Question: 4 + 3 Overdrive ratio??

[vettebob]

Hi,

Does anyone know what the gear ratio is in overdrive on the 4 + 3 setup? I am assuming it is 1:1 in four gear without the overdrive...

vettebob

 

[BADDUCK]

There are two versions. The MH5 is 0.62 and the MK2 is 0.68

 

[vettebob]

Thanks for the answer. Mine is a 1987 Corvette. How do I verify the ratio/model??

Thanks again,
vettebob

 

[BADDUCK]

It may be marked on the case of the OD unit but I have no idea where. I think I remember the MK2 being associated with the Z51 model but I'm going by distant memory on that. Ether way there is damn little difference between the two.

 

[vettebob]

Yeah, .06 is very little difference. Mine is the Z-51, which also had a touch lower differential gear.

VETTEBOB

 

[vetteoz]

Mine is the Z-51, which also had a touch lower differential gear.

3.07 was the only ratio 4+3 cars got the regardless of Z51 or not
( IIRC '84 had a one year only 3.31? option available as well )

 

[John Robinson]

Look here

Corvette Action Center | Model Center | C4 | 1987 | 1987 Corvette Specifications

This is spec sheet for the 87 model. For all those who are looking for spec information on your c4 model check out the top of the CAC page. There is a header for spec and when you go to this you can find out a lot about your particular model.

 

[vettebob]

Nice page and thanks for the referral.

Does anyone know what the camshaft specs are for the 1987 engine?

Thanks,
vettebob

 

[BADDUCK]

294/294 dur @.050 202/206 Lift (w 1.5) 403"/415" 114.5' LobSep

 

[vettebob]

Thanks for the specs.

How close can you go on the lobe separation before the computer can't be tuned with a custom chip?

For example, would a cam with 214/224 degrees .500 lift, and a 110 Lobe separation be useable with a new chip? How about 108 degree L.S.??

Thanks again,
vettebob

 

[vetteoz]

How about 108 degree L.S.??

What would be your thinking in wanting run a 108 LSA ?
A good tuner can tune 110 , but other things come into play like cam duration / overlap / etc

 

[vettebob]

A tight lobe separation gives you that cool rumpy idle, and a good boost in low to midrange torque. I would like to have a tight LSA, and lower overall duration so the cam will "behave well" at low rpms in overdrive cruise.

On all my other, (admittedly carbureted) hot rods, I have always used tight lobe separation cams and have always been happy with them.

vettebob

 

[vetteoz]

A tight lobe separation gives you that cool rumpy idle,

That " cool rumpy idle," from the cam overlap is what gives the computer a heart attack .The O2 sensor sees all that air going out the exhaust and tells the computer car is lean so computer just loads engine up with fuel at idle.
"Makes your eyes cry at idle " is a common comment around the tuning forums not to mention fouled plugs

 

[BADDUCK]

Thanks for the specs.

How close can you go on the lobe separation before the computer can't be tuned with a custom chip?

For example, would a cam with 214/224 degrees .500 lift, and a 110 Lobe separation be useable with a new chip? How about 108 degree L.S.??

Thanks again,
vettebob

I would personally not go less than 112 LS. These cars do better if you stay around that and around .525 gross lift. It's a good compromise between performance and drivability. You will STILL need a custom tune and heavier valve springs.

 

[vettebob]

Ok, thanks for the answers. Pretty much what I expected to hear, but better to research something than find out the hard way!

Does anyone know how much lift you can use on a 1987 L-98 before having piston/valve clearance issues??

Thanks again,
VETTEBOB