Bigger tyres and performance(or lack of)

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Innzy

Innzy

Active Member
Posts
104
Location
Cheshire
Ok so in prep for a weekends laning in Wales a few weeks back I replaced my worn out BFG 235/70 mud terrains with some nice new 235/85 cooper stt's.
Now being a bit taller and with new tread I was expecting some loss in performance but what I've got is from being happy pulling of in second anywhere and using overdrive in top at 45, now Its first gear pulling of everywhere and any sign of a hill it sh#ts itself. Overdrive is redundant apart from in 3rd to try and keep some momentum up hills:D
It's that bad I was convinced my engine was tired and about to pop (been running unleaded for a while), so just done a compression test and all cylinders are 158-168psi!
Anyone else had similar when moving up tyre size?
 
blue beasty said:
It's less than 9% increase in circumference and therefore gearing which is approx 1/3 of the O/D (28%) so it seems odd that it makes that much difference.
Click to expand...

Thanks, interesting to get some actual numbers to go on. I suppose new tyres with 15mm tread compared to old tyres with 5mm tread adds another 20mm diameter but still doesn't add up!
Think a closer look at the engine is in order, just seems too much of a coincidence that it's since the tyres were changed!
 
The gearing was set up for 600 section tyres, I assume you still have 4.7 diffs ? Iv'e never been tempted with bigger tyres and have never got stuck running on 205s. 88"s do look good on 750s though. It sounds like you are just a step to far for the engine to pull the tall gearing. 200 tdi will sort it.........
 
Yep still on std diffs. The reason I went for 85s was to gain a bit more ground clearance, and tbh its great off road now it seems to go anywhere my mates can go in there lifted discos, before it did struggle on the really rocky lanes.
Tdi or a turner performance head was next on the shopping list if the engine was tired as I suspected but since compression is good it would be a shame to rip out a good engine and I do like the quietness of the petrol! Tdi maybe the only option long term though I think
 
As well as a head a hi-torque cam from ACR might be worth looking at, this loses you a bit of top end power but gives a lot more low down grunt and requires fewer gear changes and will cruise up hills that before required ear splitting revs. However, before spending a lot of money it's worth checking out that your carb is in tip-top shape as a over lean or rich mixture can cause driveability issues. Worth checking your ignition out too - check advance and that everything's nice and dry. The other morning it was a bit foggy and my truck was a little truc(k)ulent first thing in the morning - a wipe out of the distributor cap was overdue.

In an 88 with a rag top you should be able to trundle up hills easily enough but a skimmed, ported head and a torquey cam are worth the investment if you drive it a lot.
 
dominicbeesley said:
it's worth checking out that your carb is in tip-top shape as a over lean or rich mixture can cause driveability issues. Worth checking your ignition out too - check advance and that everything's nice and dry
Click to expand...

Hmmm interesting it's just had a brand new weber, 100 miles before the tyre swap. Fitted the carb and it ran like a dream so didn't mess around with the settings. Possible it could be running lean and lacking power and the bigger tyres have highlighted the problem? It felt fine on the old tyres but theres no proper hills round here and was just pottering about. 350 miles laning in Wales could have highlighted the issue! Thanks I will have a look into this :)
 
If its new it should be ok though the webers do run a little leaner than some other carbs that shouldn't be a problem. It may be worth checking that the manifold gaskets are nipped up nicely, I've had problems with old nuts, studs and washers slackening off. In fact my manifold gasket is leaking now from the rear but it is an exhaust leak rather than an inlet leak.

D
 
If the compression test is Ok, I would look at the simple stuff first:

Clean fuel pick up in tank, fuel filters etc. Lift pump cold also be tired. Check handbrake isn't binding and drums also free to turn.
 
Yeah think il give all the basics a good check over and go from there. When you say fuel filters is there more than 1? I have an inline just before the carb assumed that was it?
 
Innzy said:
Yeah think il give all the basics a good check over and go from there. When you say fuel filters is there more than 1? I have an inline just before the carb assumed that was it?
Click to expand...

Yes just the inline but worth checking the gauze on the end of the in tank pick up pipe as well
 
Before you go spending bucks on stuff to fix something you know isnt really the problem, why not just put some standard wheels back on to check how it drives again? Local club or someone on here could lend out im sure?
 
ronsealdeath said:
Before you go spending bucks on stuff to fix something you know isnt really the problem, why not just put some standard wheels back on to check how it drives again? Local club or someone on here could lend out im sure?
Click to expand...

That's actually a really good idea if I can find somebody local, thanks:)
Took it for a spin yesterday checking speeds with gps and on the flat it won't pull over 55, hit a bit of a hill and it loses speed quickly so I think I do have an underlying issue though and the wheels are making it worse
 
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