performance downgrade?

[doclees]

For you combustion engine theorists, if you re-shim the intake valves to reduce the time they are open, hence reducing the amount of air/fuel going into the cylinders, would this theoretically reduce fuel consumption?
If so can the amount of shim change be calculated to dial in a theoretical reduction in HP?
Would any other remapping or re-timing be needed?
I understand that honda uses variable valve timing for car and motorcycle engines.

 

[stangeater]

I think re-shimming your valves is WAY over rated, and not necessary unless your having problems. We have a guy around here who use to work for Ford, and specialized in SHO's and Super Coupes'....he was the first to tell me what a joke this was when ppl just did it because they thought they had to. He explained in depth, and it really made sense.

 

[gmorrell]

I would agree that re-shimming the valves to affect performance, in either direction, is probably futile. Figure that typical intake lift is 0.35" (OEM MTX), and the distance to get the lobe-to-shim clearance in the right range is +/-0.003", so by playing with clearance, you're looking at being able to affect total lift by less than +/-1%.

I would say emphatically that shimming is NOT over rated in terms of maximizing valve train longevity and minimizing wear. If the clearance gap is too small, the resulting oil film may not be sufficiently robust to prevent excess cam lobe wear. If the clearance gap grows too large, the cam lobe ramps tend to slap the shim, which increases noise and wear.

This is why there's an optimal range for the clearance gap. The larger exhaust clearance gap is designed to account for higher thermal expansion of the exhaust valve stems. If you under-clearance the exhaust valves, there is danger that they won't close tightly and will burn.

 

[AREA 91]

:wave:
PA SHO SHOP
Selinsgrove, PA

 

[rubydist]

Gary is right on (as always) - exhaust valves will gladly burn if the clearance is too small.

My take on the idea of shimming them such that they spend less time open is that it will make power and mileage worse, because it will increase pumping losses. Keep in mind that most of the time (part throttle cruising) is spent with the manifold vacuum around 10-15" of mercury - shimming the valves won't change the mass of air entering for part throttle driving, it will just increase the drag on that air getting in and out of the cylinder.

 

[gmorrell]

Gary is right on (as always).

I thought I was wrong once, but I was mistaken, I really wasn't wrong...

 

[doclees]

This is obviously getting into an area where I need/want to learn more. Please assume that exhaust valves are left unchanged and don't need change.
From what I am just starting to gather on how the v-tec engine works some of the above statements would contradict Honda's work. So either I don't have the v-tec concept down or I'm not understanding some of these posts.
Maybe I should come at this a different way. We know that this engine is not tuned for economy or top performance. It is somewhere in the middle with a lean toward performance. So what would be needed to tune it for economy? I can squeeze 30 mpg out of the 89 on the highway with 87 octane. I think that is fantastic given the hp in reserve. I would love to tune one to get 35 or better on the highway. I would be happy to tweak the ATX to get better than 32 mpg highway.

 

[1993MTXSHO]

I think you are confused, the SHO engine is NOT a "vtec" (I use quotes because most people think honda invented it... they didn't).

Here is your "vtec":

http://en.wikipedia.org/wiki/Variable_Cam_Timing

The sho engine uses a variable runner control intake:

http://en.wikipedia.org/wiki/Variable_Length_Intake_Manifold

I am unsure of what your are really trying to do but I would not use any shim gaps other then what the manual says. If it is to small, as stated it will not get the oil it needs and may even stick the valves open. If you use to much the cam lob will effectively "slap" the shim, and if it is large enough it may even eject a shim. I know people who have let maintenance go (engine had a slight tapping to it because the valve lash was so far off and ejected shims destroying their cams, we will let those people remain nameles. Ford di a very good job in balancing the performance vs gas mileage in these cars, and if you tune it for gas mileage it will be running dangerously lean and be down on power, so what is the point in owning a sho with no power?

 

[HotRodKid]

reshimming your valves to reduce horsepower is just as much of a good idea as filling your tires with concrete to reduce rolling resistance

if you want less valve lift, have your cams reground, if you want better mileage from your taurus, sell the sho and buy a slo

if you want to beat the living snot out of your valve train in an attempt to make your car more fuel efficient, by all means add some slop to your valvetrain, youll eventualy destroy some shims and flatten a lobe, and as an added bonus your engine will click and clatter you right into insanity

and if you tune it for gas mileage it will be running dangerously lean and be down on power, so what is the point in owning a sho with no power?

lean is hot and heat is horsepower, which is why a car is always most powerfull when its on the edge of detonation. alex's turbo sho is tuned for better mileage on part throttle (16:1 i think) and still drives quite nice at that ratio.

my buddies turbo focus had a little hiccup when we were getting it ready for import madness at carlisle a few years back, and we made the 6+ hour drive there and back not knowing it was running at 17:1 at all times, i moved GREAT .... and somehow didnt blow up during wot highway pulls .... it just wanted to overheat repeatedly

lean only starts to **** power when its to the point that it has to start pulling timing because of ping / detonation

 

[doclees]

Thanks hotrodkid. That is more the info I was looking for. 1993mtxsho I am quiet aware this is not a V-tec engine. I was referencing it because of its ability to have different cam lobes for the valves. But I see these lobes also have different profiles along with their different lifts.
I am beginning to get the idea that the general consensus is in order to safely change the valve timing for a more fuel efficient arrangement it would be necessary to have a different cam installed. I sure wouldn't want to add clatter to this engine. Its tone and tightness is what I loved about it from our first meet.

