Hi Folks,
I want to increase the compression ratio from the stock 8.1:1 on my 1980 2 Litre Spider. Vicks have a piston set that puts the ratio at 9.9:1. while others such as IAP have Mahle Flat Top that increase the compression to 8.9:1. The question is whether increasing to 9.9:1 would be too much as there may be too much detonation requiring lots of ignition retard and therefore not allowing the theoretical performance increase to be realised. I have no experience here but I can imagine it being a problem especially with a combustion chamber that was designed for 8.1:1 compression. At the same time I want to use the larger diameter inlet and exhaust valves along with a street performance camshaft (not autocross).
I appreciate all your comments and words of wisdom.
Thanks, Brian.
Increasing the Compression Ratio
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bandit
Re: Increasing the Compression Ratio
Hi Brian
I would ask you a few Qs first before i would make the purchase on the pistons, I haven't spoken to Vicks about
the pistons they are selling or IAP but have bought many pistons in my time are they giving you a CR based on
your 2L 2000cc Head or 1800/(1756) head there is a difference and is your 1980 Fuel injected or carb model ?
You didn't mention anything about the rest of the build are you going to run dual carbs with the Larger Valves, Cams,
Pistons ?
If your willing to invest what it takes for the Valve work on the head - pistons, cams ect ect you might as well do the
larger cams, the 40/80 are ok but with mods like what your planning the 42/82s would be much better !!!
About your Q to 9.9 or 8.9:1 what you would gain vs the hassle of trying to deal with Pump Gas and make changes
through knock sensors ect ect wouldn't be remotely worth it only a power gain of about 5 HP per point up in CR is about
what you will gain or lose depending on your Piston choice with those Numbers doesn't seem like much of a loss or gain
i would choose the 8.9 on pump gas .
Daniel
I would ask you a few Qs first before i would make the purchase on the pistons, I haven't spoken to Vicks about
the pistons they are selling or IAP but have bought many pistons in my time are they giving you a CR based on
your 2L 2000cc Head or 1800/(1756) head there is a difference and is your 1980 Fuel injected or carb model ?
You didn't mention anything about the rest of the build are you going to run dual carbs with the Larger Valves, Cams,
Pistons ?
If your willing to invest what it takes for the Valve work on the head - pistons, cams ect ect you might as well do the
larger cams, the 40/80 are ok but with mods like what your planning the 42/82s would be much better !!!
About your Q to 9.9 or 8.9:1 what you would gain vs the hassle of trying to deal with Pump Gas and make changes
through knock sensors ect ect wouldn't be remotely worth it only a power gain of about 5 HP per point up in CR is about
what you will gain or lose depending on your Piston choice with those Numbers doesn't seem like much of a loss or gain
i would choose the 8.9 on pump gas .
Daniel
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BrianAz
Re: Increasing the Compression Ratio
Hi Bandit,
Thanks for the reply. My car is a 1980 2 litre injection so I am assuming the CR is the stock 8.1:1. Very valid question regarding Vicks and IAPs reference for increased CR, whether it is based on an 1800 carb or 2000 injected engine. I will contact them and find out. I owned a 1985 spider many years ago, a real garage queen, just polish polish polish. The only thing that was a bit disappointing was the torque available at 1500 - 3500 wide open throttle. This is mainly what I want to improve by going with a higher CR, 40/80 cams and larger valves. Add an allison header and larger diameter exhaust system. Then, correct the ignition curve by installing a megajole ignition with a Ford EDIS, as RacyDave is doing right now. I am not interested in having to stay at 6000 RPM to enjoy the benefits of engine modifications.
Based on the above maybe you have some recommendations, thanks.
// Brian.
Thanks for the reply. My car is a 1980 2 litre injection so I am assuming the CR is the stock 8.1:1. Very valid question regarding Vicks and IAPs reference for increased CR, whether it is based on an 1800 carb or 2000 injected engine. I will contact them and find out. I owned a 1985 spider many years ago, a real garage queen, just polish polish polish. The only thing that was a bit disappointing was the torque available at 1500 - 3500 wide open throttle. This is mainly what I want to improve by going with a higher CR, 40/80 cams and larger valves. Add an allison header and larger diameter exhaust system. Then, correct the ignition curve by installing a megajole ignition with a Ford EDIS, as RacyDave is doing right now. I am not interested in having to stay at 6000 RPM to enjoy the benefits of engine modifications.
Based on the above maybe you have some recommendations, thanks.
// Brian.
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bandit
Re: Increasing the Compression Ratio
Hello Brian
NP . I own a 1975 131 with a FI 2L Engine 8.9 Pistons, 40/80 Cams a header , K&N Air Filter, Balanced ,
ported head and FI Manifold . The car runs great starts right up after sittings for a couple months the performance
is good it feels like a Spider with 40 IDFs but once i reach about 6k RPM it starts to lose its pulling power as the FI
System runs out of Flow . About the Valves your wasting money adding them With a engine running a stock Fiat FI
system this isn't going to provide enough Flow to make gains where the valves would make a difference .
Running the Mega Jolts with the Ford EDIS ( i am building this as well ) will mean your not going to run the dizzy
and i wouldn't know how you can keep the Fiat FI system running off of that . With my Build i am changing out
the dizzy and FI system so this isn't a issue but you sound like your keeping the FI unit stock ? The Fiat Dizzy
isn't bad it's pretty good as a matter of fact if i wasn't doing the after market FI i would of left the dizzy alone !
