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Same power level after increasing compression ratio

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Hi. The car is a 4g93 dohc tuned on vp racing m5 using maxxecu. It was meant to be a 1/8 mile drag car. The first time i got the car, the engine made 208hp@5500rpm till 8500rpm. The power was really flat around 5500 to 8500. We played with the cam and we still didnt get to raise the power output. So i decided to change the piston and raise the compression ratio from 11.5 to 16:1. With the same setup, (only the piston changed) the car still made 208hp from 5500 to 8500. What did i do wrong here? The cam is toda 308 with 11mm lift. But we did gain a lot of low rpm power with the raised compression ratio. The car did improve on the 1/8 as we run 8.3sec vs 8.5 with early setup. My question is why did the compression setup did not raise any power?

Usually that gain in compression ratio have to gain some power on all the rpm range.

Maybe your engine is seriously air restricted (intake restricted) for the upper rpm range .

Anyway 208 hp is not to bad for a 1.8L engine @ 8500 rpm, your cam could rev that engine a little more ,try another intake system.

Also it is very difficult to achive 16:1 c.r in a little engine , are you sure of your numbers?

If so ,you have a very big piston dome there, maybe it is interrupting the air flow dynamics when the piston is on TDC .

There could be more things to check... but here are some things to start thinking about

I'm a little suspicious of your dyno figures to be honest - It's very unusual to have a flat power curve for such a wide rpm range. If you're on a rolling road is it possible you were having wheel spin issues? The very fact you've dropped 0.2 seconds indicates the car is now making more power than before although the mph will tell more of that story.

As Julio has mentioned above, 16:1 is hard to achieve and could be detrimental in terms of affecting the flame front propagation during combustion. While it's engine dependent, generally you'll find there's little to be gained beyond a CR of about 14:1 and you can end up going backwards with too much CR.

What intake/exhaust setup?

For reference, my 1700cc 4age made 235BHP on 13:1 compression ratio and UK pump fuel. Your 16:1 does seem excessive, even for race fuel.

What does the power/torque curve, ignition and fuel maps look like?

Im pretty sure of the compression ratio gained as ive been calculating and measuring it at least 5-6 times because i just couldnt believe it too. The piston was a mismatch aftermarket honda piston with an obviously high dome. Overbored from 81 to 84mm. Not sure about the tire spin on the rolling road but ive used some tyre glue called ‘thread bite’ just to make sure its not. Im just going to do another intake setup. As for now its running with toyota 45mm itb. With a stepped header and 2.5inch exit 20in after the collector. Im making a custom 50mm itb now using some honda tb i found in my shop. Will update it here with the dyno graph once it done. Thank you guys.

45mm itb is going to be getting close to the limit at around 215-220bhp, so increasing would be good.

2.5in system is about right (i ran 2.25in).

What are the header diameters and lengths?

What is the intake system length?

What is the cam timing?

What are the head flow figures?

Header dimension : 1.75in x 22in primary to 1.9in x 15in secondary to a 2.5in collector (4-2-1) it was off the shelf from an exhaust shop here.

intake dimension : 5inch from the throttle base to the cylinder head with 75mm trumpet on top

cam timing : intake 104° Exhaust 116° that is where it makes the most power

we dont have the head flow tested as we dont have any facilities for that here. It was just hand ported by me and my friends

may i know what dimension is your header denis?

From the top....

Header. I ran a 4-1 with 1.75" OD 26.5" long primaries into a 2.25" system. I spent a couple of years trying to get a 4-2-1 working. Generally it gave very little extra mid range and 10-20BHP less peak power than a 4-1. We gave up and went back to a 4-1.

Intake. That sounds quite short. Measured from the intake valve centre to the top of the inlet trumpet I originally ran 355mm long (kind off, if you have every seen an off the shelf big port 4a-ge "weber" manifold you will know what I mean!). Subsequently I straight the intake path, improved filtration efficiency and went upper/lower injector, which helped top end a bit and massively improved starting/idling/off cam performance.

Cam timing. Your intake sounds bang on for the profile. Your exhaust cam timing sound deeply suspicious. That is road cam timing! Can't help thinking that timing is the result of the exhaust manifold. Even on mild cams I would end up with 108 degrees max on the exhaust.

Personally I would probably look at the exhaust manifold next. If you are after power/revs then 4-1 is going to be the way to go.

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