7AG and compression ratios

[Golden Circle]

Hey all,

I'm curious to know what my compression ratio will be on my engine. My setup currently consists of;

7AFE bottom end

SPOOL forged rods

4AGE smallport pistons (.20 over)

I'm planning on running a .8mm TRD HG too

So, my question is, what compression are people getting with their same sort of setups and how much have you taken off your head etc.


Cheers
Clinton

[Matt]

General rule of thumb 7a is 1 point higher than 4a

[pen15]

there is compression ratio calculators you will need to cc test the head and find out how many cc's the piston dome is and work out your swept volume plus your tdc volume over your tdc cc volume so you will need to use a calculator just google it but its best to do the test rather then just guess

.5mm over will alter the compression the rods wont the head gasket will and milling the head and or block will, just do some research smallport piston cc's are easy to find the rest you know already

cheers
mat

[Sam-Q]

I calculated 11.98:1, also their +0.5mm pistons not .20"

[Golden Circle]

I know you have written a calculator Sam, Do you have a link for it please? Forgive my noobieness, But I know that is reletively high. Should I take some more off my head?

[Matt]

Did you want to raise it even higher?

[Sam-Q]

the calculator I used is blue and next to my keyboard, kinda hard to exlain how I worked it out and the article on my website is really hard to understand as well. If you need more calculations let me know.

[Golden Circle]

basically boys, I'm running carbs, standard cams and standard head/de-joggled and good exhaust polishing. My tuner has a dynometre and plenty of other groovy shit so he should be able to have some tunability.

So what is high compression?

Will my motor even like the extra compression?

[Matt]

the new ft86 is 12.5:1 ... Your starting to get up there, but it isn't stupidly high or uncommon today.

With that range of compression you'd benefit from bigger cams...

[Bomfunkmc]

Some of the new mazda motors run 14:1 from factory.

My experience with engines and especially with my high comp rb25de neo would suggest that about 12.5 is as far as you would want to go on street fuel (98 octane) unless the combustion chamber was originally designed to deal with effects that increased compression has on quench etc.

[sevensevenseven]

Some of the new mazda motors run 14:1 from factory.

My experience with engines and especially with my high comp rb25de neo would suggest that about 12.5 is as far as you would want to go on street fuel (98 octane) unless the combustion chamber was originally designed to deal with effects that increased compression has on quench etc.

the skyactive 2.0 has a 14:1 cr in japan
the ones that we get will have 12.5:1 simply due to fuel quality

[pen15]

leave it there clinton 12.0-1 is good enough unless you get a massive set of cams to bleed off static comp but you will want a good battery and a good starter as it is

[Delazy]

12:1 is perfect..my 20v 7a is sitting around the 11.5:1 mark..prolly regret not building it higher now..

Most of the motors mentioned in this thread soo far with high compression standard are quite modern and obviously alot higher tech than ye olde 4age..

I dnt think I'd personally build a motor for a streetcar higher than 12:1 tho..one bad batch of fuel on a seriously tuned motor (let's be honest, your not building a 12:1 motor to make 90kw and drive to the supermarket) and it could well e game over :( which was the reason I settled for a slightly lower figure..tassie is known for ordinary fuel at times and I wasnt really prepared to run race e85

[Golden Circle]

has anyone ever mixed kerosene into their fuel? lol...


Ok thanks guys I'm just going to get the minimum taken off the head to be flat. cheers boys

[Delazy]

Dunno about kero, but I'll be running 50/50 water meth injection to help the old girl along and keep it well on the safe side...

[Golden Circle]

you're injecting what now?

[timbo]

Delazy is talking about injecting water/metho into the intake side of a turbo set up to keep temperatures down.

As for kerosene, I add a little every now and then to help keep the fuel system clean. Injector cleaner is just kerosene with a fancy name and colour.

[Delazy]

Delazy is talking about injecting water/metho into the intake side of a turbo set up to keep temperatures down.

As for kerosene, I add a little every now and then to help keep the fuel system clean. Injector cleaner is just kerosene with a fancy name and colour.

Turbo? Pffftt...no turbos here lol!

[timbo]

Oh really!?

Be careful then, you don't want to make it ice cold. I experimented with it a few months ago, good results but sometimes the manifold was almost icy.

[Lurkin]

Petrol was running low on the scooter one night.. so Kero, engine start and a few other things were poured into the tank... ran ok lol.

on another note any ideas on the compression of:

- 7afe block
- 7afe standard rods
- 4age 20v BT head
- 4age 20v silvertop pistons
- thinest of either 7afe or 4age 20v silvertop head gasket?

and

what it would be if I used blacktop pistons?

(I have had a go at using calculators on the internet, but have had mixed (read: incorrect) results.)

[Delazy]

Oh really!?

Be careful then, you don't want to make it ice cold. I experimented with it a few months ago, good results but sometimes the manifold was almost icy.

Should be right dude, won't be a silly setup and running failsafes etc, just enough to cool the intake charge, keep the motor free of carbon build up and added protection from detonation when we wind plenty of timing into the tune..too much money invested in this motor now for it to shit itself..

E85 also wasn't an option when the motor was built...now it could possibly be given it being available in Hobart on pump, but it's defect territory trying to get to said service station..compression is too low to justify the e85 switch also..(11.5:1)

[timbo]

Good to hear, do it once and do it right.