Fiat use's a GM Module to control the Dizzy and coil so i started looking into the modules that would work
there is a number of after market upgraded Modules that fit something to think about .
Daniel
NP . I own a 1975 131 with a FI 2L Engine 8.9 Pistons, 40/80 Cams a header , K&N Air Filter, Balanced ,
ported head and FI Manifold . The car runs great starts right up after sittings for a couple months the performance
is good it feels like a Spider with 40 IDFs but once i reach about 6k RPM it starts to lose its pulling power as the FI
System runs out of Flow . About the Valves your wasting money adding them With a engine running a stock Fiat FI
system this isn't going to provide enough Flow to make gains where the valves would make a difference .
Running the Mega Jolts with the Ford EDIS ( i am building this as well ) will mean your not going to run the dizzy
and i wouldn't know how you can keep the Fiat FI system running off of that . With my Build i am changing out
the dizzy and FI system so this isn't a issue but you sound like your keeping the FI unit stock ? The Fiat Dizzy
isn't bad it's pretty good as a matter of fact if i wasn't doing the after market FI i would of left the dizzy alone !
Fiat use's a GM Module to control the Dizzy and coil so i started looking into the modules that would work
there is a number of after market upgraded Modules that fit something to think about .
Daniel
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BrianAz
Re: Increasing the Compression Ratio
Hi Bandit,
Thanks again for the reply. The stock FI unit running out of flow. Is that air, fuel or both? If it is fuel I can change the pressure regulator for one with a manual adjustment and raise the system fuel pressure as necessary. Yes, correct I hadn't intended changing from the stock fuel injection, but with the planned engine mods, I figured the ignition would need remapping and it will be a blast doing so! Maybe, as you say, the larger valves will be a bit of an overkill. Sent you a PM. Hope you got it.
Brian.
Thanks again for the reply. The stock FI unit running out of flow. Is that air, fuel or both? If it is fuel I can change the pressure regulator for one with a manual adjustment and raise the system fuel pressure as necessary. Yes, correct I hadn't intended changing from the stock fuel injection, but with the planned engine mods, I figured the ignition would need remapping and it will be a blast doing so! Maybe, as you say, the larger valves will be a bit of an overkill. Sent you a PM. Hope you got it.
Brian.
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So Cal Mark
Re: Increasing the Compression Ratio
the first step is to determine the rpm range you want to operate in. I believe the larger valves give a considerable boost from 5k on up. With perf cams, this would be right in the power band.
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bandit
Re: Increasing the Compression Ratio
Hi Brian
The bottleneck is the AFM but we have had talks about this issue before on the forum and if you look at the system as a
whole the Plenum seems to small IMO, the Ports on the Manifold are nice but the most restrictive out of all the spider manifolds someone looking for a lot more HP that wouldn't be a good starting point .
It looks like you would be better off doing a Mega Squirt and re mapping the FI giving you more control all the way around
with the rebuild plus Advance your Intake Cam about 3 degs .
Daniel
The bottleneck is the AFM but we have had talks about this issue before on the forum and if you look at the system as a
whole the Plenum seems to small IMO, the Ports on the Manifold are nice but the most restrictive out of all the spider manifolds someone looking for a lot more HP that wouldn't be a good starting point .
It looks like you would be better off doing a Mega Squirt and re mapping the FI giving you more control all the way around
with the rebuild plus Advance your Intake Cam about 3 degs .
Daniel
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BrianAz
Re: Increasing the Compression Ratio
Hi Guys,
Thanks for your thoughts. About the AFM. I know that for the alfa 2.5l in the GTV6 an AFM from a 1980s BMW 6 series does exactly the same job, is connector and pin compatable but has a lot less flow restriction. Does anyone know whether the same exists for the spider 2.0l AFM? Street performance cam: does it push the torque curve up the RPM scale, thereby giving less torque at lower (2500 approx) engine speeds.
Brian.
Thanks for your thoughts. About the AFM. I know that for the alfa 2.5l in the GTV6 an AFM from a 1980s BMW 6 series does exactly the same job, is connector and pin compatable but has a lot less flow restriction. Does anyone know whether the same exists for the spider 2.0l AFM? Street performance cam: does it push the torque curve up the RPM scale, thereby giving less torque at lower (2500 approx) engine speeds.
Brian.
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So Cal Mark
Re: Increasing the Compression Ratio
everything in an engine is a compromise. A stock cam sacrifices top end and hp for a smooth torque curve from idle. The more radical the cam, the more the power curve will shift upwards. With a radical cam, unless you improve the rest of the motor to work at a higher rpm you're wasting your time (and probably losing hp).
In my experience, using a mild cam results in such little loss at low rpm that it's not a concern. I'd say even a stock Fiat TC doesn't make much power below 2500rpm anyway. Pay attention to your tach the next time you're flogging the car. If the rpm drops into the 2500 range when shifting you're not revving the engine high enough
In my experience, using a mild cam results in such little loss at low rpm that it's not a concern. I'd say even a stock Fiat TC doesn't make much power below 2500rpm anyway. Pay attention to your tach the next time you're flogging the car. If the rpm drops into the 2500 range when shifting you're not revving the engine high enough
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shallene
Re: Increasing the Compression Ratio
wher can i get a set of over size piston for my 1979 spider it has the 132 a 2000 motor in it can any one help
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So Cal Mark
Re: Increasing the Compression Ratio
you can have them custom made, or buy them from IAP, Bruce's or Vicks