Working Out Compression Ratios

[JJRC]

Car - JDM AE86

motor / part in question - Bigport 16v

problem -

occurs/started when (if applicable)-

personal situation (if applicable)-

I've been using Sam-q's website on calculating theoretical compression ratios and was just looking for some advice and feedback.

Currently a stock big port so CR = 9.4:1, CH = 47.23cc, CY = 396.78

If I were to use a 0.6 head gasket CR = 10:1, CH = 44.14cc, CY = 396.78

and then bore it out to 82mm CR = 10.2:1, CH = 44.06cc, CY = 406.63

Now it comes to if i were to use an aftermarket piston say TODA's 82mm version. It states it has a crown volume of 3.96cc/3.7cc and a projection of -0.05mm.

Do I subtract or add this from the chamber volume to get the new compression ratio?

Also for my application along with cams (288 Duration 10mm lift) what sort of compression ratio should i be aiming for?

Thanks

[TODA AU]

For 4valve 4AG, with TODA 82mm high compression pistons.
The TODA 1.0mm head gasket will give you 11.3:1 C/R (You can't use stock as the bore is too big)
A 0.8mm head gasket will give you 11.57:1 C/R
A 0.6mm head gasket will give you 11.85:1 C/R
This assumes no machining off the deck or head / Any machining will take the ratio higher.
Eg: 0.010" off the head & the comp with a 0.6mm gasket quickly goes to 12.26:1 C/R
I'd use a 0.8 gasket to maintain piston to head clearence of 0.030"
Skim 0.002" off the deck for zero deck height & shave the head 0.010" from std
This will give you 12.0:1 C/R with a std chamber.
NB: If you reprofile the chambers removing the joggles etc, you'll have to measure & re calc.

To work out where you're going, input your values into this calulator

Deck height = 191mm
Rod length = 122mm
Combustion chamber volume = 38cc
Compression height = 34.45mm
The rest you can sort out yourself -

For best results with 288deg cams, you should be aiming for approx 12.0:1 C/R
I put a post here regarding this - cam selection together with required compression etc

[Sam-Q]

it's good to have you on this forum Adrian

[JJRC]

Thanks for the information, I notice however it assumes an octane rating of 100RON will there be much difference as I plan to run it on pump gas (98 RON).

Also a bit o/t I know the differences between the bigport and smallport engines such as smallport having oil squirters, more reinforced, better port sizes for mild modification.

However as I already have the bigport is it really worth it to build from a smallport instead of my bigport?

Thanks

[TODA AU]

Re octane rating, if your're going to use 98, the difference is minimal.
at worst you could revise your compression ratios down by 0.2 of a point.
Regarding bigport vs small port,
The main advantage is superior airspeed in the intake ports. As you know, though they are called small & big ports, this is only in reference to the entry portion of the port.
There is no real difference at the valve / valve pocket & the "big" portion of the big port is to accomodated the OEM TVIS system.
Further the big port can be welded to produce a small port. (Bob Holden showed me how to do that back in 1990/91)
The differnce between the two narrows considerably the more heavily the engines are modified.
So, money no object opinion... the small port head would be my choice.

Cheers Sam

[timbo]

Second that. This thread was a good read!