 

[rubydist]

if you want better mileage from your taurus, sell the sho and buy a slo

lean is hot and heat is horsepower, which is why a car is always most powerfull when its on the edge of detonation. alex's turbo sho is tuned for better mileage on part throttle (16:1 i think) and still drives quite nice at that ratio.

lean only starts to **** power when its to the point that it has to start pulling timing because of ping / detonation

A couple of corrections and/or clarifications from my perspective:

A gently driven SHO will get better mileage than any SLO that I have ever seen. The problem is being disciplined to drive a SHO gently...

Internal combustion engines produce max torque at very close to 12.5:1 a/f - this has been documented repeatedly. However, for part throttle cruising, you are not looking for max torque, you are looking to operate at the highest thermal efficiency island of the engine's performance map, and typically a little lean will help that. However, for non-"lean-burn" engines, the NOx levels climb pretty rapidly when you go to the lean side of 14.7:1, so if you have to worry about emissions, you pretty much must stay at 14.7:1 And, going lean quickly reduces power - however at part throttle this may be a good thing, because it means the throttle plate is open farther, which reduced pumping losses and therefore improves mileage.

An engine is developing max torque when its at the edge of detonation because max torque occurs when by maximizing cylinder pressure, which almost always happens with timing advanced as far as possible - even slightly into detonation. The problem is that detonation is not very repeatable, and a little too much quickly results in a dead engine, so the typical manufacturer keeps the timing safely away from that situation. However, the guys who have been doing their own tuning know that a little detonation at part throttle is not something to be scared of, since the cylinder pressures at part throttle aren't high enough to cause damage, and running there can help mileage noticeably.

So, tuning will have a much more significant effect on performance and mileage than playing with valve clearances will.

Since the factory tuning allows you to run regular gas, I would expect both performance and mileage gains could be found by adding some timing and running premium, but I haven't done that to a SHO - but the ohc engine in the Ranger sure responded well to that approach.

 

[1993MTXSHO]

Thanks hotrodkid. That is more the info I was looking for. 1993mtxsho I am quiet aware this is not a V-tec engine. I was referencing it because of its ability to have different cam lobes for the valves. But I see these lobes also have different profiles along with their different lifts.
I am beginning to get the idea that the general consensus is in order to safely change the valve timing for a more fuel efficient arrangement it would be necessary to have a different cam installed. I sure wouldn't want to add clatter to this engine. Its tone and tightness is what I loved about it from our first meet.

if you want to do this on your mtx, you could always install a set of atx cams, those have less lift, I'll gladly trade you my atx cams for your mtx cams

 

[doclees]

Ahh so that is why you swap the cams with a 3.2 swap. If I had the time I would like to switch my ATX and MTX cams and see what happens with mileage and driveability. I assume you would need to re-shim also. OK I don't have that kind of time.
I thought timing was fixed on the SHO and not adjustable? Is that why you get a LPM, to change timing and mixture?

 

[rubydist]

If you change to atx cams, you will likely need to re-shim, but I'd suggest shimming to stock clearances, not oversized clearances like you were discussing at the beginning of the thread.

The tweecer is probably the best way to change timing on the SHO - along with whatever else you might want to tweak...

 

[doclees]

Yeah I've let go of the idea of changing the clearances. But this cam change/tweecer sounds interesting.
So what will a Tweecer do for a stock engine? I read some of the info on the web site. Sounds like you can pick a setting as your mood changes? So I could set an economy mode for the week and change it in the car at the office before Friday rush hour?

 

[pjtoledo]

too bad we can't change final drive ratios, or go to significantly larger tires. that would make 35 mpg easily attainable



but these are performance sedans!!!!!!!!!!!!!!!!!!! and old ones at that. there isn't much room for inexpensive improvements.

 

[rubydist]

Looking at the torque curves, I wouldn't bet on much (if any) mileage gains by dropping the rpms on the highway...

 

[doclees]

Looking at the torque curves, I wouldn't bet on much (if any) mileage gains by dropping the rpms on the highway...

Why so? If you were able to drop 300 rpm at 65 would you lose torque and need more throttle to maintain speed?

I have been hoping to find an Avanti cheap, my middle name, with a bad engine. Would love to put the SHO engine in one. Both are about the same weight. With a rear wheel drive you could change final drive ratios easier.

 

[HotRodKid]

too bad we can't change final drive ratios, or go to significantly larger tires. that would make 35 mpg easily attainable

32 is easily attainable if you have a tweecer and dont have to worry about emissions

and by "easily" i mean "takes next to zero effort"

 

[zak]

I would agree that swapping in an ATX intake cam is your best bet for increasing low rpm torque and engine efficiency.

As far as shimming to the large end of the spec, aside from it being only a +/- 1% change on lift, and perhaps a somewhat larger effect on overall duration, I bet it would increase valvetrain friction somewhat, e.g. the cam lobe would "hit" the shim with higher force that on an engine shimmed to nominal, or tight, specs (the greater force would have a better chance of reducing or pushing through the hydrodynamic oil film in the shim). That said, I certainly did not sweat it when I had to shim a couple of intake lobes to a higher gap than nominal due to limited shim availability.

zak