Fiat twincam project coming off the stand

General engine tech -- Drag Racing to Circle Track

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Nut124
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Re: Fiat twincam project coming off the stand

Post by Nut124 »

The engine and trans are back in the car.

Did a cranking compression test: All 4 cylinders measure 158-160 PSIG. This indicates 10.5:1 static CR with intake closing at 73ABDC.

The test was done with plugs out, intakes open and with a weak battery.

My throttle cable setup is interfering w the brake cylinder. Needs some re-engineering.
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Re: Fiat twincam project coming off the stand

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Just a little project update in case anyone is interested. This is my -78 1756cc Fiat twincam.

Advice and comments at this forum were most important in guiding me during the build. It gave me reassurance to proceed when on the right track and convinced me to redo, recheck when in doubt. Thanks!

I ended up removing the hot intake cam due to lack of deceleration ramps, excessive seating velocity per advice at this site.

I re-installed my old 40/80 intake cam along with the 80/40 ex cam.

The engine runs awesome although a bit rich, which I'm working on. The engine pulls strong from 3000 to 7500 or so. I try not to exceed 7800.

I took it to the TnT at US131 track in Martin a few weeks back. Ran 15.76@85. This is an improvement of 0.5sec and 2mph from previous build. This would seem to indicate about 140HP, comparing to similar production cars (-99 Miata).

The track was very busy. Got in two just about identical passes in 3hrs: 15.80 and 15.76.

I'm looking at much more cam from Piper in England. 300deg rallye cam with a lot of LATDC. This would be a lot like the cam I took out but with proper decel ramps. Looking to get 150HP @ 7000, low 15's.

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Re: Fiat twincam project coming off the stand

Post by Wolf_Tm250 »

Nut124 wrote: Fri Oct 30, 2020 9:15 pm
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Wow, a beautiful red 124! =D> =D> =D>
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Re: Fiat twincam project coming off the stand

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This is my cousin one.... looooong restoration project... :( :( :(

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Re: Fiat twincam project coming off the stand

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I hope you do not mind me posting some track video.

https://photos.app.goo.gl/YJkKm7tsdXY2a8cv8

Took my -78 Fiat to a SCCA Track Nite at Gingerman Raceway in SW MI. This is the engine I built here last year.

Three 20min hot lap sessions. Shifting at 7500rpm.

At about 4min in the 1st video I lock up, late braking going into tough turn 10. Into the weeds.
Then, at about 8min into the 4th track vid (5th), spin out at the end of the back straight, distracted by passing.

Older Miata drivers told me I was pulling away on straights but they were gaining in corners.
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Re: Fiat twincam project coming off the stand

Post by ptuomov »

A very nice car! Thanks for posting.
Paradigms often shift without the clutch -- https://www.youtube.com/watch?v=cxn-LxwsrnU
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Re: Fiat twincam project coming off the stand

Post by allencr267 »

Thanks for the pics.
What happened to the fuel pump?
It sounds awfully ragged, it's torture listening. I can't run an engine like that, except to try clearing a plug/Italian tuneup stuff. It's gotta be fixed.
What's the compression, what's the plugs look like, whats the AFR telling you?
////
IIRC the coupes came out a year or 2 earlier then the convert, 1600cc, 5 speed, beautiful looking & inexpensive, and that the timing belt was the main thing talked about, scary even, one of the first if not first. Engine didn't look too nice though with that DOHC setup, not as elegant as Jag & Alfa & Lotus.
Also, keep your elbow off the window sill,
Good luck.
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Re: Fiat twincam project coming off the stand

Post by Nut124 »

allencr267 wrote: Fri Aug 13, 2021 3:47 pm Thanks for the pics.
What happened to the fuel pump?
It sounds awfully ragged, it's torture listening. I can't run an engine like that, except to try clearing a plug/Italian tuneup stuff. It's gotta be fixed.
What's the compression, what's the plugs look like, whats the AFR telling you?
////
IIRC the coupes came out a year or 2 earlier then the convert, 1600cc, 5 speed, beautiful looking & inexpensive, and that the timing belt was the main thing talked about, scary even, one of the first if not first. Engine didn't look too nice though with that DOHC setup, not as elegant as Jag & Alfa & Lotus.
Also, keep your elbow off the window sill,
Good luck.
The fuel pump is fine. In order to avoid the buffeting wind noise, the GoPro microphone is in the trunk, where it picks up the pump noise when the car is idling.

CR is right around 10:1. Running a bit rich, 12.5 at WOT, top end. High 11's at the start of main ckt. There must be a rule about the elbow as I was asked about it by the staff. Plugs are light brown. NGK 7.
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Re: Fiat twincam project coming off the stand

Post by Nut124 »

Took my pandemic Fiat twin cam project to a chassis dyno for some benchmarking, base line for winter upgrades. The engine runs stronger now than it did last fall at the 1/4mi track.

The little 1756cc Fiat made 141HP @ 7000-7200rpm at the wheels on a Mustang dyno. This was about 15-20HP more than I had expected. He estimated about 155HP at the crank.

That comes to about 1.45HP per cubic inch.

Is there anything to conclude from the shape of the TQ/HP curves?

Should I bother with the bigger cams?

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Re: Fiat twincam project coming off the stand

Post by dannobee »

You gotta like that flat torque curve.

Good job on the build!
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Re: Fiat twincam project coming off the stand

Post by ptuomov »

Remind us what’s the highest redline rpm you’re comfortable with?
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Re: Fiat twincam project coming off the stand

Post by Nut124 »

ptuomov wrote: Wed Oct 06, 2021 10:19 am Remind us what’s the highest redline rpm you’re comfortable with?
I have cast pistons. I think I could occasionally hit 7800rpm.

I was hoping the bigger cam would lift the torque curve up a bit all along, not just at the very top rpm. The bigger cam has about 10deg more duration at 0.050", only about 0.010 more lift, seat to seat duration just marginally longer than this current cam.
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Re: Fiat twincam project coming off the stand

Post by hoffman900 »

Nut124 wrote: Wed Oct 06, 2021 10:47 am
ptuomov wrote: Wed Oct 06, 2021 10:19 am Remind us what’s the highest redline rpm you’re comfortable with?
I have cast pistons. I think I could occasionally hit 7800rpm.

I was hoping the bigger cam would lift the torque curve up a bit all along, not just at the very top rpm. The bigger cam has about 10deg more duration at 0.050", only about 0.010 more lift, seat to seat duration just marginally longer than this current cam.
If it were my car, I would be thinking how to lift the power curve in that 4100-5500rpm range without hurting the top.

Love the car / build though.
-Bob
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Re: Fiat twincam project coming off the stand

Post by dannobee »

If the Miata guys said you were pulling them on the straights, that should be your clue to now work on the handling.
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Re: Fiat twincam project coming off the stand

Post by Nut124 »

Last nite I drove the car with a much bigger intake cam, briefly. Initial impression is that power/torque is improved across the rpm range.

Is that even possible? I expected some sacrifice.

If my first impression is correct, does that mean the engine was majorly under cammed?

Idle quality is unchanged, fine.

This cam has a pretty similar seat to seat duration but over 10% more lift and 16 deg more 0.050" duration @256. I timed the cam, based on advice here, for the same IVC, based on actual closing, at 80 ABDC. ICL 106.

Below are the cams plotted, as timed. The "Old build" is the previous cam, incorrectly timed, retarded almost 10 deg. That build had audible flow reversal, cool snorty sound, and little power below 4000rpm.

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This shows the IVC events:
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