a bit low, it
ll always feel a bit lazy off boost.

You just put on a GT3076R didn't you Jase? That would be about the perfect turbo for your combination I reckon :-).

JAS · Post by **JAS** » Sun Oct 06, 2013 4:50 pm

ProZac wrote:Doesn't matter what turbo chosen if the static compression is a bit low, it
ll always feel a bit lazy off boost.

You just put on a GT3076R didn't you Jase? That would be about the perfect turbo for your combination I reckon :-).

Yip, thats the turbo Ive got Zac. And your right, should be perfect for my setup. If I was using a GT35 type turbo Id understand it being a bit lazy.

Any thoughts on a good compression ratio? Keeping in mind E85 is only available in a drum and not from any pumps at this stage. I reckon around 9.0/1.

And Ill be needing a re-tune Zac. Defiantly.

Starion VR4 · Post by **Starion VR4** » Sun Oct 06, 2013 5:40 pm

Jas definitely go up with compression. What Pistons did you build it with? VR4 units? If so you will be around 7.2-7.6/8:1 Compression. As Prozac said changing the turbo will make very little difference. Bite the bullet & do it correctly & get a set of Customs pistons made to suit.. aim for 9.0:1 compression. :beer

JAS · Post by **JAS** » Sun Oct 06, 2013 6:01 pm

I used Wiesco forged pistons that were supposed to be 8.5/1

I guess it wouldn't cost much to replace the pistons. And as you say Zac, I could sell my old ones.

fugazi · Post by **fugazi** » Sun Oct 06, 2013 6:54 pm

There is not much you can shave off the head and the comp would probably be about the same anyway...
The 4g61 head has a smaller chamber... that would up the comp.
The intake-ports are a bit smaller though...

But the best way is probably new pistons!

ProZac · Post by **ProZac** » Sun Oct 06, 2013 7:01 pm

Evo's make really good power with an 8.5:1 compression ratio, and seem pretty responsive, I'd aim for that :-). It's not much of a comparison, but that's what I've set my SOHC motor up to make.

I wonder why your compression is low though... Are combustion chamber volumes different between VR4 and EVO 1-3 heads? Did you use a multi layer steel head gasket? They are usually a bit thicker when torqued down, might be a contributing factor?

Ethanol would be awesome, but its just too much of a pain in the ass in New Zealand. You'd probably have to upgrade your injectors and fuel pump too, as ethanol is must less energy dense, so you have to spray heaps more to make the power...

8.5:1, BP Ultimate 98, it'll be a monster :-).

JAS · Post by **JAS** » Mon Oct 07, 2013 5:38 am

8.5/1 is what I wanted when I had the engine built. And yes it does have a multi layer head gasket.

OLD FART · Post by **OLD FART** » Mon Oct 07, 2013 5:46 am

Jas what is the AR of the rear housing ? IF it's too large it will be laggy'
Your other option is to sell the 3076 to me @ mates rates :D OR put a 71 compressor wheel and cover on it.

ProZac · Post by **ProZac** » Mon Oct 07, 2013 7:07 am

I think you should take it apart just so we can figure out what's going on! Did you buy the pistons new? I'd really like to measure the dish on them.

All the early 16v heads (VR4, EVO I, II, III, RVR, etc...) should have the same 47cc combustion chamber volume, so I don't think that'll be the issue.

The wiseco sites uses an assumed 1.1mm headgasket compressed thickness when specifying the compression ratio their pistons generate. The Permaseal MLS-R gaskets compress to 1.3mm, which is a bit thicker, but not enough to drop the compression more than 0.2 or so.

Post by **Alspos** » Mon Oct 07, 2013 7:37 am

Glue some coins on top of the pistons (in the middle so you don't upset the balance) that should bump it up a little and it's cheap.

Post by **Alspos** » Mon Oct 07, 2013 7:55 am

Glue some coins on top of the pistons (in the middle so you don't upset the balance) that should bump it up a little and it's cheap.

JAS · Post by **JAS** » Mon Oct 07, 2013 8:55 am

Its a .82 Rad. So should be fine for the motor. Hehe what turbo are you running at the moment?

Maybe the pistons I ordered were the wrong ones to start with. I never actually layed eyes on them before the builder put it together. I just ordered them and had them sent straight to the engine builder.

Who would you recommend Zac?
Im going to leave it as it is for the time being. Ive got the South Island 4 and Rotors next month so it can stay like it is until next year I reckon.

Austarion

Best way to up my compression ratio

Best way to up my compression ratio

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