Jump to content

Sizes Verification

Recommended Posts

Hi smart people,

Looking to verify the re-sizing of my engine for use in my ECU map.


If someone can verify my results it would be appreciated. Started with a 1.8L MX-5 engine. Stock bore 83mm, stroke 85mm = 1839.606cc.

Bored cylinders out .020" = .3277cc.

So new numbers 83.33 bore, stroke 85mm = 1854.2638cc


Used calculator from http://revetec.com/Calculators/Engine_Displacement_Calculator-2.htm but would just like some confirmation.




Link to post
Share on other sites

Well this is all new to me to coming from the era of carb also. But as I understand it the ECU calculations are based on the engine as an air pump. So the ECU calculates the (VE) volumetric efficiency of the pump for fuel and air ratio. Hence, if you put in the most accurate theoretical volume for the engine, in the ECU, it can calculate the best efficiency. It sounds like I know what I'm talking about but actually I'm just parroting what I have read, hardly understanding it all. Yes, close to a dead Polly.


My actual confusion is weither the .3277 should be diameter or radius? Does it all really matter, maybe not? Just part of this ol' dog learned something new.

Hopefully someone in the know can get us straight.


Thanks for the inquiry.


  • Like 1
Link to post
Share on other sites

I only got O level maths by the skin of my teeth & despite working in finance all my life,

where we used calculators and computers, I think there is an error in the maths.....not much,

but if youre looking for accuracy........

I assume you are boring out to plus 20 thou?

20thou is 0.508mm per google. so I think the new bore is 83.508???

That would tie in with what we do here in the UK we have overbore of .5mm, 1.0mm, 1.5mm etc.

Then using your linked formula, displacement comes out as 1862.19

Quite prepared to be shot down here, but this site seems to confirm.....


Link to post
Share on other sites

You seem to have caught one of my errors. When I ran 0.020" through Google it took it onto itself to convert inch to cubic inch which I had not noticed.

Thanks for the catch and save. Gold star in maths for you. :good:


Link to post
Share on other sites

Its a very small over bore and wouldnt effect things much or even at all. A wideband 02 would show your AFRs.


To be 100% accurate,its not the volume but the weight of air as density that changes the weight of fuel required.


A 'Stoichiometric' AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric, A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel.


You can see that by increasing your volume you "could" increase the amount of fuel, however with the restriction of head to air cleaner, it probably wouldnt see any requirements to be changed.


You would only in reality be providing a small increase in available oxygen and therefore be burning what you have more efficiently,

Link to post
Share on other sites

Well, since I am self learning this EFI stuff all information is held in equal value of importance at this point. This engine volume question maybe insignificant but I didn't know that. Thanks for the education. A good exercise of the gray matter in any event.


I am setting up to have a wideband O2, air temp, MAP sensor, TPS sensor and an ECU on-board barometric sensor (in an attempt to compensate for elevation changes). So hopefully will have plenty of control over air density and AFR. But I maybe barking up the wrong tree again?


I'm not even sure the ECU uses the engine size, just a box on the setup screen. Will check with a tech at the ECU company (Haltech). Feel free to keep the education flowing in case I maybe missing something important. :D

Link to post
Share on other sites

Nice..whats to 302 running? You got that tricked up?


Looked at your choice of ecu, which model you got?


The wide band and kpa sensor should work out the fueling requirements and if you have inteligent ignition that should be able to advance and learn within parameters.

Link to post
Share on other sites

302 is carb and MSD box. Some engine mod but nothing extreme, .020 over also (I like to clean up the cylinder walls), Alum heads and intake, mildly aggressive cam but street driveable. Just changed from 650 Edlebrock (vac. secondaries) to 600 Holley DP, should have better throttle response. Optimistic guess ~350 HP. Not as fuel efficient as I had hoped. 3.55 diff., 17" wheels, .64 5th speed, overdrive, even with unbroken egg under right foot hardly getting 20 mpg.


Haltech ECU is the 1500. Had to go to the more expensive series for VVT control and knock sensor, which I can't see not using. When purchasing ECU they had a sale of 50% off wide band if purchased at same time. ECU has both the short and long term learning for many functions also. And it has 3 level Diagnosis ability which I found quite surprising. Record, throw-up warning light, put car into limp mode. User settable and OBD2 output capable. More stuff for me to learn during my travels. Will not set any up any at the beginning.


Thanks for the interest.


Link to post
Share on other sites

600dp, id rather have gone 750 vac sec. But your 600vs should have been fine.

Ive ran 600vac sec 1850 and 750 3310's on midly tuned and fairly trick 302 and 351 windsors and clevelands. Easy to set up too. Ignition is a main cause of stubbling too so make sure its set properly.


Spent many years looking after UK Cobras

Link to post
Share on other sites

Went through the exercise of changing out the dizzy springs to much lighter weight, which made worlds of difference.


Most on the FFR forum say the car is too lightweight for the vac. secondaries but that was not my experience or issue. This was the 2nd carb on the car to start flooding through the needle valve. 1st time was during 1st start and under warranty. This time same thing but out of warranty. Same failure for a second time? I don't like being bite 3x. Also got the Holley for free from a good friend, so price was right, just needed rebuild. 302 (306 actually) specs out to a 575 carb on the charts, and this is my street car so it should be fine.


Did you own a UK cobra? Which side is the steering on?

Link to post
Share on other sites

Ive owned SR, Southern Roadcraft, in fact it was THE one they used for TUV testing. Ian and Brian stayed with our familly in Germany while it was tested.


DJ Sports cars Dax, still got a few kit bits left over,


GD Gardener Douglas, still got that one being upgraded. 350 Chevy being swapped out for a 383 stroker.


All right hand drive replicas. I built them for a bit of fun back in the days of my youth.


Generally now i build specialist engines for people, bit of a part time hobby. Any thing from a 1600 crossflow right up to a BMW Macca F1 V12 engine. Last v8 for a customer was a 429 4v that ended up with just over 800ft/lbs after a bit of bore and stroke work. The parts bill was horendous.


I like to build things by using the maths to set a base line and work from there, the maths equations are very close out of the box. For yours, a 306 at 85% volumetric efficiency using a red line of 6000 will only require at 100% a flow of 530cfm, therefore at 85% ve, 450cfm


The same calcs give a red line of close to 8000rpm to use 600cfm at 85% ve.


I know its getting away from your origonal post so please forgive the off piste rambles.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Create New...