7A compression (pics)

[94_Rolla_Guy]

heres the deal i got some pistons 2 sets lets just say the price was right i payed a 1cent a piston no really only 1 cent a piston



the 7A-FE on the left (12cc dish), 4A-FE on the right (8cc dish)




heres my issue i wanna build up a Hi-compression 7A-FE with a street Port and Polish and make 135-140 hp (crank)

Now heres the $3.2 million dollar question stock its 9.5:1 and needs 87octane i wanna bump it to 10.4 to 10.5:1 (still trying to accurately calculate compression right on) assuming this what will the octane requirement be?

people have suggested anywhere from 89 to 94+ and i dont see the reasoning behind these opinions so please tell me why youd suggest a certain octane

The other question is that the 4A-FE pistons are quite a bit heavier and im concerned with that extra weight on the 7A-FE's rods

[toyotaspeed90]

91 octane

ugh.... now i gotta explain myself......

what is octane? it really is just how easily the fuel burns.....the lower the octane, the easier the fuel is to ignite. If you have lower compression or not boosted, the lower the octane you can run the better the engine will run (and i'm sure someone on here will disagree because their car 'miraculously runs better on high octane'). What you want to do is run the lowest octane that doesn't cause your motor to ping/knock/detonate (whichever word you chose to use, though they really aren't the same).

when you have higher compression, however.... if you run too low of octane the extra heat made by the engine and the extra compression can cause the 'easier to burn' (ie lower octane) gas to pre-ignite.

[Flashmn]

What you want to do is run the lowest octane that doesn't cause your motor to ping/knock/detonate (whichever word you chose to use, though they really aren't the same).

You should get the highest octane you can afford, its always better.

[punter]

Originally Posted by toyotaspeed90

(and i'm sure someone on here will disagree because their car 'miraculously runs better on high octane')

Originally Posted by Flashmn

You should get the highest octane you can afford, its always better.

I agree with toyotaspeed90 because he is correct.

[toyotaspeed90]

Originally Posted by Flashmn

You should get the highest octane you can afford, its always better.

and your actual basis on this is.....? other than the aforementioned 'its better in my car, so it must be true!'

[canadianae92]

I can use 87 octane on my high comp(10.3:1) but I use 92 octane because the gas station near me sells it most places are 91 but I'm trying to find a place with 94 oct, higher octane gas seems like it makes my engine feel more responsive and a little more powerful, not really noticeable tho.

[94_Rolla_Guy]

thats what i was hoping that it would run on 91 because you can't always get 94 in canada becuase Sunoco is the only company to offer it.

i figure i could run it on 87 if i want, pull timing out and richen it up... but it wont make power the goal of this is to make some power so i need it to run good on a max of 91

Im still learning alot but from what i have read you wanna heat the mixture via compression to 99% of its flash point (or ping point) then let the sparkplug finish the job, if i run 110 octane then it will only heat it lets say 70% to the flash point, and then when it sparks it burns slower and with less power because the flame front is heating the excess fuel to flash before it can ignite causing the slow, weak burn, thus reduced power

thanks guys thats some good news for the week...

[Flashmn]

other than the aforementioned 'its better in my car, so it must be true!'

It resists knock better, you can run more timing.

[toyotaspeed90]

Originally Posted by Flashmn

It resists knock better, you can run more timing.

right, but after a certain point adding more timing will just yield better gas mileage..... & what's the point of that if you're spending another $0.30 per gallon

[Flashmn]

When you're engine is running a much stronger tune than stock, you WONT be able to run on low octane. For example, higher compression, turbocharging etc can cause the necessity to run the higher octane grade.
If you're gonna tune your car, one thing you gotta learn is to stop bitching about fuel consumption

[punter]

Originally Posted by Flashmn

When you're engine is running a much stronger tune than stock, you WONT be able to run on low octane. For example, higher compression, turbocharging etc can cause the necessity to run the higher octane grade.
If you're gonna tune your car, one thing you gotta learn is to stop bitching about fuel consumption

Agreed, but all he said was that you should run the lowest possible octane (As in no knock) for the best results, If your running agresive timing or more compression then your requirements will obviously go up.

But if your car runs fine on 91 octane then 94 will make no difference to power.
(Unless you can anvance the timing to suit)

[Flashmn]

But if your car runs fine on 91 octane then 94 will make no difference to power.
(Unless you can anvance the timing to suit)

Right, unless your ECU can determine the octane rating and adjusts the timing automatically to the optimal range. VW 1.8t does that Afaik, therefor you do get performance increase with higher octane.

[toyotaspeed90]

um, this isn't a VW forum.... and the only other car that i know of that has that is the SC mr2's which have a gas selection switch (which toyota & owners even reccomend to never use low grade gas unless you have to)

[Flashmn]

Yeah, so what? Are you sure newer toyotas dont have learning ecu's? Alot of newer cars are getting them.